Ultrastructure of Giant Pigment Granules (Macromelanosomes) in the Cutaneous Pigmented Macules of Neurofibromatosis

Electron microscopy was used to elucidate the nature of the giant granules in the light-brown, or cafe-au-lait, macules from 6 patients with neurofibromatosis. It was shown that these giant granules were surrounded by a membrane system similar to that of normal melanosomes in melanocytes and keratinocytes. These giant granules contained several subunits: (i) highly electron-dense amorphous material; (ii) electron-lucent globular bodies (400 Å in diameter), characteristic also of the normal melanosomes in heavily pigmented skin and hair; (iii) electron-dense globular bodies (400–500 Å in diameter) with fine grains inside; and (iv) fine granular subtances, or grains, of moderate to high electron density. Several subtypes were found, suggesting that a morphologic sequence may exist in the development of these granules in melanocytes. The giant granules were found also in keratinocytes as a single unit, indicating that they were apparently transfered from melanocytes to keratinocytes as a single unit after the maturation. Because of many similarities with normal melanosomes, we suggest that these giant granules be called "macromelanosomes". In addition to these macromalanosomes small ellipsoidal melanosomes that were normal and small spherical granules melanosomes like those found in melanosomes were also found in the cafe-au-lait lesions.