Equine cutaneous mastocytoma: morphology, biological behaviour and evolution of the lesion.

Thirty equine cutaneous mastocytomas were examined histologically and two were studied ultrastructurally. Lesions were characterized by distinct sheets of well-differentiated mast cells with variable degrees of eosinophil infiltration, collagen degeneration, necrosis, granulomatous inflammation and fibrosis. Twenty-two of 25 growths did not recur for up to 6 years after surgical excision, two recurred at the surgical site and one spontaneously regressed less than 3 months after obtaining a biopsy sample. Equine cutaneous mastocytoma is a benign proliferative lesion which seldom recurs after excision. The varied histological presentation of equine mastocytoma can be attributed to a sequence of events initiated by a cutaneous mast cell proliferation. It is suggested that these mast cells release chemotactic factors for eosinophils which accumulate and degranulate, initiating collagen degeneration and cellular necrosis with subsequent granulomatous inflammation and fibrosis. The focal spontaneous nature of the primary mast cell proliferation is typical of neoplasia.