The Melanocytes of Mammals

Melanocytes may be regarded as unicellular, pigment-secreting glands, largely though not entirely confined to the epidermis, of which they are a constant though often un-recognized cellular component. Their numerical incidence, and their branching form, with numerous dichotomizing processes, are such as to form a reticular system within the epidermis. Evidence is evaluated that they are a race or lineage of cells sui generis and form a self-maintaining system within the epidermis. Their product-melanin granules-is secreted directly into the cytoplasm of neighboring Malpighian cells upon which the processes of the melanocytes terminate in the form of small swellings or end-caps. The several types of branched cells demonstrable within the epidermis by various techniques are discussed in relation to their identity as living melanocytes, or as effete cells of this lineage which, having lost or discharged their pigment, participate in the general out-ward movement of epidermal cells to be lost from the skin surface. The color variants of skin and hair, including spotting and albinism, are the direct result of melanocyte activity, and have been shown to be determined by the action and interaction of multiple genes, some of which operate via the milieu in which the pigment cells reside and others of which appear to act intracellularly and to control the biochemical steps of melanogenesis. The problem of pigment spread is discussed and related to the anatomy of the melanocyte system and to the co-existence of variant types of melanocytes within a single individual.