development of cns nervous tissue

8/6/2019 Development of Cns Nervous Tissue

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Nervous Tissue : CNS

Consists of cells, fibres, and blood vessels.



Formation of Neurons and Neuroglial cells

� Both are formed in the neural tube.



Formation of neural tube

� At the beginning of the 3rd week, developing notochordinduces overlying ectoderm to become neuroectoderm.Thus forming an elongated, slipper-shaped plate of thickened ectoderm ± the neural plate.

� Neural plate invaginates and forms neural tube.



A) Early 3rd week:

Formation of neuralplate (thickening of the

ectodermal layer)

B) End of 3rd week:

Lateral edges of

neural plate elevate asfolds with groove in

the midline

C) 4th week: Continue to

elevate and fusion of

neural foldsD) End of 4th week:

Neural tube formation

complete



Neural tube

� At first, lined by a single layer of cells, which later proliferate to formseveral layers.

� There are 3 layers;

a) Neuroepithelial layer

b) Mantle layer

c) Marginal layer



± Neuroepithelial layer (neuroblasts)

� innermost

� It is a thick layer of pseudo-stratified epithelium lining the lumen of the neural tube.

� Its component cells undergo rapid cell division to form new cells ±neuroblasts

± divide into - apolar ±> bipolar ±> multipolar

± which migrates to the adjacent mantle zone

± Forms:» Gliablasts

» astrocytes

» oligodendrocytes

» Mesenchymal cells

» Microglial cells



� Mantle layer (gray matter)

± middle

± The neuroblasts migrated from the neuroepithelium forms a zonearound the neuroepithelial layer

± The neuroblasts are characterized by

� large round nuclues

� pale nucleoplasm

�

dark staining nucleolus ± Mantle zone later forms the gray mater of spinal cord



± Marginal layer (white matter)

� outermost

� predominantly composed of nerve fibers (processes of cells in the

mantle zone)

� Marginal zone later forms the white mater of the spinal cord.



Stages in formation of a typical neuroblast.



Formation of myelin sheath



� Schwann cells myelinate the peripheral nerves with each cellmyelinating only a single axon.

� These cells originate from neural crest, migrate peripherally, andwrap themselves around axons, forming neurilemma sheath.

� Nerve fibers take on whitish appearance:- due to deposition of myelin, formed by repeated coiling of Schwann cell membranearound axons.

Fig. 1 (a) A myelinated axon in the peripheral

nervous system and (b) its development. Each

Schwann cell myelinates a single axon, to which

it is directly apposed. During development (anticlockwise) Schwann cells loosely ensheath

axons and the myelin sheath grows around the

axon to form concentric layers, which become

tightly apposed



� Myelin sheath surrounding nerve

fibers in the spinal cord (central

nervous system) originate from

oligodendroglial cells.

� Unlike Schwann cells, a singleoligodendrocyte can myelinate up

to 50 axons.

� (a) in section, concentric layers of

myelin are seen to spiral around the

axon (b). Myelin sheaths are arranged

along axons in segments 1 mm long

separated by short nodes, and would

appear as large sheets if they were

unwrapped from around the axon



� Development of myelinated axons in the central nervous system, as seen by electron microscopy in transverse section. (a) newborn.Occasional axons are loosely ensheathed by primitive, undifferentiated glial cells, g, but myelin is not yet present. × 40, 000. (b) Adult. Theaxons are fully myelinated. × 20, 000.

development of cns nervous tissue

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