Central Nervous System Development-Embryology

The central nervous system develops from the neural tube which arises from the ectoderm. It appears in the 3rd week as a thickening of the ectoderm known as the neural plate.

Extending from the prechordal plate in front to the caudal end of the embryonic disc behind.

The neural plate will form the neural groove which has 2 elevated edges called the neural folds. The neural folds fuse together transforming the neural groove into neural tube lying beneath the ectoderm in the median plane.

The fusion of the folds is absent at the anterior and posterior ends of the tube leaving 2 openings on the ectoderm called the anterior and the posterior neuropores.

Later on, the anterior neuropore closes at the 23rd day while the posterior neuropore closes at the 25th day. The closed neural tube develops into the spinal cord & the brain

The Neural Crest

Formation

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central nervous system neural tube

Transverse sections through successively older embryos showing formation of the neural groove, neural tube, and neural crest.[/caption]

The neural crest arises as a strip of ectoderm cells situated along the lateral edge of the neural groove. As the 2 edges of the neural groove fuse together forming the neural tube,the neural crests separates as 2 longitudinal cords that migrate ventrally to lie one on each side of the neural tube.

Derivatives:

The neural crest becomes segmented into masses which give the following derivatives:

Sensory ganglia of the cranial nerves(5,7,9 & 10)
Automatic ganglia (both sympathetic & parasympathetic)
Dorsal root ganglia of all spinal nerves.
(Schwann) cell of peripheral nerves:
The medulla of the suprarenal gland (Chromaffin cells).
Melanoblasts of the skin which produce melanin pigment
Arachnoid & pia mater which are ectodermal (but not dura)

Development of The Spinal Cord

Another component of the central nervous system that we shall cover its development is the spinal cord. The spinal cord develops from the caudal part of the neural tube as follows:

At first , the neural tube is formed of one layer of simple columnar epithelium surrounding an oval central canal. Later on ,this layer proliferates and the neural tube becomes formed of the 2 thick lateral walls by a thin roof plate and a thin floor plate.

The lateral. Walls differentiates into 3 layers:

(a) inner ependymal layer: the cells of which gives rise to
- the epindymal cells lining the central canal of the spinal cord.
- the primitive nerve cells (nueroblasts)which migrate to the mantle layer.
(b) middle mantle layer: formed of nerve cells (neuroblasts) & neuroglial cells(spongioblasts) which form the grey matter.
(c)Outer marginal layer: formed of the nerve fibres (ascending& descending tracts) which constitute the white matter.

A groove called sulculus limitations appear on the inner surface of the wall on either side dividing it into:

(a) An alar plate posteriority which contains sensory cells and forms the posterior .horn of the spinal cord.
(b)A basal plate anteriorly, contains motor cells & forms the anterior Horn the of spinal cord.

spinal cord development

Enlargements are formed in the cervical & lumbar regions of the spinal cord & the central canal becomes marrow.

Growth of the spinal cord

Until the 3rd month the spinal cord fills the vertebral canal completely. The vertebral column then grows at a faster rate than the spinal cord producing the following changes:

(a ) the lower end of the cord shifts upwards to lie at the level of l3 at the time of birth.

(N.B) in the adult, the spinal cord ends at the disc between L1 & L2

Myelination of the nerve fibres in the spinal cord begins in the 4th month of intrauterine life & is completed by the end of the 1st year of post natal life.

Development of the spinal meninges:

Another component of the central nervous system are the meninges.

(a)The dura mater develops from the sclerotomes which form the vertebral column.

(b) The arachnoid & pia mater develop from the neural crest (ectodermal in origin)

Congenital Anomalies of the Spinal Cord

There are some various congenital anomalies that can occur in te central nervous system specifically on the spinal cord;

1)Spinal bifida occulta occurs due to failure of fusion of the dorsal parts of one of the vertebrate around the spinal cord.this condition occurs commonly in the lumbosacral region & the affected site is covered by hairy skin.

2)Meningocele occurs due to failure of fusion of the dorsal parts of 2 or 3 vertebrate. In this anomaly, the meninges bulge through the defect & the conditions is accompanied by some nuerological symptoms.

3)Meningo-myelocele: like the previous condition but here he spinal cord bulges through the defect. This normally is accompanied by severe neurological symptoms.

4)Myelocele results due to failure of closure of the neural tube & the affected pat of the spinal cord remains exposed to the surface through the defect in the vertebral canal. It is the most serious anomally.

5)Rachischisis

Development of the brain

The major component of the central nervous system is the brain. Here we shall look at how it develops.

The brain develops from the cranial end of the neural tube as follows.

(1) The brain develops from the neural tube as it expands to form the brain swelling.

(2) Two constrictions appear in the brain swelling,dividing it into 3 parts called brain vesicles:

(a) fore brain prosencephalon

(b) mid brain or mesencephalon

(3 ) The 3 brain vesicles differentiate as follows:

(A) The forebrain: gives 2 optic vesicles (the future eyes) then divides into:

-a median part called the diencephalon

- lateral diverticula called telencephalic vesicles(the future cerebral hemispheres)

(B) the midbrain: remains undivided

(c)the hind brain: gives rise to the following derivatives:

(1)metencephalon which forms the pons & cerebellum

(2)myelencephalon which forms the medulla oblongata.

Development of the brain system

As the development of the spinal cord ,the Lateral walls of the brain stem are connected by a thin roof plate, a thin floor plate& and will have a ventral basal lamina ( containing motor nuclei) & dorsal alar lamina ( containing sensory nuclei).

Each wall shows a sulcus limitations internally, separating the alar lamina from the basal lamina.

Differentiation of the basal & alar lamina into columns of nuclei

The alar lamina differentiate into 4 column of sensory nuclei.
The basal lamina differentiates into 3 columns of motor nuclei.

(A) COLUMNS OF BASAL LAMINA:

(1)Somatic efferent column (most medial)

-it lies to the middle line, in line with the Ant.horn cells of the spinal cord.

-its efferent fibres supply somatic somatic striated muscles (i.e derived from the somites).

- it differentiates into the following nuclei {-12th n-nucleus medulla

{-6th n.nucleus in pons

{-3rd & 4th n.nuclei in the midbrain

(2)special visceral (branchian) efferent column:

-it lies just lateral to the somatic efferent column.

-its efferent fibres supply the muscles derived from the branchial (Pharyngeal) arches.

It differentiates into the following nuclei: -5th & 7th nerves motor nuclei:in the pons.

-9th ,10th & 11th nerver motor n: in the medulla(which join together forming nucleus ambiguous)

(3)general visceral efferent column:

- lies just lateral to he special visceral efferent column,in lin with autonomic lat. horn grey matter of the spinal cord.

- its efferent fibres are parasymphathetic supplying visceral smooth muscles & glands it differentials into the following nuclei:

- (1)dorsal nucleus of vagus: in the medulla
- (2)inferior salivary nucleus(of glossopharyngeal n.): in the medulla
- (3)superior salivary nucleus(of facial n.):in the pons.
- (4) edinger-westphal nucleus(of occulomotor n.) in he midbrain.
- (3) The column of the alar lamina:

The alar lamina 4 sensory(afferent) columns as follows:

1.General visceral afferent column:

It is the most medial Of the sensory fibres from the viscera.it is the represented by sensory component

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