HETEROGENEITY OF EPITHELIAL-CELLS IN THE RAT THYMUS

The morphological heterogeneity of the thymic epithelium has been well . documented both at the light and electron microscopic level. Immunoh istochemistry has revealed four broad classes of epithelial cells (EC) : subcapsule/perivascular, cortical, medullary EC, and medullary Hassa ll's corpuscles. Ultrastructural analysis has revealed further heterog eneity. In the cortex, four EC subtypes have been described ultrastruc turally: subcapsular/perivascular, ''pale,'' ''intermediate,'' and ''d ark'' EC. These subtypes are also present in the medulla. Two addition al EC subtypes are restricted to the medulla: an undifferentiated subt ype, and a subtype displaying signs of high metabolic activity. Based on the morphological features of the epithelium, it has been hypotheti zed that the thymic EC subtypes represent a process of differentiation . (C) 1997 Wiley-Liss, Inc.