Primary leptomeningeal melanoma: an ultrastructural study.

A primary leptomeningeal melanoma in a 19-year-old white women has been shown by electron microscopy to contain light and dark cells. The light cells fill the subarachnoid space, contain mature melanosomes and exhibit intracytoplasmic fine fibrils. The dark cells are found within the collagen network of pia-arachnoid. They possess a small rim of ribosomal-rich cytoplasms, occasional premelanosomes and no intracytoplasmic fibrils. The tumor cells appear to arise from these dark activated melanoblasts in the pia-arachnoid and proliferate into more differentiated light cells. A similar pattern of two types of cells has not been described in primary CNS melanomas. The possibility of differentiation from cutaneous melanoma is discussed.