DEVELOPMENT OF CNS

Lecture 1

Formation of neural tube

 At the beginning of the third week of development, the ectodermal germ layer has the shape of a disc that is broader in the cephalic than the caudal region .

 Appearance of the notochord and prechordal mesoderm induces the overlying ectoderm to thicken and form the

neural plate

.

 Appearance of the notochord and prechordal mesoderm induces the overlying ectoderm to thicken and form the

neural plate

.

By the end of the third week, the lateral edges of the neural plate become more elevated to form

neural folds,

and the depressed mid region forms the

neural groove

 Gradually, the neural folds approach each other in the midline, where they fuse .

 cells along the margin of the neural groove is called the neural crest  develop into sensory and sympathetic neurons and schwann cells

 Fusion begins in the cervical region and proceeds cranially and caudally . As a result, the

neural tube

is formed. Until fusion is complete, the cephalic and caudal ends of the neural tube communicate with the amniotic cavity by way of the

cranial

and

caudal neuropores,

respectively

 Neurulation is then complete, and the central nervous system is represented by a closed tubular structure with a narrow caudal portion, the

spinal cord,

and a much broader cephalic portion characterized by a number of dilations, the

brain vesicles

 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 . Junctional complexes at the lumen connect them. During the neural groove stage and immediately after closure of the tube,they divide rapidly, producing more and more neuroepithelial cells. Collectively they constitute the

neuroepithelial layer

or

neuroepithelium.

 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.

 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

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

.

 A longitudinal groove, the

sulcus limitans,

marks the boundary between the two.

 The dorsal and ventral midline portions of the neural tube, known as the

roof

and

.floor plates,

respectively, do not contain neuroblasts; they serve primarily as pathways for nerve Fibers crossing from one side to the other.

 In addition to the ventral motor horn and the dorsal sensory horn, a group of neurons accumulates between the two areas and forms a small

intermediate horn

. This horn, containing neurons of the sympathetic portion of the autonomic nervous system, is present only at thoracic (T1–T12) and upper lumbar levels (L2 or L3) of the spinal cord.

HISTOLOGICAL DIFFERENTIATION

 Nerve Cells

Neuroblasts,

or primitive nerve cells, arise exclusively by division of the neuroepithelial cells. further development becomes the adult nerve cell or

neuron.

Axons of neurons in the basal plate break through the marginal zone and become visible on the ventral aspect of the cord. Known collectively as the

ventral motor root of the spinal nerve,

they conduct motor impulses from the spinal cord to the muscles .

Axons of neurons in the dorsal sensory horn (alar plate) behave differently from those in the ventral horn. They penetrate into the marginal layer of the cord, where they ascend to either higher or lower levels to form

association neurons.

  Glial Cells The majority of primitive supporting cells, the

gliablasts,

are formed by neuroepithelial cells and migrate from the neuroepithelial layer to the mantle and marginal layers. In the mantle layer, they differentiate into

protoplasmic astrocytes

and

.fibrillar astrocytes

Another type of supporting cell possibly derived from gliablasts is the

oligodendroglial cell.

axons in the marginal layer.

This cell, which is found primarily in the marginal layer, forms myelin sheaths around the ascending and descending

 In the second half of development, a third type of supporting cell, the CNS.

microglial cell,

appears in the  Neural Crest Cells During elevation of the neural plate, a group of cells appears along each edge (the crest) of the neural folds Crest cells migrate laterally and give rise to

sensory ganglia (dorsal root ganglia)

of the spinal nerves and other cell types

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

and

Dorsal nerve roots

form as collections of fibers originating from cells in

dorsal root ganglia (spinal ganglia)

.

Nerve cells in anterior gray column

 nerve cells are large multipolar known as alpha which innervate skeletal muscle and gamma efferents which innervate intrafusal fibers of neuromucula spindles.

Nerve cells in posterior gray column

 At apex-substantia gelatinosa  Nucleus prorius  Nucleus dorsalis-c8-L4

 The dorsal region of the gray matter, called the

dorsal or posterior horn

, is associated with the incoming (

afferent

) dorsal root, and is thus related to sensory functions.

The cell body of these sensory fibers is located in the

dorsal root ganglion

.

The dorsal horn is quite prominent in this region because of the very large sensory input to this segment of the cord from the upper limb, particularly from the hand.

 The ventral gray matter, called the

ventral or anterior horn

, is the motor portion of the gray matter. The ventral horn has the large motor neurons, the anterior horn cells,which are

efferent

to the muscles . These neurons, because of their location in the spinal cord, which is “below” the brain, are also known as

lower motor neurons

.