Naevogenesis: a hypothesis concerning the control of proliferation of melanocytes with special reference to the growth of intradermal naevi.

The pigmentary function of epidermal melanocytes depends on the donation of melanin granules to the surrounding surface structures. This involves transfer of cytoplasm (cytocrine transfer) from melanocytes to keratocytes, a process which requires competence of the donor cells and the availability of adjacent competent recipient cells. Donor cell competence involves the extension of dendrites and recipient cell competence consists of the ability of these cells to phagocytose peripheral portions of the melanocyte cytoplasm. Since there is a highly regulated mechanism for the control of cellular size which operates by inhibiting proliferation of cells that are below a critical volume, it is proposed that the continual removal of portions of the melanocyte cytoplasm by cytocrine transfer is responsible for inhibiting growth of the epidermal melanocyte population, accounting for their relatively low population density. It is proposed that inhibition of cytocrine transfer permits the proliferation of melanocytes. Cytocrine transfer may be inhibited by loss of competence of donor or recipient cells or by their relative displacement. Displacement of melanocytes into the dermis, out of range of potential recipient keratocytes, would, according to this hypothesis, result in melanocyte proliferation leading to the generation of localized aggregations of melanocytes (melanocytomas). It is proposed that this is the origin of acquired benign pigmented moles.