A fine-structure study of the human epidermal melanosome complex and its acid phosphatase activity

Abstract

Observations by electron microscopy have revealed that phagocytosing melanophages aggregate melanosomes in special membrane-delimited organelles containing acid-phosphatase activity; their ultrastructure resembles that of lysosomes. Here we report: (a) the presence of a “melanosome complex” associated with acid-phosphatase activity in epidermal keratinocytes; (b) that UV radiation induces larger complexes with more melanosomes. A “melanosome complex”, each an aggregation of one or more melanosomes, surrounded by a membrane and occurring in the keratinocytes, was studied by electron microscopy in biopsies of Caucasians, an Oriental, and a Negro. Modified Gomori's technique was used for the demonstration of acid phosphatase activity. Melanosome complexes in Caucasian and Oriental keratinocytes from unexposed sites usually contain 1–3 melanosomes and some small, about 100-A diameter, less electron dense, particles. In Negro keratinocytes, however, melanosomes usually are singly dispersed. Caucasian subjects biopsied 24 hours after 60-min exposure to sun showed an increase in the size and the quantity of the melanosome complexes. Both in the irradiated and unirradiated biopsies, there are more melanosome complexes in the basal and suprabasal cells than in the upper cells. Electron microscopic study of the biopsies in conjunction with histochemical technique has shown that melanosome complexes exhibit a positive acid phosphatase reaction. Since the melanosome is an entity extrinsic to the keratinocyte, the presence of acid phosphatase activity in melanosome complexes and other observations described in this report appear to have significance in regard to the fate of melanosomes after their discharge from the melanocytes.