HUMAN EPIDERMAL EMBRYOGENESIS

The events in epidermal development are distinctive for each trimester. During the first trimester, the epidermis changes from a single-layered covering to a stratified epithelium. Many of the structural changes that occur establish cellular integration and support to stabilize this barrier between the embryo and its environment. The morphologic and physiologic evidence, indicates, however, that the epidermis is not a functional barrier to diffusible compounds during this period, a time when the environment is comparable and perhaps important in providing substances needed for development. This is also a period in which all of the primordia of the epidermal appendages are initiated and the immigrant cells begin to migrate in among the keratinocytes. In the second trimester, the system is established and the pattern for the adult epidermis is laid out. Further development and differentiation are required to refine the pattern into a functional system which will acquire the properties of the adult tissue. The work of the second trimester then is to expand and elaborate certain structures and to change and modify others. The functional state is altered as glands become secretory and the epidermis differentiates into a functional barrier, first by regression of the periderm and secondly with keratinization of the epidermis proper. Exclusion of the amniotic fluid occurs at a time when this formerly compatible environment becomes hostile. The epidermis, thus formed in nearly its final form, ceases, new developmental events and subsists throughout the third trimester until near birth in basically the same condition. Most of the data to support these generalizations have come from an impressive body of morphologic information. From the present review, it should be quite apparent that the quantitative and functional aspects of epidermal development are areas where inquiry is needed so that speculation may be substantiated by data and replaced with fact.