Nervous System

Central Nervous System Development-Embryology

  • Physiology
  • Nervous System
  • 2020-07-05 06:11:04
  • 11 minutes, 19 seconds

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

[caption id="attachment_6330" align="aligncenter" width="403"]

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:

  1. Sensory ganglia of the cranial nerves(5,7,9 & 10)
  2. Automatic ganglia (both sympathetic & parasympathetic)
  3. Dorsal root ganglia of all spinal nerves.
  4. (Schwann) cell of peripheral nerves:
  5. The medulla of the suprarenal gland (Chromaffin cells).
  6. Melanoblasts of the skin which produce melanin pigment
  7. 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

     (c) hindbrain or rhombencephalon.

(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 of the dorsal vagal nucleus.

2.Special visceral afferent column:

It lies just lat. to the general visceral afferent column. It receive taste sensation from the tongue & epiglottis.

It is represented by the nucleus of tracus solitarius.

3.General somatic afferent column:

Lies lat. to he special visceral afferent column.

It receives afferent sensory fibres from the skin of the face of the face & scalp + prioceptive sensation from the muscles of mastication this column differentiates into the 3 sensory nuclei of trigeminal n - spinal

-main sensory

- mesencephalic.

Special somatic afferent column:

-It is the most lat. column of the alar lamina..

-It receives afferent auditory & vestibular fibres of the vestibulocochlea nerve .

-it is represented by the vesibular & cichlear nuclei

Migration of nuclei (nuerobiotaxis):

Some of the motor nuclei of he cranial nerves migrate from their original position onwards he chief fibre tracts from which it receives maximal impulses.this is called nuerobiotaxis & is manifested by the migration of the facial motor nucleus & the vagus motor (nuclus ambiguus)resulting in bending of the axons emerging from hse nuclei.

Some of the nuclei of the dorsal alar lamina ( mostly extrapyramidal) migrate ventrally. Examples of these nuclei are the following:

(a)the olivary in the medulla

(b) the pantine nuclei in the pons.

(c)the red nucleus & substantia nigra in the midbrain.

Development of the lumen of the brain stem

In the central nervous system there are some pathways where cerebrospinal fluid flows and bathes the brain to nourish it. On e of them is the fourth ventricle. Lets have a look on its development.

  • Formation of the 4th ventricle:

in the rhombencephation,the alar lamine.of  both sides move away from each other (as in the way of opening a book )leading to widening of the lumen thus forming the 4th ventricle.

-the thin roof plate becomes stretched & rhomboid in shape, extending laterally to form the lat. recess of the 4th ventricle on each side.

-the vascular mesenchyme lying in contact with the outer surface of the roof plate forms the pia  mater & the 2 layers form the tela choroidae which sends vascular projections into the cavity of the 4th ventricle to form the choroid plexus.

- local resorption of the roof plate leads o the formation of a median foramen(of magendi) & 2 lateral foramina(of luschka)

2) formation of the cerebral acqueduct:

The lumen of the mesencephalon (midbrain) becomes much reduced (due to thickening in the walls) & is transformed into a central canal called the cerebral a queduct(of sylvius)

Development of the cerebellum

The cerebellum is formed of the dorsal pat of the alar laminae of the metencephalon as follows:

  • the alar laminae of both sides bend medially to form the rhombic lips of arch of which grows to form media & lateral bulges.
  • The med-budges of both sides meet each over the roof plate of the 4th ventricle & unite forming the vermis.
  • The lat. bulges grow to form the cerebellar hemispheres.
  • The cereballar cortex is formed by migration of the nueroblasts from the mantle layer to enter the marginal layer.
  • The dentate nucleus develops as a collection of nueroblasts which remains deeply situated in the mantle layer.
  • The cerebelar peduncles develop later as the axons of the neurons of the cerebellar nuclei grow out of the cerebellum to reach the brain stem.

Development of the forebrain (prosencephalon)

It is the most cranical of the 3 brain vesicles.

A lateral diverticulum appears on each side of the forebrain(called he optic vesicle) subdividing the forebrain into:

(a)telencephalon: the cranical part of the forebrain, including the lateral diverticula. It gives 2 lateral outpockeings forming the celebral hemispheres while each optic vesicle & its stalk will form the retina & optican.

(b) dieencephalon: the part immediately caudal o the septic vesicles.it develops into the thalamus, hypothalamus, epithalamus & pineal body.

Development of the cerebral hemispheres

The 2 cerebral hemispheres arise as ivaginations from the lateral wall of the forebrain (prosencephalon).

The cavity of each evagination expands forming lateral ventricle.

The wall of each hemisphere is formed of the following layers:

(a)outer marginal layer formed of white matter.

(b)middle mantle layer formed of the nerve cells (nueroblasts)

( c) inner ependylmal layer lining the cavity of the lateral ventricle

As development proceeds the following changes he following changes occur:

(1) the nueroblasts of the wall of the hemisphere (except its base)migrate from the mantle layer to the marginal layer forming the grey matter of the cortex & the axons of these nueroblasts invade the rest of he wall forming the white matter of the cerebral hemisphere.

(2)the nueroblasts of the mantle layer at the base of the hemisphere do not migrate to the marginal layer but remain deeply situated forming masses of the grey matter (corpus striatum).

(3)the part of the medial wall of the hemisphere just above the roof of the 3rd ventricle is invaginated by choroid plexus forming the choroid fissure.

(4)Expansion of the cerebral cortex: the cortex expands in all directions resulting in the following changes.

(a)the medial surfaces of the 2 hemispheres come very close to each other.

(b) the corpus striatum at the base of the hemispheres is pushed towards the thalamus but kept seperated from it by the fibres of the internal capsule.

(c ) the insula which appears on the lat. surface as an area of less active growth becomes buried deep to the surface  by the surrounding actively growing parts.

(d)the cerebral hemispheres overlap the brain stem & cerebellum.

After having looked at all these we hope you have gotten a clue on the basic embryology of the central nervous system.


References:
author

Daniel Ogera

Medical educator, passionate about simplifying difficult medical concepts for easier understanding and mastery by nursing and medical students.

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About this article:
  • Topic:Physiology
  • Duration:11 minutes, 19 seconds
  • Subtopic:Nervous System

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