Cutaneous mast cell tumours in a lion (Panthera leo): a light and transmission electron microscopical study.

Cutaneous mast cell tumours of a 16-year-old female Indian lion (Panthera leo) were studied histologically and ultrastructurally. The proliferation index detected with an antibody against the nuclear antigen Ki-67 was 16.5%. A cytochemical test for chymase activity was negative. Mast cell tumours are well known in domestic animals and in cats are of two distinct histological types. The present paper is the first report of poorly differentiated cutaneous mast cell tumours in a lion. In their histological and ultrastructural appearance, and in their lack of chymase activity, the neoplastic mast cells resembled tumour mast cells in cats.