The two compartment model of the stratum corneum: biochemical aspects and pathophysiological implications.

"La raison d'être" of the epidermis, as beautifully portrayed by Kathy Madison in The Journal of Investigative Dermatology (2003;121:231-41), describes the primary function of the skin: The terminal differentiation of an initially multipotent stem cell produces a protective, semi-permeable SC that permits terrestrial life. The permeability barrier function of the SC is provided by lipid bilayers lamellae surrounding anucleated apoptotic corneocytes. This structure has been described by Peter Elias as the "bricks and mortar" model where the bricks are the cornecytes and the mortar refers to the adjoining lipids. How important is the barrier to the skin? Certain genetic defects are characterized by profound barrier abnormaities due to alterations in either stratum corneum lipid metabolism or protein components of the corneocytes, producing ichthyotic genodermatosis. In this review, the mechanisms that drive barrier formation are summarized and this in the light of our recent scientific findings and publications in the domain of epidermal structure and function.