Ultrastructure of epithelial thymic cysts of the chicks.

The medulla of the chick thymus contains variable numbers of intercellular and intracellular cysts. The epithelium varies from squamous to columnar or stratified and contains cystic, endocrine-like, and lymphoid cells. The lumen-lining cystic cells contains microvilli, moderately developed rough- and smooth-surfaced endoplasmic reticulum, and Golgi complex. Mitochondria, free ribosomes, and varying number of microfilaments are observed. Endocrine-like cells are separated from the lumen by the cystic cells, which in places are extremely attenuated. The cytoplasm is characterized by the presence of membrane-limited secretory granules. Cleft-like spaces, partially lined by microvilli, are also encountered. Numerous vesicles, some in tight association with the core rootlets, are evident. Cystic cells with single or multiple cysts, lined with microvilli, are also observed. The lumina of these cystic elements contain flocculent or dense granular material, amorphous masses, cellular elements, or debris. Morphological evidence suggests synthetic and secretory activities of the cystic cells in the young chicks.