An unusual neoplasm of adipose tissue in a rat.

The light microscopic features of a spontaneous neoplasm of adipose tissue from a rat were suggestive of a mixed liposarcoma with a myxoid matrix. However, ultrastructurally, the cell characteristics were those of a hibernoma. These characteristics included cells containing lipid droplets of variable size, numberous pleomorphic mitochondria and close apposition to blood vessels. Lipofuscin granules and subplasmalemmal condensations were not observed ultrastructurally.