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. The growing tips of neuroblasts have been studied extensively in tissue culture. 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 can occur. These processes are stabilized in one direction, determining the direction of future growth, and after 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, along with a core of centrally located microtubules and sometimes neurofilaments. The assembly of these components, as well as the synthesis of new membrane, occurs in segments distal to the cell body and behind the growth cone, although 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 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. It is possible that 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, secretomotor 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 inhibitory. Short-range cues require factors that are displayed on cell surfaces or in the extracellular matrix; for example, axon extension requires a permissive, physical substrate, the molecules of which are actively recognized by the growth cone. They also require negative cues that 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 toward 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 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 the release of these factors, 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 cortices, for example, start out with a 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 that 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; for instance, 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 to be remarkably stable, and partial deafferentation affects only dendritic spines or similar small details. As development proceeds, plasticity is lost, and soon after birth a neurone 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 or even the whole soma. Different regions of the nervous system vary quantitatively in their responses 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, such as in the developing limb, results in reduced numbers of motor and sensory neurones. The specific factor produced by these target organs is termed nerve growth factor (NGF). NGF is taken into nerve endings and transported back to the neuronal somata. It is necessary for the survival of many types of neurones during early development and for the growth of their axons and dendrites, and it promotes the synthesis of neurotransmitters and enzymes. Antibodies to NGF cause the death of neuronal subsets when they reach their targets, and added NGF rescues neurones that would otherwise die. Since the discovery of NGF, several other trophic factors have been identified, including brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and NT-4/5.

Neurotrophins 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, sensory proprioceptive and placode-derived neurones, such as those of the nodose ganglion, which are unresponsive to NGF. NT-3 has effects on motor neurones and both placode- and neural crest–derived sensory neurones. Other growth factors that 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 tyrosine kinase receptors bind with higher affinity and display binding preferences for particular neurotrophins. However, the presence of a tyrosine kinase 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, not vice versa.

Spinal Nerves

Each spinal nerve is connected to the spinal cord by a ventral root and a dorsal root (Fig. 3.18). 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; 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 (S2–4).

Fig. 3.18 Transverse sections through the developing spinal cord of human embryos. A, Approximately 6 weeks old. B, Approximately 3 months old.

The fibres of the dorsal roots extend from cell somata in dorsal root ganglia into the spinal cord and also 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; see Fig. 3.12). 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, whereas 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, which are initially 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, and 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; those of sensory neuroblasts are derived from the neural crest, with additional contributions in the head from ectodermal placodes (Fig. 3.19).

Fig. 3.19 Brain and cranial nerves of a human embryo, 10.2 mm long. Note the ganglia (stippled) associated with the trigeminal, facial, vestibulocochlear, glossopharyngeal, vagus and spinal accessory nerves. Froriep’s ganglion, an occipital dorsal root ganglion, is inconstant and soon disappears.

The motor fibres of the cranial nerves that 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 divided into two pathways (see Figs. 3.2B, 3.19). 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, the 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. All these nerves have nerve exit points more dorsally placed than in the somatic motor system.

The cranial nerves also contain general visceral efferent neurones (parasympathetic preganglionic neurones) that 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, and the prechordal mesenchyme gives rise to the extrinsic ocular muscles. Special branchial efferent nerves supply the striated muscles developing within the pharyngeal (branchial) arches, 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, because of 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 with the somatic efferent nerves, the distinction between the two is of some value.

General visceral efferent neurones (parasympathetic preganglionic neurones) innervate the 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 (see Figs. 3.13, 3.19). Generally, neurones distal to the brain are derived from placodes, and proximal ones are derived from the neural crest (see Fig. 3.19). 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. The same applies to the more caudal cranial nerves (facial, glossopharyngeal, vagus), but the two cell populations form separate ganglia in the case of each nerve. The proximal series of ganglia is derived from neural crest (forming the proximal ganglion of the facial nerve, the superior ganglion of the glossopharyngeal nerve and the jugular ganglion of the vagus nerve); 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 nerve). These ganglia contain neurones that mediate special, general visceral and somatic afferent functions. The vestibulocochlear nerve has a vestibular ganglion that contains both crest and placodal cells and an acoustic ganglion from placodal neurones only; it conveys special somatic afferents.

The neurones and supporting cells of the cranial autonomic ganglia in the head and trunk originate from neural crest cells. Caudal to the ganglion of the vagus nerve, 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; they subsequently regress. Ganglion cells are found on the developing spinal root of the accessory nerve, and these are believed to persist in the adult. The central processes of the cells of these various ganglia, where they persist, form some sensory roots of the cranial nerves and 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 are described: cranial neural crest, cardiac neural crest and vagal neural crest. Migration of the sacral neural crest and 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.

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, among other things, 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 to 7 are collectively termed vagal neural crest; they migrate to the gut along with the sacral neural crest.

Sympathetic Ganglia

Neural crest cells migrate ventrally within the body segments to penetrate the underlying somites and continue to the region of the future paravertebral and prevertebral plexuses, notably forming the sympathetic chain of ganglia as well as the major ganglia around the ventral visceral branches of the abdominal aorta (see Figs. 3.12, 3.20).

There is cell-specific recognition of postganglionic neurones and the growth cones of sympathetic preganglionic neurones. They meet during growth, and this may be important in terms of 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 apparently does not influence the appearance of the cell processes.

The local environment is the major factor that controls the appropriate differentiation of the presumptive autonomic ganglion neurones. The factors responsible for subsequent adrenergic, cholinergic or peptidergic phenotype have yet to be identified, although it has been proposed that fibronectin and basal lamina components initiate adrenergic phenotypical expression at the expense of melanocyte numbers. Cholinergic characteristics are acquired relatively early, and the appropriate phenotypical expression may be promoted by cholinergic differentiation factor and CNTF.

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. Preganglionic fibres that supply the intestine, and therefore modulate the enteric neurones, are much fewer.

The enteric nervous system is derived from the neural crest. The axial levels of crest origin are shown in Figure 3.20. 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 that may be controlled by the local splanchnopleuric mesenchyme. Cranial neural crest from somite levels 1 to 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, or somite 28 onward, form components of the pelvic plexus after migrating through the somites toward the level of the colon, rectum and cloaca. Initially the cells 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 outward 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 associated with other defects of neural crest development, including Waardenburg’s syndrome type II, 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 (see Fig. 3.6). Their disposition may be determined in part by contact guidance along the earliest radial array of glial fibres that 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 produced. As growth proceeds, these enlargements, which are further increased by 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 (see Fig. 3.18). 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 that 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 it regresses at other levels.

Axons arising from ventral horn neurones—that is, 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 ventral column cells, which characterizes the mature cord, occurs early; by the fourteenth 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 less thick until it ultimately forms the single-layered ependyma that lines the ventral part of the central canal of the spinal cord.

Posterior (Dorsal) Grey Column

The posterior (dorsal) column develops later; consequently, for a time, the ventricular zone is much thicker in the dorsolateral lamina (alar plate) than it is in the ventrolateral lamina (basal plate) (see Fig. 3.6).

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 (see Fig. 3.18). 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, derived from neuroblasts in the intermediate zone, and fibres of dorsal roots of spinal nerves that pass into the spinal cord, derived 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 (see Figs. 3.6, 3.18). This bundle increases in size and, spreading toward the median plane, forms the primitive, fine-calibre posterior funiculus. 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—the ventral and dorsal nerve roots about the fifth month, and 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 it is formed 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 their respective limb buds develop.

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 (Fig. 3.21). Most of this relative rostral migration occurs during the first half of intrauterine life. By the twenty-fifth week the terminal ventricle of the spinal cord has altered in level from the second coccygeal vertebra to the third lumbar vertebra, a distance of nine segments. Because 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 occasionally gives rise to congenital cysts in the neighbourhood of the coccyx. In the definitive state, the upper cervical spinal nerves retain their position at roughly right angles to the cord. Proceeding caudally, the nerve roots lengthen and become progressively more oblique.

Fig. 3.21 Meningomyelocele due to failure of closure of the caudal neural tube.

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.

When performing a lumbar puncture, it is important to enter the spinal canal below the level of the tip of the conus medullaris. Although this is usually at or above the level of the second lumbar vertebra, in some individuals the cord may rarely extend as low as the third lumbar vertebra. It is therefore advisable for the needle to enter the canal below this level.

Rhombencephalon

By the time the midbrain flexure appears, the length of the hindbrain is greater than that of the combined extent of the other two brain vesicles. Rostrally it exhibits a constriction, the isthmus rhombencephali (see Fig. 3.2B), which is 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 maximal convexity, so the outline of the roof plate becomes rhomboidal. Due to 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. 3.23); at the same time, the lateral angles of the cavity extend to form the lateral recesses of the fourth ventricle.

At about the end of the fourth week, when the pontine flexure is first discernible, a series of seven transverse rhombic grooves appears in the ventrolateral laminae (basal plate) of the hindbrain. Between the grooves, the intervening masses of neural tissue are termed rhombomeres. These are closely associated with the pattern of the underlying motor nuclei of certain cranial nerves. The general pattern of distribution of motor nuclei seems to be as follows: rhombomere 1 contains the trochlear nucleus, rhombomeres 2 and 3 the trigeminal nucleus, rhombomeres 4 and 5 the facial nucleus, rhombomere 5 the abducens nucleus, rhombomeres 6 and 7 the glossopharyngeal nucleus and rhombomeres 7 and 8 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. With further morphogenesis, however, the obvious constrictions of the rhombomere boundaries disappear, and the medulla once again assumes a smooth contour. 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; 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 but interrupted columns positioned ventrally and dorsally, with an intermediate column between (Fig. 3.22). 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, which are somatic efferent nuclei. The intermediate column is represented in the upper part of the spinal cord and caudal brain stem (medulla oblongata and pons) and is for the supply of branchial (pharyngeal) and postbranchial musculature. It is interrupted and forms the elongated nucleus ambiguus in the caudal brain stem, which gives fibres to the ninth, tenth and eleventh cranial nerves. The latter 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. These three nuclei are termed branchial (special visceral) efferent nuclei. The most dorsal column of the basal plate (represented in the spinal cord by the lateral grey column) innervates viscera. It too is interrupted, with its large caudal part forming some of the dorsal nucleus of the vagus and its cranial part the salivatory nucleus. These are termed general visceral (general splanchnic) efferent nuclei, and their neurones give rise to preganglionic, parasympathetic nerve fibres.

Fig. 3.22 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 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 lamina columns.

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

Myelencephalon

The caudal slope of the embryonic hindbrain constitutes the myelencephalon, which develops into the medulla oblongata (see Fig. 3.2). The nuclei of the ninth, tenth, eleventh and twelfth cranial nerves develop in the positions already indicated, and afferent fibres from the ganglia of the ninth and tenth nerves form an oval marginal bundle in the region overlying the alar (dorsolateral) lamina. Throughout the rhombencephalon, the dorsal edge of this lamina is attached to the thin expanded roof plate and is termed the rhombic lip. (The inferior rhombic lip is confined to the myelencephalon; the superior rhombic lip to the metencephalon.) As the walls of the rhombencephalon spread outward, the rhombic lip protrudes as a lateral edge that becomes folded over the adjoining area. The rhombic lip may later become adherent to this area, and its cells migrate actively into the marginal zone of the basal plate. In this way the oval bundle that forms the tractus solitarius becomes buried. Alar plate cells that migrate from the rhombic lip are believed to give rise to the olivary and arcuate nuclei and the scattered grey matter of the nuclei pontis. While this migration is in progress, the floor plate is invaded by fibres that cross the median plane (accompanied by neurones that cluster in and near this plane), and it becomes thickened to form the median raphe. Some of the migrating cells from the rhombic lip in this region do not reach the basal plate and form an oblique ridge: the corpus pontobulbare (nucleus of the circumolivary bundle) across the dorsolateral aspect of the inferior cerebellar peduncle.

The lower (caudal half) part of the myelencephalon has no role in the formation of the fourth ventricle, and in its development it closely resembles the spinal cord. The gracile and cuneate nuclei, and some reticular nuclei, are derived from the alar plate, and their efferent arcuate fibres and interspersed neurones play a large part in the formation of the median raphe.

At about the fourth month the descending corticospinal fibres invade the ventral part of the medulla oblongata to initiate formation of the pyramids. Contemporaneously, the inferior cerebellar peduncle is formed dorsally by ascending fibres from the spinal cord and by olivocerebellar and parolivocerebellar fibres, external arcuate fibres and two-way reticulocerebellar and vestibulocerebellar interconnections. (The reticular nuclei of the lower medulla probably have a dual origin from both basal and alar plates.) In the neonate the brain stem is more oblique and has a distinct bend as it passes through the foramen magnum to become the spinal cord.

Metencephalon

The rostral slope of the embryonic hindbrain is the metencephalon, from which both the cerebellum and the 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 (Fig. 3.23). Accentuation of the flexure approximates the cranial angle of the ventricle to the caudal angle, and the alar plates of the metencephalon now lie almost horizontally.

Fig. 3.23 A, Cerebellum of a fetus in the fifth month. B, Dorsal aspect of the hindbrain of a human fetus approximately 3 months old, viewed partly from the right side.

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, at about the fourth month the pons is invaded by corticopontine, corticobulbar and corticospinal fibres; it becomes proportionately thicker and takes on its adult appearance. It is relatively smaller in a full-term neonate.

The region of the isthmus rhombencephali undergoes a series of changes that are notoriously difficult to interpret but 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 is 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.

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 that project partly into the ventricle at first (see Fig. 3.23), 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 (dumbbell shaped) swelling stretched across the rostral part of the fourth ventricle (see Fig. 3.23). 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 that characterize the surface of the mature cerebellum (Fig. 3.24).

Fig. 3.24 Median sagittal sections through the developing cerebellum, showing four different stages.

The first fissure to appear on the cerebellar surface (see Fig. 3.24) 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 (see Fig. 3.24). The cerebellum now grows dorsally, rostrally, caudally and laterally, and the hemispheres expand much more than does the inferior vermis, which 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 and the former has only three. However, whereas in the cerebral cortex neuroblasts originate from the ventricular zone and migrate ventriculofugally toward 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: 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 that will give rise to the radial glia, cerebellar nuclei and efferent neurones of the cerebellar cortex (Purkinje cells) (Fig. 3.25). 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. Extroversion of the cerebellum begins later, when cells of the external granular layer, also termed the superficial matrix, begin proliferating and migrating. These cells produce the granule cells, which migrate inward along the radial glia and through the layers of Purkinje cells, settling deep to them in the granular layer. This stage coincides with the emergence of the transverse folial pattern. Proliferation and migration of granule cells lead 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 efferent neurones of the cerebellar cortex and nuclei precedes the development of its afferent organization.

Fig. 3.25 Four stages in the histogenesis of the cerebellar cortex and cerebellar nuclei. A, Purkinje cells and cells of the cerebellar nuclei are produced by the ventricular epithelium and are in the process of migrating to their future positions. The cells of the superficial matrix (external granular layer) originate 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 granular layer. Cell production in the ventricular epithelium has stopped. The remaining cells transform into ependymal cells. C, Granule cells are produced by the external granular layer and migrate inward through the Purkinje cell layer to their position in the 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 is subdivided into clusters that form columns extending rostrocaudally (Fig. 3.26). The medial Purkinje cell clusters develop into the future vermis. These Purkinje cells will grow axons that 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 that project to a single vestibular or cerebellar nucleus.

Fig. 3.26 Coronal section through the cerebellum and brain stem of a 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. B, brain stem; D, dentate nucleus; EGL, external granular layer; m, midline; 4, fourth ventricle.

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

Mesencephalon

The mesencephalon or midbrain is derived from the intermediate primary cerebral vesicle. 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. 3.2, 3.27). 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 small at first 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 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 giving rise to the trigeminal mesencephalic nucleus arise on either side of the dorsal midline, from the isthmus rhombencephali 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. 3.27 A, Brain of a human embryo, approximately 10.2 mm long. B, Medial surface of the right half of the brain of a human embryo, 13.6 mm long. The roof of the hindbrain has been removed. C, Medial surface of the right half of the brain of a human fetus, approximately 3 months old.

The cells of the dorsal part of the alar (dorsolateral) plates proliferate and invade the roof plate, which 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, are not 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 displayed in similar sections of the spinal cord and medulla oblongata: 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 (see Fig. 3.27). This sulcus ends anteriorly 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 is 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 closure of the rostral neuropore, two lateral diverticula—the optic vesicles—appear, one on each side, at about the level of the prosencephalon. For a time they communicate with the cavity of the prosencephalon by relatively wide openings. The distal parts of the optic vesicles expand, and the proximal parts become the tubular optic stalks. The optic vesicles (which are described in the section on the development of the eye) are derived from the lateral walls of the prosencephalon before the telencephalon can be identified. They are usually regarded as derivatives of the diencephalon, and the optic chiasma is often regarded as the boundary between the diencephalon and telencephalon.

As the most rostral portion of the prosencephalon enlarges, it curves ventrally, and two additional diverticula rapidly expand from it, one on each side. These diverticula are rostrolateral to the optic stalks and 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 that ultimately become the interventricular foramina. The anterior limit of the median part of the forebrain consists of a thin sheet, the lamina terminalis (see Fig. 3.27), which stretches from the interventricular foramina to the recess at the base of the optic stalks. The anterior part of the forebrain, including the rudiments of the cerebral hemispheres, is the telencephalon (endbrain) and the posterior part of the diencephalon (between brain). Both contribute to the formation of the third ventricle, although the latter predominates. The fate of the lamina terminalis is described later.

Diencephalon

The diencephalon is broadly divided by the hypothalamic sulcus into dorsal (pars dorsalis diencephali) and ventral (pars ventralis diencephali) parts; these, however, are composite, and 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 (see Fig. 3.27) is first visible as a thickening that involves the anterior part of the dorsal area. Caudal to the thalamus, the lateral and medial geniculate bodies, or metathalamus, are first recognizable as surface depressions on the internal aspect and as elevations on the external aspect of the lateral wall. As the thalami enlarge to become smooth ovoid masses, the wide interval between them gradually narrows into a vertically compressed cavity that 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. 3.28) 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. 3.28 Development of the basal nuclei and internal capsule.

(Redrawn by permission from Hamilton, W.J., Boyd, J.D., Mossman, H.W. 1972. Human Embryology: Prenatal Development of Form and Function. Williams and Wilkins, Baltimore.)

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 to 20 mm crown–rump 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, ultimately, structures of epithalamic origin are 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 is two-fold. The anterior outgrowth (parapineal organ) develops into the pineal or parietal eye, whereas the posterior outgrowth is glandular in character. The posterior outgrowth is homologous with the pineal gland in humans. The anterior 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 that 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 the infundibulum of the hypophysis. The mammillary bodies arise as a single thickening, which becomes divided by a median furrow during the third month. Anterior to them, the tuber cinereum develops as a cellular proliferation that extends forward as far as the infundibulum. In front of the tuber cinereum, a wide-mouthed diverticulum forms in the floor of the diencephalon. It grows toward 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 (see Fig. 3.15). 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 subsequently invaginated by the choroid plexuses of the third ventricle (Fig. 3.29). 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 carry numerous microvilli on their ventricular surfaces; basally, the plasma membrane becomes complexly folded into the cell. 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 becomes increasingly vascularized, the histochemical reactions of the cuboidal cells and the character of the fluid change to the adult type. The remaining lining of the third ventricle does not simply form generalized ependymal cells. Many regions become highly specialized, developing concentrations of tanycytes or other modified cells (e.g. those of the subfornical organ), the organum vasculosum (intercolumnar tubercle) of the lamina terminalis, the subcommissural organ and those lining the pineal, suprapineal and infundibular recesses, which are collectively termed the circumventricular organs.

Fig. 3.29 Coronal section of the left cerebral hemisphere in a 73-mm fetus.

(Redrawn by permission from Hamilton, W.J., Boyd, J.D., Mossman, H.W. 1972. Human Embryology: Prenatal Development of Form and Function. Williams and Wilkins, Baltimore.)

Telencephalon

The telencephalon (endbrain) consists of two lateral diverticula connected by a median region (the telencephalon impar). The anterior 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; these 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 (see Fig. 3.27). The anterior 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 backward 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 the initially relatively large interventricular foramina, except caudally, where they are continuous with the anterior part of the lateral wall of the diencephalon (see Figs. 3.2, 3.27). As growth proceeds, the hemisphere enlarges forward, upward and backward 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 that fill the median longitudinal fissure (see Fig. 3.29). 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 larger remainder of the hemisphere forms the anlage of the primitive corpus striatum and amygdaloid complex, including its associated rim of lateral and medial walls; this is the striate part of the hemisphere. The rest of the hemisphere—the medial, lateral, dorsal and caudal regions—is the suprastriate part of the hemisphere. Although largest in terms of surface area, it initially possesses comparatively thin walls. The rostral end of the 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 toward 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. 3.30). The great expansion of the cerebral hemispheres is characteristic of mammals and especially of humans. In their subsequent growth they 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.

Fig. 3.30 Formation of the basal nuclei and lateral ventricles as the telencephalon develops.

(Redrawn by permission from Hamilton, W.J., Boyd, J.D., Mossman, H.W. 1972. Human Embryology: Prenatal Development of Form and Function. Williams and Wilkins, Baltimore.)

Olfactory nerve, limbic lobe and hippocampus

The growth changes in the temporal lobe that help 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 other archaeocortical areas anterior 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, when present, the intermediate stria terminate in the rostral parts of the piriform area. This includes the olfactory trigone and tubercle, anterior perforated substance and uncus (hook) and entorhinal area of the anterior part of the future parahippocampal gyrus. Its lateral limit 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, with the summit of the convexity lying at the anteroinferior corner of the developing insula (Fig. 3.31). During the fourth and fifth months, much of the piriform area becomes submerged by the adjoining neocortex, and in the adult, only part of it remains visible on the inferior aspect of the cerebrum.

Fig. 3.31 A–G, Superolateral surfaces of human fetal cerebral hemispheres at the ages indicated, showing the changes in size and profile and the emerging pattern of cerebral sulci with increasing maturation. Note the changing prominence and relative positions of the frontal, occipital and particularly temporal poles 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 (G), all the features that 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 (bordering) lobe is the first part of the cortex to differentiate. 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 part of the piriform area. The portion outside the curve of the choroid fissure (Fig. 3.32) constitutes the hippocampal formation. In this region the neural progenitors of the developing cortex proliferate and migrate, and the wall of the hemisphere thickens and produces an elevation that projects into the medial side of the ventricle: this 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 backward, 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 neuroblasts to form the dentate gyrus. Both extend from the paraterminal area backward, above the choroid fissure, and follow its curve downward and forward toward 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.

Fig. 3.32 Medial aspect of the left half of the brain of a human fetus, 16 weeks old.

The efferent fibres from the cells of the hippocampus collect along its medial edge and run forward immediately above the choroid fissure. Anteriorly 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.

Development of commissures

The development of the commissures causes a profound alteration in 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 from hemisphere to hemisphere across the median plane; these fibres therefore pass through the anterior wall of the interventricular foramen, 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 anterior part of the anterior commissure (Figs. 3.32, 3.33). In addition, the two hippocampi become interconnected by transverse fibres that cross from fornix to fornix in the upper part of the lamina terminalis as the commissure of the fornix. 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. 3.33 Formation of the commissures. The telencephalon gives rise to commissural tracts that integrate the activities of the left and right cerebral hemispheres. These include the anterior and hippocampal commissures and the corpus callosum. The small posterior and habenular commissures arise from the epithalamus. A, Ten weeks. B, Sixteen weeks.

(By permission from Larsen.)

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 (see Figs. 3.32, 3.33).

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 term has been rejected here because of increasing use of the adjective ‘precommissural’ to denote the position of parts of the limbic lobe—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 backward to form the trunk of the corpus callosum. The rostrum of the corpus callosum develops later and separates part 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 backward, it extends above the choroidal fissure, carrying the commissure of the fornix on its undersurface. 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 archaeocortical hippocampal formation, and the corresponding parts of the dentate gyrus and hippocampus are reduced to vestiges, the indusium griseum and the longitudinal striae (see Figs. 3.32, 3.33). However, the posteroinferior (temporal) archaeocortical regions of both the dentate gyrus and the 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, 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 that migrate to and mature in their final positions. These phases of proliferation, migration, differentiation and maturation overlap one another in space and time and are not precisely sequential.

The earliest migration of neuronal precursors from the ventricular and intermediate zones occurs radially until they approach, but do not reach, the pial surface. Their somata become arranged as a transient cortical plate. Subsequently, proliferation wanes in the ventricular zone but persists for considerable periods in the immediately subjacent subventricular zone. From the pial surface inward, the following zones may be defined: marginal, cortical plate, subplate, intermediate, subventricular and ventricular (see Fig. 3.5). The marginal zone gives rise to the outermost layer of the cerebral cortex, and the neuroblasts of the cortical plate and subplate form the neurones of the remaining cortical laminae (the complexity varies 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 that 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 phases of proliferation vary spatiotemporally with location 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 that is most prominent during mid-gestation, contains neurones surrounded by a dense neuropil; it is the site of the most intense synaptogenesis in the 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. 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.)

Mechanisms of cortical development

Rakic (2003) initially demonstrated the migration of neuroblasts along radial glial processes, and this has subsequently been seen to occur in three phases. First, the neuroblasts 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 neuroblasts migrate past them toward the pial surface (see Fig. 3.5, Figs. 3.34, 3.35). 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.

Fig. 3.34 Dynamics of neuroblast migration during transformation of the early cranial neural tube to form the cerebral neocortex of the rat through days 10 to 28. Note the successive waves of migration. Symbolic metaphase chromosomes, mitotic cells; full black discs, ventricular and subventricular zone neuroblasts; full yellow discs, infragranular neuroblasts destined for lamina VI; full magenta discs, infragranular neuroblasts destined for lamina V; open black circles, granular neuroblasts destined for lamina IV; full blue discs, supragranular neuroblasts destined for laminae III and II.

(Redrawn and colour-coded from data provided by Professor M. Berry [1974], Anatomy Department, Guy’s Hospital Medical School, London.)

Fig. 3.35 Initial stages of the formation of apical and basal dendrites of pyramidal neurones and of stellate neurone dendrites in the cortical plate. Note radial glial cells (black) extending from the internal to external limiting membrane; these provide contact guidance paths for neuroblasts. 1, Migration of a presumptive pyramidal neurone (magenta); 2, migration of a presumptive stellate neurone (purple). Time increments are from left to right.

(After Berry, M. 1982. Cellular differentiation. Neurosci. Res. Prog. Bull. 20, 451–461.)

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 an 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 that remain apposed to the ventricular surface but develop an appropriate phenotype and efferent projections. Transplantation of labelled cells suggests that commitment to a particular cortical lamina occurs shortly after S phase. Neurones of preexisting laminae that have begun axonogenesis may provide 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 into a number of areas, displaying a hierarchy of organization. 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 areas concerned with functions that are even less well 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 to 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% to 85% of cells 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 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 lateral ganglionic eminence into the developing cortex.

Two models have been proposed to explain the development of this complex cortical organization. The ‘protocortex’ model assumes that the proliferative ventricular epithelium is a ‘tabula rasa’ that generates homogeneous layers of neurones that are patterned solely by the ingrowth of processes from the thalamus. The ‘protomap’ hypothesis proposes that the intrinsic differences between the various areas are specified prior to cell 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 coordinates 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 adulthood. 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; the medial edge or cortical hem expresses signalling molecules of the WNT and BMP families. Any or all of these may be the components of short-range signalling centres along the edges of the cortex. Coupled with the gradients of transcription factors such as Emx2 and Lhx2, there is evidence to support the protomap hypothesis (Donoghue and Rakic 1999).

However, studies of cell migration are consistent with the idea that cortical areas might not be rigidly determined. 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 and 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, and receptive field and response properties resembled those seen in 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 in terms of both 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 that 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 extend axons via the internal capsule to the thalamus and superior colliculus 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, whereas 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 neurones at various times in development has shown that rather than being generated de novo, these patterns seem to arise by the 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 by 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 were labeled later 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. Lissencephaly constitutes a broad class of neuronal migration disorders in which the cortex has a normal thickness but a decreased number of neurones and a smooth 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 levels of the lipid messenger platelet-activating factor. How this translates into a cell migration defect remains 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 is established, but each column contains fewer neurones, implying either a reduced rate of proliferation and cell migration or an enhanced level of cell death.

Neonatal Brain and Reflexes

The brain of the full-term neonate ranges from 300 to 400 g, with an average of 350 g; the brains of male neonates are slightly heavier than those of females. Because the head is large at birth, measuring one-quarter of the total body length, the brain is also proportionally larger and constitutes 10% of body weight, compared with 2% in the adult. At birth the volume of the brain is 25% of its adult volume. The greatest increase occurs during the first year, at the end of which the volume of the brain has increased to 75% of its adult volume. This growth can be accounted for partly by the increased size of nerve cell somata, by the profusion and dimensions of their dendritic trees, axons and collaterals and by the growth of neuroglial cells and cerebral blood vessels; however, it mainly reflects the acquisition of myelin sheaths by the axons. The sensory pathways—visual, auditory and somatic—myelinate first; the motor fibres myelinate later. During the second and subsequent years, growth proceeds much more slowly. The brain reaches 90% of its adult size by age 5 years and 95% by 10 years. The brain attains adult size by the seventeenth or eighteenth year. This is largely due to continued myelination of various groups of nerve fibres.

The sulci of the cerebral hemispheres appear from the fourth month of gestation (see Fig. 3.31), and at full term the general arrangement of sulci and gyri is present, but the insula is not completely covered. The central sulcus is situated farther 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 and the optic 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 the skull increases in volume until the adult brain occupies only 92.5% of the cranial cavity. Although the cerebral ventricles are larger in the neonate than in the adult, the newborn has a total of 10 to 15 ml of cerebrospinal fluid when delivered vaginally and 30 ml when delivered by caesarean section.

Meninges

The meningeal layers originate from paraxial mesenchyme in the trunk and caudal regions of the head and from the neural crest in regions rostral to the mesencephalon (the prechordal plate may also make a contribution). Those skull bones formed from neural crest, such as the base of the skull rostral to the sella turcica and the frontal, parietal and squamous temporal bones, overlie meninges that are also formed from crest cells.

During development the meninges can be divided into the pachymeninx (dura mater) and the leptomeninges (arachnoid mater, subarachnoid space with arachnoid cells and fibres and pia mater). All meningeal layers are derived from loose mesenchyme that surrounds the developing neural tube, termed the meninx primitiva or primary meninx. (For a detailed account of development of the meninges in the human, consult O’Rahilly and Muller 1986.)

The first indication of pia mater, consisting of a plexus of blood vessels that forms on the neural surface, is seen at stage 11 (24 days) around the caudalmost part of the medulla; this extends to the mesencephalic level by stage 12. Mesenchymal cells projecting from the rostral end of the notochord, and those in the region of the prechordal plate, extend rostrally into the mesencephalic flexure and form the earliest cells of the tentorium cerebelli; at the beginning of its development, the medial part of the tentorium is predominantly leptomeningeal. By stage 17 (41 days), dura mater can be seen in the basal areas where the future chondrocranium is also developing. Precursors of the venous sinuses lie within the pachymeninx at stage 19 (48 days), and by stage 20, cell populations in the region of the future falx cerebri are proliferating, although the dorsal regions of the brain are not yet covered with putative meninges.

By stage 23 (57 days), the dura is almost complete over the rhombencephalon and mesencephalon but is present only laterally around the prosencephalon. Subarachnoid spaces and most of the cisternae are present from this time, after the arachnoid mater becomes separated from the primitive dura mater by the accumulation of cerebrospinal fluid (which now has a net movement out of the ventricular system). The medial part of the tentorium is becoming thinner. A dural component of the tentorium is seen from stage 19. The earlier medial portion disappears, leaving an incomplete partition that separates a subarachnoid area containing the telencephalon and diencephalon from one containing the cerebellum and rhombencephalon.

There is a very close relationship, during development, between the mesenchyme from which the cranial dura mater is formed and that which is chondrified and ossified, or ossified directly, to form the skull. These layers are clearly differentiated only as the venous sinuses develop. The relationship between the developing skull and the underlying dura mater continues during postnatal life while the bones of the calvaria are still growing.

Growth of the cranial vault is initiated from ossification centres within the desmocranial mesenchyme. A wave of osteodifferentiation moves radially outward from these centres, stopping when adjacent bones meet at regions where sutures are induced to form. Once sutures are formed, a second phase of development occurs in which growth of the cranial bones occurs at the sutural margins. This growth forms most of the skull. A number of hypotheses have been generated to explain the process of suture morphogenesis. It has been suggested that the dura mater contains fibre tracts that extend from fixed positions in the cranial base to sites of dural reflection underlying each of the cranial sutures, and the tensional forces so generated dictate the position of the sutures and locally inhibit precocious ossification. Other hypotheses support the concept of local factors in the calvaria that regulate suture morphogenesis. Following removal of the entire calvaria, the skull regenerates and sutures and bones develop in anatomically correct positions, suggesting that the dura can dictate suture position at least in regeneration of the neonatal calvaria. In transplants of sutures in which the fetal dura mater was left intact, a continuous fibrous suture remained between developing vault bones, whereas in transplants in which the fetal dura mater was removed, bony fusion occurred (Opperman et al 1993).

The presence of fetal dura is not required for initial suture morphogenesis, which appears to be controlled by mesenchymal cell proliferation and fibrous extracellular matrix synthesis induced by overlapping of the advancing osteoinductive fronts of the calvarial bones. It is thought that following overlap of the bone fronts, a signal is transferred to the underlying dura that induces changes in localized regions beneath the sutures. Once a suture has formed, it serves as a primary site for cranial bone growth, but constant interaction with the dura is required to avoid ossiferous obliteration.

Meningeal Arteries

At stages 20 to 23 (7 to 8 weeks’ gestation), further expansion of the cerebral hemispheres completes the circle of Willis, with development of the anterior communicating arteries by 8 weeks’ gestation. An anular network of meningeal arteries originates, mainly from each middle cerebral artery, and passes over each developing cerebral hemisphere. Caudally, similar meningeal branches arise from the vertebral and basilar arteries and embrace the cerebellum and brain stem. The further development of the telencephalon somewhat obscures this early pattern over the cerebrum.

The meningeal arteries so formed have been classified into three groups: paramedian, short circumferential and long circumferential arteries. They can be described both supratentorially and infratentorially: all give off fine side branches and end as penetrating arteries. Of the supratentorial vessels, the paramedian arteries have a short course prior to penetrating the cerebral neuropil (e.g. branches of the anterior cerebral artery); the short circumferential arteries have a slightly longer course before becoming penetrating arteries (e.g. the striate artery); the long circumferential arteries reach the dorsal surface of the hemispheres. Infratentorial meningeal arteries are very variable. The paramedian arteries, after arising from the basilar or vertebral arteries, penetrate the brain stem directly. The short circumferential arteries end at the lateral surface of the brain before penetration, and the long circumferential arteries later form the range of cerebellar arteries. These vessels, arranged as a series of loops over the brain, arise from the circle of Willis and brain stem vessels on the base of the brain.

At 16 weeks’ gestation the anterior, middle and posterior cerebral arteries contributing to the formation of the circle of Willis are well established. The meningeal arteries arising from them display a simple pattern, with little tortuosity and very few branches. With increasing age of the fetus and acquisition of the gyral pattern on the surface of the brain, their tortuosity, diameter and number of branches all increase. The branching pattern is complete by 28 weeks’ gestation, and the number of branches does not increase further. Numerous anastomoses (varying in size from 200 to 760 µm) occur between the meningeal arteries in the depths of the developing sulci, nearly always in the cortical boundary zones of the three main cerebral arteries supplying each hemisphere. The number, diameter and location of these anastomoses change as fetal growth progresses, reflecting the regression and simplification of the complex embryonic cerebral vascular system. The boundary zones between the cerebral arteries may be the sites of inadequate perfusion in the premature infant.

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 floor plate. Subsequently, additional sprouts penetrate the inter-rhombomeric regions on the walls and floor of the hindbrain. Branches from the latter elongate toward and join the branches in the inter-rhombomeric junctions, forming primary vascular channels between rhombomeres and longitudinally on each side of the median floor plate. Later, additional sprouts invade the hindbrain within the rhombomeres, anastomosing in all directions.

The meningeal perforating branches pass into the brain parenchyma as cortical, medullary and striate branches (Fig. 3.37). The cortical vessels supply the cortex via short branches that may form precapillary anastomoses, whereas the medullary branches supply the white matter. The latter converge toward the ventricle but rarely reach it; they often follow a tortuous course as they pass around bundles of nerves. The striate branches, which penetrate 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 ones 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. 3.37 Development of cerebral blood vessels. A, The brain is surrounded by a system of leptomeningeal arteries from afferent trunks at the base of the brain. Intracerebral arteries arise from this system and converge (ventriculopetally) toward 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 toward the ventriculopetal vessels without making anastomoses. C, Arrangement of ventriculopetal and ventriculofugal vessels around a cerebral hemisphere. D, 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, from Hambleton, G., Wigglesworth, J.S. 1976. Origin of intraventricular haemorrhage in the preterm infant. Arch. Dis. Child. 51, 651–659, by permission from the BMJ Publishing Group.)

The cortical and medullary branches irrigate a series of corticosubcortical cone-shaped areas, centred around a sulcus containing an artery. They supply a peripheral portion of the cerebrum and are grouped as ventriculopetal arteries. Striate branches, in contrast, 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. The latter supply the ventricular zone (germinal matrix of the brain) and send branches toward the cortex. The ventriculopetal and ventriculofugal arteries run toward each other but do not make any connections or anastomoses (see Fig. 3.37); however, the ventriculopetal arteries form networks of small arterioles. The ventriculopetal vessels supply relatively more mature regions of the brain compared with the ventriculofugal vessels, which are subject to constant remodelling and do not develop tunicae mediae until ventricular zone proliferation is complete. The boundary zone between these two systems (outer centripetal and inner centrifugal) has practical implications related to the location of ischaemic lesions (periventricular leukomalacia [PVL]) in the white matter of premature infants’ 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 no longer 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 bloodflow 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, bypassing 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 there is a relatively well-developed blood supply to the basal nuclei and internal capsule through a prominent Heubner’s artery (arteria recurrens anterior), a branch of the anterior cerebral artery. The cortex and the white matter regions are rather poorly vascularized at this stage. The distribution of arteries and veins on the lateral aspects of the cerebral hemispheres is affected by formation of the lateral fissure and development of the 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 with the territories supplied by the anterior and posterior cerebral arteries. Early arterial anastomoses appear around 16 weeks’ gestation and increase in size with age. The sites of anastomoses between the middle and anterior cerebral arteries move from the convexity of the brain toward the superior sagittal sinus. Anastomotic connections between the middle and posterior cerebral arteries shift toward the basal aspect of the brain.

By 32 to 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 (see Fig. 3.37), and toward the end of the third trimester, the balance of cerebral circulation shifts from one that is central and basal nuclei oriented to one that 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, that is, in the periventricular white matter. In the full-term infant the cortical boundary zones and watershed areas between different arterial blood supplies are similar to those in adults.

Veins of the Head

The earliest vessels form a transitory primordial hindbrain channel that drains into the precardinal vein (Fig. 3.38). 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, then medial to the vagus nerve, to become continuous with the precardinal vein. A lateral anastomosis subsequently brings it lateral to the vagus nerve. The cranial part of the precardinal vein forms the internal jugular vein.

Fig. 3.38 Successive stages in the development of the veins of the head and neck. A, At approximately 8 mm crown–rump length. B, At approximately 24 mm crown–rump length.

The primary capillary plexus of the head becomes separated into three fairly distinct strata by the differentiation of the skull and meninges. The superficial vessels, draining the skin and underlying soft parts, eventually discharge in large part into the external jugular system. They retain some connections with the deeper veins through so-called emissary veins. Deep to this is the venous plexus of the dura mater, from which the dural venous sinuses differentiate. This plexus converges on each side into anterior, middle and posterior dural stems (see Fig. 3.37). 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, while 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.

The vessels of the dural plexus undergo profound changes, largely to accommodate the growth of the cartilaginous otic capsule of the membranous labyrinth and expansion of the cerebral hemispheres. With growth of the otic capsule, the primary head vein is gradually reduced, and a new channel joining the anterior, middle and posterior dural stems appears dorsal to the cranial nerve ganglia and the capsule. Where this new vessel joins the middle and posterior stems, together with the posterior dural stem itself (see Fig. 3.37B), the adult sigmoid sinus is formed.

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, and it drains, most commonly, 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 to the transverse sinus. With expansion of the cerebral hemispheres and, in particular, the emergence of the temporal lobe, the tentorial sinus becomes elongated, attenuated and eventually disappears, and its territory is drained by enlarging anastomoses of pial vessels. The latter become the basal veins, which are 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 in 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. Meanwhile, the pro-otic sinus has developed a new and more caudally situated stem draining into the sigmoid sinus; this new stem is the petrosquamosal sinus. With progressive ossification of the skull, the pro-otic sinus becomes diploic in position. Development of the venous drainage and portal system of the hypophysis cerebri is closely associated with that of the venous sinuses.

Neural Retina

The neural retina comprises an outer nuclear zone and an inner marginal zone that is devoid of nuclei. At around 36 days, the cells of the nuclear zone invade the marginal zone, and by 44 days, the nervous stratum of the retina consists of inner and outer neuroblastic layers. The inner neuroblastic layer gives rise to the ganglion cells, the amacrine cells and the somata of the ‘fibrous’ sustentacular cells (of Müller); the outer neuroblastic layer is the source of the horizontal and rod-and-cone bipolar neurones and probably the rod-and-cone cells, which first appear in the central part of the retina. By the eighth month, all the named layers of the retina can be identified. However, the retinal photoreceptor cells continue to form after birth, generating an array of increasing resolution and sensitivity.

The divergent differentiation of the pigmented and sensory layers of the retina depends on interactions mediated by diffusible molecules. For example, soluble factors from the retina elicit the polarized distribution of plasma membrane proteins and the formation of tight junctions in the pigmented epithelium. Neural retinal differentiation appears to be mediated by FGFs. However, the pigmented epithelium retains the potential to become neural retina and will do so if the embryonic retina is wounded.

Optic Nerve

The optic nerve develops from the optic stalk. The centre of the optic cup, where the optic fissure is deepest, later forms the optic disc. Here the neural retina is continuous with the corresponding invaginated cell layer of the optic stalk; consequently, the developing nerve fibres of the ganglion cells pass directly into the wall of the stalk and convert it into the optic nerve. The fibres of the optic nerve begin to acquire their myelin sheaths shortly before birth, but the process is not completed until sometime later. The optic chiasma is formed by the meeting and partial decussation of the fibres of the two optic nerves in the ventral part of the lamina terminalis at the junction of the telencephalon and the diencephalon in the floor of the third ventricle. Beyond the chiasma, the fibres are continued backward as the optic tracts and pass principally to the lateral geniculate bodies and to the superior tectum.

Ciliary Body

The ciliary body is a compound structure. Its epithelial components are the region of the inner layer of the retina between the iris and the neural retina and the adjacent outer layer of pigmented epithelium. The cells here differentiate in close association with the surrounding mesenchyme to form highly vascularized folds that secrete fluid into the globe of the eye. The inner surface of the ciliary body also forms the site of attachment of the lens, whereas the outer layer is associated with smooth muscle derived from mesenchymal cells in the choroid lying between the anterior scleral condensation and the pigmented ciliary epithelium.

Iris

The iris develops from the tip of the optic cup, where the two layers remain thin and are associated with vascularized, muscular connective tissue. The muscles of the sphincter and dilator pupillae are unusual, being of neuroectodermal origin, and develop from the cells of the pupillary part of the optic cup. The mature colour of the iris develops after birth and is dependent on the relative contributions of the pigmented epithelium on the posterior surface of the iris and the chromatophore cells in the mesenchymal stroma of the iris. If only epithelial pigment is present, the eye appears blue; if there is an additional contribution from the chromatophores, the eye appears brown.

Lens

The lens develops from the lens vesicle (see Fig. 3.39A). Initially, this is a ball of actively proliferating epithelium that encloses a clump of disintegrating cells; by 37 days, there is a discernible difference between the thin anterior (outward-facing) epithelium and the thickened posterior epithelium. Cells of the posterior wall lengthen and fill the vesicle (see Fig. 3.39B, C) and reduce the original cavity to a slit by about 44 days. The posterior cells become filled with a very high concentration of proteins (crystallins), which renders them transparent. They also become densely packed within the lens as primary lens fibres. Cells at the equatorial region of the lens elongate and contribute secondary lens fibres to the body of the lens in a process that continues into adult life, sustained by continued proliferation of cells in the anterior epithelium. The polarity and growth of the lens appear to depend on the differential distribution of soluble factors that promote either cell division or lens fibre differentiation and are present in the anterior chamber and vitreous humour, respectively.

The developing lens is surrounded by a vascular mesenchymal condensation, the vascular capsule; the anterior part is called the pupillary membrane. The posterior part of the capsule is supplied by branches from the hyaloid artery, and the anterior part is supplied by branches from the anterior ciliary arteries. During the fourth month, the hyaloid artery gives off retinal branches. By the sixth month, all the vessels have atrophied except the hyaloid artery. The latter becomes occluded during the eighth month of intrauterine life, although its proximal part persists in the adult as the central artery of the retina. Atrophy of the hyaloid vasculature and of the pupillary membrane appears to be an active process of programmed tissue remodelling that is macrophage dependent. The hyaloid canal, which carries the vessels through the vitreous, persists after the vessels have become occluded. In the newborn it extends more or less horizontally from the optic disc to the posterior aspect of the lens, but when the adult eye is examined with a slit lamp, it can be seen to follow an undulating course, sagging downward as it passes forward to the lens. With the loss of its blood vessels, the vascular capsule disappears, and the lens becomes dependent for nutrition on diffusion via the aqueous and vitreous humours. The lens remains enclosed in the lens capsule, a thickened basal lamina derived from the lens epithelium. Sometimes the pupillary membrane persists at birth, which gives rise to congenital atresia of the pupil.

Vitreous Body

The vitreous body develops between the lens and the optic cup as a transparent, avascular gel of extracellular substance. The precise derivation of the vitreous remains controversial. The lens rudiment and the optic vesicle are in contact at first; they draw apart after closure of the lens vesicle and formation of the optic cup but remain connected by a network of delicate cytoplasmic processes. This network, derived partly from cells of the lens and partly from those of the retina, is the primitive vitreous body. At first, these cytoplasmic processes are connected to the whole of the neuroretinal area of the cup; later, they become limited to the ciliary region, where, by a process of condensation, they form the basis of the suspensory ligaments of the ciliary zonule. The vascular mesenchyme that enters the cup through the choroidal fissure and around the equator of the lens associates locally with this reticular tissue and thus contributes to formation of the vitreous body.

Aqueous Chamber

The aqueous chamber of the eye develops in the space between the surface ectoderm and the lens that is invaded by mesenchymal cells of neural crest origin. The chamber initially appears as a cleft in this mesenchymal tissue. The mesenchyme superficial to the cleft forms the substantia propria of the cornea, and the mesenchyme deep to the cleft forms the mesenchymal stroma of the iris and the pupillary membrane. Tangentially, this early cleft extends as far as the iridocorneal angle, where communications are established with the sinus venosus sclerae. When the pupillary membrane disappears, the cavity continues to form between the iris and the lens capsule as far as the zonular suspensory fibres. In this way, the aqueous chamber is divided by the iris into anterior and posterior chambers that communicate through the pupil. The walls of these chambers furnish both the sites of production of aqueous humour and the channels for its circulation and reabsorption.

Cornea

The cornea is induced in front of the anterior chamber by the lens and optic cup. The corneal epithelium is formed from surface ectoderm, and the epithelium of the anterior chamber is formed from mesenchyme. A regular array of collagen fibres is established between these two layers, and these serve to reduce the scattering of light entering the eye.

Choroid and Sclera

The choroid and sclera differentiate as inner vascular and outer fibrous layers from the mesenchyme that surrounds the optic cup. The blood vessels of the choroid develop from the fifteenth week and include the vasculature of the ciliary body. The choroid is continuous with the internal sheath of the optic nerve, which is pia-arachnoid mater, and the sclera is continuous with the outer sheath of the optic nerve and thus with the dura mater.

Extraocular Muscles

The extrinsic ocular muscles derive from prechordal mesenchyme that ingresses at the primitive node very early in development. The prechordal cells lie at the rostral tip of the notochordal process and remain mesenchymal after the notochordal process becomes epithelial and gains a basal lamina. The prechordal mesenchyme migrates laterally toward the paraxial mesenchyme. Although this is a singular origin for muscle, the early myogenic properties of these cells have been demonstrated experimentally; moreover, if transplanted into limb buds, the cells are able to develop into muscle tissue (Wachtler and Jacob 1986).

Early embryos develop bilateral premandibular, intermediate and caudal cavities in the head, previously described as preotic somites. The walls of the premandibular head cavities are lined by flat or cylindrical cells that do not exhibit the characteristics of a germinal epithelium. As the oculomotor nerve grows down to the level of the head cavity, a condensation of premuscle cells appears at its ventrolateral side, which later subdivides into the blastemata of the different muscles supplied by the nerve. Similar events occur with respect to the intermediate head cavity (trochlear nerve and superior oblique muscle) and the caudal head cavity (abducens nerve and lateral rectus muscle).

There is no doubt that the head cavities are formed by a mesenchymal–epithelial shift similar to that seen in the somites. However, the epithelial plate of the somite is a germinal centre that produces postmitotic myoblasts destined for epaxial regions and migratory premitotic myoblasts destined for the limbs and body wall. The head cavities may serve a similar purpose if a mesenchymal–epithelial shift is part of the maturation process for putative myoblasts. However, there may be no need to provide a centre for cell replication, because premitotic myoblasts differentiated directly from the prechordal mesenchyme may form the premuscular masses.

Eyelids

The eyelids are formed as small cutaneous folds of surface ectoderm and neural crest mesenchyme (see Fig. 3.39C). During the middle of the third month, their edges come together and unite over the cornea to enclose the conjunctival sac, and they usually remain united until about the end of the sixth month. When the eyelids open, the conjunctivae lining their inner surfaces and covering the white (scleral) region of the eye fuse with the corneal epithelium. The eyelashes and the lining cells of the tarsal (meibomian), ciliary and other glands that open onto the margins of the eyelids are all derived from the tarsal plate. Orbicularis oculi develops from skeletal myoblasts that invade the eyelids from the second pharyngeal arch. Levator palpebrae superioris develops from the prechordal mesenchyme and is attached to the upper eyelids by tendons derived from the neural crest. Smooth muscle also develops within the eyelids.

Lacrimal Apparatus

The epithelium of the alveoli and the ducts of the lacrimal gland arise as a series of tubular buds from the ectoderm of the superior conjunctival fornix. These buds are arranged in two groups: one forms the gland proper, and the other forms its palpebral process (de la Cuadra-Blanco, Peces-Peňa and Mèrida-Velasco 2003). The lacrimal sac and nasolacrimal duct are derived from ectoderm in the nasomaxillary groove between the lateral nasal process and the maxillary process of the developing face. This thickens to form a solid cord of cells, the nasolacrimal ridge, which sinks into the mesenchyme. During the third month, the cord becomes canalized to form the nasolacrimal duct. The lacrimal canaliculi arise as buds from the cranial extremity of the cord, which establish openings (puncta lacrimalia) on the margins of the lids. The inferior canaliculus isolates a small part of the lower eyelid to form the lacrimal caruncle and plica semilunaris.