Abstract
The epidermis of the skin is a multi-layered epithelial structure, with a layer of rapidly proliferating cells at the base of the structure. These cells are attached to a basement membrane that separates the epidermis from the underlying connective tissue. Above the basal layer are cells that have begun the process of differentiation. As these cells are pushed further toward the surface, differentiation progresses until the uppermost layer of the skin consists primarily of cross-linked, high-molecular-weight proteins that are eventually sloughed from the skin. Hyperproliferative skin diseases, of which psoriasis is a prototype, result in excess keratinocyte proliferation and a correspondingly thickened epidermis. Abnormal growth is accompanied by abnormalities in the differentiation process. Understanding how proliferation is regulated in the epidermis is important, since it may be possible to utilize this information to interdict events that give rise to abnormal growth without compromising physiological proliferation. The EGF receptor and its ligands are important in regulating hyperplastic epidermal growth. The EGF receptor system’s role in normal proliferation is much less clear. Factors elaborated in the dermis and that act through other receptors may be more important in physiological growth control. This review summarizes some of the current knowledge concerning growth-regulating mechanisms in the homeostatic epidermis and the role of the EGF receptor in the hyperplastic condition.