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.