The nature and significance of macromelanosomes in pigmented skin lesions: their morphological characteristics, specificity for their occurrence, and possible mechanisms for their formation.

The nature and significance of macromelanosomes, or giant melanosomes, in various pigmented skin lesions are characterized in this paper in terms of a) their morphological features, b) the specificity for their occurrence, and c) the possible mechanisms responsible for their formation. These granules occur most frequently in the skin and eyes of persons with x-linked ocular albinism of the Nettleship-Fall type, in the café-au-lait macules of neurofibromatosis, and in the pigmented macules of xeroderma pigmentosa. In these genetic disorders, skin biopsy for detection of the macromelanosomes could be valuable for confirming the diagnosis and for establishing who in asymptomatic families are carriers of the disorders. The macromelanosomes seem not to be formed solely by autophagic degradation of melanosomes, as has been proposed recently in studies of melanocytic nevi and lentigo simplex where aggregation of the macromelanosomes and melanosomes often do occur through autophagocytosis. The macromelanosomes in these genetic disorders may be formed by a more complex process in which the melanocytes are coded for aberrant melanogenesis with formation of precursor vacuoles similar to those of pheomelanosomes and with continuous accumulation of vesiculoglobular bodies. We suggest that vesiculoglobular bodies are crucial to the formation of macromelanosomes.