Cutaneous melanocytoma in a cynomolgus monkey (Macaca fascicularis): cytologic and histologic characteristics

Abstract

A 4-year old, male cynomolgus monkey (Macaca fascicularis) imported from China was acquired by Wyeth Research from a commercial source. On physical examination the animal had a 2 cm in diameter pigmented raised mass at the dorsal aspect of the neck. The mass was surgically removed for cytologic and histopathologic evaluations. Touch imprints of the mass stained with Wright–Giemsa revealed pleomorphic neoplastic cells arranged individually and in small clusters, admixed with extracellular pigment granules. The neoplastic cells were mostly round to oval and were filled with green-black to blue-black pigment granules obscuring the nucleus. The pigment granules were about 1 μm, oval to elongate, and consistent with melanin granules. Histologically, this nonencapsulated mass was composed of poorly demarcated, heavily pigmented melanocytes diffusely infiltrating the dermis. In the bleached sections, the neoplasm was composed of fairly uniform round to polygonal cells arranged in packets and supported by a fibrovascular stroma. The cells had discrete cell borders, and the nuclei were mostly round with finely stippled chromatin and lacked nuclear atypia. The cytologic and histologic findings supported a final diagnosis of melanocytoma. To the authors’ knowledge, the present case is the first cytologic description of a cutaneous melanocytoma in a nonhuman primate.