Ultrastructural examination of a balloon cell nevus confirmed previous findings that balloon cells form by progressive vacuolization of melanocytes and nevus cells. Most granules of non-vacuolated nevus cells were round, membrane-bound structures with a uniform particulate matrix. Early changes leading to the formation of vacuoles in epidermal melanocytes included enlargement of melanosomes with separation of their fibers. Early changes leading to formation of vacuoles in dermal nevus cells included microvacuole formation in nevus granules and diffuse loss of particulate matrix. Loss of particulate matrix allowed visualization of discrete ellipsoid melanosomes in some nevus granules. Some of the epidermal balloon cells were keratinocytes. These may have developed by transfer of altered melanosomes from keratinocytes. Numerous mast cells were present; some of these showed cytoplasmic vacuoles and granules with microvacuoles. This observation, as well as the presence of ultrastructural intergrades betwten mast cells and nevus cells, supports the proposal that mast cells and nevus cells (as well as melanocytes) are histogenetically related.
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Okun et al. (1974) studied this question.