Osteoma in the skull of a sheep.

An osteoma located in the frontal bone and nasal cavity is described in a 4-year-old dairy sheep with respiratory difficulty and progressive emaciation. The mass was hard, formed by bone, and caused compression of the ethmoidal labyrinth. Microscopically, the tumour was characterized by trabeculae of woven bone, frequently coexisting with lamellar bone composed of fine fibres, lined by osteoblasts and surrounded by a fibrous stroma. A severe chronic inflammation was seen in the nasal mucosa. Differential diagnosis is discussed, particularly in relation to ossifying fibroma.