The epidermis accounts for only one-fortieth of the thickness of the skin, yet it must keep microoganisms out and essential body fluids in, and must also survive the physical and chemical traumas of the environment. The epidermis satisfies both these requirements by continually rejuvenating itself and producing a resilient, impenetrable mass of stable proteins and lipids. How does it accomplish these spe&&ed functions?

Kejuvenation evolves from the organization of epidermis (Fig. la). Of. the l&20 layers of epidermal cells present in humans, only the innermost, basal layer has the capacity for DNA synthesis and mitosis, and consequently, this is the layer that supplies progenitor cells to the epidermis. In response to a trigger as yet unidentified, a basal cell will move towards the skin surface while undergoing a series of morphological and biochemical changes. These, culminate in the production of dead, enucleated squames, which are sloughed from the epidermal surface and continually replaced by inner cells ditferentiating outwards (Fig. la). In this way, the progenitor cells are tucked safely under an armour of dead cells.