A slow growing progressive head skin tumour in a 6-year-old “Serra da Estrela” (black variety) male sheep without previous history of ultraviolet radiation exposure or illness is described. The tumour consisted of a large (5.4cm) irregular mass subdivided into two polypoid-like regions and small alternating pigmented and non-pigmented areas in the surrounding thick epidermis. By light microscopy the tumour disclosed morphologic and immunohistochemistry features consistent with a melanocytoma type tumour, without signs of vascular and perineural invasion. The tumour cells contained Masson-Fontana positive pigment, expressed S100 protein and vimentin, and display a low proliferative (Ki67<1%) labelling index. No metastases were found in the autopsy study. Analyses of the homologous regions of the hotspot mutational exons of BRAF and NRAS (the most often mutated genes in human melanocytic tumours) did not reveal alterations, but silent polymorphic variations, and no variation was observed in GNAQ gene sequence, that is found mutated in human melanocytomas.
In summary, we report the phenotypic and genotypic features of, as far as we are aware, the first case of a head skin melanocytoma type tumour with low-grade malignant potential in a black sheep non-exposed to environmental radiation
