Growth cones

Ramón y Cajal (1890) was the first to recognize that the expanded end of an axon, the growth cone, is the principal sensory organ of the neurone. Classically, the growth cone is described as an expanded region that is constantly active, changing shape, extending and withdrawing small filopodia and lamellipodia that apparently ‘explore’ the local environment for a suitable surface along which extension may occur. These processes are stabilized in one direction, determining the direction of future growth, and following consolidation of the growth cone, the exploratory behaviour recommences. This continuous cycle resembles the behaviour at the leading edge of migratory cells such as fibroblasts and neutrophils. The molecular basis of this behaviour is the transmission of signals external to the growth cone via cell surface receptors to the scaffolding of microtubules and neurofilaments within the axon. Growing neuroblasts have a cortex rich in actin associated with the plasma membrane, and a core of centrally located micro-tubules and sometimes neurofilaments. The assembly of these components, along with the synthesis of new membrane, occurs in segments distal to the cell body and behind the growth cone, though some assembly of microtubules may take place near the cell body.

The driving force of growth cone extension is uncertain. One possible mechanism is that tension applied to objects by the leading edge of the growth cone is mediated by actin, and that local accumulations of F-actin redirect the extension of microtubules. Under some culture conditions, growth cones can develop mechanical tension, pulling against other axons or the substratum to which they are attached. Possibly, tension in the growth cone acts as a messenger to mediate the assembly of cytoskeletal components. Adhesion to the substratum appears to be important for consolidation of the growth cone and elaboration of the cytoskeleton in that direction.

During development, the growing axons of neuroblasts navigate with precision over considerable distances, often pursuing complex courses to reach their targets. Eventually they make functional contact with their appropriate end organs (neuromuscular endings, secreto-motor terminals, sensory corpuscles or synapses with other neurones). During the outgrowth of axonal processes the earliest nerve fibres are known to traverse appreciable distances over an apparently virgin landscape, often occupied by loose mesenchyme. A central problem for neurobiologists, therefore, has been understanding the mechanisms of axon guidance (Gordon-Weeks 2000). Axon guidance is thought to involve short-range, local guidance cues and long-range diffusible cues, any of which can be either attractive and permissive for growth, or repellent and hence inhibitory. Short-range cues require factors which are displayed on cell surfaces or in the extracellular matrix, e.g. axon extension requires a permissive, physical substrate, the molecules of which are actively recognized by the growth cone. They also require negative cues which inhibit the progress of the growth cone. Long-range cues come from gradients of specific factors diffusing from distant targets, which cause neurones to turn their axons towards the source of the attractive signal. The evidence for this has come from in vitro co-culture studies. The floor plate of the developing spinal cord exerts a chemotropic effect on commissural axons that later cross it, whereas there is chemorepulsion of developing motor axons from the floor plate. These forces are thought to act in vivo in concert in a dynamic process to ensure the correct passage of axons to their final destinations and to mediate their correct bundling together en route.

Dendritic tree

Once growth cones have arrived in their general target area, they then have to form terminals and synapses. In recent years, much emphasis has been placed on the idea that patterns of connectivity depend on the death of inappropriate cells. Programmed cell death or apoptosis occurs during the period of synaptogenesis if neurones fail to acquire sufficient amounts of specific neurotrophic factors. Coincident firing of neighbouring neurones that have found the appropriate target region might be involved in eliciting release of factor(s), thus reinforcing correct connections. Such mechanisms may explain the numerical correspondence between neurones in a motor pool and the muscle fibres innervated. On a subtler level, pruning of collaterals may give rise to mature neuronal architecture. The projections of pyramidal neurones from the motor and visual cortex, for example, start out with similar architecture: the mature repertoire of targets is produced by the pruning of collaterals leading to loss of projections to some targets.

The final growth of dendritic trees is also influenced by patterns of afferent connections and their activity. If deprived of afferents experimentally, dendrites fail to develop fully and, after a critical period, may become permanently affected even if functional inputs are restored, e.g. in the visual systems of young animals which have been visually deprived. This is analogous to the results of untreated amblyopia in infants. Metabolic factors also affect the final branching patterns of dendrites, e.g. thyroid deficiency in perinatal rats results in a small size and restricted branching of cortical neurones. This may be analogous to the mental retardation of cretinism.

Once established, dendritic trees appear remarkably stable and partial deafferentation affects only dendritic spines or similar small details. As development proceeds plasticity is lost, and soon after birth a neuron is a stable structure with a reduced rate of growth.

Neurotrophins

If neurones lose all afferent connections or are totally deprived of sensory input, there is atrophy of much of the dendritic tree and even the whole soma. Different regions of the nervous system vary quantitatively in their response to such anterograde transneuronal degeneration. Similar effects occur in retrograde transneuronal degeneration. Thus neurones are dependent on peripheral structures for their survival. Loss of muscles or sensory nerve endings, e.g. in the developing limb, will result in reduction in numbers of motor and sensory neurones. Specific factors which these target organs produce, such as the neurotrophins, are taken into nerve endings and transported back to the neuronal somata: they are necessary for the survival of many types of neurone during early development, and for the growth of their axons and dendrites, and also promote the synthesis of neurotransmitters and enzymes.

The neurotrophins, nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3) and NT-4/5 exert their survival effects selectively on particular subsets of neurones. NGF is specific to sensory ganglion cells from the neural crest, sympathetic postganglionic neurones and basal forebrain cholinergic neurones. BDNF promotes the survival of retinal ganglion cells, motor neurones, and some placode-derived neurones, such as those of the nodose ganglion, that are unresponsive to NGF. NT-3 has effects on motor neurones, limb proprioceptive neurones and both placode and neural crest-derived sensory neurones. Other growth factors found to influence the growth and survival of neural cells include the fibroblast growth factors (FGFs) and ciliary neurotrophic factor (CNTF), all of which are unrelated in sequence to the NGF family. Members of the FGF family support the survival of embryonic neurones from many regions of the CNS. CNTF may control the proliferation and differentiation of sympathetic ganglion cells and astrocytes.

Each of the neurotrophins binds specifically to certain receptors on the cell surface. The receptor termed p75^NTR binds all the neurotrophins with similar affinity. By contrast, members of the family of receptor tyrosine kinases (Trks) bind with higher affinity and display binding preferences for particular neurotrophins. However, the presence of a Trk receptor seems to be required for p75^NTR function.

Nervous tissue influences the metabolism of its target tissues. If during development a nerve fails to connect with its muscle, both degenerate. If the innervation of slow (red) or fast (white) skeletal muscle is exchanged, the muscles change structure and properties to reflect the new innervation, indicating that the nerve determines muscle type and not vice versa.

Segmentation in the neural tube

The early neural tube is visibly divided into segments, termed neuromeres, by shallow transverse folds which extend perpendicular to its long axis. Primary neuromeres can be identified at stage 9, and 16 secondary neuromeres are present at stage 14. They are especially noted in the rhombencephalon, where they are termed rhombomeres; they have now been shown to constitute crucial units of pattern formation. Domains of expression of developmental control genes abut rhombomere boundaries; single cell labelling experiments have revealed that cells within rhombomeres form segregated non-mixing populations (Fig. 24.16). The neural crest also shows intrinsic segmentation in the rhombencephalon, and is segregated into streams at its point of origin in the dorsal neural tube. This may represent a mechanism whereby morphogenetic specification of the premigratory neural crest cells is conveyed to the pharyngeal arches (see Fig. 12.4). Although these segmental units lose their morphological prominence with subsequent development, they represent the fundamental ground plan of this part of the neuraxis, creating a series of semi-autonomous units within which local variations in patterning can then develop. The consequences of early segmentation for events later in development, such as the formation of definitive neuronal nuclei within the brain stem, and of peripheral axonal projections remain to be explored.

Fig. 24.16 Hox gene expression domains in the branchiorhombomeric area in the mouse embryo at E9.5. The arrows indicate neural crest cells migrating from the rhombencephalon and midbrain. At the former level they are shaded to indicate the Hox genes they express. The same combination of Hox genes is expressed in the rhombomeres and in the superficial ectoderm of the pharyngeal arches at the corresponding rostrocaudal levels. The four Hox clusters are represented below.

(Modified by permission from the Annual Review of Cell and Developmental Biology, Volume 8, 1992 by Annual Reviews www.annualreviews.org.)

Other brain regions are not segmented in quite the same way as the hindbrain. However, morphological boundaries, domains of cell lineage restriction and of cell mixing, and regions of gene expression that abut sharp boundaries, are found in the diencephalon and telencephalon. It is thus likely that compartmentation of cell groups with some, if not all, the features of rhombomeres plays an important role in the formation of various brain regions.

The significance of intrinsic segmentation in the hindbrain is underlined by the absence of overt segmentation of the adjacent paraxial mesenchyme. There is no firm evidence for intrinsic segmentation in the spinal cord. Instead, segmentation of the neural crest, motor axons, and thus eventually the spinal nerves, is dependent on the segmentation of the neighbouring somites. Both neural crest cell migration and motor axon outgrowth occur through only the rostral and not the caudal sclerotome of each somite, so that dorsal root ganglia form only at intervals. The caudal sclerotome possesses inhibitory properties that deter neural crest cells and motor axons from entering. This illustrates the general principle that the nervous system is closely interlocked, in terms of morphogenesis, with the ‘periphery’, i.e. surrounding non-nervous structures, and each is dependent upon the other for its effective structural and functional maturation.

Genes such as the Hox and Pax gene families, which encode transcription factor proteins, show intriguing expression patterns within the nervous system. Genes of the Hox-b cluster, for example, are expressed throughout the caudal neural tube, and up to discrete limits in the hindbrain that coincide with rhombomere boundaries. The ordering of these genes within a cluster on the chromosome (5′-3′) is the same as the caudal to rostral limits of expression of consecutive genes. This characteristic pattern is surprisingly similar in fish, frogs, birds and mammals. Hox genes play a role in patterning not only of the neural tube but also of much of the head region, consistent with their expression in neural crest cells, and within the pharyngeal arches. Disruption of Hox a-3 gene in mice mimics DiGeorge’s syndrome, a congenital human disorder characterized by the absence (or near absence) of the thymus, parathyroid and thyroid glands, by the hypotrophy of the walls of the arteries derived from the aortic arches, and by subsequent conotruncal cardiac malformations. Some Pax genes are expressed in different dorsoventral domains within the neural tube. Pax-3 is expressed in the alar lamina, including the neural crest, while Pax-6 is expressed in the intermediate plate. The Pax-3 gene has the same chromosomal localization as the mouse mutation Splotch and the affected locus in the human Waardenburg’s syndrome, both of which are characterized by neural crest disturbances with pigmentation disorders and occasional neural tube defects. Both Hox and Pax genes have restricted expression patterns with respect to the rostrocaudal and the dorsoventral axes of the neural tube, consistent with roles in positional specification. (For reviews of the expression patterns of these genes see Krumlauf et al 1993.)

While craniocaudal positional values are probably conferred on the neuroepithelium at the neural plate or early neural tube stage, dorsoventral positional values may become fixed later. The development of the dorsoventral axis is heavily influenced by the presence of the underlying notochord. The notochord induces the ventral midline of the neural tube, the floor plate. This specialized region consists of a strip of non-neural cells with distinctive adhesive and functional properties. Notochord and floor plate together participate in inducing the differentiation of the motor columns. Motor neurone differentiation occurs early, giving some grounds for the idea of a ventral to dorsal wave of differentiation. The notochord/floor plate complex may also be responsible for allotting the values of more dorsal cell types within the tube (Fig. 24.7). For example, the dorsal domain of expression of Pax-3 extends more ventrally in embryos experimentally deprived of notochord and floor plate, while grafting an extra notochord adjacent to the dorsal neural tube leads to a repression of Pax-3 expression.

Spinal nerves

Each spinal nerve is connected to the spinal cord by a ventral root and a dorsal root (Fig. 24.17). The fibres of the ventral roots grow out from cell bodies in the anterior and lateral parts of the intermediate zone. These pass through the overlying marginal zone and external limiting membrane. Some enter the myotomes of the somites, and some penetrate the somites, reaching the adjacent somatopleure, and in both sites they ultimately form the α-, β- and γ-efferents. At appropriate levels these are accompanied by the outgrowing axons of preganglionic sympathetic neuroblasts (segments T1–L2), or preganglionic parasympathetic neuroblasts (S2–S4).

Fig. 24.17 The developing spinal cord of human embryos: A, approximately 6 weeks; B, approximately 3 months. Transverse sections.

The fibres of the dorsal roots extend from cell somata in dorsal root ganglia (DRG) into the spinal cord and also extend into the periphery. Neural crest cells are produced continuously along the length of the spinal cord, but gangliogenic cells migrate only into the rostral part of each somitic sclerotome where they condense and proliferate to form a bilateral series of oval-shaped primordial spinal ganglia (dorsal root ganglia) (Fig. 24.11). Negative factors in the caudal sclerotome deter neural crest from entering. The rostral sclerotome has a mitogenic effect on the crest cells that settle within it. From the ventral region of each ganglion a small part separates to form sympathochromaffin cells, while the remainder becomes a definitive spinal ganglion (dorsal root ganglion). The spinal ganglia are arranged symmetrically at the sides of the neural tube and, except in the caudal region, are equal in number to the somites. The cells of the ganglia, like the cells of the intermediate zone of the early neural tube, are glial and neuronal precursors. The glial precursors develop into satellite cells (which become closely applied to the ganglionic nerve cell somata), Schwann cells, and possibly other cells. The neuroblasts, at first round or oval, soon become fusiform, and their extremities gradually elongate into central and peripheral processes. The central processes grow into the neural tube as the fibres of dorsal nerve roots, while the peripheral processes grow ventrolaterally to mingle with the fibres of the ventral root, thus forming a mixed spinal nerve. As development proceeds the original bipolar form of the cells in the spinal ganglia changes and the two processes become approximated until they ultimately arise from a single stem to form a unipolar cell. The bipolar form is retained in the ganglion of the vestibulocochlear nerve.

Cranial nerves

Cranial nerves may contain motor, sensory or both types of fibres. With the exception of the olfactory and optic nerves, the cranial nerves develop in a manner similar in some respects to components of the spinal nerves. The somata of motor neuroblasts originate within the neuroepithelium, while those of sensory neuroblasts are derived from the neural crest with the addition in the head of contributions from ectodermal placodes (Fig. 24.18; see Fig. 12.4).

Fig. 24.18 The brain and cranial nerves of a human embryo, 10.2 mm long. Note the derivation of the ganglia associated with the trigeminal, facial, vestibulocochlear, glossopharyngeal, vagus and spinal accessory nerves.

The motor fibres of the cranial nerves which project to striated muscle are the axons of cells originating in the basal plate of the midbrain and hindbrain. The functional and morphological distinction between the neurones within these various nerves is based on the types of muscle innervated. In the trunk, the motor roots of the spinal nerves all emerge from the spinal cord close to the ventral midline to supply the muscles derived from the somites.

In the head the motor outflow is traditionally segregated into two pathways (Figs 24.3B and 24.18). General somatic efferent neurones exit ventrally in a similar manner to those of the spinal cord. Thus the oculomotor, trochlear, abducens and hypoglossal nerves parallel the organization of the somatic motor neurones in the spinal cord. The second motor component, special branchial efferent, consists of the motor parts of the trigeminal, facial, glossopharyngeal and vagus nerves which supply the pharyngeal (branchial) arches, and the accessory nerve. These nerves all have nerve exit points more dorsally placed than the somatic motor system.

The cranial nerves also contain general visceral efferent neurones (parasympathetic preganglionic) which travel in the oculomotor, facial, glossopharyngeal and vagus nerves and leave the hindbrain via the same exit points as the special branchial efferent fibres. All three categories of motor neurones probably originate from the same region of the basal plate, adjacent to the floor plate. The definitive arrangement of nuclei reflects the differential migration of neuronal somata. It is not known whether all these cell types share a common precursor within the rhombencephalon, however, in the spinal cord somatic motor and preganglionic autonomic neurones are linearly related.

These motor neurone types have been designated according to the types of muscles or structures they innervate. General somatic efferent nerves supply striated muscle derived from the cranial (occipital) somites and prechordal mesenchyme. Myogenic cells from the ventrolateral edge of the epithelial plate of occipital somites give rise to the intrinsic muscles of the tongue, while the prechordal mesenchyme gives rise to the extrinsic ocular muscles. Special branchial efferent nerves supply the striated muscles developing within the pharyngeal (branchial) arches (see Fig. 12.4) which are derived from parachordal mesenchyme between the occipital somites and the prechordal mesenchyme. All the voluntary muscles of the head originate from axial (prechordal) or paraxial mesenchyme which renders the distinction between somatic efferent supply and branchial efferent supply somewhat artificial. However, the obviously special nature of the arch musculature, its patterning by the neural crest cells, its particularly rich innervation for both voluntary and reflex activity, and the different origins from the basal plate of the branchial efferent nerves compared to the somatic efferent nerves, make the retention of a distinction between the two of some value.

General visceral efferent neurones (parasympathetic preganglionic) innervate glands of the head, the sphincter pupillae and ciliary muscles, and the thoracic and abdominal viscera.

The cranial sensory ganglia are derived in part from the neural crest, and in part from cells of the ectodermal placodes (Figs 24.12 and 24.18). Generally, neurones distal to the brain are derived from placodes while proximal ones are derived from the neural crest (Fig. 24.18). Supporting cells of all sensory ganglia arise from the neural crest. The most rostral sensory ganglion, the trigeminal, contains both neural crest and placode-derived neurones that mediate general somatic afferent functions. In the case of more caudal cranial nerves (the facial, glossopharyngeal and vagus), the same applies, but the two cell populations form separate ganglia in the case of each nerve. The proximal series of ganglia is neural crest derived (forming the proximal ganglion of the facial nerve, the superior ganglion of the glossopharyngeal nerve and the jugular ganglion of the vagus) while the distal series is derived from placodal cells (forming the geniculate ganglion of the facial nerve, the petrosal ganglion of the glossopharyngeal nerve and the nodose ganglion of the vagus). These ganglia contain neurones that mediate special, general visceral and somatic afferent functions. The vestibular ganglion contains both crest and placodal cells and the acoustic ganglion contains only placodal neurones: the axons from these cells are special somatic afferents and they all travel in the vestibulocochlear nerve.

The neurones and supporting cells of the cranial autonomic ganglia in the head and the trunk originate from neural crest cells. Caudal to the vagal ganglion, the occipital region of the neural crest is concerned with the ‘ganglia’ of the accessory and hypoglossal nerves. Rudimentary ganglion cells may occur along the hypoglossal nerve in the human embryo, but they subsequently regress. Ganglion cells are found on the developing spinal root of the accessory nerve and are believed to persist in the adult. The central processes of the cells of these various ganglia, where they persist, enter the alar lamina of the hindbrain. Their peripheral processes join the efferent components of the nerve to be distributed to the various tissues innervated. Some incoming fibres from the facial, glossopharyngeal and vagus nerves collect to form an oval bundle, the tractus solitarius, on the lateral aspect of the myelencephalon. This bundle is the homologue of the oval bundle of the spinal cord, but in the hindbrain it becomes more deeply placed by the overgrowth, folding and subsequent fusion of tissue derived from the rhombic lip on the external aspect of the bundle.

Parasympathetic ganglia

Neural crest cells migrate from the region of the mesencephalon and rhombencephalon prior to neural tube closure. From rostral to caudal, three populations of neural crest have been noted: cranial neural crest, cardiac neural crest and vagal neural crest. The migration of the sacral neural crest and the formation of the caudal parasympathetic ganglia have attracted little research interest.

Neural crest cells from the caudal third of the mesencephalon and the rostral metencephalon migrate along or close to the ophthalmic branch of the trigeminal nerve and give rise to the ciliary ganglion. Cells migrating from the nucleus of the oculomotor nerve may also contribute to the ganglion; a few scattered cells are always demonstrable in postnatal life along the course of this nerve. Preotic myelencephalic neural crest cells give rise to the pterygopalatine ganglion, which may also receive contributions from the ganglia of the trigeminal and facial nerves. The otic and submandibular ganglia are also derived from myelencephalic neural crest and may receive contributions from the glossopharyngeal and facial cranial nerves respectively (see Fig. 12.4).

Neural crest from the region located between the otic placode and the caudal limit of somite 3 has been termed cardiac neural crest. Cells derived from these levels migrate through pharyngeal arches 3, 4, and 6 where they provide, inter alia, support for the embryonic aortic arch arteries, cells of the aorticopulmonary septum and truncus arteriosus. Some of these neural crest cells also differentiate into the neural anlage of the parasympathetic ganglia of the heart. Sensory innervation of the heart is from the inferior ganglion of the vagus, which is derived from the nodose placodes. Neural crest cells migrating from the level of somites 1–7 are collectively termed vagal neural crest; they migrate to the gut along with sacral neural crest.

Sympathetic ganglia

Neural crest cells migrate ventrally within the body segments, penetrate the underlying somites and continue to the region of the future paravertebral and prevertebral plexuses, where they form the sympathetic chain of ganglia and the major ganglia around the ventral visceral branches of the abdominal aorta (Figs 24.11 and 24.19). Neural crest cells are induced to differentiate into sympathetic neurones by the dorsal aorta through the actions of the signaling molecules, Bmp-4 and Bmp-7.

There is cell specific recognition of postganglionic neurones and the growth cones of sympathetic preganglionic neurones. They meet during their growth, and this may be important in guidance to their appropriate target. The position of postganglionic neurones, and the exit point from the spinal cord of preganglionic neurones, may influence the types of synaptic connections made, and the affinity for particular postganglionic neurones. When a postganglionic neuroblast is in place it extends axons and dendrites and synaptogenesis occurs. The earliest axonal outgrowths from the superior cervical ganglion occur at about stage 14: although the axon is the first cell process to appear, the position of the neurones does not apparently influence the appearance of the cell processes.

The local environment plays a major role in controlling the appropriate differentiation of the presumptive autonomic ganglion neurones. The identity of the factors responsible for subsequent adrenergic, cholinergic or peptidergic phenotype has yet to be elucidated: it has been proposed that fibronectin and basal lamina components initiate adrenergic phenotypic expression at the expense of melanocyte numbers. Cholinergic characteristics are acquired relatively early and the appropriate phenotypic expression may be promoted by cholinergic differentiation factor and ciliary neurotrophic factor.

Neuropeptides are expressed by autonomic neurones in vitro and may be stimulated by various target tissue factors in sympathetic and parasympathetic neurones. Some neuropeptides are expressed more intensely during early stages of ganglion formation.

Enteric nervous system

The enteric nervous system is different from the other components of the autonomic nervous system because it can mediate reflex activity independently of control by the brain and spinal cord. The number of enteric neurones that develop is believed to be of the same magnitude as the number of neurones in the spinal cord, whereas the number of preganglionic fibres that supply the intestine, and therefore modulate the enteric neurones, is much fewer.

The enteric nervous system is derived from the neural crest. The axial levels of crest origin are shown in Fig. 24.19. Premigratory neural crest cells are not prepatterned for specific axial levels, rather they attain their axial value as they leave the neuraxis. Once within the gut wall there is a regionally specific pattern of enteric ganglia formation which may be controlled by the local splanchnopleuric mesenchyme. Cranial neural crest from somite levels 1–7 contributes to the enteric nervous system, forming both neuroblasts and glial support cells.

The most caudal derivatives of neural crest cells, from the lumbosacral region, somites 28 onwards, form components of the pelvic plexus after migrating through the somites towards the level of the colon, rectum and cloaca. Initially the cells come to lie within the developing mesentery, then transiently between the layers of the differentiating muscularis externa, before finally forming a more substantial intramural plexus characteristic of the adult enteric nervous system.

Of the neural crest cells that colonize the bowel, some in the foregut may acquire the ability to migrate outwards and colonize the developing pancreas.

Hirschsprung’s disease appears to result from a failure of neural crest cells to colonize the gut wall appropriately. The condition is characterized by a dilated segment of colon proximally and lack of peristalsis in the segment distal to the dilatation. Infants with Hirschsprung’s disease show delay in the passage of meconium, constipation, vomiting and abdominal distension. In humans, Hirschsprung’s disease is often seen associated with other defects of neural crest development, e.g. Waardenburg type II syndrome, which includes deafness and facial clefts with megacolon.

Anterior (ventral) grey column

The cells of the ventricular zone are closely packed at this stage and arranged in radial columns (Fig. 24.6). Their disposition may be determined in part by contact guidance along the earliest radial array of glial fibres which cross the full thickness of the early neuroepithelium. The cells of the intermediate zone are more loosely packed. They increase in number initially in the region of the basal plate. This enlargement outlines the anterior (ventral) column of the grey matter and causes a ventral projection on each side of the median plane: the floor plate remains at the bottom of the shallow groove so produced. As growth proceeds these enlargements, which are further increased by the development of the anterior funiculi (tracts of axons passing to and from the brain), encroach on the groove until it becomes converted into the slit-like anterior median fissure of the adult spinal cord (Fig. 24.17). The axons of some of the neuroblasts in the anterior grey column cross the marginal zone and emerge as bundles of ventral spinal nerve rootlets on the anterolateral aspect of the spinal cord. These constitute, eventually, both the α-efferents which establish motor end plates on extrafusal striated muscle fibres and the γ-efferents which innervate the contractile polar regions of the intrafusal muscle fibres of the muscle spindles.

Lateral grey column

In the thoracic and upper lumbar regions some intermediate zone neuroblasts in the dorsal part of the basal plate outline a lateral column. Their axons join the emerging ventral nerve roots and pass as preganglionic fibres to the ganglia of the sympathetic trunk or related ganglia, the majority eventually myelinating to form white rami communicantes. The axons within the rami synapse on the autonomic ganglionic neurones, and axons of some of the latter pass as postganglionic fibres to innervate smooth muscle cells, adipose tissue or glandular cells. Other preganglionic sympathetic efferent axons pass to the cells of the suprarenal medulla. An autonomic lateral column is also laid down in the midsacral region. It gives origin to the preganglionic parasympathetic fibres which run in the pelvic splanchnic nerves.

The anterior region of each basal plate initially forms a continuous column of cells throughout the length of the developing cord. This soon develops into two columns (on each side): one is medially placed and concerned with innervation of axial musculature, and the other is laterally placed and innervates the limbs. At limb levels the lateral column enlarges enormously, but regresses at other levels.

Axons arising from ventral horn neurones, i.e. α-, β- and γ-efferent fibres, are accompanied at thoracic, upper lumbar and midsacral levels by preganglionic autonomic efferents from neuroblasts of the developing lateral horn. Numerous interneurones develop in these sites (including Renshaw cells): it is uncertain how many of these differentiate directly from ventrolateral lamina (basal plate) neuroblasts and how many migrate to their final positions from the dorsolateral lamina (alar plate).

In the human embryo, the definitive grouping of the ventral column cells, which characterizes the mature cord, occurs early, and by the 14th week (80 mm) all the major groups can be recognized. As the anterior and lateral grey columns assume their final form the germinal cells in the ventral part of the ventricular zone gradually stop dividing. The layer becomes reduced in thickness until ultimately it forms the single-layered ependyma which lines the ventral part of the central canal of the spinal cord.

Posterior (dorsal) grey column

The posterior (dorsal) column develops later; consequently the ventricular zone is for a time much thicker in the dorsolateral lamina (alar plate) than it is in the ventrolateral lamina (basal plate) (Fig. 24.6).

While the columns of grey matter are being defined, the dorsal region of the central canal becomes narrow and slit-like, and its walls come into apposition and fuse with each other (Fig. 24.17). In this way the central canal becomes relatively reduced in size and somewhat triangular in outline.

About the end of the fourth week advancing axonal sprouts invade the marginal zone. The first to develop are those destined to become short intersegmental fibres from the neuroblasts in the intermediate zone, and fibres of dorsal roots of spinal nerves which pass into the spinal cord from neuroblasts of the early spinal ganglia. The earlier dorsal root fibres that invade the dorsal marginal zone arise from small dorsal root ganglionic neuroblasts. By the sixth week they form a well-defined oval bundle near the peripheral part of the dorsolateral lamina (Figs 24.6 and 24.7). This bundle increases in size and, spreading towards the median plane, forms the primitive posterior funiculus of fine calibre. Later, fibres derived from new populations of large dorsal root ganglionic neuroblasts join the dorsal root: they are destined to become fibres of much larger calibre. As the posterior funiculi increase in thickness, their medial surfaces come into contact separated only by the posterior medial septum, which is ependymal in origin and neuroglial in nature. It is thought that the displaced primitive posterior funiculus may form the basis of the dorsolateral tract or fasciculus (of Lissauer).

Maturation of the spinal cord

Long intersegmental fibres begin to appear at about the third month and corticospinal fibres are seen at about the fifth month. All nerve fibres at first lack myelin sheaths. Myelination starts in different groups at different times, e.g. the ventral and dorsal nerve roots about the fifth month, the corticospinal fibres after the ninth month. In peripheral nerves the myelin is formed by Schwann cells (derived from neural crest cells) and in the CNS by oligodendrocytes (which develop from the ventricular zone of the neural tube). Myelination persists until overall growth of the CNS and PNS has ceased. In many sites, slow growth continues for long periods, even into the postpubertal years.

The cervical and lumbar enlargements appear at the time of the development of their respective limb buds.

In early embryonic life, the spinal cord occupies the entire length of the vertebral canal and the spinal nerves pass at right angles to the cord. After the embryo has attained a length of 30 mm the vertebral column begins to grow more rapidly than the spinal cord and the caudal end of the cord gradually becomes more cranial in the vertebral canal. Most of this relative rostral migration occurs during the first half of intrauterine life. By the 25th week the terminal ventricle of the spinal cord has altered in level from the second coccygeal vertebra to the third lumbar, a distance of nine segments. As the change in level begins rostrally, the caudal end of the terminal ventricle, which is adherent to the overlying ectoderm, remains in situ, and the walls of the intermediate part of the ventricle and its covering pia mater become drawn out to form a delicate filament, the filum terminale. The separated portion of the terminal ventricle persists for a time, but it usually disappears before birth. It does, however, occasionally give rise to congenital cysts in the neighbourhood of the coccyx. In the definitive state, the upper cervical spinal nerves retain their position roughly at right angles to the cord. Proceeding caudally, the nerve roots lengthen and become progressively more oblique.

During gestation the relationship between the conus medullaris and the vertebral column changes, such that the conus medullaris gradually ascends to lie at higher vertebral levels. By 19 weeks of gestation the conus is adjacent to the fourth lumbar vertebra, and by full term (40 weeks) it is at the level of the second lumbar vertebra. By 2 months postnatally the conus medullaris has usually reached its permanent position at the level of the body of the first lumbar vertebra.

Rhombencephalon

By the time the midbrain flexure appears, the length of the rhombencephalon is greater than that of the combined extent of the mesencephalon and prosencephalon. Rostrally it exhibits a constriction, the isthmus rhombencephali (Fig. 24.3B), best viewed from the dorsal aspect. Ventrally the hindbrain is separated from the dorsal wall of the primitive pharynx only by the notochord, the two dorsal aortae and a small amount of mesenchyme; on each side it is closely related to the dorsal ends of the pharyngeal arches.

The pontine flexure appears to ‘stretch’ the thin, epithelial roof plate which becomes widened. The greatest increase in width corresponds to the region of maximum convexity, so that the outline of the roof plate becomes rhomboidal. By the same change the lateral walls become separated, particularly dorsally, and the cavity of the hindbrain, subsequently the fourth ventricle, becomes flattened and somewhat triangular in cross-section. The pontine flexure becomes increasingly acute until, at the end of the second month, the laminae of its cranial (metencephalic) and caudal (myelencephalic) slopes are opposed to each other (see Fig. 24.21) and, at the same time, the lateral angles of the cavity extend to form the lateral recesses of the fourth ventricle.

Fig. 24.21 A, The developing brainstem of a human fetus approximately 3 months, left dorsolateral aspect. B, The developing cerebellum in the fifth month, dorsal aspect.

At about four and a half weeks of development, when the pontine flexure is first discernible, the association between the rhombomeres and the underlying motor nuclei of certain cranial nerves can be seen. The general pattern of distribution of motor nuclei is as follows: rhombomere 1 contains the trochlear nucleus, rhombomeres 2 and 3 contain the trigeminal nucleus, rhombomeres 4 and 5 contain the facial nucleus, rhombomere 5 contains the abducens nucleus, rhombomeres 6 and 7 contain the glossopharyngeal nucleus, and rhombomeres 7 and 8 contain the vagal, accessory and hypoglossal nuclei. Rhombomeric segmentation represents the ground plan of development in this region of the brain stem and is pivotal for the development of regional identity (see Fig. 12.4). However, with further morphogenesis the obvious constrictions of the rhombomere boundaries disappear, and the medulla once again assumes a smooth contour. The differentiation of the lateral walls of the hindbrain into basal (ventrolateral) and alar (dorsolateral) plates has a similar significance to the corresponding differentiation in the lateral wall of the spinal cord, and ventricular, intermediate and marginal zones are formed in the same way.

Cells of the basal plate (ventrolateral lamina)

Cells of the basal plate form three elongated, discontinuous, columns that are positioned ventrally and dorsally with an intermediate column between (Fig. 24.20).

Fig. 24.20 Transverse section through the developing hindbrain of a human embryo 10.5 mm long, showing the relative positions of the columns of grey matter from which the nuclei associated with the different varieties of nerve components are derived. Postganglionic neurones are associated with the general visceral efferent column, bipolar neurones are associated with the otocyst and unipolar afferent neurones are associated with the other alar plate columns.

The most ventral column is continuous with the anterior grey column of the spinal cord and will supply muscles considered ‘myotomic’ in origin. It is represented in the caudal part of the hindbrain by the hypoglossal nucleus, and it reappears at a higher level as the nuclei of the abducens, trochlear and oculomotor nerves (somatic efferent nuclei). The intermediate column is represented in the upper part of the spinal cord and caudal brainstem (medulla oblongata and pons) and its neurones supply branchial (pharyngeal) and postbranchial musculature. It is discontinuous, forming the elongated nucleus ambiguus in the caudal brainstem, which gives fibres to the ninth, tenth and eleventh cranial nerves, and continues into the cervical spinal cord as the origin of the spinal accessory nerve. At higher levels, parts of this column give origin to the motor nuclei of the facial and trigeminal nerves. The nucleus ambiguus and the facial and trigeminal motor nuclei are termed branchial (special visceral) efferent nuclei. Neurones in the most dorsal column of the basal plate (represented in the spinal cord by the lateral grey column) innervate viscera. The column is discontinuous, its large caudal part forms some of the dorsal nucleus of the vagus and its cranial part forms the salivatory nucleus. These nuclei are termed general visceral (general splanchnic) efferent nuclei, and their neurones give rise to preganglionic, parasympathetic nerve fibres.

It should be noted that the neurones of the basal plate and their three columnar derivatives are only motor in the sense that some of their number form either motor neurones or preganglionic parasympathetic neurones. The remainder, which greatly outnumber the former, differentiate into functionally related interneurones and, in some loci, into neuroendocrine cells.

Metencephalon

The rostral slope of the embryonic hindbrain is the metencephalon, from which both the cerebellum and pons develop. Before formation of the pontine flexure, the dorsolateral laminae of the metencephalon are parallel with one another. After its formation the roof plate of the hindbrain becomes rhomboidal and the dorsal laminae of the metencephalon lie obliquely. They are close at the cranial end of the fourth ventricle, but widely separated at the level of its lateral angles (see Fig. 24.21). Accentuation of the flexure approximates the cranial angle of the ventricle to the caudal, and the alar plates of the metencephalon now lie almost horizontally.

The basal plate of the metencephalon becomes the pons. Ventricular, intermediate and marginal zones are formed in the usual way, and the nuclei of the trigeminal, abducens and facial nerves develop in the intermediate layer. It is possible that the grey matter of the formatio-reticularis is derived from the basal plate and that of the nuclei pontis from the alar plate by the active migration of cells from the rhombic lip. However, about the fourth month the pons is invaded by corticopontine, corticobulbar and corticospinal fibres, becomes proportionately thicker, and takes on its adult appearance: it is relatively smaller in the full-term neonate.

The region of the isthmus rhombencephali undergoes a series of changes that are notoriously difficult to interpret, but which result in the incorporation of the greater part of the region into the caudal end of the midbrain. Only the roof plate, in which the superior medullary velum is formed, and the dorsal part of the alar plate, which becomes invaded by converging fibres of the superior cerebellar peduncles, remain as recognizable derivatives in the adult. Early in development, the decussation of the trochlear nerves is caudal to the isthmus, but as growth changes occur it is displaced rostrally until it reaches its adult position.

Fourth ventricle and choroid plexus

Caudal to the developing cerebellum the roof of the fourth ventricle remains epithelial, and covers an approximately triangular zone from the lateral angles of the rhomboid fossa to the median obex (see Fig. 24.21). Nervous tissue fails to develop over this region and vascular pia mater is closely applied to the subjacent ependyma. At each lateral angle and in the midline caudally the membranes break through forming the lateral (Luschka) and median (Magendie) apertures of the roof of the fourth ventricle. These become the principal routes by which cerebrospinal fluid, produced in the ventricles, escapes into the subarachnoid space. The vascular pia mater (tela choroidea), in an inverted V formation cranial to the apertures, invaginates the ependyma to form vascular fringes which become the vertical and horizontal parts of the choroid plexuses of the fourth ventricle.

Cerebellum

The cerebellum develops from the rhombic lip, the dorsal part of the alar plate of the metencephalon, which constitutes the rostral margin of the diamond-shaped fourth ventricle. Two rounded swellings develop which at first project partly into the ventricle (Fig. 24.21), forming the rudimentary cerebellar hemispheres. The most rostral part of the roof of the metencephalon originally separates the two swellings, but it becomes invaded by cells derived from the alar plate, which form the rudiments of the vermis. At a later stage, extroversion of the cerebellum occurs, its intraventricular projection is reduced and the dorsal extraventricular prominence increases. The cerebellum now consists of a bilobar (dumb-bell shaped) swelling stretched across the rostral part of the fourth ventricle (Fig. 24.21). It is continuous rostrally with the superior medullary velum, formed from the isthmus rhombencephali, and caudally with the epithelial roof of the myelencephalon. With growth, a number of transverse grooves appear on the dorsal aspects of the cerebellar rudiment: these are the precursors of the numerous fissures which characterize the surface of the mature cerebellum (Fig. 24.22).

Fig. 24.22 Median sagittal sections through the developing cerebellum, at four chronologically later stages.

The first fissure to appear on the cerebellar surface (Fig. 24.22) is the lateral part of the posterolateral fissure which forms the border of a caudal region corresponding to the flocculi of the adult. The right and left parts of this fissure subsequently meet in the midline, where they form the boundary between the most caudal vermian lobule, the nodule, and the rest of the vermis. The flocculonodular lobe can now be recognized as the most caudal cerebellar subdivision at this stage and it serves as the attachment of the epithelial roof of the fourth ventricle. Because of the expansion of the other divisions of the cerebellum, the flocculonodular lobe comes to occupy an anteroinferior position in adults. At the end of the third month a transverse sulcus appears on the rostral slope of the cerebellar rudiment and deepens to form the fissura prima. This cuts into the vermis and both hemispheres, and forms the border between the anterior and posterior lobes. Contemporaneously, two short transverse grooves appear in the caudal vermis. The first is the fissura secunda (postpyramidal fissure), which forms the rostral border of the uvula; the second, the prepyramidal fissure, demarcates the pyramid (Fig. 24.22). The cerebellum now grows dorsally, rostrally, caudally and laterally, and the hemispheres expand much more than the inferior vermis, which therefore becomes buried at the bottom of a deep hollow, the vallecula. Numerous other transverse grooves develop, the most extensive being the horizontal fissure.

Cellular development of the cerebellum

The cerebellum consists of a cortex beneath which are buried a series of deep nuclei. The organization of the cerebellar cortex is similar to that of the cerebral cortex, except that the latter has six layers, while the former has only three. However, whereas in the cerebral cortex neuroblasts originate from the ventricular zone and migrate ventriculofugally towards the pial surface (in an ‘inside-out’ fashion), early in cerebellar development a layer of cells derived exclusively from the metencephalic rhombic lip initially migrates ventriculofugally to form a layer beneath the glia limitans over the surface of the developing cerebellum. These cells form the external germinative layer and later in development their progeny will migrate ventriculopetally (in an ‘outside-in’ manner), into the cerebellum. Thus, the cerebellum has an intraventricular portion (cells proliferating from the ventricular zone) and an extraventricular portion (cells proliferating from the external germinative layer) during development. The extraventricular portion becomes larger at the expense of the intraventricular part, the so-called extroversion of the cerebellum. Before the end of the third month the main mass of the cerebellum is extraventricular.

The developed cerebellar cortex contains three layers, namely the molecular layer, the Purkinje layer, and the granular layer. The early bilateral expansion of the ventricular surface reflects the production, by the metencephalic alar plate ventricular epithelium, of neuroblasts which will give rise to the radial glia, cerebellar nuclei, and efferent neurones of the cerebellar cortex (the Purkinje cells) (Fig. 24.23). The radial glia play a role in guiding the Purkinje cells to the meningeal surface of the cerebellar anlage. During this early stage of cerebellar development, which is dominated by the production and migration of efferent cerebellar neurones, the surface of the cerebellar anlage remains smooth. The extroversion of the cerebellum begins later when cells of the external germinative layer, also termed the superficial matrix, begin proliferation and migration. These cells produce the granule cells, which migrate inward along the radial glia, through the layers of Purkinje cells, settling deep to them in the granular layer. This stage coincides with the emergences of the transverse folial pattern. Proliferation and migration of granule cells leads to a great rostrocaudal expansion of the meningeal surface of the cerebellum, forming the transverse fissures and transforming the multicellular layer of Purkinje cells into a monolayer. Purkinje cells and nuclear cells are formed prior to the granule cells, and granule cells serve as the recipient of the main afferent (mossy fibre) system of the cerebellum. Thus the development of the efferent neurones of the cerebellar cortex and nuclei precedes the development of its afferent organization.

Fig. 24.23 The histogenesis of the cerebellar cortex and the cerebellar nuclei. A, Purkinje cells and cells of the cerebellar nuclei are produced by the ventricular zone and migrate to their future positions. The cells of the superficial matrix (the external granular layer) take their origin from the ventricular epithelium at the caudal pole of the cerebellar anlage and migrate rostrally over its surface. B, After migration the Purkinje cells constitute a multicellular layer beneath the external germinative layer. Cell production in the ventricular epithelium has stopped. The remaining cells transform into ependymal cells. C, Granule cells produced by the external germinative layer migrate inwards through the Purkinje cell layer to their position in the internal granular layer. Purkinje cells spread into a monolayer. D, Adult position of cortical and nuclear neurones.

The early bilateral cerebellar anlage is changed into a unitary structure by fusion of the bilateral intraventricular bulges and the disappearance of the ependyma at this site, the merging of the left and right primitive cerebellar cortex over the midline, and the development of the cerebellar commissure by ingrowth of afferent fibres and outgrowth of efferent axons of the medial cerebellar nucleus.

When the external germinative layer is initially formed, the multicellular Purkinje cell layer beneath is not uniform, but subdivided into clusters which form rostrocaudally extending columns (Fig. 24.24). The medial Purkinje cell clusters develop into the future vermis. These Purkinje cells will grow axons which connect to neurones in the vestibular nuclei and the fastigial nucleus. The lateral clusters belong to the future hemispheres and will grow axons terminating in the interposed and dentate nuclei. The sharp border in the efferent projections from the vermis and hemispheres is thus established at an early age. These clusters will give rise to Purkinje cell zones in the adult cerebellum which project to a single vestibular or cerebellar nucleus.

Fig. 24.24 Coronal section through the cerebellum and the brain stem of 65 mm human fetus. The Purkinje cells are located in five multicellular clusters (stars) on both sides of the midline. The anlage of the dentate nucleus occupies the centre of the most lateral Purkinje cell cluster. Abbreviations: B, brain stem; D, dentate nucleus; EGL, external granular layer; m, midline; 4, fourth ventricle.

(By courtesy of the Schenk Collection, Dr Johan M Kros, Division of Neuropathology, Department of Pathology, Erasmus Medical Centre, Rotterdam, The Netherlands.)

In the developing human brain only the external germinative layer can be seen at 17–18 weeks; the Purkinje cells become apparent between 20–23 weeks. After 30 weeks four layers can be recognized, the extermal germinative layer (external granular layer) is formed by 6–8 rows of densely packed small round cells; the Purkinje cell layer is formed by 5–6 layers of larger, round immature neurones external to the internal granular layer; the molecular layer contains cells resembling external granular layer cells in migration (Lavezzi et al 2006). The external granular layer involutes between 5 and 7 months after birth: it is only a discontinuous layer at 10 months, and is totally absent by 12 months, after which time the cerebellar cortex shows a three-layered structure. From 5 to 7 months the Purkinje cells are reduced in number, more widely spaced and display mature polygonal somata with evident axon and dendrites.

Mesencephalon

The mesencephalon or midbrain is subdivided early in development into two neuromeres, mesencephalon 1 and mesencephalon 2. It persists for a time as a thin-walled tube enclosing a cavity of some size, separated from that of the prosencephalon by a slight constriction and from the rhombencephalon by the isthmus rhombencephali (Figs 24.2 and 24.25). Later, its cavity becomes relatively reduced in diameter, and in the adult brain it forms the cerebral aqueduct. The basal (ventrolateral) plate of the midbrain increases in thickness to form the cerebral peduncles, which are at first of small size, but enlarge rapidly after the fourth month, when their numerous fibre tracts begin to appear in the marginal zone. The neuroblasts of the basal plate of mesencephalon 2 give rise to the nuclei of the oculomotor nerve and some grey masses of the tegmentum, while the nucleus of the trochlear nerve remains in the region of the isthmus rhombencephali. The cells which give rise to the trigeminal mesencephalic nucleus arise either side of the dorsal midline, from the isthmus rhobencephali rostrally across the roof of the mesencephalon. Recent studies have shown that the progenitors of these cells do not express neural crest cell markers.

Fig. 24.25 The shape of the developing ventricular system and the expansion of the medial portion of the telencephalon. A, Human embryo, approximately 10.2 mm long, left lateral surface of the diencephalon and telencephalon removed. B, Human embryo 13.6 mm long, median section. C, Human fetus, approximately 3 months old, median section. The roof of the metencephalon and myelencephalon has been removed in each case.

The cells of the dorsal part of the alar (dorsolateral) plates proliferate and invade the roof plate, which therefore thickens and is later divided into corpora bigemina by a median groove. Caudally this groove becomes a median ridge, which persists in the adult as the frenulum veli. The corpora bigemina are later subdivided into the superior and inferior colliculi by a transverse furrow. The red nucleus, substantia nigra and reticular nuclei of the midbrain tegmentum may first be defined at the end of the third month. Their origins are probably mixed from neuroblasts of both basal and alar plates.

The detailed histogenesis of the tectum and its main derivatives, the colliculi, will not be followed here, but in general the principles outlined for the cerebellar cortex, the palaeopallium and neopallium also apply to this region. A high degree of geometric order exists in the developing retinotectal projection (the equivalent of the retinogeniculate projection), and in the tectospinal projection.

Prosencephalon

At an early stage, a transverse section through the forebrain shows the same parts as are displayed in similar sections of the spinal cord and medulla oblongata, i.e. thick lateral walls connected by thin floor and roof plates. Moreover, each lateral wall is divided into a dorsal area and a ventral area separated internally by the hypothalamic sulcus (Fig. 24.25). This sulcus ends rostrally at the medial end of the optic stalk. In the fully developed brain it persists as a slight groove extending from the interventricular foramen to the cerebral aqueduct. It is analogous to, if not the homologue of, the sulcus limitans. The thin roof plate remains epithelial, but invaginated by vascular mesenchyme, the tela choroidea of the choroid plexuses of the third ventricle. Later, the lateral margins of the tela undergo a similar invagination into the medial walls of the cerebral hemispheres. The floor plate thickens as the nuclear masses of the hypothalamus and subthalamus develop.

At a very early period, before the closure of the rostral neuropore, the subdivision of the prosencephalon into the most rostral telencephalon and two subdivisions of the diencephalon, D1 and D2, is heralded (Fig. 24.2). At this early time two eye fields are separated by the future neurohypophysis in the floor of the future D1. After head folding, the eye fields expand as two lateral optic evaginations which become optic vesicles, one on each side of the early brain. For a time they communicate with the cavity of the prosencephalon by relatively wide openings. The distal parts of the optic vesicles expand, while the proximal parts become the tubular optic stalks. The optic vesicles (which are described with the development of the eye in Ch. 41) are thus derived from the lateral walls of the D1 subdivision of the prosencephalon before the telencephalon can be clearly identified. The optic chiasma is often regarded as the boundary between diencephalon and telencephalon.

As the most rostral portion of the prosencephalon enlarges it curves ventrally, and two further diverticula expand rapidly from it, one on each side. These diverticula, which are rostrolateral to the optic stalks, subsequently form the cerebral hemispheres. Their cavities are the rudiments of the lateral ventricles and they communicate with the median part of the forebrain cavity by relatively wide openings which ultimately become the interventricular foramina. The rostral limit of the median part of the forebrain consists of a thin sheet, the lamina terminalis (Fig. 24.25A–C), which stretches from the interventricular foramina to the recess at the base of the optic stalks. The rostral part of the forebrain, including the rudiments of the cerebral hemispheres, consists of the telencephalon and the caudal part of the diencephalon: both contribute to the formation of the third ventricle, although the latter predominates. The fate of the lamina terminalis is described below.

Diencephalon

The diencephalon, D2, is broadly divided by the hypothalamic sulcus into dorsal (pars dorsalis diencephali) and ventral (pars ventralis diencephali) parts: each contributes to diverse neural structures. The dorsal part develops into the (dorsal) thalamus and metathalamus along the immediate suprasulcal area of its lateral wall, while the highest dorsocaudal lateral wall and roof form the epithalamus. The thalamus (Fig. 24.25A–C) is first visible as a thickening that involves the rostral part of the dorsal area. Caudal to the thalamus, the lateral and medial geniculate bodies, or metathalamus, are recognizable at first as surface depressions on the internal aspect and as elevations on the external aspect of the lateral wall. As the thalami enlarge as smooth ovoid masses, the wide interval between them gradually narrows into a vertically compressed cavity which forms the greater part of the third ventricle. After a time these medial surfaces may come into contact and become adherent over a variable area, the connection (single or multiple) constituting the interthalamic adhesion or massa intermedia. The caudal growth of the thalamus excludes the geniculate bodies from the lateral wall of the third ventricle.

At first the lateral aspect of the developing thalamus is separated from the medial aspect of the cerebral hemisphere by a cleft, but with growth the cleft becomes obliterated (Fig. 24.26) as the thalamus fuses with the part of the hemisphere in which the corpus striatum is developing. Later, with the development of the projection fibres (corticofugal and corticopetal) of the neocortex, the thalamus becomes related to the internal capsule, which intervenes between it and the lateral part of the corpus striatum (lentiform nucleus). Ventral to the hypothalamic sulcus, the lateral wall of the diencephalon, in addition to median derivatives of its floor plate, forms a large part of the hypothalamus and subthalamus.

Fig. 24.26 The development of the basal nuclei and internal capsule, coronal views

(Redrawn by permission from Hamilton WJ, Boyd JD, Mossman HW 1972 Human Embryology: Prenatal Development of Form and Function. Baltimore: Williams and Wilkins.)

The epithalamus, which includes the pineal gland, the posterior and habenular commissures and the trigonum habenulae, develops in association with the caudal part of the roof plate and the adjoining regions of the lateral walls of the diencephalon. At an early period (12–20 mm CR length), the epithalamus in the lateral wall projects into the third ventricle as a smooth ellipsoid mass, larger than the adjacent mass of the (dorsal) thalamus and separated from it by a well-defined epithalamic sulcus. In subsequent months, growth of the thalamus rapidly overtakes that of the epithalamus and the intervening sulcus is obliterated. Thus, structures of epithalamic origin are ultimately topographically relatively diminutive.

The pineal gland arises as a hollow outgrowth from the roof plate, immediately adjoining the mesencephalon. Its distal part becomes solid by cellular proliferation, but its proximal stalk remains hollow, containing the pineal recess of the third ventricle. In many reptiles the pineal outgrowth consists of a rostral outgrowth (parapineal organ) that develops into the pineal or parietal eye and a glandular caudal outgrowth: the caudal outgrowth is homologous with the pineal gland in man. The rostral outgrowth also develops in the human embryo but soon disappears entirely.

The nucleus habenulae, which is the most important constituent of the trigonum habenulae, develops in the lateral wall of the diencephalon and is at first in close relationship with the geniculate bodies, from which it becomes separated by the dorsal growth of the thalamus. The habenular commissure develops in the cranial wall of the pineal recess. The posterior commissure is formed by fibres which invade the caudal wall of the pineal recess from both sides.

The ventral part of the diencephalon forms the subsulcal lateral walls of the third ventricle and takes part in the formation of the hypothalamus, including the mammillary bodies, the tuber cinereum and infundibulum of the hypophysis. The mammillary bodies arise as a single thickening which becomes divided by a median furrow during the third month. The tuber cinereum develops rostral to the mammillary bodies as a cellular proliferation that extends forwards as far as the infundibulum. In front of the tuber cinereum, a wide-mouthed diverticulum forms in the floor of the diencephalon, grows towards the stomodeal roof and comes into contact with the posterior aspect of a dorsally directed ingrowth from the stomodeum (Rathke’s pouch). These two diverticula together form the hypophysis cerebri (Fig. 24.14). An extension of the third ventricle persists in the base of the neural outgrowth as the infundibular recess. The remaining caudolateral walls and floor of the ventral diencephalon are an extension of the midbrain tegmentum, the subthalamus. This forms the rostral limits of the red nucleus, substantia nigra, numerous reticular nuclei and a wealth of interweaving, ascending, descending and oblique nerve fibre bundles, which have many origins and destinations.

Third ventricle and choroid plexus

The roof plate of the diencephalon rostral to the pineal gland, and continuing over the median telencephalon, remains thin and epithelial in character and is invaginated by the choroid plexuses of the third ventricle (Fig. 24.27). Before the development of the corpus callosum and the fornix it lies at the bottom of the longitudinal fissure, between and reaching the two cerebral hemispheres. It extends as far rostrally as the interventricular foramina and lamina terminalis. Here, and elsewhere, choroid plexuses develop by the close apposition of vascular pia mater and ependyma without intervening nervous tissue. With development, the vascular layer is infolded into the ventricular cavity and develops a series of small villous projections, each covered by a cuboidal epithelium derived from the ependyma. The cuboidal cells display numerous microvilli on their ventricular surfaces and complex folding of their basal plasma membranes. The early choroid plexuses secrete a protein-rich cerebrospinal fluid into the ventricular system which may provide a nutritive medium for the developing epithelial neural tissues. As the latter become increasingly vascularized the histochemical reactions of the cuboidal cells and the character of the fluid change to the adult type. Many regions of the lining of the third ventricle become highly specialized, and develop concentrations of tanycytes or other modified cells that are collectively termed the circumventricular organs, e.g. the subfornical organ, the organum vasculosum (intercolumnar tubercle) of the lamina terminalis, the subcommissural organ and the linings of the pineal, suprapineal, and infundibular recesses.

Fig. 24.27 Coronal section of the left cerebral hemisphere in a 73 mm fetus.

(Redrawn by permission from Hamilton WJ, Boyd JD, Mossman HW 1972 Human Embryology: Prenatal Development of Form and Function. Baltimore: Williams and Wilkins.)

Telencephalon

The telencephalon consists of two lateral diverticula connected by a median region, the telencephalon impar. The rostral part of the third ventricle develops from the impar, and is closed below and in front by the lamina terminalis. The lateral diverticula are outpouchings of the lateral walls of the telencephalon, which may correspond to the alar lamina, although this is uncertain. Their cavities are the future lateral ventricles, and their walls are formed by the presumptive nervous tissue of the cerebral hemispheres. The roof plate of the median part of the telencephalon remains thin and is continuous behind with the roof plate of the diencephalon (Fig. 24.25). The rostral parts of the hypothalamus, which include the optic chiasma, optic recess and related nuclei, develop in the floor plate and lateral walls of the prosencephalon, ventral to the primitive interventricular foramina. The chiasma is formed by the meeting, and partial decussation, of the optic nerves in the ventral part of the lamina terminalis. The optic tracts subsequently grow backwards from the chiasma to end in the diencephalon and midbrain.

Cerebral hemispheres

The cerebral hemispheres arise as diverticula of the lateral walls of the telencephalon, with which they remain in continuity around the margins of initially relatively large interventricular foramina, except caudally, where they are continuous with the rostral part of the lateral wall of the diencephalon (Figs 24.2 and 24.25). As growth proceeds each hemisphere enlarges forwards, upwards and backwards and acquires an oval outline, medial and superolateral walls, and a floor. As a result the medial surfaces approach, but are separated by, a vascularized mesenchyme and pia mater which fills the median longitudinal fissure (Fig. 24.27). At this stage the floor of the fissure is the epithelial roof plate of the telencephalon, which is directly continuous caudally with the epithelial roof plate of the diencephalons.

At the early oval stage of hemispheric development, regions are named according to their future principal derivatives. The rostromedial and ventral floor becomes linked with the forming olfactory apparatus and is termed the primitive olfactory lobe. The floor (ventral wall, or base) of the remainder of each hemisphere forms the anlage of the primitive corpus striatum and amygdaloid complex, including its associated rim of lateral and medial walls (the striate part of the hemisphere). The rest of the hemisphere i.e. the medial, lateral, dorsal and caudal regions, is the suprastriate part of the hemisphere. Although it is the largest in terms of surface area, initially it possesses comparatively thin walls. The rostral end of each oval hemisphere becomes the definitive frontal pole. As the hemisphere expands, its original posterior pole moves relatively in a caudoventral and lateral direction, following a curve like a ram’s horn: it curves towards the orbit in association with the growth of the caudate nucleus and other structures to form the definitive temporal pole. A new posterior part persists as the definitive occipital pole of the mature brain (Fig. 24.28).

Fig. 24.28 Lateral views of developing brains show the formation of the basal nuclei and lateral ventricles as the telencephalon develops.

(Redrawn by permission from Hamilton WJ, Boyd JD, Mossman HW 1972 Human Embryology: Prenatal Development of Form and Function. Baltimore: Williams and Wilkins.)

The migration and differentiation of neural progenitors to form nuclei is either minimal or limited throughout the brain stem and spinal cord: the progeny of these progenitors remain immediately extra-ependymal or partially displaced towards the pial exterior, and are arrested deeply embedded in the myelinated fibre ‘white matter’ of the region. In marked contrast, proliferation and migration of neuroblasts in the cerebral hemispheres produces a superficial layer of grey matter in both the striate and suprastriate regions, but not in the central areas of the original medial wall (where secondary fusion with the diencephalon occurs). The superficial layer of grey matter consists of neuronal somata, dendrites, the terminations of incoming (afferent) axons, the stems of (or the whole of) efferent axons, and glial cells and endothelial cells. Successive generations of neuroblasts migrate through the layers of earlier generations to attain subpial positions (see below),which means that the surface of the cerebral hemispheres expands at a rate greater than that of the hemispheres as a whole: the great expansion of the cerebral hemispheres is characteristic of mammals and especially of man. Neuroblast differentiation produces a highly organized subpial surface coat of grey matter termed the cortex or pallium. The growing hemispheres subsequently overlap, successively, the diencephalon and the mesencephalon, and then meet the rostral surface of the cerebellum. The temporal lobes embrace the flanks of the brain stem.

The terminology used to describe regions of the cortex is based on evolutionary concepts. The oldest portions of cortex receive information concerned with olfaction; they are termed the archicortex (archipallium) and paleocortex (paleopallium), and both are subdivisions of an overall allocortex. The archicortex is the forerunner of the hippocampal lobe, and the paleocortex gives rise to the piriform area. The remaining cortical surface expands greatly in mammals forming the neocortex (young cortex) which displaces the earlier cortices so that they come to lie partially internally in each hemisphere.

Olfactory bulb

A longitudinal groove appears in the anteromedial part of the floor of each developing lateral ventricle about the fifth week of embryonic development. It deepens and forms a hollow diverticulum which is continuous with the hemisphere by a short stalk. The diverticulum becomes connected on its ventral or inferior surface to the olfactory placode. Placodal cells give rise to afferent axons which terminate in the walls of the diverticulum. As the head increases in size, the diverticulum grows forwards, loses its cavity and becomes converted into the solid olfactory bulb. The forward growth of the bulb is accompanied by elongation of its stalk, which forms the olfactory tract. The part of the floor of the hemisphere to which the tract is attached constitutes the piriform area.

Lateral ventricles and choroid plexus

The early diverticulum or anlage of the cerebral hemisphere initially contains a simple spheroidal lateral ventricle which is continuous with the third ventricle via the interventricular foramen. The rim of the foramen is the site of the original evagination. The expanding ventricle develops the ram’s horn shape of the surrounding hemisphere, becoming first roughly ellipsoid and then a curved cylinder which is convex dorsally (Fig. 24.28). The ends of the cylinder expand towards, but do not reach, the frontal and (temporary) occipital poles; differentiating and thickening neural tissues separate the ventricular cavities and pial surfaces at all points, except along the line of the choroidal fissure. Pronounced changes in ventricular form accompany the emergence of a temporal pole. The original caudal end of the curved cylinder expands within its substance and the temporal extensions in each hemisphere pass ventrolaterally to encircle both sides of the upper brain stem. Another extension may develop from the root of the temporal extension in the substance of the definitive occipital pole and pass caudomedially; it is quite variable in size, often asymmetrical on the two sides, and one or both may be absent. Although the lateral ventricle is a continuous system of cavities, specific parts are now given regional names. The central part (body) extends from the interventricular foramen to the level of the posterior edge (splenium) of the corpus callosum. Three cornua (horns) diverge from the body: anterior towards the frontal pole, posterior towards the occipital pole, and inferior towards the temporal pole.

At these early stages of hemispheric development the term pole is preferred, in most instances, to lobe. Lobes are defined by specific surface topographical features which will appear over several months, and differential growth patterns persist for a considerable period.

The pia mater which covers the epithelial roof of the third ventricle at this stage is itself covered with loosely arranged mesenchyme and developing blood vessels. These vessels subsequently invaginate the roof of the third ventricle on each side of the median plane to form its choroid plexuses. The lower part of the medial wall of the cerebral hemisphere, which immediately adjoins the epithelial roof of the interventricular foramen and the rostral extremity of the diencephalon, also remains epithelial. It consists of ependyma and pia mater; elsewhere the walls of the hemispheres are thickening to form the pallium. The thin part of the medial wall of the hemisphere is invaginated by vascular tissue which is continuous in front with the choroid plexus of the third ventricle and constitutes the choroid plexus of the lateral ventricle. This invagination occurs along a line which arches upwards and backwards, parallel with and initially limited to, the rostral and upper boundaries of the interventricular foramen. This curved indentation of the ventricular wall, where no nervous tissue develops between ependyma and pia mater, is termed the choroidal fissure (Figs 24.25C and 24.26). The subsequent assumption of the definitive form of the choroidal fissure depends on related growth patterns in neighbouring structures. Of particular importance are the relatively slow growth of the interventricular foramen, the secondary ‘fusion’ between the lateral diencephalon and medial hemisphere walls, the encompassing of the upper brain stem by the forward growth of the temporal lobe and its pole towards the apex of the orbit, and the massive expansion of two great cerebral commissures (the fornix and corpus callosum). The choroidal fissure is now clearly a caudal extension of the much reduced interventricular foramen, which arches above the thalamus and is here only a few millimetres from the median plane. Near the caudal end of the thalamus it diverges ventrolaterally, its curve reaching and continuing in the medial wall of the temporal lobe over much of its length (i.e. to the tip of the inferior horn of the lateral ventricle). The upper part of the arch will be overhung by the corpus callosum and, throughout its convexity, it is bordered by the fornix and its derivatives.

Basal nuclei

At first growth proceeds more actively in the floor and the adjoining part of the lateral wall of the developing hemisphere, and elevations formed by the rudimentary corpus striatum encroach on the cavity of the lateral ventricle (Figs 24.25 and 24.26). The head of the caudate nucleus appears as three successive parts, medial, lateral and intermediate, which produce elevations in the floor of the lateral ventricle. Caudally these merge to form the tail of the caudate nucleus and the amygdaloid complex, which both remain close to the temporal pole of the hemisphere. When the occipital pole grows backwards, and the general enlargement of the hemisphere carries the temporal pole downwards and forwards, the tail of the caudate is continued from the floor of the central part (body) of the ventricle into the roof of its temporal extension, the future inferior horn. The amygdaloid complex encapsulates its tip. Rostrally the head of the caudate nucleus extends forwards to the floor of the interventricular foramen, where it is separated from the developing rostral end of the thalamus by a groove; later, the head expands in the floor of the anterior horn of the lateral ventricle. The lentiform nucleus develops from two laminae of cells, medial and lateral, which are continuous with both the medial and lateral parts of the caudate nucleus. The internal capsule appears first in the medial lamina and extends laterally through the outer lamina to the cortex. It divides the laminae into two, the internal parts join the caudate nucleus and the external parts form the lentiform nucleus. In the latter, the remaining medial lamina cells give rise mainly to the globus pallidus and the lateral lamina cells to the putamen. The putamen subsequently expands concurrently with the intermediate part of the caudate nucleus.

Fusion of diencephalic and telencephalic walls

As the hemisphere enlarges, the caudal part of its medial surface overlaps and hides the lateral surface of the diencephalon (thalamic part), from which it is separated by a narrow cleft occupied by vascular connective tissue. At this stage (about the end of the second month) a transverse section made caudal to the interventricular foramen would pass from the third ventricular cavity successively through the developing thalamus, the narrow cleft just mentioned, the thin medial wall of the hemisphere, and the cavity of the lateral ventricle, with the corpus striatum in its floor and lateral wall (Fig. 24.26).

As the thalamus increases in extent it acquires a superior surface in addition to medial and lateral surfaces. The lateral part of its superior surface fuses with the thin medial wall of the hemisphere so that this part of the thalamus is finally covered with the ependyma of the lateral ventricle immediately ventral to the choroidal fissure. As a result the corpus striatum is approximated to the thalamus and is separated from it only by a deep groove which becomes obliterated by increased growth along the line of contact. The lateral aspect of the thalamus is now in continuity with the medial aspect of the corpus striatum so that a secondary union between the diencephalon and the telencephalon is affected over a wide area, providing a route for the subsequent passage of projection fibres to and from the cortex (Fig. 24.26).

Formation of the insula

At the end of the third month, while the corpus striatum is developing, there is a relative restriction of growth between the frontal and temporal lobes. The region lateral to the striatum becomes depressed to form a lateral cerebral fossa with a portion of cortex, the insula, at its base (Fig. 24.28). As the temporal lobe continues to protrude towards the orbit, and with more rapid growth of the temporal and frontal cortices, the surface of the hemisphere expands at a rate greater than the hemisphere as a whole and the cortical areas become folded, forming gyri and sulci. The insula is gradually overgrown by these adjacent cortical regions, and they overlap it forming the opercula, the free margins of which form the anterior part of the lateral fissure. This process is not completed until after birth. The lentiform nucleus remains deep to and coextensive with the insula.

Olfactory nerve, limbic lobe and hippocampus

The growth changes in the temporal lobe which help to submerge the insula produce important changes in the olfactory and neighbouring limbic areas. As it approaches the hemispheric floor, the olfactory tract diverges into lateral, medial and (variable) intermediate striae. The medial stria is clothed with a thin archaeocortical medial olfactory gyrus. This curves up into further archaeocortical areas rostral to the lamina terminalis (paraterminal gyrus, prehippocampal rudiment, parolfactory gyrus, septal nuclei) and these continue into the indusium griseum. The lateral stria, clothed by the lateral olfactory gyrus, and the intermediate stria (when present), terminate in the rostral parts of the piriform area, including the olfactory trigone and tubercle, anterior perforated substance, uncus and entorhinal area of the anterior part of the future parahippocampal gyrus. The lateral limit of the lateral stria is indicated by the rhinal sulcus. The forward growth of the temporal pole and the general expansion of the neocortex cause the lateral olfactory gyrus to bend laterally, the summit of the convexity lying at the anteroinferior corner of the developing insula (Fig. 24.29). During the fourth and fifth months, much of the piriform area becomes submerged by the adjoining neocortex and in the adult only a part of it remains visible on the inferior aspect of the cerebrum.

Fig. 24.29 A–G, The superolateral surfaces of human fetal cerebral hemispheres at the ages indicated, showing the changes in size, profile and the emerging pattern of cerebral sulci with increasing maturation. Note the changing prominence and relative positions of the frontal, occipital and particularly the temporal pole of the hemisphere. At the earliest stage (A) the lateral cerebral fossa is already obvious; its floor covers the developing corpus striatum in the depths of the hemisphere and progressively matures into the cortex of the insula. The fossa is bounded by overgrowing cortical regions, the frontal, temporal and parietal opercula, which gradually converge to bury the insula; their approximation forms the lateral cerebral sulcus. By the sixth month the central, pre- and postcentral, superior temporal, intraparietal and parieto-occipital sulci are all clearly visible. In the subsequent stages shown all the remaining principal and subsidiary sulci rapidly appear and by 40 weeks all the features which characterize the adult hemisphere in terms of surface topography are present in miniature.

(Photographs provided by Dr Sabina Strick, The Maudsley Hospital, London.)

The limbic lobe is the first part of the cortex to differentiate and at first it forms a continuous, almost circular strip on the medial and inferior aspects of the hemisphere. Below and in front, where the stalk of the olfactory tract is attached, it constitutes a part of the piriform area. The portion outside the curve of the choroid fissure (Fig. 24.30) constitutes the hippocampal formation. In this region the neural progenitors of the developing cortex proliferate and migrate. The wall of the hemisphere thickens, producing an elevation that projects into the medial side of the ventricle. The elevation is the hippocampus; it appears first on the medial wall of the hemisphere in the area above and in front of the lamina terminalis (paraterminal area) and gradually extends backwards, curving into the region of the temporal pole where it adjoins the piriform area. The marginal zone in the neighbourhood of the hippocampus is invaded by neurones to form the dentate gyrus. Both extend from the paraterminal area backwards above the choroid fissure and follow its curve downwards and forwards towards the temporal pole, where they continue into the piriform area. A shallow groove, the hippocampal sulcus, crosses the medial surface of the hemisphere throughout the hippocampal formation. The efferent fibres from the cells of the hippocampus collect along its medial edge and run forwards immediately above the choroid fissure. Rostrally they turn ventrally and enter the lateral part of the lamina terminalis to gain the hypothalamus, where they end in and around the mammillary body and neighbouring nuclei. These efferent hippocampal fibres form the fimbria hippocampi and the fornix.

Fig. 24.30 The anterior and posterior commissures in the brain of a 16 week human fetus; medial aspect of left half of the brain.

Projection fibres, internal capsule

The growth of the neocortex, and its enormous expansion during the latter part of the third month, is associated with the initial appearance of corticofugal and corticopetal projection fibres and the internal capsule. The fibres follow the route provided by the apposition of the lateral aspect of the thalamus with the medial aspect of the corpus striatum. They divide the latter, almost completely, into a lateral part, the lentiform nucleus, and a medial part, the caudate nucleus: these two nuclei remain confluent only in their anteroinferior regions (Figs 24.26 and 24.28). The corticospinal tracts begin to develop in the ninth week of fetal life and reach their caudal limits by the twenty-ninth week. The fibres destined for the cervical and upper thoracic regions, which innervate the upper limbs, are in advance of those which innervate the lower limbs, and these, in turn, are in advance of fibres that innervate the face: the timing of the appearance of reflexes associated with these three parts of the body is similarly staggered.

The majority of subcortical nuclear masses receive terminals from descending fibres of cortical origin. These are joined by thalamocortical, hypothalamocortical and other afferent ascending bundles. The internal capsular fibres pass lateral to the head and body of the caudate nucleus, the anterior cornu and central part of the lateral ventricle, the rostroventral extensions and body of the fornix, the dorsal thalamus and dorsal choroidal fissure, and medial to the lentiform nucleus (Fig. 24.26).

Formation of gyri and sulci

Apart from the shallow hippocampal sulcus and the lateral cerebral fossa, the surfaces of the hemisphere remain smooth and uninterrupted until early in the fourth month (Fig. 24.29). The parieto-occipital sulcus appears about that time on the medial aspect of the hemisphere. Its appearance seems associated with an increase in the number of splenial fibres in the corpus callosum. Over the same period the posterior part of the calcarine sulcus appears as a shallow groove extending forwards from a region near the occipital pole. It is a true infolding of the cortex in the long axis of the striate area and produces an elevation, the calcar avis, on the medial wall of the posterior horn of the ventricle.

During the fifth month the cingulate sulcus appears on the medial aspect of the hemisphere, and sulci appear on the inferior and superolateral aspects in the sixth month. The central, precentral and postcentral sulci appear, each in two parts, upper and lower, which usually coalesce shortly afterwards, although they may remain discontinuous. The superior and inferior frontal, the intraparietal, occipital, superior and inferior temporal, occipitotemporal, collateral and rhinal sulci all make their appearance during the same period. By the end of the eighth month all the important sulci can be recognized (Fig. 24.29).

Development of commissures

The development of the commissures causes a very profound alteration of the medial wall of the hemisphere. At the time of their appearance the two hemispheres are connected to each other by the median part of the telencephalon. The roof plate of this area remains epithelial, while its floor becomes invaded by the decussating fibres of the optic nerves and developing hypothalamic nuclei. These two routes are thus not available for the passage of commissural fibres passing from hemisphere to hemisphere across the median plane, and these fibres therefore pass through the rostral wall of the interventricular foramen, i.e. the lamina terminalis. The first commissures to develop are those associated with the palaeocortex and archicortex. Fibres of the olfactory tracts cross in the ventral or lower part of the lamina terminalis and, together with fibres from the piriform and prepiriform areas and the amygdaloid bodies, form the rostral part of the anterior commissure (Figs 24.30 and 24.31). In addition the two hippocampi become interconnected by transverse fibres which cross from fornix to fornix in the upper part of the lamina terminalis as the commissure of the fornix (hippocampal commissure). Various other decussating fibre bundles (known as the supraoptic commissures, although they are not true commissures) develop in the lamina terminalis immediately dorsal to the optic chiasma, between it and the anterior commissure.

Fig. 24.31 Median sections showing formation of the commissures. The telencephalon gives rise to commissural tracts that integrate the activities of the left and right cerebral hemispheres. Tracts include the anterior and hippocampal commissures and the corpus callosum. The small posterior and habenular commissures arise from the epithalamus.

The commissures of the neocortex develop later and follow the pathways already established by the commissures of the limbic system. Fibres from the tentorial surface of the hemisphere join the anterior commissure and constitute its larger posterior part. All the other commissural fibres of the neocortex associate themselves closely with the commissure of the fornix and lie on its dorsal surface. These fibres increase enormously in number and the bundle rapidly outgrows its neighbours to form the corpus callosum (Figs 24.30 and 24.31).

The corpus callosum originates as a thick mass connecting the two cerebral hemispheres around and above the anterior commissure. (This site has been called the precommissural area, but this use has been rejected here because of the increasing use of the adjective precommissural to denote the position of parts of the limbic lobe, i.e. prehippocampal rudiment, septal areas and nuclei and strands of the fornix, in relation to the anterior commissure of the mature brain.) The upper end of this neocortical commissural area extends backwards to form the trunk of the corpus callosum. The rostrum of the corpus callosum develops later and separates some of the rostral end of the limbic area from the remainder of the cerebral hemisphere. Further backward growth of the trunk of the corpus callosum then results in the entrapped part of the limbic area becoming stretched out to form the bilateral septum pellucidum. As the corpus callosum grows backwards it extends above the choroidal fissure, carrying the commissure of the fornix on its under surface. In this way a new floor is formed for the longitudinal fissure, and additional structures come to lie above the epithelial roof of the third ventricle. In its backward growth the corpus callosum invades the area hitherto occupied by the upper part of the archaeo-cortical hippocampal formation, and the corresponding parts of the dentate gyrus and hippocampus are reduced to vestiges, the indusium griseum and the longitudinal striae. However, the posteroinferior (temporal) archaeocortical regions of both dentate gyrus and hippocampus persist and enlarge.

Cellular development of the cerebrum

The wall of the earliest cerebral hemisphere consists of a pseudostratified epithelium, whose cells exhibit interkinetic nuclear migration as they proliferate to form clones of germinal cells. The columnar cells elongate and their non-nucleated peripheral processes now constitute a marginal zone (but see Fig. 24.32A legend), while their nucleated, paraluminal and mitosing regions constitute the ventricular zone. Some of their progeny leave the ventricular zone and migrate to occupy an intermediate zone. The proliferative phase continues for a considerable period of fetal life. Ultimately, groups of progenitor cells form, at first, generations of definitive neurones and, later, glial cells which migrate to, and mature in, their final positions. These phases of proliferation, migration, differentiation and maturation overlap each other in space and time, and are not precisely sequential.

Fig. 24.32 A, Two distinct programmes of cell division are observed in the ventricular zone. Asymmetric division where radial glia give rise to neurones directly; symmetric division where neurones arise via intermediate progenitors after arrest in the subventricular zone. B, The dynamics of neuroblast proliferation and successive migrations to form the cerebral neocortex. Cells divide initially in the ventricular and subventricular zones and successive progeny migrate towards the pia. The first cells to migrate form deep cortical laminae; later cells pass through these layers to form successively superficial laminae. The following revisions to the 1970 Bolder Committee nomenclature for development of the human cerebral cortex given by Bystron, Blakemore and Rakic (2008) are included here, although not shown on the above diagram. Within the developing cerebral cortex: there is no distinct marginal zone beneath the pial layer before the cortical plate forms; the subventricular zone appears as a distinct proliferative layer before the cortical plate forms; a transient preplate layer forms before the cortical plate appears; the intermediate zone denotes a heterogenous compartment lying between the proliferative layers (ventricular and subventricular zones) and the postmigratory cells above; there is a transient subplate zone directly below the cortical plate.

(Adapted from Noctor et al 2004.)

The migration of neuronal precursors from the ventricular and intermediate zones occurs radially towards the pial surface Radial glial cells are now considered as neural progenitors (Kriegstein & Gotz 2003). Some early progeny from the ventricular zone form a primordial plexiform layer, or preplate. Their somata become arranged as a transient dense cortical plate from which subsequently generated cortical plate cells arise to form a subplate region. At later stages a considerable proportion of cortical progenitors divide in a subventricular zone (Fig. 24.32A). Proliferation in the ventricular zone, which produces excitatory pyramidal neurones of the infragranular layers of the cortex, wanes, whereas in the subventricular zone proliferation persists for a considerable period: the symmetrical terminal division of these cells gives rise to neurones which migrate to form the supragranular layers of the cortex (Kriegstein & Noctor 2004). From the pial surface inwards, the following zones may be defined: marginal, cortical plate, subplate, intermediate, subventricular and ventricular (Fig. 24.32B). (For a review of the nomenclature of the developing zones see Bystron et al 2008.) The marginal zone, the outermost layer of the developing cerebral cortex, will form layer 1, and the neurones of the cortical plate and subplate form the neurones of the remaining (2–6) cortical laminae, the complexity varying in different locations and with further additions of neurones from the deeper zones. The intermediate zone gradually transforms into the white matter of the hemisphere. Meanwhile other deep progenitor cells produce generations of glioblasts which also migrate into the more superficial layers. As proliferation wanes and finally ceases in the ventricular and subventricular zones, their remaining cells differentiate into general or specialized ependymal cells, tanycytes or subependymal glial cells.

The time of the proliferation of different cortical neurones vary according to their laminar destination and cell type. The first groups of cells to migrate are destined for the deep cortical laminae and later groups pass through them to more superficial regions. The subplate zone, a transient feature containing some of the earliest generated cortical neurones is most prominent during mid-gestation. It contains neurones surrounded by a dense neuropil and is the site of the most intense synaptogenesis in the embryonic cortex. The cumulative effect of this radial and tangential growth is evident in a marked increase in cortical thickness and surface area.

In the pallial walls of the mammalian cerebral hemisphere, the phylogenetically oldest regions, which are the first to differentiate during ontogeny, are those that border the interventricular foramen and its extension the choroidal fissure, the lamina terminalis and the piriform lobe. The pallidum also contributes interneurones to the formation of the cerebral cortex. A large portion of GABAergic inhibitory (nonpyramidal) cells are not generated in the cortical ventricular zone, but migrate tangentially through the striatocortical junction to reach the cortex (Wonders & Anderson 2006; Metin et al 2006; Fig. 24.33). An increasingly complex level of organization, from three to six tangential laminae, is encountered in passing from the dentate gyrus and cornu ammonis through the subiculum to the general neocortex. (Many investigators find the simple progression from three to six major laminae a gross oversimplification, and numerous subdivisions have been proposed.)

Fig. 24.33 (A) The ganglionic eminence (GE) of the subpallium is subdivided into three regions: lateral (LGE), medial (MGE), and caudal (CGE) according to the expression patterns of various transcription factors (Dlx2, Mash1, Nksx2.1 and Lhx6) and of the cellular retinol-binding protein 1 (CRBP1). (B) Main migratory paths of interneurones derived from the three subdivisions of the GE. MGE cells tend to migrate laterally and spread throughout the cortex, CGE cells migrate predominantly towards the caudal telencephalon, whereas LGE cells migrate rostrally and contribute to interneurones in the olfactory bulb. It has been suggested that most MGE cells differentiate into parvalbumin (PV)- and somatostatin (SS)-immunoreactive interneurones, whereas most CGE cells differentiate into calretinin (CR) immunoreactive interneurones.

Mechanisms of cortical development

Rakic (1971) initially demonstrated the migration of neurones along radial glial processes, and this has subsequently been seen to occur in three phases. First, the neurones become apposed to the radial glial cells and establish an axis of polarity away from the ventricular surface. Next, they are propelled along the glial surface until they ‘recognize’ their final destination, whereupon they cease locomotion and detach from the glial processes. They then continue to differentiate according to their final position, and later-born neurones migrate past them towards the pial surface (Fig. 24.5). Cortical neurones or cerebellar granule cells appear equally capable of migrating on hippocampal or cerebellar Bergmann glia, indicating conservation of migration mechanisms in different brain regions. These observations are still valid, although it is now recognized that radial glia represent the major neuronal progenitor population of the cortex.

Various lines of evidence support the proposal that the laminar fate of neurones is determined prior to migration. In the mutant reeler mouse, laminar formation is inverted so that layers form in outside-in rather than inside-out array, yet axonal connections and neuronal properties appear normal, suggesting that the cells differentiate according to their time of origin rather than their location. Likewise, the prevention of neuronal migration by irradiation leads to the production of cells which remain apposed to the ventricular surface but which develop an appropriate phenotype and efferent projections. Transplantation of labelled cells has suggested that commitment to a particular cortical lamina occurs shortly after S-phase. Neurones of pre-existing laminae that have begun axonogenesis may provide a feedback on the forming cortical layers, providing a sort of developmental clock for histogenesis.

In a plane perpendicular to its laminae, i.e. tangentially/circumferentially, the cortex is divided up into a number of areas, displaying a hierarchy of organization (see Ch. 23). These include primary areas, such as the motor cortex, unimodal association areas concerned with the integration of information from one of the primary areas, and multimodal association areas that integrate information from more than one modality. There are also the areas concerned with functions that are even less understood, such as the frontal lobes, concerned with goal-orientation, responsibility and long-term planning. The primary areas are further divided into somatotopic maps. At the finest level, the cortex is known to consist of a series of ‘columns’, 50–500 μm wide, within which cells on a vertical traverse display common features of modality and electrophysiological responses to stimuli, e.g. the ocular dominance columns of the visual cortex. Despite the precise stacking of neurones in these columns, only 80–85% of cells are thought to migrate radially along the glial cells: a subpopulation is thought to move tangentially in the intermediate zone (O’Rourke et al 1995). Moreover, some neurones may also migrate tangentially on the radial glial cells, as a result of glial cell branching in the cortical plate. The ventricular zone is not the only source of cortical neurones, because striatal and GABAergic neurones are known to migrate from the medial, lateral and caudal ganglionic eminences into the developing cortex. The proportion of locally generated and tangentially migrating GABAergic neurones it believed to be 65–35% in human and 5–95% in mouse.

Two models have been proposed to explain the development of the complexity of cortical organization. The ‘protocortex’ model assumes that the proliferative ventricular epithelium is a ‘tabula rasa’ generating homogeneous layers of neurones that are patterned solely by their future connectivity, especially by the ingrowth of processes from the thalamus. The ‘protomap’ hypothesis proposes that the intrinsic differences between the different areas are specified prior to cell generation and migration (Rakic 1988, 2003). The radial glial cells translate this map from the ventricular zone to the cortical plate where the pattern is refined by innervating axons. In this ‘radial unit’ model, the tangential co-ordinates of the different areas are determined by the position of their ventricular ancestors, whereas their radial position is determined by their time of birth and rate of migration.

But what would constitute such a ‘protomap’? The investigation of gene expression patterns shows that the early cortex is not homogeneous, and that it expresses some markers that are transient and some that persist into the adult. For example, the mouse gene Id2 marks the transition between the motor and somatosensory cortices in the embryo, whereas limbic associated membrane protein (LAMP) delineates the limbic cortex throughout life. LAMP expression is regulated by transforming growth factor-α, which is expressed by the lateral ganglionic eminence at the lateral edge of the cortex, whereas the medial edge or cortical hem expresses signalling molecules of the Wnt and BMP families. Any, if not all, of these may be the components of short range signalling centres along the edges of the cortex (e.g. FGF8) (Fig. 24.34). Coupled with the gradients of transcription factors such as Emx2 and Lhx2, there is therefore evidence to support the ‘protomap’ hypothesis (Donoghue & Rakic 1999). The challenge is to relate the gradients of gene expression patterns to formation of sharp cytoarchitectonic boundaries.

Fig. 24.34 Frontolateral view of the telencephalon showing the patterning centres as marked by expression of the genes indicated in the key, the cross regulation between the Fgf8 and Bmp4/Wnt3a-expressing centres, and the positive interactions between the Fgf8 and Shh-expressing domains.

However, studies of cell migration are consistent with the idea that cortical areas might not be rigidly determined. Experimental manipulations of the developing cortex by deafferentation or manipulation of inputs give some indication of the state of commitment of cortical areas. In two independent sets of experiments, somatosensory or auditory cortex was induced to process visual information by misrouting retinal axons to somatosensory thalamus or auditory thalamus (von Melchner, Pallas & Sur 2000). When the lateral geniculate nucleus and the visual cortex were ablated and space was created in the medial geniculate by ablating the inferior colliculus, cells in the somatosensory or auditory cortex were visually driven due to the subcortical rewiring, and receptive field and response properties resembled those typical of the visual cortex. These results suggest that the modality of a sensory thalamic nucleus or cortical area can be specified by inputs during development.

The development of cortical projections has been investigated both in terms of laminar and area-specific connectivity. Recently, attention has focused on the idea that connections might be influenced by the existence of a transient population of subplate neurones, which later dies. The cortex develops within a preplate, consisting of corticopetal nerve fibres and the earliest generated neurones. This zone is then split into two zones, the subplate underneath the cortical plate, and the marginal zone at the pial surface, by the arrival of cortical neurones. Subplate neurones integrate into the intra- and extra-cortical circuitry, they extend axons via the internal capsule towards the thalamus and superior colliculus at times before other cortical neurones have been born.

How are region-specific projections generated? Layer 5 neurones in various cortical areas extend axons to different repertoires of targets. For instance, layer 5 neurones of the visual cortex project to the tectum, pons and mesencephalic nuclei, while those in the motor cortex project to mesencephalic and pontine targets, the inferior olive and dorsal column nuclei and the spinal cord. An interesting feature of these cortical projections is that they arise by collateral formation rather than by projection of the primary axon, or by growth cone bifurcation. In the case of the corticopontine projection, collaterals are elicited by a diffusible, chemotrophic agent. Retrograde labelling of layer 5 neurones at various times in development has shown that rather than being generated de novo, these patterns seem to arise by pruning of collaterals from a more widespread projection. Visual cortical neurones possess a projection to the spinal cord early in development, which is later eliminated. This late emergence of the specificity of projections could be driven by intrinsic programming of the neurones to be pruned, or a response to position-dependent factors. There is evidence that the latter is the case. When pieces of visual cortex were transplanted into motor areas, and the resulting layer 5 projections labelled at later times in development, projections to the spinal cord persisted, rather than being eliminated as in normal development. Thus position plays an important role in the modelling of cortical projections, implying that the same classes of neurones exist in different tangential regions of the cortex. Regressive events such as axon and synapse elimination and neuronal death thus play an important part in modelling the cortex. For example in rodents approximately 30% of cortical neurones die, and the number of cells in layer 4 is governed by thalamic input.

Human cortical malformations are thought to arise as neuronal migration disorders (NMDs). A broad class of NMDs is lissencephaly, in which the cortex has a normal thickness, but a decreased number of neurones and a smoothened surface with a decreased number of gyri. The mutated protein in some forms of the disorder, LIS-1, is expressed in the ventricular neuroepithelium and is responsible for regulating the levels of the lipid messenger platelet activating factor (PAF). How this translates into a cell migration defect remains as yet obscure. Conversely, polymicrogyria manifests as a highly convoluted cerebrum, with a nearly normal surface area but a thinner cortex. It is thought that the normal number of proliferative units and thus ontogenetic columns are established, but each column contains fewer neurones, implying either a reduced rate of proliferation and/or cell migration, or an enhanced level of cell death.

Neonatal brain and reflexes

The brain of the full-term neonate ranges from 300–400 g with an average of 350 g; the brains of neonatal males are slightly heavier than those of females. Because the head is large at birth, measuring one quarter the total body length, the brain is also proportionally larger, and constitutes 10% of the body weight compared with 2% in the adult. At birth the volume of the brain is 25% of its volume in adult life. The greater part of the increase occurs during the first year, at the end of which the volume of the brain has increased to 75% of its adult volume. The growth can be accounted for partly by increase in the size of nerve cell somata, the profusion and dimensions of their dendritic trees, axons and collaterals, and by the growth of the neuroglial cells and cerebral blood vessels, but mostly it reflects the myelination of many of the axons: the sensory pathways, visual, auditory and somatic, myelinate first, and the motor fibres later. During the second and subsequent years, growth proceeds much more slowly. The brain reaches 90% of its adult size by the fifth year, 95% by 10 years, and attains adult size by the 17th or 18th year, largely as a result of the continuing myelination of various groups of nerve fibres.

The sulci of the cerebral hemispheres appear from the fourth month of gestation (Fig. 24.29). At full term, the general arrangement of sulci and gyri is present, but the insula is not completely covered, the central sulcus is situated further rostrally, and the lateral sulcus is more oblique than in the adult. Most of the developmental stages of sulci and gyri have been identified in the brains of premature infants. Of the cranial nerves, the olfactory nerve and the optic nerve at the chiasma are much larger than in the adult, whereas the roots of the other nerves are relatively smaller.

The brain occupies 97.5% of the cranial cavity from birth to 6 years of age, after which the space between the brain and skull increases in volume until the adult brain occupies only 92.5% of the cranial cavity. The cerebral ventricles are larger in the neonatal brain than they are in the adult. The newborn has a total of 10–15 ml of cerebrospinal fluid when delivered vaginally and 30 ml when delivered by Caesarean section. As the head moves down the birth canal and is compressed, the cerebrospinal fluid is pushed out into the venous sinuses; this does not happen in a Caesarean delivery.

Cranial arteries

The internal carotid artery is formed progressively from the third arch artery is the dorsal aorta cranial to this, and a further forward continuation which differentiates, at the time of regression of the first and second aortic arches, from the capillary plexus extending to the walls of the forebrain and midbrain. At its anterior extremity this primitive internal carotid artery divides into cranial and caudal divisions. The former terminates as the primitive olfactory artery, and supplies the developing regions implied (see Fig. 35.8). The latter sweeps caudally to reach the ventral aspect of the midbrain, its terminal branches are the primitive mesencephalic arteries. Simultaneously bilateral longitudinal channels differentiate along the ventral surface of the hindbrain from a plexus fed by intersegmental and transitory presegmental branches of the dorsal aorta and its forward continuation. The most important of the presegmental branches is closely related to the fifth nerve, the primitive trigeminal artery. Otic and hypoglossal presegmental arteries occur and may persist. The longitudinal channels later connect cranially with the caudal divisions of the internal carotid arteries (each of which gives rise to an anterior choroidal artery supplying branches to the diencephalon, including the telae choroideae and midbrain) and caudally with the vertebral arteries through the first cervical intersegmental arteries. Fusion of the longitudinal channels results in the formation of the basilar artery, while the caudal division of the internal carotid artery becomes the posterior communicating artery and the stem of the posterior cerebral artery. The remainder of the posterior cerebral artery develops comparatively late, probably from the stem of the posterior choroidal artery which is annexed by the caudally expanding cerebral hemisphere, its distal portion becoming a choroidal branch of the posterior cerebral artery. The posterior choroidal artery supplies the tela choroidea at the future temporal end of the choroidal fissure; its rami advance through the tela to become confluent with branches of the anterior choroidal artery. The cranial division of the internal carotid artery gives rise to anterior choroidal, middle cerebral and anterior cerebral arteries. The stem of the primitive olfactory artery remains as a small medial striate branch of the anterior cerebral artery. The cerebellar arteries, of which the superior is the first to differentiate, emerge from the capillary plexus on the wall of the rhombencephalon.

The source of the blood supply to the territory of the trigeminal nerve varies at different stages in development. When the first and second aortic arch arteries begin to regress, the supply to the corresponding arches is derived from a transient ventral pharyngeal artery, which grows from the aortic sac. It terminates by dividing into mandibular and maxillary branches.

Vascularization of the brain

The brain becomes vascularized by angiogenesis (angiotrophic vasculogenesis) rather than by direct invasion by angioblasts. Blood vessels form by sprouting from vessels in the pial plexus that surrounds the neural tube from an early stage. These sprouts form branches that elongate at the junction between the ventricular and marginal zones; the branches project laterally within the inter-rhombomeric boundaries and longitudinally adjacent to the median floorplate. Subsequently, additional sprouts penetrate the inter-rhombomeric regions on the walls and floor of the hindbrain. Branches from the latter elongate towards and join the branches in the inter-rhombomeric junctions, forming primary vascular channels between rhombomeres and longitudinally on each side of the median floorplate. Later additional sprouts invade the hindbrain within the rhombomeres, anastomosing in all directions.

The leptomeningeal perforating branches pass into the brain parenchyma as cortical, medullary and striate branches (Fig. 24.35). The cortical vessels supply the cortex via short branches which may form precapillary anastomoses, whereas the medullary branches supply the white matter. The latter converge towards the ventricle but rarely reach it; they often follow a tortuous course as they pass around bundles of nerves. The striate branches which penetrate into the brain through the anterior perforated substance, supply the basal nuclei and internal capsule via a sinuous course: they are larger than the medullary branches and the longest of them reach close to the ventricle. The periventricular region and basal nuclei are also supplied by branches from the tela choroidea, which develops from the early pial plexus but becomes medially and deeply placed as the telencephalon enlarges.

Fig. 24.35 Development of cerebral blood vessels. A, The brain is surrounded by a system of leptomeningeal arteries from afferent trunks at its base. Intracerebral arteries arise from this system and converge (ventriculopetally) towards the ventricle (the inner circle in this diagram). B, A few deep penetrating vessels supply the brain close to the ventricle and send ventriculofugal arteries towards the ventriculopetal vessels without making anastomoses. C, The arrangement of ventriculopetal and ventriculofugal vessels around a cerebral hemisphere. D, The similar arrangement of vessels around the cerebellum. E, Changes in the arterial pattern of the human cerebrum between 24 and 34 weeks’ gestation. F, Arterial supply to the basal nuclei at 30 weeks’ gestation.

(A–D, by permission from Van den Bergh R, Van der Eecken H 1968 Anatomy and embryology of cerebral circulation. Prog Brain Res 30: 1–25; E and F, by permission from the BMJ Publishing Group from Hambleton G, Wigglesworth J S 1976 Origin of intraventricular haemorrhage in the preterm infant. Arch Dis Child 51: 651–659.)

The cortical and medullary branches irrigate a series of corticosubcortical cone-shaped areas, each centred around a sulcus containing an artery. They supply a peripheral portion of the cerebrum and are grouped as ventriculopetal arteries. In contrast, striate branches arborize close to the ventricle and supply a more central portion of the cerebrum; together with branches from the tela choroidea, they give rise to ventriculofugal arteries which supply the ventricular zone (germinal matrix of the brain) and send branches towards the cortex. The ventriculopetal and ventriculofugal arteries run towards each other but they do not make any connections or anastomoses: the ventriculopetal arteries form networks of small arterioles (Fig. 24.35). The ventriculopetal vessels supply relatively more mature regions of the brain compared to the ventriculofugal vessels, which are subject to constant remodelling and do not develop tunicae mediae until ventricular zone proliferation is completed. The boundary zone between these two systems (an outer centripetal and inner centrifugal) has practical implications related to the location of ischaemic lesions (periventricular leukomalacia, PVL) in the white matter of premature infant brains. Although it was thought that the distribution of ischaemic lesions in PVL coincided with the demarcation zone between the centrifugal and centripetal vascular arterial systems, this is now not thought to provide the complete answer. Three major interacting factors contribute to the pathology seen in PVL: the incomplete state of development of the vasculature in the ventricular zone, the maturation-dependent impairment of cerebral blood flow regulation in premature infants, and the vulnerability of oligodendroblasts in the periventricular region (which are particularly affected by swings in cerebral ischaemia and reperfusion) (Volpe 2001).

The same pattern of centripetal and centrifugal arteries develops around the fourth ventricle. The ventriculofugal circulation is more extensive in the cerebellum than in the telencephalon. The arteries arise from the various cerebellar arteries and course, with the cerebellar peduncles, directly to the centre of the cerebellum, by-passing the cortex. The ventriculopetal arteries are derived from the meningeal vessels over the cerebellar surface, and most terminate in the white matter.

At 24 weeks’ gestation, the blood supply to the basal nuclei and internal capsule is relatively well-developed, via a prominent Heubner’s artery (arteria recurrens anterior), a branch of the anterior cerebral artery. In contrast, the cortex and the white matter regions are rather poorly vascularized at this stage. The distribution of arteries and veins on the lateral aspect of the cerebral hemispheres is affected by the formation of the lateral fissure and development of cerebral sulci and gyri. Between 12 and 20 weeks’ gestation the middle cerebral artery and its branches are relatively straight, branching in an open-fan pattern. At the end of 20 weeks, the arteries become more curved as the opercula begin to appear and submerge the insular cortex. The area supplied by the middle cerebral artery becomes dominant when compared to the territories supplied by the anterior and posterior cerebral arteries. Early arterial anastomoses appear around 16 weeks of gestation and increase in size with advancing age. The sites of anastomoses between the middle and anterior cerebral arteries move from the convexity of the brain towards the superior sagittal sinus. Anastomotic connections between the middle and posterior cerebral arteries shift towards the basal aspect of the brain.

By 32–34 weeks, marked involution of the ventricular zone (germinal matrix) has occurred and the cortex acquires its complex gyral pattern and an increased vascular supply. Ventricular zone capillaries are gradually remodelled to blend with the capillaries of the caudate nucleus. Heubner’s artery eventually supplies only a small area at the medial aspect of the head of the caudate nucleus. In the cortex there is progressive elaboration of cortical blood vessels (Fig. 24.35) and towards the end of the third trimester the balance of cerebral circulation shifts from one which is central and basal nuclei-oriented, to one which predominantly serves the cortex and white matter. These changes in the pattern of cerebral circulation are of major significance in the pathogenesis and distribution of hypoxic/ischaemic lesions in the developing human brain. In a premature brain, the majority of ischaemic lesions occur in the boundary zone between the centripetal and centrifugal arteries, i.e. in the periventricular white matter, whereas in a full-term infant the cortical boundary zones and watershed areas between different arterial blood supplies are similar to those in adults and so presumably are the risks of ischaemic lesion.

Vessels of the ventricular zone (germinal matrix)

The germinal matrix (ventricular zone) is the end zone or border zone between the cerebral arteries and the collection zone of the deep cerebral veins. The germinal matrix is probably particularly prone to ischaemic injury in the immature infant because of its unusual vascular architecture. The subependymal veins (septal, choroidal, thalamostriate and posterior terminal) flow towards the interventricular foramen. There is a sudden change of flow at the level of the foramen, and the veins recurve at an acute angle to form the paired internal cerebral veins. The capillary channels in the germinal matrix open at right angles directly into the veins, and it has been postulated that these small vessels may be points of vascular rupture and the site of subependymal haemorrhage.

The capillary bed in the ventricular zone is supplied mainly by Heubner’s artery and terminal branches of the lateral striate arteries from the middle cerebral artery. The highly cellular structure of the ventricular zone is a temporary feature, and the vascular supply to this area displays some primitive features: it has the capacity to remodel when the ventricular zone cells migrate and the remaining cells differentiate as ependyma towards the end of gestation.

Vessel density is relatively low in the ventricular zone, suggesting that this area may normally have a relatively low blood flow. Immature vessels, without a complex basal lamina or glial sheet, have been described up to 26 weeks’ gestation in the zone: the endothelium of these vessels is apparently thinner than in the cortical vessels. In infants of less than 30 weeks’ gestation, the vessels in the ventricular zone contain no smooth muscle, collagen or elastic fibres. Collagen and smooth muscle are seen in other regions after 30 weeks, but are not detected in the remains of the germinal matrix. The lack of these components could make the vessels in this zone vulnerable to changes in intraluminal pressure, and the lack of smooth muscle would preclude them from participating in autoregulatory processes. Cerebral vessels in premature infants lack elastic fibres and have a disproportionately small number of reticulin fibres. Comparison of the cortical and ventricular zone blood vessels shows that in infants of between 25 and 32 weeks’ gestation the walls of vessels in the germinal matrix consist commonly of 1–2 endothelial cells with an occasional pericyte, and the capillary lumina are larger than those of the vessels in the cortex. In more mature infants the basal lamina surrounding germinal matrix vessels is thicker and more irregular when compared to cortical vessels.

Glial fibrillary acidic protein positive cells have been detected around blood vessels in the germinal matrix from 23 weeks’ gestation. Glial cells may contribute to changes in the nature of endothelial intercellular junctions in brain capillaries.

Cerebral veins

From 16 weeks onwards, cerebral veins can be identified. The superior, middle, inferior, anterior and posterior cerebral veins appear more tortuous than meningeal arteries. Veins draining the cortex, white matter and deeper structures are recognized in the midtrimester. Subcortical veins drain the deep white matter, deep cortical and subcortical superficial tissue: they terminate together with cortical veins which drain the cortex in the meningeal veins. The deep white matter and central nuclei are drained by longer veins which meet and join subependymal veins from the ventricular zone. Anastomoses between various groups of cortical veins can be recognized by 16 weeks’ gestation. The inferior anastomotic vein (of Labbé), an anastomosis between the middle cerebral and inferior cerebral veins, becomes recognizable at 20 weeks, but the superior anastomotic vein (of Trolard), connecting the superior and middle cerebral veins, does not appear before the end of 30 weeks.

Rapid cortical development is correlated with the regression of the middle cerebral vein and its tributaries, and development of ascending and descending cortical veins and intraparenchymal (medullary) arteries and veins.

Cerebral venous drainage in a full-term baby is essentially composed of two principal venous arrays, the superficial veins and the deep Galenic venous system: anastomoses between these two systems persist into adult life.

Veins of the head

The earliest vessels form a transitory primordial hindbrain channel which drains into the precardinal vein. This is soon replaced by the primary head vein which runs caudally from the medial side of the trigeminal ganglion, lateral to the facial and vestibulocochlear nerves and otocyst, and then medial to the vagus nerve, to become continuous with the precardinal vein. An anastomosis to other more lateral venous channels developing over the hindbrain ultimately brings the primary head vein lateral to the vagus nerve. The cranial part of the precardinal vein forms the internal jugular vein.

The primary capillary plexus of the head is separated into three fairly distinct strata by the differentiation of the skull and meninges. The superficial vessels drain the skin and underlying soft parts, and eventually discharge in large part into the external jugular system, although they retain some connections with the deeper veins through so-called emissary veins. The next layer of vessels is the venous plexus of the dura mater, from which the dural venous sinuses differentiate: vessels from the plexus converge on each side into anterior, middle and posterior dural stems (Fig. 24.36). The anterior stem drains the prosencephalon and mesencephalon and enters the primary head vein rostral to the trigeminal ganglion. The middle stem drains the metencephalon and empties into the primary head vein caudal to the trigeminal ganglion, and the posterior stem drains the myelencephalon into the start of the precardinal vein. The deepest capillary stratum is the pial plexus from which the veins of the brain differentiate. It drains at the dorsolateral aspect of the neural tube into the adjacent dural venous plexus. The primary head vein also receives, at its cranial end, the primitive maxillary vein which drains the maxillary prominence and region of the optic vesicle.

Fig. 24.36 Successive stages in the development of the veins of the head and neck. A, At approximately 8 mm CR length; B, at approximately 24 mm CR length.

The vessels of the dural plexus undergo profound changes, largely accommodating the growth of the cartilaginous otic capsule of the membranous labyrinth and the expansion of the cerebral hemispheres. As the otic capsule grows, the primary head vein is gradually reduced and a new channel joining anterior, middle and posterior dural stems appears dorsal to the cranial nerve ganglia and the capsule. The adult sigmoid sinus is formed where this channel joins the middle and posterior stems, together with the posterior dural stem itself (Fig. 24.36B).

A curtain of capillary veins, the sagittal plexus, forms between the growing cerebral hemispheres and along the dorsal margins of the anterior and middle plexuses, in the position of the future falx cerebri. Rostrodorsally this plexus forms the superior sagittal sinus. It is continuous behind with the anastomosis between the anterior and middle dural stems, which forms most of the transverse sinus. Ventrally the sagittal plexus differentiates into the inferior sagittal and straight sinuses and the great cerebral vein: it commonly drains into the left transverse sinus.

The vessels along the ventrolateral edge of the developing cerebral hemisphere form the transitory tentorial sinus, which drains the convex surface of the cerebral hemisphere and basal ganglia, and the ventral aspect of the diencephalon, into the transverse sinus. With expansions of the cerebral hemispheres, and in particular the emergence of the temporal lobe, the tentorial sinus becomes elongated, attenuated and eventually disappears; its territory is drained by enlarging anastomoses of pial vessels which become the basal veins, radicles of the great cerebral vein.

The anterior dural stem disappears and the caudal part of the primary head vein dwindles: it is represented in the adult by the inferior petrosal sinus. The cranial part of the primary head vein, medial to the trigeminal ganglion, persists and still receives the stem of the primitive maxillary vein. The latter has now lost most of its tributaries to the anterior facial vein, and its stem becomes the main trunk of the primitive supraorbital vein, which will form the superior ophthalmic vein of the adult. The main venous drainage of the orbit and its contents is now carried via the augmented middle dural stem, the pro-otic sinus, into the transverse sinus and, at a later stage, into the cavernous sinus. The cavernous sinus is formed from a secondary plexus derived from the primary head vein and lying between the otic and basioccipital cartilages. The plexus forms the inferior petrosal sinus which drains through the primordial hindbrain channel into the internal jugular vein. The superior petrosal sinus arises later from a ventral metencephalic tributary of the pro-otic sinus and it communicates secondarily with the cavernous sinus. The pro-otic sinus meanwhile has developed a new and more caudally situated stem, the petrosquamosal sinus, which drains into the sigmoid sinus. With progressive ossification of the skull, the pro-otic sinus becomes diploic in position.

The development of the venous drainage and portal system of the hypophysis cerebri is closely associated with that of the venous sinuses.