Canine malignant melanotic schwannomas: a light and electron microscopic study of two cases.

Malignant, melanotic schwannoma of the spinal cord was diagnosed in two two-year-old dogs. The neoplastic spindle cells were arranged in a herringbone or storiform pattern; the intracytoplasmic melanin varied in amount in different areas of the two neoplasms. Ultrastructural features including cytoplasmic lamellar premelanosomes and melanosomes, rudimentary cell junctions, scattered pinocytotic vesicles, and remnants of external lamina supported the diagnosis of melanotic schwannoma.