Each thymic lobule comprises a darker staining outer cortex and a lighter staining inner medulla. The cortex is composed of dense lymphoid tissue that lacks nodules. The medulla stains more lightly than the cortex because the medullary stroma is less heavily infiltrated with lymphocytes than is the cortex.
The thymic medulla contains Hassall's corpuscles, which are flattened, concentrically arranged keratinizing epithelial cells thatare strongly acidophilic (staining pink with H & E). Degenerate cells are found at the center of the Hassall's bodies. Thymic stromal cells are not reticular cells, and they differ from those of other lymphatic tissue in being epithelial (endodermal) in origin. They are large, pale cells (epithelial-reticular cells) within the medulla. They do not make reticular fibers and are attached to each other through desmosomes.