Neuroepithelial Mantle And Marginal Layers

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 pseu-dostratified epithelium (Fig. 19.6). 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 neuro-blasts (Fig. 19.7). They form the mantle layer, a zone around the neuroepithelial layer (Fig. 19.8). 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 (Fig. 19.8).

Forebrain Midbrain Hindbrain Mouse Stage
Figure 19.4 Scanning electron micrograph of a sagittal section through a mouse embryo at approximately 27 days of human development. Three brain vesicles represent the forebrain (F), midbrain (M), and hindbrain (H).
Sagittal Section Mouse Embryo Brain

Figure 19.5 Scanning electron micrograph of a sagittal section through a mouse embryo at approximately 32 days of human development. The three brain vesicles have segregated into the telencephalon (T), diencephalon (D), mesencephalon (M), meten-cephalon (Mt), and myelencephalon (My). Asterisk, outpocketing of the telencephalon; arrow, rhombencephalic isthmus; arrowheads, roof ofthe fourth ventricle; o, optic stalk.

Figure 19.5 Scanning electron micrograph of a sagittal section through a mouse embryo at approximately 32 days of human development. The three brain vesicles have segregated into the telencephalon (T), diencephalon (D), mesencephalon (M), meten-cephalon (Mt), and myelencephalon (My). Asterisk, outpocketing of the telencephalon; arrow, rhombencephalic isthmus; arrowheads, roof ofthe fourth ventricle; o, optic stalk.

Neuroepithelium
Junctional complexes at lumen

Figure 19.6 A. Section of the wall of the recently closed neural tube showing neuroepithelial cells, which form a pseudostratified epithelium extending over the full width of the wall. Note the dividing cells at the lumen of the tube. B. Scanning electron micrograph of a section of the neural tube of a mouse embryo similar to that in A.

Figure 19.6 A. Section of the wall of the recently closed neural tube showing neuroepithelial cells, which form a pseudostratified epithelium extending over the full width of the wall. Note the dividing cells at the lumen of the tube. B. Scanning electron micrograph of a section of the neural tube of a mouse embryo similar to that in A.

Pia cell

External limiting .£_ membrane

Differentiating neuroblast

Intermitotic neuroepithelial cells

Dividing neuroepithelial cell

Pia cell

External limiting .£_ membrane

Differentiating neuroblast

Intermitotic neuroepithelial cells

Dividing neuroepithelial cell

Germinal Neuroepithelium

Junctional complexes

Figure 19.7 Section of the neural tube at a slightly more advanced stage than in Figure 19.6. The major portion of the wall consists of neuroepithelial cells. On the periphery, immediately adjacent to the external limiting membrane, neuroblasts form. These cells, which are produced by the neuroepithelial cells in ever-increasing numbers, will form the mantle layer.

Junctional complexes

Figure 19.7 Section of the neural tube at a slightly more advanced stage than in Figure 19.6. The major portion of the wall consists of neuroepithelial cells. On the periphery, immediately adjacent to the external limiting membrane, neuroblasts form. These cells, which are produced by the neuroepithelial cells in ever-increasing numbers, will form the mantle layer.

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