1. DEVELOPMENT OF THE CNS
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3. INTRODUCTION
The central nervous system (CNS) appears at the beginning of the third week as a slipper-shaped plate of thickened ectoderm, the neural plate, in the middorsal region in front of the primitive node. Its lateral edges soon elevate to form the neural folds
With further development, the neural folds continue to elevate, approach each other in the midline, and finally fuse, forming the neural tube
The notochord (a midline rod of cells) stimulates the overlying ectoderm to differentiate into neuroectoderm through a process known as induction.
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5. THE NEURAL TUBE…..
NOTE: Stages of formation of neural tube do not proceed simultaneously all over the length of the neural plate.
The middle part is the first to become tubular, so for some time the neural tube is open cranially and caudally to form (neuropores)
Once fusion is initiated, the open ends of the neural tube form the cranial and caudal neuropores that communicate with the overlying amniotic cavity
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6. THE NEURAL TUBE…..
Closure of anterior (cranial) neuropore occurs on 25th day, where as the posterior (caudal) neuropore closes on 27th day.
Even before the neural tube has completely closed, it is divided into an enlarged cranial part and a caudal tubular part.
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8. CONGENITAL ANOMALIES OF THE SPINAL CORD (Spina bifida)
Spina bifida (Latin: "split spine") is a developmental congenital disorder caused by the incomplete closing of the embryonic neural tube.
spina bifida occulta,
spina bifida cystica and
spina bifida rachischisis.
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9. Spina bifida occulta
Occulta is Latin for "hidden". Many people with this type of spina bifida do not even know they have it, as the condition is asymptomatic in most cases
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10. Spina bifida cystica
This is a developmental defect of the central nervous system in which a hernial cyst containing meninges (meningocele), spinal cord (myelocele), or both (myelomeningocele) protrudes through a congenital cleft in the vertebral column
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15. Spina bifida with myeloschisis (rachischisis).
This is often a severe or complete defect involving the entire spine from the cervical region through to the sacrum
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16. CONGENITAL ANOMALIES
ANENCEPHALY- Is failure of closure of the anterior neuropore. The brain substance is exposed to the surface as an irregular degenerated mass of nervous tissue with no bony covering. The appearance is characterized by; prominent eyes which bulge forwards and the chin is continuous with the chest due to the absence of the neck. The condition is incompatible with life
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18. THE SPINAL CORD NEUROEPITHELIAL, MANTLE, AND MARGINAL LAYERS
The spinal cord is developed from the caudal part of the neural tube, caudally to the fourth pair of somites.
The wall of a recently closed neural tube consists of neuroepithelial cells. These cells extend over the entire thickness of the wall and form a thick pseudostratified epithelium (neuroepithelium).
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19. THE SPINAL CORD…
Once the neural tube closes, neuroepithelial cells begin to give rise to another cell type characterized by a large round nucleus with pale nucleoplasm and a dark-staining nucleolus. These are the primitive nerve cells, or neuroblasts. They form the mantle layer, a zone around the neuroepithelial layer . The mantle layer later forms the gray matter of the spinal cord.
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20. THE SPINAL CORD…
The outermost layer of the spinal cord, the marginal layer, contains nerve fibers emerging from neuroblasts in the mantle layer. As a result of myelination of nerve fibers, this layer takes on a white appearance and therefore is called the white matter of the spinal cord
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21. BASAL, ALAR, ROOF, AND FLOOR PLATES
THE SPINAL CORD…
BASAL, ALAR, ROOF, AND FLOOR PLATES
As a result of continuous addition of neuroblasts to the mantle layer, each side of the neural tube shows a ventral and a dorsal thickening. The ventral thickenings, the basal plates, which contain ventral motor horn cells, form the motor areas of the spinal cord; the dorsal thickenings, the alar plates, form the sensory areas
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22. THE SPINAL CORD…
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24. THE SPINAL CORD… GLIAL CELLS
The majority of primitive supporting cells, the gliablasts, are formed by neuroepithelial cells after production of neuroblasts ceases. Gliablasts migrate from the neuroepithelial layer to the mantle and marginal layers. In the mantle layer, they differentiate into protoplasmic astrocytes and fibrillar astrocytes
In the second half of development, a third type of supporting cell, the microglial cell, appears in the CNS. This highly phagocytic cell type is derived from mesenchyme.
Another type of supporting cell possibly derived from gliablasts is the oligodendroglial cell. This cell, which is found primarily in the marginal layer, forms myelin sheaths around the ascending and descending axons in the marginal layer.
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25. THE SPINAL CORD…
When neuroepithelial cells cease to produce neuroblasts and gliablasts, they differentiate into ependymal cells lining the central canal of the spinal cord.
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26. Neural Crest Cells & Myelination of PNS
THE SPINAL CORD…
Neural Crest Cells & Myelination of PNS
During elevation of the neural plate, a group of cells appears along each edge (the crest) of the neural folds. These neural crest cells are ectodermal in origin and extend throughout the length of the neural tube. Crest cells migrate laterally and give rise to sensory ganglia (dorsal root ganglia) of the spinal nerves and other cell types
Schwann cells myelinate the peripheral nerves. These cells originate from neural crest, migrate peripherally, and wrap themselves around axons, forming the neurilemma sheath.
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27. THE SPINAL CORD… Spinal Nerves
Motor nerve fibers begin to appear in the fourth week, arising from nerve cells in the basal plates (ventral horns) of the spinal cord. These fibers collect into bundles known as ventral nerve roots.
Dorsal nerve roots form as collections of fibers originating from cells in dorsal root ganglia (spinal ganglia). Central processes from these ganglia form bundles that grow into the spinal cord opposite the dorsal horns.
Distal processes join the ventral nerve roots to form a spinal nerve. Almost immediately, spinal nerves divide into dorsal and ventral primary rami. Dorsal primary rami innervate dorsal axial musculature, vertebral joints, and the skin of the back. Ventral primary rami innervate the limbs and ventral body wall and form the major nerve plexuses (brachial and lumbosacral).
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28. POSITIONAL CHANGES OF THE CORD
In the third month of development the spinal cord extends the entire length of the embryo, and spinal nerves pass through the intervertebral foramina at their level of origin.
With increasing age, the vertebral column and dura lengthen more rapidly than the neural tube, and the terminal end of the spinal cord gradually shifts to a higher level.
Note: The dura remains attached to the vertebral column at the coccygeal level
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29. Positional Changes of the Spinal Cord
8- weeks – Entire length of vertebral canal
24-weeks – S1
Newborn – L3
Adult – L1/L2
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30. Positional Changes …
In the adult, a threadlike extension of the pia mater forms the filum terminale, which is attached to the periosteum of the first coccygeal vertebra and which marks the tract of regression of the spinal cord.
Nerve fibers below the terminal end of the cord collectively constitute the cauda equina. When cerebrospinal fluid is tapped during a lumbar puncture, the needle is inserted at the lower lumbar level, avoiding the lower end of the cord.
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31. THE BRAIN (primary brain vesicles)
The brain is developed from the anterior end of the neural tube cranial to the fourth pair of somites.
Fusion of the neural folds in the cranial region and closure of the rostral neural pore form the three primary brain vesicles from which the brain develops.
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32. Development of the brain…
During the fifth week, the fore brain partly divides into two secondary brain vesicles, the telencephalon and diencephalons;
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38. Brain flexures
During the fourth week the embryonic brain grows rapidly and bends ventrally with the head fold. This produces the three brain flexures which are; mid brain (cephalic) flexure in the mid brain region, pontine flexure, and the cervical flexure at the junction of the hind brain and spinal cord.
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39. HOLOPROSENCEPHALY
The forebrain does not properly divide into two hemispheres as it should. A child born with holoprosecephaly will have a single lobed brain with severe defects of the skull and face. Babies with this condition have abnormalities of the eyes nose and upper lip.
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40. Myelencephalon/ medulla oblongata
The myelencephalon is a brain vesicle that gives rise to the medulla oblongata. It differs from the spinal cord in that its lateral walls are everted. Alar and basal plates separated by the sulcus limitans can be clearly distinguished.
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42. Myelencephalon…The basal plate
The most lateral is the somatic afferent group: receives impulses from the ear and surface of the head via the staticoacoustic (VIII) and bulbospinal part of the trigeminal (V) nerves
The intermediate is the special visceral afferent group: receives impulses from the taste buds of the tongue and from the palate, oropharynx, and epiglottis. These neurons later form the nucleus of the solitary tract
The medial is the general visceral afferent group: represented by the dorsal sensory nucleus of the vagus (X) nerve with its neurons receiving interoceptive information from the heart and gastrointestinal tract.
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43. Myelencephalon …The alar plate
The roof plate of the myelencephalon consists of a single layer of ependymal cells which is later covered by vascular mesenchyme, the pia mater. Together they make up the tela choroidea. As a result of active proliferation of vascular mesenchyme, the tela choroidea forms a series of saclike invaginations that project into the underlying ventricular cavity in the region of the pontine flexure, forming the choroid plexus.
At about month 4, areas of the roof plate of the rhombencephalon thin out, bulge outward, and finally disappear. The apertures formed are the 2 lateral foramina of Luschka and a median foramen of Magendie which allow the cerebrospinal fluid to move freely between the ventricles and the surrounding subarachnoid space
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44. Metencephalon
The metencephalon is the embryonic part of the hindbrain that differentiates into the pons and the cerebellum. It contains a portion of the fourth ventricle and the trigeminal nerve (CN V), abducens nerve (CN VI), facial nerve (CN VII), and a portion of the vestibulocochlear nerve (CN VIII)
The marginal layer of the basal plates of the metencephalon expands as it makes a bridge for nerve fibers connecting the cerebral cortex and cerebellar cortex with the spinal cord. Hence this portion of the metencephalon is known as the pons (bridge).
In addition to nerve fibers, the pons contains the pontine nuclei, which originate in the alar plates of the metencephalon and myelencephalon
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45. Metencephalon…
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46. Cerebellum
The dorsolateral parts of the alar plates bend medially and form the rhombic lips. In the caudal portion of the metencephalon, the rhombic lips are widely separated, but immediately below the mesencephalon they approach each other in the midline.
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47. Cerebellum…
A transverse fissure soon separates the nodule from the vermis and the lateral flocculus from the hemispheres. This flocculonodular lobe is phylogenetically the most primitive part of the cerebellum.
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52. MESENCEPHALON (midbrain)
The mesencephalon is morphologically the most primitive of the brain vesicles. Its basal and alar plates, separated by the sulcus limitans.
Each basal plate contains 2 groups of motor nuclei
A medial somatic efferent group is represented by the oculomotor (III) and trochlear (IV) cranial nerves, which innervate the preoptic (eye) muscles. A small general visceral efferent group is represented by the Edinger-Westphal nucleus, also associated with the oculomotor (III) nerve, and innervates the sphincter pupillary muscle
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53. MESENCEPHALON… THE ALAR OR ROOF PLATE AND THE COLLICULI
The alar plates initially appear as 2 longitudinal elevations separated by a shallow midline depression. The (superior) and a posterior (inferior) colliculus. The colliculi are formed by waves of neuroblasts produced by the neuroepithelial cells that migrate into the overlying marginal zone and become arranged in stratified layers
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54. Diencephalon
The diencephalon develops from the median portion of the prosencephalon. The alar plates form the lateral walls of the diencephalon. A groove, the hypothalamic sulcus, divides the plate into a dorsal (thalamus ) and a ventral region (hypothalamus).
The hypothalamus, forming the lower portion of the alar plate, differentiates into a number of nuclear areas that regulate the visceral functions, including sleep, digestion, body temperature, and emotional behavior.
One of these groups, the mamillary body, forms a distinct protuberance on the ventral surface of the hypothalamus on each side of the midline
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55. Diencephalon…
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56. DEVELOPMENT OF THE PITUITARY GLAND
Just anterior to the buccopharyngeal membrane, a midline diverticulum known as the Rathke pouch develops in the oral ectoderm of the roof of the primitive oral cavity.
This evaginating pouch comes in contact with a pouch developing from the floor of the diencephalon. Further development of these two opposed structures gives rise to the pituitary gland.
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59. Hypophyseal Defects
Occasionally a small portion of Rathke’s pouch persists in the roof of the pharynx as a pharyngeal hypophysis.
Craniopharyngiomas arise from remnants of Rathke’s pouch. They may form within the sella turcica or along the stalk of the pituitary but usually lie above the sella. They may cause hydrocephalus and pituitary dysfunction (e.g., diabetes insipidus, growth failure).
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60. HYDROCEPHALUS
Hydrocephalus is a significant enlargement of the head due to excessive accumulation of CSF within the skull.
Hydrocephalus results from obstruction of CSF passage through which it circulates or interference with the absorption of the CSF.
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61. Telencephalon
The telencephalon, the most rostral of the brain vesicles, consists of two lateral out-pocketings, the cerebral hemispheres, and a median portion, the lamina terminales.
The cavities of the hemispheres, the lateral ventricles, communicate with the lumen of the diencephalon through the interventricular foramina of Monro
The cerebral hemispheres arise at the beginning of the fifth week of development as bilateral evaginations of the lateral wall of the prosencephalon. Simultaneously side to side expansion of the cerebral cortex results into greatly increased surface area as a result, the cerebral cortex becomes folded on itself.
The sulci and gyri are formed as a result of this folding.
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63. CRANIUM BIFIDUM
This is a defect in the formation of the cranium. Large protrusion from the occipital region of the skull
Cranium bifidum with meningocele- in this condition the gap in the occipital bone is small, only the cranial meninges protrude or herniate filled with CSF.
Cranium bifidum with meningo-encephalocele- in this condition the gap in the occipital bone is large, making the protrution to consist the portion of the cerebellum that is covered by the meninges and the skin.
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