THE ROLE OF EPIDERMAL LYSOSOMES IN MELANIN PHYSIOLOGY

SUMMARY.— Melanosomes are formed within melanocytes and transferred via dendrites to keratinocytes. In caucasoids, most melanosomes are aggregated in membrane‐bound organelles (melanosome complexes). These aggregates have phagocytic capacity, stain positively with aryl sulphatase and acid phosphatase, contain melanosomes apparently undergoing degradation, and are lysosomes. In addition to melanosome complexes, individual melanosomes, both in melanocytes and keratinocytes, also exhibit lysosomal properties of phagocytosis and hydrolytic enzyme activity. In contrast, negro melanosomes are mostly singly dispersed and are rarely aggregated in complexes. Melanosome dispersal contributes to the darker colour and superior sunlight protection of negro skin. Degradation by lysosomal enzymes explains the limitation of melanin principally to the basal layer. In skin with severe solar degeneration, particularly in xeroderma pigmentosum, pigment production is often apparently adequate since hypertrophic melanocytes and dendrites are conspicuous. However, pigment transfer from melanocytes to keratinocytes does not occur normally. Some keratinocytes contain little melanin while others show an increase. Melanosomes within keratinocytes of actinically damaged skin tend to be concentrated within larger than normal complexes. Skin colour and sunlight protection appear to depend not only upon the rate of melanin formation but also upon distribution and rate of degradation.