The epithelial cell component of the thymuses of aged female BN/Bi rats: a light microscopic, electron microscopic, and autoradiographic study.

Thymuses of virgin female BN/Bi rats of various ages were studied by light microscopy, electron microscopy, and autoradiography. The thymuses appeared to undergo a normal involution during the first 2 years of life. In rats 2 to 4 years of age, there was an apparent proliferation of epithelial cords and tubules suggestive of a true hyperplasia. The appearance of these epithelial cords and tubules in the otherwise atrophied thymuses of old animals (more than 30 months of age) is described. The predominant epithelial cell type, containing dark apical granules, showed morphologic evidence of secretion. 3H-leucine labeling confirmed active protein synthesis and secretion by these cells. Lymphocytes and macrophages were identified within the lumina and between the epithelial cells of the epithelial cords and tubules suggesting that they were able to migrate into and probably out of these structures.