Effects of hydrocortisone on the ultrastructure of the thymic cysts of chicks.

Thymi from 10-day old male chicks injected subcutaneously with 5 mg of hydrocortisone for 2 consecutive days and sacrificed on the 4th day were examined by electron microscopy. With depletion of the lymphoid cells, cystic structures, consisting of inter- and intracellular cysts are more apparent in the hydrocortisone-treated chicks. The intercellular cysts vary in size and shape and are lined by cystic, endocrine-like and lymphoid cells. Luminal surfaces of the cystic cells contain microvilli. The lateral plasma membranes show complex interdigitations. The prominent terminal web contains vesicles and numerous core rootlets. Frequent membrane invaginations are apparent at the bases of the microvilli. The endoplasmic reticulum and the Golgi complex are moderately developed. Mitochondria are variable in number but are numerous in some cells. The cytoplasm also contains dense bodies, vacuoles and myelin figures. Endocrine-like cells, containing numerous secretory granules and bordering directly on the follicular lumen suggest that their secretion may be discharged into the lumen. Cystic cells containing intracellular cysts are observed. Presence of cleft-like spaces, microvilli closely opposed to adjacent cytoplasmic processes or lateral microvilli may reflect responses to increase in absorptive or transport function. The present data suggest that hydrocortisone may modulate the structure and function of the cystic structures in the chick thymus.