Early Vesicles and Flexures of the Neural Tube

Prior to closure of the neural tube, the neural folds become considerably expanded in the head region—the first indication of a brain. After the rostral neuropore closes, these regional expansions form three primary cerebral vesicles (Fig. 3.2). The term ‘vesicle’ may be a misnomer, because it suggests an exaggerated view of these localized accelerations of growth in the wall of the brain. The bulging is not initially marked, and the vesicles are more like gently fusiform tubes. The three regions are the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain), the last being continuous caudally with the spinal cord. As a result of unequal growth of their different regions, the prosencephalon and rhombencephalon enlarge more than the mesencephalon and can be subdivided. The prosencephalon gives rise to a midline diencephalon and bilateral telencephalon; the rhombencephalon gives rise to the metencephalon and myelencephalon. A summary of the derivatives of the cerebral vesicles is given in Table 3.1.

Fig. 3.2 A, Right side of the brain of a human embryo, 9 mm long. B, Right lateral surface of the brain of a human embryo, approximately 10.2 mm long. C, Right side of the brain of a human embryo, 13.6 mm long. The roof of the hindbrain has been removed.

Table 3.1 Derivatives of the cerebral vesicles from caudal to rostral

Elongation of the brain occurs at the same time as the appearance of three flexures that are also developing prior to closure of the neural tube; two are concave ventrally, and one is concave dorsally. During stages 13 and 14 the brain bends at the mesencephalon (mesencephalic flexure), so the prosencephalon bends in a ventral direction around the cephalic end of the notochord and foregut until its floor lies almost parallel to that of the rhombencephalon (see Fig. 3.2). A bend also appears at the junction of the rhombencephalon and spinal cord (cervical flexure). This increases from the fifth to the end of the seventh week, by which time the rhombencephalon forms nearly a right angle to the spinal cord. However, after the seventh week, extension of the head takes place, and the cervical flexure diminishes and eventually disappears. The third bend, the pontine flexure, is directed ventrally between the metencephalon and myelencephalon. It does not substantially affect the outline of the head. In this region the roof plate thins until it is composed of only a single layer of cells and pia mater, the tela choroidea. The flexure of the neural tube at this point produces a rhombic shape in the roof that later forms the medullary velum.

In addition to these gross divisions, a number of ridges and depressions are present transiently on the inner surface of the brain. Prominent among these are the serial bulges that appear very early in the rhombencephalon, before the main flexures of the neural tube develop (Fig. 3.3). These bulges are termed rhombomeres.

Fig. 3.3 Rhombomeric segmentation. Rostral is toward the right.

(Courtesy of Professor Lumsden.)

Early Cellular Arrangement of the Neural Tube

Histologically, the early neural tube is composed of a pseudostratified neuroepithelium. It extends from the inner aspect of the tube to the outer limiting basal lamina and surrounding neural crest, which will form the pia mater. The epithelium contains stem cells that will give rise to populations of neuroblasts and glioblasts. A population of radial glia differentiates very early and provides a scaffold for later cells to follow. As development proceeds, three zones or layers develop (Figs. 3.4–3.6): an internal ventricular zone (variously termed the germinal, primitive ependymal or matrix layer), in which mitosis occurs and that contains the nucleated parts of the columnar cells and rounded cells undergoing mitosis; a middle mantle zone (also termed the intermediate zone), which contains the migrant cells from the divisions occurring in the ventricular zone; and an outer marginal zone, which initially consists of the external cytoplasmic processes of the radial glia and the neuroepithelial stem cells. The last is soon invaded by tracts of axonal processes that grow from neuroblasts developing in the mantle zone, together with varieties of non-neuronal cells (glial cells and later vascular endothelium and perivascular mesenchyme).

Fig. 3.4 Transverse section through the developing spinal cord of a 4-week-old human embryo. Historical terminology is retained; more recent terms are in parentheses.

Fig. 3.5 Early development of the neural tube. Three layers can be delineated in the spinal cord and brain stem.

Fig. 3.6 Transverse section of the developing spinal cord in the cervical region of a human embryo early in the sixth week (crown–rump length, 8 mm).

At first the neural tube caudal to the brain is oval in transverse section, and its lumen is narrow and slit-like (see Fig. 3.4). The original floor plate and the dorsal site of fusion of the tube initially contain non-neural cells. With cellular proliferation, the lateral walls thicken and the lumen, now the central canal, widens in its dorsal part and is somewhat diamond shaped on cross-section (see Fig. 3.6). Widening of the canal is associated with the development of a longitudinal sulcus limitans on each side. This divides the ventricular and mantle (intermediate) zones in each lateral wall into a ventrolateral lamina or basal plate and a dorsolateral lamina or alar plate. This separation indicates a fundamental functional difference.

Throughout the neural tube there is a generic pattern in the position of the neurones, specified by the juxtaposition of the notochord to the neural tube. Lateral or dorsal grafting of a notochord results in the induction of a floor plate overlying the grafted notochord and the induction of ectopic dorsal motor neurones. Similarly, lateral or dorsal grafts of a floor plate also result in the induction of a new floor plate overlying the graft and the induction of ectopic dorsal motor neurones. Removal of the notochord results in the elimination of the floor plate and motor neurones and the differentiation of dorsal cell types in the ventral region of the cord (Fig. 3.7).

Fig. 3.7 A–D, Successive stages in the development of the neural tube and spinal cord. A, The neural plate consists of epithelial cells. Cells in the midline of the neural plate are contacted directly by the notochord. More lateral regions of the neural plate overlie the paraxial mesenchyme (not shown). B, During neurulation, the neural plate bends at its midline, which elevates the lateral edges of the plate as the neural folds. Contact between the midline of the neural plate and the notochord is maintained at this stage. C, The neural tube is formed when the dorsal tips of the neural folds fuse. Cells in the region of fusion form the roof plate, which is a specialized group of dorsal midline cells. D, Cells at the ventral midline of the neural tube retain proximity to the notochord and differentiate into the floor plate. After neural tube closure, neuroepithelial cells continue to proliferate and eventually differentiate into defined classes of neurones at different dorsoventral positions within the spinal cord. For example, sensory relay, commissural and other classes of dorsal neurones (D) differentiate near the roof plate (R), and motor neurones (M) differentiate ventrally near the floor plate (F), which by this time is no longer in contact with the notochord (N). E–H, Summary of the results of experiments in chick embryos in which the notochord or floor plate is grafted to the dorsal midline of the neural tube or the notochord is removed before neural tube closure. E, The normal condition, showing the ventral location of motor neurones (M) and the dorsal location of sensory relay neurones (D). F, Dorsal grafts of a notochord result in induction of a floor plate in the dorsal midline and ectopic dorsal motor neurones (M). G, Dorsal grafts of a floor plate induce a new floor plate in the dorsal midline and ectopic dorsal motor neurones (M). H, Removal of the notochord results in the elimination of the floor plate and motor neurones and the expression of dorsal cells types (D) in the ventral region of the spinal cord.

(After Jessell, Dodd. 1992. WB Saunders.)

The basal plate is normally concerned predominantly with motor function and contains the cell bodies of motor neurones of the future anterior and lateral grey columns. The alar plate receives sensory inflow from external dorsal root ganglia. Motor and sensory axons combine to form the mixed nerves.

Failure of Neurulation

Failure of neurulation produces the conditions of craniorachischisis totalis (the entire neural tube is unfused in the dorsal midline), cranioschisis or anencephaly (the neural tube is fused dorsally to form the spinal cord but is not fused dorsally in the brain) and spina bifida (local regions of the spinal neural tube are unfused, or there is failure of formation of the vertebral neural arches) (Fig. 3.8). Anencephalic fetuses display severe disturbances in the shape, position and ossification of the basichondrocranium and in the course of the intracranial notochord (Fig. 3.9).

Fig. 3.8 Defects caused by failure of neural tube formation. A, Total failure of neurulation. B, Failure of rostral neurulation. C, Failure of caudal neurulation.

Fig. 3.9 Anencephaly.

Histogenesis of the Neural Tube

The wall of the early neural tube consists of an internal ventricular zone (sometimes termed the germinal matrix) abutting the central lumen. It contains the nucleated parts of the pseudostratified columnar neuroepithelial cells and rounded cells undergoing mitosis. The early ventricular zone also contains a population of radial glial cells whose processes pass from the ventricular surface to the pial surface, thus forming the internal and external glia limitans (glial limiting membrane). As development proceeds, the early pseudostratified epithelium proliferates, and an outer layer (the marginal zone), devoid of nuclei but containing the external cytoplasmic processes of cells, is delineated. Subsequently, a middle mantle layer (the intermediate zone) forms as the progeny from the ventricular zone migrate ventriculofugally (see Fig. 3.5).

Most CNS cells are produced in the proliferative zone adjacent to the future ventricular system, and in some regions this area is the only actively mitotic zone. According to the monophyletic theory of neurogenesis, it is assumed to produce all cell types. The early neural epithelium, including the deeply placed ventricular mitotic zone, consists of a homogeneous population of pluripotent cells whose varying appearances reflect different phases in a proliferative cycle. The ventricular zone is considered to be populated by a single basic type of progenitor cell and to exhibit three phases. The cells show an ‘elevator movement’ as they pass through a complete mitotic cycle, progressively approaching and then receding from the internal limiting membrane (Fig. 3.16). DNA replication occurs while the cells are extended and their nuclei approach the pial surface; they then enter a premitotic resting period as the cells shorten and their nuclei pass back toward the ventricular surface. The cells now become rounded close to the internal limiting membrane and undergo mitosis. They then elongate, and their nuclei move toward the outer edge during the postmitotic resting period, after which DNA synthesis commences once more, and the cycle is repeated. The cells so formed may either start another proliferative cycle or migrate outward (i.e. radially) and differentiate into neurones as they approach and enter the adjacent stratum. This differentiation may be initiated as they pass outward during the postmitotic resting period. The proliferative cycle continues with the production of clones of neuroblasts and glioblasts. This sequence of events has been called interkinetic nuclear migration, and it eventually declines. At the last division, two postmitotic daughter cells are produced, and they differentiate at the ventricular surface into ependyma.

Fig. 3.16 Cell cycle in the ventricular zone of the developing neural tube. The nuclei of the proliferating stem cells show interkinetic migration.

(From Fujita, S. 1963. J. Comp. Neurol. 120, 37–42. Reprinted by permission from Wiley-Liss Inc.)

The progeny of some of these divisions move away from the ventricular zone to form an intermediate zone of neurones. The early spinal cord and much of the brain stem shows only these three main layers: ventricular, intermediate and marginal zones. However, in the telencephalon, the region of cellular proliferation extends deeper than the ventricular zone, where the escalator movement of interkinetic migration is seen, and a subventricular zone appears between the ventricular and intermediate layers (see Fig. 3.5). Here cells continue to multiply to provide further generations of neurones and glia, which subsequently migrate into the intermediate and marginal zones. In some regions of the nervous system (e.g. the cerebellar cortex) some mitotic subventricular stem cells migrate across the entire neural wall to form a subpial population and establish a new zone of cell division and differentiation. Many cells formed in this site remain subpial in position, but others migrate back toward the ventricle through the developing nervous tissue and finish their migration in various definitive sites where they differentiate into neurones or macroglial cells. In the cerebral hemispheres, a zone termed the cortical plate is formed outside the intermediate zone by radially migrating cells from the ventricular zone. The most recently formed cells migrate to the outermost layers of the cortical plate, so that earlier formed and migrating cells become subjacent to those migrating later. In the forebrain there is an additional transient stratum deep to the early cortical plate, the subplate zone.

Lineage and Growth in the Nervous System

Neurones come from two major embryonic sources: CNS neurones originate from the pluripotential neural plate and tube, whereas ganglionic neurones originate from the neural crest and ectodermal placodes. The neural plate also provides ependymal and macroglial cells. Peripheral Schwann cells and chromaffin cells arise from the neural crest. The origins and lineages of cells in the nervous system have been determined experimentally by the use of autoradiography, by microinjection or retroviral labelling of progenitor cells and in cell culture.

During development, neurones are formed before glial cells. The timing of events differs in various parts of the CNS and between species. Most neurones are formed prenatally in mammals, but some postnatal neurogenesis does occur (e.g. the small granular cells of the cerebellum, olfactory bulb and hippocampus, and neurones of the cerebral cortex). Gliogenesis continues after birth in periventricular and other sites. Autoradiographic studies have shown that different classes of neurones develop at specific times. Large neurones, such as principal projection neurones, tend to differentiate before small ones, such as local circuit neurones. However, their subsequent migration appears to be independent of the time of their initial formation. Neurones can migrate extensively through populations of maturing, relatively static cells to reach their destination; for example, cerebellar granule cells pass through a layer of Purkinje cells en route from the external pial layer to their final central position. Later, the final form of their projections, their cell volume and even their continuing survival depend on the establishment of patterns of functional connection.

Initially, immature neurones, termed neuroblasts, are rotund or fusiform. Their cytoplasm contains a prominent Golgi apparatus, many lysosomes, glycogen and numerous unattached ribosomes. As maturation proceeds, cells send out fine cytoplasmic processes that contain neurofilaments, microtubules and other structures, often including centrioles at their bases where microtubules form. Internally, endoplasmic reticulum cisternae appear, and attached ribosomes and mitochondria proliferate, whereas the glycogen content progressively diminishes. One process becomes the axon, and other processes establish a dendritic tree. Axonal growth, studied in tissue culture, may be as much as 1 mm per day.

Induction and Patterning of the Brain and Spinal Cord

The generation of neural tissue involves an inductive signal from the underlying chordamesoderm (notochord), termed the organizer. The observation by Spemann in 1925 that, in intact amphibian embryos, the presence of an organizer causes ectodermal cells to form nervous tissue, whereas in its absence they form epidermis, led to the discovery of neural induction. However, experiments performed much later in the century revealed that when ectodermal cells are dissociated, they also give rise to neural tissue. The paradox was resolved by the finding that intact ectodermal tissue is prevented from becoming neural by an inhibitory signal that is diluted when cells are dissociated. Many lines of evidence now indicate that this inhibitory signal is mediated by members of a family of secreted proteins, the bone morphogenetic proteins (BMPs). These molecules are found throughout ectodermal tissue during early development, and their inhibitory effect is antagonized by several neural inducers that are present within the organizer: noggin, chordin and follistatin. Each of these factors is capable of blocking BMP signalling, in some cases by preventing it from binding to its receptor.

The regional pattern of the nervous system is induced before and during neural tube closure. Early concepts about regional patterning envisaged that regionalization within mesenchymal populations that transmit inductive signals to the ectoderm imposes a similar mosaic of positional values on the overlying neural plate. For example, transplantation of caudal mesenchyme beneath the neural plate in Amphibia induced spinal cord, whereas rostral mesenchyme induced brain, as assessed by the morphology of the neuroepithelial vesicles. However, later work indicated a more complex scenario in which organizer grafts from early embryos induced mainly head structures, whereas later grafts induced mainly trunk structures. Subsequent molecular data tend to support a model in which neural-inducing factors released by the organizer, such as noggin, chordin and follistatin, neuralize the ectoderm and promote a mainly rostral neural identity. Later secreted signals then act to caudalize this rostral neural tissue, setting up an entire array of axial values along the neural tube. Candidates for these later caudalizing signals include retinoic acid, FGFs, and the WNT secreted proteins, which are present in the paraxial mesenchyme and later in its derivatives, the somites. This combination of signals does not seem to be sufficient to produce the most rostral forebrain structures. Other secreted proteins resident in the rostralmost part of the earliest ingressing axial populations of endoderm and mesenchyme are also capable of inducing markers of forebrain identity from ectodermal cells (Withington, Beddington and Cooke 2001).

As the neural tube grows and its shape is modified, a number of mechanisms refine the crude rostrocaudal pattern imposed during neurulation. Molecules that diffuse from tissues adjacent to the neural tube, such as the somites, have patterning influences. The neural tube possesses a number of intrinsic signalling centres, such as the midbrain–hindbrain boundary, which produce diffusible molecules capable of influencing tissue development at a distance. In this way extrinsic and intrinsic factors serve to subdivide the neural tube into a number of fairly large domains, on which local influences can then act. Domains are distinguished by their expression of particular transcription factors, which in many cases have been causally related to the development of particular regions. Examples of such genes are the Hox family, which are expressed in the spinal cord and hindbrain, and the Dlx, Emx and Otx families of genes, which are expressed in various regions of the forebrain. These are all developmental control genes that lie high up in the hierarchy and are capable of initiating cascades of expression of other genes to create a more fine-grained pattern of cellular differentiation. In contrast to the aforementioned secreted molecules, these genes encode proteins that are retained in the cell nucleus and thus can act on DNA to induce or repress further gene expression.

Segmentation in the Neural Tube

One mechanism involved in the process of regional differentiation of cell populations within the neural tube is segmentation, which is conspicuous in humans and other vertebrates in the serial arrangement of the vertebrae and axial muscles and in the periodicity of the spinal nerves. In the last century, the possibility that the neural tube might be divided into segments or neuromeres was entertained, but some contended that the bulges observed in the lateral walls of the neural tube were artifacts or were caused by mechanical deformation of the tube by adjacent structures. Recent years have seen a resurgence of interest in this subject and a detailed evaluation of the significance of neuromeres. A series of eight prominent bulges that appear bilaterally in the rhombencephalic wall early in development have been termed rhombomeres (see Fig. 3.3). (Whereas the term neuromere applies generally to putative ‘segments’ of the neural tube, the term rhombomere applies specifically to the rhombencephalon.) Many aspects of the patterning of neuronal populations and the elaboration of their axon tracts conform to a segmental plan, and rhombomeres have now been shown to constitute crucial units of pattern formation. Domains of expression of developmental control genes abut rhombomere boundaries, and perhaps most importantly, single-cell labelling experiments have revealed that cells within rhombomeres form segregated non-mixing populations (Fig. 3.17). The neural crest also shows intrinsic segmentation in the hindbrain and is segregated into streams at its point of origin in the dorsal neural tube. This may represent a mechanism whereby morphogenetic specification of the premigratory neural crest cells is conveyed to the pharyngeal arches. Although these segmental units lose their morphological prominence with subsequent development, they represent the fundamental ground plan of this part of the neuraxis, creating a series of semiautonomous units within which local variations in patterning can develop. The consequences of early segmentation for later developmental events, such as the formation of definitive neuronal nuclei within the brain stem, and of peripheral axonal projections remain to be explored.

Fig. 3.17 Hox gene expression domains in the branchiorhombomeric area in the mouse embryo at stage 9.5. The arrows indicate neural crest cells migrating from the rhombencephalon and midbrain. At the former level, they are shaded to indicate the Hox genes they express. The same combination of Hox genes is expressed in the rhombomeres and in the superficial ectoderm of the pharyngeal arches at the corresponding rostrocaudal levels. The four Hox clusters are represented below.

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

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

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

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

Whereas craniocaudal positional values are probably conferred on the neuroepithelium at the neural plate or early neural tube stage, dorsoventral positional values may become fixed later. The development of the dorsoventral axis is heavily influenced by the presence of the underlying notochord. The notochord induces the ventral midline of the neural tube, the floor plate. This specialized region consists of a strip of non-neural cells with distinctive adhesive and functional properties. Notochord and floor plate together participate in inducing the differentiation of the motor columns. Motor neurone differentiation occurs early, giving some support to the idea of a ventral-to-dorsal wave of differentiation. The notochord–floor plate complex may also be responsible for allotting the values of more dorsal cell types within the tube (see Fig. 3.7). For example, the dorsal domain of Pax-3 expression extends more ventrally in embryos experimentally deprived of notochord and floor plate, whereas grafting an extra notochord adjacent to the dorsal neural tube leads to the repression of Pax-3 expression.

Somatic Nerves

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.

Autonomic Nervous System

Autonomic nerves, apart from the preganglionic motor axons arising from the CNS, are formed by the neural crest. The autonomic nervous system includes the sympathetic and parasympathetic neurones in the peripheral ganglia and their accompanying glia, the enteric nervous system and glia and the suprarenal medulla.

In the trunk at neurulation, neural crest cells migrate from the neural epithelium to lie transitorily on the fused neural tube. Thereafter, crest cells migrate laterally and then ventrally to their respective destinations (see Fig. 3.12). In the head, the neural crest cells migrate prior to neural fusion, producing a vast mesenchymal population as well as autonomic neurones.

The four major regions of neural crest cell distribution to the autonomic nervous system are cranial, vagal, trunk and lumbosacral. The cranial neural crest gives rise to the cranial parasympathetic ganglia, whereas the vagal neural crest gives rise to the thoracic parasympathetic ganglia. The trunk neural crest gives rise to the sympathetic ganglia, mainly the paravertebral ganglia, and suprarenomedullary cells. This category is often referred to as the sympathoadrenal lineage.

Neurones of the enteric nervous system are described as arising from the vagal crest—that is, the neural crest derived from somite levels 1 to 7, and the sacral crest caudal to the twenty-eighth somite. At all these levels the crest cells also differentiate into glial-like support cells alongside the neurones (Fig. 3.20).

Fig. 3.20 Derivatives of neural crest cells in the trunk. Somites are indicated on the right, and vertebral levels are indicated on the left. The fate of crest cells arising at particular somite levels is shown.