The epidermis, the outermost layer of all individuals, is a point of contact between individuals and their surroundings, especially insofar as it mediates sensory stimuli, but it also has the function of separating them from their surroundings and providing protection from harmful environmental influences. This paper traces the adaptation of the epidermis of vertebrates to these multiple functions via various methods of differentiation. In the human, epidermal differentiation is divided into synthesis, transformation, and terminal stages, and the stratum corneum is the final product of this differentiation. The differentiation products (tonofilaments, keratohyalin granules, membrane coating granules, cornified envelopes) are discussed with reference to their molecular bases (especially cytokeratin polypeptides, filaggrin, involucrin, lipids). The importance of these new cell-biological results for the pathogenesis of disorders of keratinization is discussed.
