Mediastinal teratoma in a free-ranging American black bear (Ursus americanus).

A 2.75-yr-old female American black bear (Ursus americanus) was found emaciated and unable to rise. It was euthanized due to the perceived poor prognosis. An approximately 18 x 8 x 10-cm, multiloculated, well-demarcated mass that extended from the bifurcation of the trachea, laterally displacing the lung lobes and caudally displacing the heart. The mass contained fibrous connective tissue, cartilage, bone, nervous tissue, smooth muscle, and a variety of epithelial structures. A mediastinal teratoma was diagnosed based on the microscopic features of the neoplasm.