Canine cutaneous histiocytoma. A light and electron microscopic study.

Cutaneous histiocytomas from 4 dogs were examined by light and electron microscopy. A large (up to 10μ in diameter) mononuclear cell with prominent filiform processes of the plasma membrane predominated. Its cytoplasm contained relatively small amounts of endoplasmic reticulum and mitochondria, only occasional lysosomes, fibrils, most obvious in the perinuclear region, and small amounts of cytoplasmic debris. Acid phosphatase was not detected. Fibroblasts and collagen formed a small part of the lesion, except at the junction with surrounding dermis, where fibers were plentiful. The morphologic features of the lesion are compatible with the suggestion that the predominant cell is of histiocytic type.