Ultrastructure of giant pigment granules in lentigo simplex.

Electron microscopic observation of lentigo simplex on the sole revealed the presence in the lesion of giant pigment granules in melanocytes and keratinocytes. The giant granules were membrane-bound bodies containing electron-dense amorphous substances and less electron-dense microvesicles. It was also revealed that compound melanosomes, similar in size and shape to giant pigment granules, were present in melanocytes. Within the compound melanosomes, melanosomes showed disintergration into electron-dense fine particles concomitant with the release of less electron-dense microvesicles. These fine particles then aggregated to the mass of electrondense amorphous substances which eventually embedded the microvesicles. This evidence strongly suggests that the giant pigment granules are formed by complete degradation of melanosomes in the large compound melanosomes which can arise within melanocytes by autophagy.