Melanocyte-keratinocyte interactions in vivo: the fate of melanosomes.

Studies on pigment donation in tissue culture (1-5) indicate that melanosome transfer is a cytophagic process during which a portion of a melanocyte dendrite is pinched off by the epidermal cell so that melanosomes and melanocyte cytoplasm are incorporated into the keratinocyte. At the ultrastructural level, one would ex-rect to see, at this stage, a cluster of melanosomes embedded in a cytoplasmic matrix surrounded by two membranes: one derived from the melanocyte and one belonging to the epidermal cell (Fig. 1). Although such images have not been published in reports on the electron microscopy of tissue culture (which readily reveals "cytophagocytosis"-phenomena at the light microscope level) they have been observed in in vivo specimens of hair bulbs (6), developing fowl feathers (7), and epidermis (Fig. 1). The melanosome complex, however, as it usually appears within keratinocytcs, is limited by only one membrane. It has been suggested (6, 7) that the inner (the melanocytic) membrane is rapidly decomposed but since double membrane-delimited structures occur only in the cell periphery they could equally well represent cross-sectioned dendrites of melanocytes bulging into the cytoplasm of the epidermal cell (Fig. 1). Also, if the matrix of a melanosome complex represents melanocyte cytoplasm it is surprising that it never contains identifiable cytoplasmic residues, such as mitochondria, microfilaments, or similar structures. In heterophagic and autophagic vacuoles such organelles may persist for a considerable time before they are decomposed (8, 9). The question also arises how cytophagocytosis explains the simultaneous occurrence of both melanosome complexes, which supposedly contain melanocyte cyto-plasm, and dispersed melanosomes, which do not. It has been suggested that melanosome complexes break and thus release the individual melanosomes which are dispersed throughout the cell (6, 7). Accordingly, the melanosome complex is considered a precursor of singly dispersed melanosomes and this could correspond to the dispersion of melanin granules observed, at the light microscope level, in tissue culture (4). It does not, however, explain the truly uniform (complexed or dispersed) distribution patterns of melanosomes that are seen in some species, All rights of reproduction in any form reserved.