Melanotic neurofibroma in a steer.

A melanotic neurofibroma in a steer was investigated histologically, immunohistochemically and ultrastructurally. A very large tumor mass was located in the region of the head and right cheek. The tumor tissue consisted of an admixture of cells resembling Schwann cells and spindle-shaped cells, and they frequently contained melanin granules. Neoplastic Schwann cells were positive for S100 protein, with variation in intensity of staining, but most spindled cells were S100 negative. The tumor cells displayed ultrastructural features similar to those of Schwann cells or perineurial cells. The presence of melanosomes in varying stages of melanization in both cell types suggests that they have a common origin. This is a tumor of neural crest origin showing schwannian and perineurial differentiation, with ectopic production of melanin granules.