• Cutaneous melanomas;
  • Pigment cell tumors;
  • Gray-skinned horse;
  • Pathology;
  • Apocrine sweat glands

The clinical and pathological characteristics of cutaneous melanomas occurring in Camargue-type gray-skinned horses are reported. Examination of 83 tumor-bearing horses revealed that the tumors occurred most frequently underneath the tail (93.9%) and at high rates in the peri-anal region (43.0%), the lips (33.0%), and the eyelids (24.0%), but rarely in the vulva (3.8%). Tumorous lesions were characterized by the presence of either hemispheric nodules or large infiltrated plaques, or their combinations. Microscopic examination indicated that tumorous lesions were composed mostly of melanocytes and numerous melanophages and that component cells manifested a remarkable cellular atypia with anisocytosis and anisokarinosis. Pathological examination of lesions corresponding to earlier stages of the tumors disclosed that tumor formation and its related melanogenesis occurred in close topographical association with apocrine sweat glands, but not at the dermal–epidermal junction. Larger nodules of the tumors were often composed of a concentric deposition of cell layers, each being separated by dermal components from the epidermis. Because of the absence of their tropism toward the epidermis and their multiloculation, horse melanomas are considered to be different in their pathological characteristics from human melanomas.