The hereditary disorder called psoriasis is characterized by certain histologic features. These include (1) increased and abnormal production of keratin by rapidly proliferating epidermal cells, (2) the migration of polymorphonuclear leukocytes from the dermal blood vessels into the horny layer of the epidermis, and (3) dilatation of the dermal capillaries in the papillae between the rete ridges of the epidermis. The clinical appearance of the psoriatic lesion is determined by the predominance of one or another of these features.
An abnormal keratinous scale accumulates on the surface from enhanced proliferation of epidermal cells and gives the most distinctive characteristic of the ordinary psoriatic lesion (Figure, left). The stimulus for such mitotic activity is not known, but the turnover time, ie, the time required for a basal cell to reach the surface and be cast off, is reduced from the normal 28 days to about 4 days in this accelerated "epidermopoiesis."