A DESMOPLASTIC MALIGNANT MELANOMA IN A DOG

A desmoplastic malignant melanoma appearing on the forelimb in a dogwas histopathologically examined. The tumor cells, having an oval nucleus and a spindle-shaped abundant cytoplasm, showed perivascular whorled and storiformed proliferative pattern and rarely formed syncytium. They also loosely proliferatedin area of myxomatous matrix occupying a part of the tumor. No melanin granules were visible in the neoplastic tissue of HE stain, but a small quantity of melanin could be seen by Fontana-Masson stain in storiformed areas and locally by DOPA reaction. These granules were never recognized in the whorled proliferative area. The tumor cells were immunohistochemically positive for S-100 protein, and ultrastructurally contained cell organellas such as rER, dense body, and microfilament. Immature melanin, however, could not be detected in the tumor cells. Numerous collagen fibers were present around the neoplastic cells. These findings described above appeared to be essentially similar to those of desmoplastic melanoma of human.