Branched cells in the epidermis: an overview.

Current knowledge concerning the nature, lineage, and function of the Langerhans cell, Merkel cell, and, to a lesser extent, the melanocyte, are reviewed under headings that emphasize the confederate constitution of the epidermis as a compound tissue composed of a variety of cellular elements; the role of the lymphocyte as a component of normal epidermis is also considered. It appears that the function of the Langerhans cell has finally been established, i.e., it serves as a front-line element in immune reactions of the skin. Developmentally, it is of mesenchymal origin. The Merkel cell still presents a number of problems centering around questions of its lineage, the nature of its characteristic granules, and the "synaptic" relationship between it and the associated neurite. The melanocyte continues to hold the attention of investigators, mainly from the point of view of the chemistry of melanin and the rational treatment of pigmentary disorders based upon findings derived from fundamental research into all aspects of its biology.