Thymus cortex (rat, young adult)

Electron microscopy. Within the thymic cortex a type II epithelioreticular cell or so-called (sometimes multinucleated) thymic nurse cell (TNC) shows a characteristic electron-light nucleus and nucleolus (1). The branches are squeezed between the thymocytes (2). In the cytoplasm a variety of empty as well as granules-containing vacuoles (*) are present, probably containing thymulin. Background: usually four morphological types of cortical thymic epithelial cell types are described (cTECs): EC1 subcapsular and perivascular; pale EC2 with a high MHC-II expression; intermediate EC3 and dark EC4. Epithelioreticular cells produce thymosin, thymulin and other humoral factors, they also express MHC I and MHC II molecules for thymocyte education. Both the developing thymocytes and epithelioreticular cells affect each other during differentiation (so-called crosstalk).