Regeneration of dog gingival epithelium following gingivectomy was studied electron microscopically. The wounds were sampled at 24 hour intervals until 14 days following the operation. From a relatively undifferentiated migrating epithelium the process of differentiation resulted in the following sequence of events. In areas where the cells made contact with the fibrin of the blood clot, a basal lamina and hemidesmosomes appeared, the number of cytofilaments increased as did the number of desmosomes and free ribosomes. Further differentiation in the masticatory epithelium was related to keratinization. The cytofilaments in the cells of the upper layers became grouped into bundles, membrane‐coating granules and keratohyalin granules appeared, the intercellular spaces in the superficial layers became narrowed and finally a stratum corneum was formed by the seventh day. In the epithelium lining the crevice a stratum corneum was not formed, the intercellular spaces in all the layers remained wide and the cells in the upper layers developed a relatively prominent Golgi apparatus and numerous membrane‐bound granules throughout their cytoplasm.
The result of the differentiation in the regenerated gingiva was the formation of masticatory and crevicular epithelium which were ultrastructurally similar to the two epithelia originally present.
A study of the ultrastructural changes associated with the detachment of the presumptive neural crest cells from the neuroepithelium in the midbrain region in mouse embryos at 9 and 9 1/2 days of gestation was carried out. The first sign of neural crest cell formation occurred in this region before fusion of the neuroepithelium had occurred. Neural crest cells arose from both the neural plate and the adjoining surface ectoderm. Initially, the cells of the neural plate and the surface ectoderm were attached to each other by zonula occludens and zonula adherans at their apical surfaces however, these junctions disappeared just prior to the beginning of the migration of the crest cells. The first sign of migration of the crest cells was the disappearance of the basal lamina in the region of the presumptive crest cells. Once the basal lamina was lost, cell junctions were formed between the epithelial cells and the underlying mesenchymal cells. Once the crest cells had migrated into the underlying mesenchyme, they tended to form clumps of closely related, irregularly shaped cells. Phagosomes and accumulations of glycogen particles were found within some crest cells when they were still within 50 to 100 microns of the epithelium.
The effect of hyaluronidase on embryonic chicken skin and mouse oral mucosa has been studied in vitro. Autoradiographic studies showed that .05% hyaluronidase increased the epithelial mitotic index while liquid scintillation techniques demonstrated that hyaluronidase increased the uptake of tritiated thymidine in chicken skin in vitro. Further experiments demonstrated that .05% hyaluronidase interferes with the keratinization process in embryonic chicken skin and mouse oral mucosa in vitro.
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