File:Wen1928-Fig08.jpg

Fig. 8. Photomicrograph from section 24 of H951 (17 somites)

Showing the site of the anterior neuropore. Zeiss apo. imm. 'X ,’ Homal III. X 700.

The section cut through about the middle of the site of the closing anterior neuropore is shown in figure 8. It is to be noted there that the surface epithelium is greatly thickened in the middle as a wedge-shaped plug of cell mass filling up the gap between the lips of the neural folds. The arrangement of cells there gives the impression that they are piled up one layer upon another, with exceedingly flattened cells on the outer surface, spindle—shaped in the intermediate layer, and two to three round cells in the innermost stratum. Under a higher magnification, the flattened cells of the outermost layer appear to be but cytoplasmic processes thrown across the midline to form a bridge between two sides; whereas the cells lateral to this region begin to assume definite forms arranging from elongated epithelial to low cuboidal cells. Next to this layer the cells, which are really not clearly demarcated from the layer just described and the one to be dealt with presently, but arbitrarily divided according to their shape for descriptive purposes, are arranged in an obliquely directed radiation toward the midline. This radial arrangement gives place to a few round cells with oval nuclei, bounding the ventricle. This type of cellular arrangement is responsible for the conical appearance of the cell mass on cross-sections. Now, turning to the neural epithelium, the neural folds have not yet fused and the arrangement of cells is clearly marked as contrasted with that of the head epithelium. But at the tip of the right neural lip (fig. 8), where the head ectoderm is continuous with the neural epithelium, the cells of the former bend inward upon the latter to form the brain wall. Tracing the sections forward (cephalic to fig. 8, i.e., ventral in fig. 5), it is seen that the lips of the neural fold gradually approach each other and then fuse. The plug of the head ectodermal cells in the meantime changes from its radial conical arrangement into an irregular cell mass between the flattened head epithelial cells and the epithelium of the neural tube, until it is completely separated from the nervous tissue. Now, let us proceed with the examination of sections immediately caudal to figure 8 (i.e., dorsal in fig. 5); the ectoderm and neural folds cannot be separated, the former still indicating an inward bending upon the latter to form the brain wall. Still caudal to these sections, the neural tube becomes closed by the joining of the two lips of the neural fold, and the head ectoderm is distinctly separated from the neural epithelium.

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Reference

Wen IC. The anatomy of human embryos with seventeen to twenty-three pairs of somites (1928) J. Comp. Neural., 45: 301-376.

Cite this page: Hill, M.A. (2024, April 24) Embryology Wen1928-Fig08.jpg. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/File:Wen1928-Fig08.jpg

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