ULTRASTRUCTURE OF SKIN OF HUMAN EMBRYOS .3. FORMATION OF NAIL IN 16-18 WEEKS OLD EMBRYOS

[摘要] An electron microscopic study of the matrix primordium of 16 to 18 weeks old human embryos revealed that it consisted of 2 horizontal layers, i. e., ventral and dorsal matrices. Proximal to the vertical level of keratinization, which is about 1/4 of the distance from the apex of the matrix to the cuticle, matrix cells did not undergo keratinization. Some of these proximal matrix cells were even vacuolated and degenerated. Distal to the vertical level of keratinization, the basal cells of both the ventral and dorsal matrices became mature squamous cells, accumulating tonofibrils, growing into granular cells, containing keratohyaline granules, and finally being transformed into horny cells (nail cells) with densely packed keratin which did not appear to differ from the keratin of the epidermis. This process of maturation took place while the basal cells of both ventral and dorsal halves traveled distally towards the axis of the matrix. At the vertical level of keratinization, horny cells independently produced by ventral and dorsal matrices met on the axis (the geometrical center line) of the matrix primordium. As this band of horny cells, i. e., the nail plate, increased in thickness distally the ventral half contributed more horny cells than its dorsal counterpart, and subsequently the nail plate became localized above the axis. Beyond the vertical level of keratinization the dorsal matrix began to show gly-cogen and melanocytes in increased amounts nearer the cuticle. The thickness of the ventral matrix gradually increased and reached the plateau beyond the level of the cuticle where the ventral matrix transformed itself into the nail bed and continued alone to contribute nail cells to the nail plate. At the vertical level of keratinization SH [sulfhydryl] groups were stained with DDD[dihydroxy-dinaphthyl-disulfide] stain in the initial formation of the nail plate. At the same level a PAS [Periodic Acid Schiff] -positive, diastase-resistant substance also began to appear in the vicinity of the nail plate. By electron microscopy numerous membrane-coating granules were detected at the same level. Silver methenamine stain performed on ultrathin sections was positive in these granules while lead citrate stain did not reveal glycogen particles, thus allowing the conclusion that these granules contained mucopolysaccha-rides which probably are responsible for the PAS-positive, diastase-resistant substance.