Canine cutaneous histiocytoma: ultrastructural and cytochemical observations.

Five canine cutaneous histiocytomas were studied by electron microscopy and esterase cytochemistry. The tumor cells contained irregular nuclei, characteristic lysosomal granules, and perinuclear microfilaments. The cells showed activity with alpha-naphthyl acetate esterase stains. These characteristics are evidence that this tumor originates from the mononuclear phagocyte system.