The fine structure of developing human epidermis: light, scanning, and transmission electron microscopy of the periderm.

Eight stages in the development of the human embryonic and fetal periderm have been established, primarily on the basis of surface morphology, major changes in epidermal stratification, and differentiation. The changes in the periderm observed with the scanning electron microscope have been correlated with and supplemented by cytologic studies with the transmission electron microscope in the periderm and other epidermal layers. Light microscopy was used to determine what proportion of the epidermal thickness is accounted for by the periderm and what relationship individual periderm cells have with underlying cells. The results yield a comprehensive, three-dimensional image of the human epidermis during development and support a concept of the periderm as a layer of "dynamic" cells which project superficial blebs, expand in surface area, then regress at the onset of keratinization, leaving only cellular remnants associated with the adult type epidermis.