[Comparative investigations of depigmented and melanomatous lesions in gray horses of the lipizzaner breed (author's transl)].

The morphological substrates of pigmented and depigmented skin as well as the structural characteristics of spontaneously developing melanomas were revealed by clinical, light- and electron microscopic methods in gray horses (Lipizzaner breed) from the Vienna Spanish Riding School. On clinical investigations in a group of 31 older horses (more than 10 years old) 20 exhibited melanomas, whereas 23 younger animals (less than 10 years of age) had no evidence for visuable melanotic tumors. Concomitantly with the progressive graying of the hair a depigmentation of the skin was frequently observed. Light and electron microscopic studies of skin biopsies revealed that in pigmented areas melanin is produced by DOPA-positive melanocytes and stored in form of large single melanosomes within keratinocytes. In depigmented areas melanocytes and melanosomes are completely lacking, but a high number of indeterminated cells is present in the basal layer. Melanotic tumors from the root of the tail, the lips, the perianal region, the sholder and intestinal lymph nodes exhibited either encapsulated nodules or diffusely infiltrating melanomatous structures similar to blue nevi in the dermis. Junctional activity could never be observed. A differentiation between melanin-producing tumor cells and melanophages was difficult in light microscopy but possible according to ultrastructural characteristics.