Book - Aids to Embryology (1948) 7
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter VII The Nervous System
The Central Nervous System
The primordium of the central nervous system appears in pre-somite embryos as a mid-lme thickening of the ectoderm in front of Hensen's node. This is the neural plate The margins of the thickening grow more rapidly than the central portion, and they become raised a ove the general level of the surrounding ectoderm as the neural folds ; the groove between them is the neural groove. With further growth fusion between e neural folds commences, first towards the middle of the embryo and extending from there to the cranial and caudal extremities. Thus is formed the neural tube which presently loses its connection with the ecto erm and occupies a mid-line position just dorsal to the notochord. A chain of ectodermal cells appears m the angle between the neural tube and the general body ectoderm. It is known as the neural crest and provides material for the sensory ganglia of the cranial and spinal nerves. The cells of the neural tube are at first columnar in shape, and proliferation of them makes the wall of the tube several cells thick and the cells are so arranged that the nuclei are aggregated near the central canal while the outer part of the neural tube is made up mainly of cytoplasm. With further development the neural tube shows three zones ; from within outwards there are ependymal, the mantle and the marginal zones, proliferation of the ependymal cells gives rise to neuroblasts which form the definitive neurons and spongioblasts from which the neuroglia, or supporting issue of the central nervous system, arises.
Myelination of the nerve fibres begins in the human foetus of fourteen weeks gestation, and is not completed until eight months or more after birth (Keene and Hewer, 1931). It proceeds from the nerve cell along the length of the fibre. It is first observed in the sensory pathways and the motor fibres of the cranial and spinal nerves. In the foetus of twenty eight weeks all the important sensory tracts are myelinated with the exception of such connections as Lissauer's bundle and the external arcuate fibres from the nuclei gracilis and cuneatus to the cerebellum. At this stage, the long motor tracts are not myelinated, but as the association paths in both the brain stem and spinal cord, and also the motor nerves, are all myelinated, the nervous activities of the foetus at this time must be purely reflex with no central control.
Fig. 10. - Two Stages in the Development of the Neural Tube.
1, Neural plate ; 2, neural fold ; 3, notochord ; 4, somite ; 5, intermediate cell mass ; 6, neural tube ; 7, neural crest ; 8, dorsal aorta.
At birth the descending motor tracts commence to exhibit myelin ; so do the cortico-pontine-cerebellar pathways and cerebral control is initiated. The aberrant pyramidal fibres and olivo-spinal tract do not commence to myelinate until about eight months after birth. Flechsig (1895) first suggested that there is a relationship between myelination and the acquisition of function in the various tracts and many other workers subscribe to this view.
The Spinal Cord
As was stated above, the neural tube is formed by the fusion of the neural folds. This commences about the level of the sixth somite. Fusion extends cranially and caudally from this level, but for some time the extremities of the tube remain open and are called the anterior and posterior neuropores. The anterior neuropore closes about the twenty somite stage and the posterior neuropore shortly afterwards. The spinal cord is considered to arise from that portion of the neural tube which lies caudal to the fourth somite. At first the central canal is quite large and of an elongated oval form. Cell proliferation causes the lateral walls to thicken while the roof and floor remain thin. The cavity takes on a rhomboidal shape in transverse section and a well-marked longitudinal groove on each side, the sulcus limitans, marks off a dorsal alar (sensory) lamina, and a ventral basal (motor) lamina in the lateral wall. The two laminae increase in size and the alar laminae bulge medially, obliterating the dorsal portion of the central canal. The site of their apposition persists in the adult as the posterior median septum. The ventral portion of the canal thus forms the central canal of the adult. This reduction in the size of the central canal is not so great at the caudal end of the neural tube and so a little dilation of the central canal, the terminal ventricle, is found here in the adult. The basal lamina bulges ventro-laterally in its further development so that an anterior median furrow is formed in the mid-line of the cord.
The cord extends the whole length of the embryo until the third month. After this time the vertebral column grows much more rapidly in length than does the cord and so the caudal extremity of the latter appears to recede up the vertebral canal. At birth the lower extremity is found opposite the third lumbar vertebra, and in the adult at the disc between the first and second lumbar vertebrae. A nonnervous strand of tissue, the filum terminale, passes from the caudal tip of the spinal cord to its original termination at the dorsal aspect of the coccyx. Another result of this upward retreat of the cord is that the lumbar, sacral and coccygeal nerves must descend for varying distances before they reach their respective intervertebral foramina through which they pass. This leash of nerves is termed the cauda equina.
Summary of the Development of the Spinal Cord
- The spinal cord is developed from a simple tube of ectoderm derived from the mid-dorsal line of the embryo, termed the neural tube.
- That portion of the neural tube caudal to the
- At first single-layered, the tube soon presents three zones, ependymal, mantle and marginal.
- Cells of the ependymal layer proliferate and migrate into the mantle layer, where they differentiate into neuroblasts and spongioblasts. The neuroblasts give rise to nerve cells and their processes while the spongioblasts form the supporting framework, the neuroglia.
- The nerve cells arrange themselves in two groups in the cord, a dorsal sensory group, and a ventral motor group. These form respectively the alar and basal laminae separated by the longitudinal running sulcus limitans.
- The central canal is narrowed by the apposition in the mid-line of the alar laminae to form the posterior median septum. The ventral portion of the original lumen persists as the central canal of the adult.
Anomalies of Development of the Spinal Cord
These are considered with the anomalies of the brain on p. 59.
The Brain
The brain is derived from that part of the neural tube in front of the fourth somite. In early stages there are three parts here where rapid growth takes place and so, even before the neural folds have united, three bulgings are found in this region. When the anterior neuropore has closed these bulgings are seen as three dilatations, the fore, mid and hind-brain vesicles. The closed anterior neuropore is represented by the lamina terminalis. The fore-brain region very early shows an evagination on either side. These are the optic bulbs, and their further development is considered in the section dealing with the special senses (p. 63). Much later the antero-lateral aspects of the original vesicle present two swellings which increase rapidly in size, and form the two cerebral hemispheres. These, along with that part of the original vesicle lying between them, constitute the telencephalon. The remainder of the original fore-brain vesicle is termed the diencephalon. The second of the primary brain vesicles remains as the mesencephalon. The hind brain vesicle becomes divided into two portions, the metencephalon which gives rise to the cerebellum and pons, and the myelencephalon from which develops the medulla oblongata. The cavities of the three primary brain vesicles are represented in the adult as follows :
- The cavity of the third primary brain vesicle becomes the fourth ventricle.
- The cavity of the second primary vesicle is transformed into the aqueduct of the mid-brain.
- The cavity of the first primary vesicle becomes the third ventricle, and is continuous through the interventricular foramina with the lateral ventricles.
Fig. 11. Diagram to show Nervous System before of the Neural Groove the Form of the Central Fusion of the Extremities. The three swellings at the cephalic end (c) indicate the form of the so-called primary brain vesicles.
Fig. 12. Three Figures to show the Changes produced in the Form of the Developing Brain by the Brain Flexures.
i, Cerebral hemisphere ; 2, optic evagination ; 3, diencephalon ; 4, mesencephalon ; 5, metencephalon ; 6, myelencephalon.
A. Mid-brain flexure ; B, cervical flexure ; C, pontine flexure.
During its early growth the future brain region of the neural tube becomes bent upon itself at three places (see Fig. 12). These are the three primary brain flexures. The first appears in the region of th e mid-brain during the fourth week and is called the mid-brain flexure. As a result of this flexure, the first primary brain vesicle takes up a position at right angles to the axis of the second brain vesicle. Shortly after this (fifth week) the second or cervical flexure occurs in the region of the future medulla. It is caused by ventral bending of the whole head region of the embryo, so that the axis of the midbrain forms almost a right angle with the medulla., and the axis of the first brain vesicle takes up a position almost parallel with the latter. The third, or pontine flexuie, occurs in the region of the metencephalon during the sixth week and is the reverse direction to the other two, being a ventral bending of the floor of the hind brain.
The first two brain flexures are the expression of changes taking place during the formation of the head end of the embryo, but the third flexure (pontine) is peculiar to the brain itself, and does not influence the external form of the head. The pontine flexure is not apparent in the adult brain because of the development of the transverse fibres of the pons, but the mid-brain and cervical flexures persist, although reduced in acuteness.
The Myelencephalon
When the pontine flexure occurs it causes the roof plate of both the myelencephalon and metencephalon to become thinned out and expanded laterally. Thus the alar and basal laminae lie in the ventral part of the myelencephalon. The cells of the basal lamina become segregated in several groups. Most medially are the cells of the hypoglossal nerve. More laterally are cells (branchiomotor) which form the nucleus ambiguus, lateral to which again lie visceral efferent cells of the dorsal nucleus of the vagus. The alar lamina cells give rise to the gracile and cuneate nuclei, the nucleus of the tractus solitarius (branchial afferent) and part of the dorsal nucleus of the vagus (visceral afferent). A ventro-medial migration of alar lamina cells forms the olivary nuclei.
About the fifth week a transverse groove appears in the roof plate into which mesenchymal cells wander ; this groove deepens, blood vessels are differentiated from the mesenchyme and the choroid plexus of the fourth ventricle is formed. A mid-line evagination of the roof plate (Weed, 1917) becomes perforated as the foramen of Majendie and similar foramina arise in like manner in the lateral recesses of the fourth ventricle.
The Metencephalon
The most medial cells in the basal lamina of this part of the neural tube form the nucleus of the sixth nerve. The intermediate (branchio-motor) cells make up the motor part of the seventh and fifth nerves while the lateral group of the basal lamina form the salivatory nucleus of the seventh. From the alar lamina come the cochlear and vestibular nuclei, and the sensory nucleus of the fifth nerve. The pontine nuclei are derived by migration of cells from the alar lamina both of the metencephalon and the myelencephalon.
The dorsal portions of the alar laminae thicken on each side to form the primordium of the cerebellum. This thickened portion is termed the rhombic lip. The thickenings bulge into the fourth ventricle and also project dorso-laterally. Fusion in the mid-line gives rise to a thickening, the vermis, while by expansion dorso-laterally the cerebellar hemispheres are formed. The cerebellar cortex is formed from the cells in the mantle zone of the rhombic lip which proliferate and migrate into the marginal zone. The dentate and other deep cerebellar nuclear masses arise by proliferations of the mantle layer in situ.
The Mesencephalon
At a stage when the brain flexures have all been formed (sixth week) the mid brain is a prominent feature of the neural tube ; but with further growth of the brain this portion increases less than adjacent parts and so becomes relatively less distinctive. The roof and floor plates as well as the lateral zones become greatly thickened and the cavity is thus reduced to a narrow channel, the cerebral aqueduct. The basal lamina cells form the nuclei of the third and fourth nerves, which are somatic motor, the Edinger-Westphal nucleus of the third nerve, however, representing visceral efferent cells. The cells of the alar laminae migrate into the thickened roof plate to form the superior and inferior colliculi. The red nucleus is also believed to arise from alar lamina cells that migrate ventrally into the basal lamina.
The Diencephalon
In the beginning of the sixth week, the diencephalon consists of roof and floor plates with thickened lateral walls. The lateral wall on each side shows a longitudinally running groove on the inner surface, the sulcus hypothalamicus. This was formerly believed to be a continuation of the sulcus limitans but it seems more probable that the latter terminates at the cephalic end of the mesencephalon and that the basal lamina of the neural tube is not represented in the diencephalon. A thickening on each lateral wall above the hypothalmic sulcus represents the primordium of the thalamus which grows rapidly and bulges medially into the diencephalic cavity, sometimes fusing across the middle line with its fellow of the opposite side. Above, an epithalmic sulcus limits the thalamus. An evagination in the mid-line of the roof near its junction with the tectum of the mid-brain is the primordium of the pineal gland, and commissural fibres develop in the roof plate adjacent to this as the posterior and habenular commissures. Anterior to this region (epithalamus), the roof plate becomes thinned out (velum interpositum), and invaginated by vascular mesenchyme into the cavities of the third and lateral ventricles, as the choroid plexuses of these cavities.
Fig. 13. - Diagram of the Early Development of the Hypophysis.
1, Infundibulum ; 2, fore-brain ; 3, optic chiasma ; 4, Rathke’s pouch ; 5, foregut ; 6, notochord.
The external surface of the lateral wall of the diencephalon becomes largely hidden from view by the great overgrowth of the telencephalic vesicle, the only parts of it remaining in view in the adult being the geniculate bodies and the pulvinar of the thalamus.
The Hypophysis (Pituitary Gland)
In early somite stages a small ectodermal evagination is found in the roof of the stomatodaeum in front of the buccopharyngeal membrane. This evagination, Rathke's pouch, extends towards the floor of the diencephalon.
During the sixth week a down-growth occurs from the floor of the diencephalon and comes into contact with the posterior surface of Rathke’s pouch. This downgrowth, the infundibulum, gives rise to the posterior lobe and stalk of the adult hypophysis. Rathke’s pouch loses its connection with the ectoderm and its cells proliferate and differentiate. The posterior wall of the original pouch forms the pars intermedia ; the anterior wall becomes thickened and extends backwards as two wings on the lateral aspect of the posterior lobe, and the former lumen of the pouch is represented in the adult by a cleft in the gland. Some cells of the pars anterior grow along the stalk as the pars tuberalis. Differentiation of the cells in the anterior lobe commences about the third month when some acidophils may be detected. Basophils appear about a month later but full histological differentiation is not completed until after birth.
The Telencephalon
The evaginations which form the primordia of the cerebral hemispheres are first seen during the sixth week of development and originally are in wide communication with the cavity of the third ventricle. The cerebral vesicle expands in all directions but least in the downward direction because the lower wall soon becomes thickened as the primordium of the corpus striatum. This expansion soon causes the diencephalon almost completely to be concealed from view, and the cerebral vesicles are separated from each other dorsally by a cleft filled with mesodermal tissue, the future falx cerebri. Here the medial wall of each vesicle is thin and becomes invaginated into the cavity by ingrowth of vascular mesoderm to form the choroid plexus of the lateral ventricle. This invagination extends backwards from the interventricular foramen as the choroidal fissure. The expanding hemispheres at first grow in a caudal direction, but soon they are checked by the limiting tissue of the brain capsule, and the posterior poles change their direction and grow downwards and forwards as the temporal poles. Each growing temporal pole carries with it the choroid plexus of its own side, together with the fissure through which the latter enters the ventricle, and the choroidal fissure is thus continued around to the under and inner aspect of the temporal pole. The expansion of the hemisphere is further reflected in the formation of the inferior and posterior horns of the lateral ventricle.
The Corpus Striatum
The corpus striatum first appears as a thickening of the floor of the cerebral vesicle at the end of the sixth week. The thickening bulges into the ventricle, and as that cavity becomes drawn out to attain its adult form so the striatal thickening elongates. Its original posterior part will thus later be found in the roof of the inferior horn. Next, axis cylinder processes running to and from the developing cerebral cortex split up the corpus striatum into two parts ; dorso-medially is a mass of cells from which the caudate nucleus is formed, while the ventro-lateral portion gives rise to the lentiform nucleus. The fibres traversing the corpus striatum will thus be the internal capsule. Continued thickening of the ventro-medial wall of the hemisphere causes fusion of it with the lateral aspect of the expanding thalamus in the diencephalon, which thus appears as a medial relation of part of the internal capsule and the caudate nucleus.
The Cerebral Commissures
The principal cerebral commissures are the anterior commissure, the commissure of the fornix (hippocampal) and the corpus callosum. These are all developed in the lamina terminalis which, it will be remembered, is the closing plate of the anterior neuropore. The anterior commissure is the first to develop as a compact bundle of fibres in the ventral part of the lamina terminalis connecting the olfactory tract and hippocampal gyrus of one side with corresponding regions on the other.
Dorsal to the anterior commissure a second fibre bundle appears in the lamina terminalis to unite the two hippocampi. It is closely associated with the fornix which is a fibre system connecting the hippocampus with the hypothalamus, and later growth changes associated with the expansion of the corpus callosum cause it to shift caudalwards.
The corpus callosum appears as a small bundle of fibres at the dorsal end of the lamina terminalis. It connects the non-olfactory parts of the cerebral cortices, and as these increase in size more fibres are added to the corpus callosum and it expands both anteriorly and posteriorly. With this expansion, the upper part of the lamina terminalis is thinned out to form the septum lucidum.
The Cerebral Cortex
In the early stages of its existence, the wall of the developing cerebral hemissphere presents ependymal, mantle, and marginal zones, but with migration of cells from the mantle layer into the superficial part of the marginal zone the primordium of the definitive cortex appears. This process does not occur simultaneously all over the hemisphere ; the cortex of the hippocampus appears first, then that of the pyriform area and lastly that of the neopallium.
The outer surface of the hemisphere is at first smooth but with increasing development and growth a pattern of convolutions bounded by sulci or fissures becomes established. The lateral sulcus commences about the fourth month of foetal life in the following way. Part of the lateral surface of the hemisphere lying in relation to the corpus striatum appears to lag behind the rest in its growth. It is termed the insula and becomes overgrown by opercular coverings from the frontal, parietal and temporal lobes and the lateral sulcus is the cleft between these opercula. About the sixth foetal month the central, parietooccipital, calcarine, and collateral sulci make their appearance and secondary and tertiary sulci become visible shortly before birth.
The Meninges
There is no positive information regarding the origin of the inner meninges, the arachnoid and pia. The transplantation experiments of Har\ey and Burr (1926) on the cerebrum of amphibian larvae suggested these meninges to be of neural crest origii , but this has been denied. It seems fairly certain that the dura mater is developed by condensation in the mesenchyme surrounding the neural tube.
Summary of the Development of the Brain
(1) The brain develops from that part of the neural tube cephalic to the fourth somite. Three primary swellings or brain vesicles form here.
(2) The original straight neural tube becomes bent by the mid-brain, cervical and pontine flexures.
(3) The first primary brain vesicle gives rise to two optic evaginations at a very early stage of development.
(4) At a later stage two further evaginations occur which form the cerebral hemispheres. Thickenings in the floor of these are the corpora striata.
(5) Most of the first primary brain vesicle forms the diencephalon. A thickening in each lateral wall is the thalamus. The anterior boundary is the lamina terminalis where the cerebral commissures are laid down.
(6) The second primary brain vesicle forms the mid-brain and its cavity becomes the cerebral aqueduct. H
(7) The third primary brain vesicle forms the pons, medulla, and cerebellum, and its cavity is the fourth ventricle.
Anomalies of Development of the Central Nervous System
(i) Anencephaly is a condition where the neural folds fail to develop in the future brain region and the exposed nervous tissue undergoes degeneration. The cranial skeleton also fails to develop.
(2) Encephalocele is a hernia of brain tissue through a defect in the skull vault.
(3) Rachischisis is a condition where there is a cleft in the vertebral column with varying degree of defect in the underlying spinal cord.
(4) Meningocele is a condition where there is a herniation of the membranes through a cleft in the skull or vertebral column.
(5) Microcephaly is a condition where the brain is abnormally small.
(6) Congenital hydrocephalus, where cerebro-spinal fluid accumulates within the ventricles, may be due to anomalies in development of the pathway for circulation of this fluid such as obliteration of the cerebral aqueduct, or non-appearance of the foramina in the ependymal roof of the fourth ventricle. Failure of the mechanism for absorption of the fluid may be another cause.
The Spinal Nerves
The anterior and posterior nerve rootlets arise from different sources. The anterior nerve root fibres grow out from basal lamina cells in the developing cord about the fifth week ; the tips of these fibres perforate the surface of the cord and grow through the surrounding mesenchyme to unite with a myotome. During the sixth week the posterior nerve rootlets appear as centrally growing processes from neural crest cells which become arranged in groups along the spinal cord. These central processes enter the cord opposite the posterior horn of grey substance. The cells of the posterior root ganglia are at first bipolar and later become T-shaped. Their distal processes grow out to unite with the anterior nerve root to form the spinal nerve The neurilemmal sheaths of the spinal nerves are derived from migrating cells of the neural crest.
The spinal nerves are segmen tally arranged and w en the limb buds grow out from the body, each is invaded by nerve fibres of the body segments opposite which it arose. It is the anterior primary rami which are concerned in this ingrowth, and at the base of the limb bud loops occur between the successive anterior primary rami to form the brachial and lumbo-sacral plexuses.
The Cranial Nerves
The cranial nerves may be subdivided into three groups on the basis of their developmental origin.
(1) The first group consists of the nerves of the special senses— olfactory, optic and auditory nerves. Since these develop in connection with the special sense organs and in a peculiar manner, they will be considered later (p. 63).
(2) A second group is made up of the trigeminal, facial, glossopharyngeal, vagus and accessory nerves! These nerves contain several components. Special visceral efferent fibres innervate the musculature derived from the branchial arches. In this way, the mandibular division of the trigeminal innervates the muscles of mastication derived from the first (mandibular) arch, and so on (see p. 7b). General visceral efferent fibres pass to visceral muscle and glands. Sensory fibres correspondingly transmit general and special visceral afferent impulses, and there are a few somatic afferent fibres in the auricular branch of the vagus.
(3) The oculomotor, trochlear, abducent and hypoglossal nerves form a group of nerves which are essentially somatic efferent in nature. The first three are distributed to the muscles of the orbit which are deemed, on comparative grounds, to be derived from three pre-otic myotomes. The hypoglossal nerve innervates the tongue musculature which is derived from the occipital somites.
The Autononic Nervous System
The primordia of the thoraco-lumbar gangliated sympathetic chains are laid down in the fifth week dorso-lateral to the aorta. The cells forming these chains migrate along the anterior primary rami. They are ectodermal and their site of origin is variously ascribed to the neural crest or the neural tube. Migration of cells from the thoraco-lumbar chains towards the heart, intestine, etc., gives rise to the visceral sympathetic ganglia. There are also extensions of the primitive chains upwards into the neck and downwards to the sacral region. The fibres of distribution from the sympathetic to the viscera and to the skin are arranged essentially in a segmental fashion.
Some of the ectodermal cells which are found in the primitive sympathetic chains become transformed into cells which have a characteristic reaction with chrome salts and hence are called chromaffin cells. These produce the sympatho-mimetic hormone adrenalin and are found related to many parts of the embryonic sympathetic system especially around the abdominal aorta. In post-natal life, practically the only chromaffin tissue that persists forms the medulla of the adrenal gland (see below).
The cells which form the para-sympathetic ganglia apparently arise within the neural tube and migrate peripherally along the nerves with which these ganglia are associated.
The Adrenal Gland
The adrenal gland consists of two parts, cortex and medulla, which are embryologically quite distinct. The cortex is mesodermal in origin and is first seen at the 9 mm. stage (sixth week) as a proliferation of the coslomic mesothelium between the root of the mesentery and the upper end of the mesonephros. This mass of cells sink into the underlying mesenchyme and a few days later has added to its surface a second mesothelial proliferation. This second group of cells will form the permanent cortex of the gland, the first proliferation being the transitory or foetal cortex which regresses during the rst year after birth. Due to its presence the human adrenal is relatively large at birth. Histological differentiation of the permanent cortex into the classical zones, glomerulosa, f asciculata, and reticularis is not completed until some time after birth. Cells derived from the sympathetic system congregate at the medial side of the developing gland during the sixth and seventh weeks and then become surrounded by the cortical cells. They mainly become transformed into chromaffin cells but some give rise to ganglionic cells of the sympathetic system.
Summary of Development of the Peripheral Nervous System
(i) The motor nerve fibres sprout out from the neural tube and secondarily connect up with the myotomes.
(2) As the myotomes resolve themselves into the various muscles, each remains connected with its primitive segmental nerve.
(3) The posterior root ganglia are formed from cells of the neural crest which develop central processes growing into the neural tube and peripheral processes passing to the end organs.
(4) The cranial nerves (apart from the special sense nerves) develop as : (a) branchial arch nerves - trigeminal, facial, glossopharyngeal, vagus, and accessory; (b) somatic motor nerves - oculomotor trochlear, abducent, and hypoglossal, which supply muscles derived from the "pre-otic" and occipital somites.
(5) The sympathetic system and the chromaffin bodies are formed from ectodermal migratory cells from either the neural crest or the neural tube.
(6) The adrenal gland has a double origin ; the medulla is ectodermal being derived from chromaffin cells and the cortex mesodermal since it arises from the coelomic mesothelium.
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