Electron microscopy. Ultrastructural observations of the extracellular sheath of dermal melanocytes in the nevus of Ota.

Under the electron microscope, we studied dermal melanocytes in lesions of six cases of nevus of Ota and also those in common blue nevus and in nevus caeruleus tardus associated with progressive systemic sclerosis. An extracellular sheath surrounded most of the dermal melanocytes in nevus of Ota in four patients ages 21, 22, 25, and 30, but not those in two younger patients ages 9 months and 10 years. Some dermal melanocytes possessed extracellular sheath as well as basal laminae. Some possessed only an extracellular sheath. The thickness of the extracellular sheath varied among dermal melanocytes. Not all dermal melanocytes possessed extracellular sheaths. In particular, only a small number of dermal melanocytes in the two younger patients possessed extracellular sheaths and these were thinner than those of the three older patients. Fibrous long-spacing collagen was recognized in some parts of the extracellular sheaths of dermal melanocytes. In contrast, dermal melanocytes in common blue nevus were not surrounded by either extracellular sheaths or basal laminae. Dermal melanocytes in nevus caeruleus tardus in patients with progressive systemic sclerosis were surrounded by thin extracellular sheaths, but not by basal laminae. The extracellular sheath is composed of fine filaments, 20-40 A in diameter, and appears to be derived from collagen.