As their name implies, epithelioreticular cells have features of both epithelial and reticular cells. They provide a framework for the developing T cells; thus, they correspond to the reticular cells and their associated reticular fibers in other lymphatic tissues and organs. Reticular connective tissue cells and their fibers, however, are not present in the thymic parenchyma. Epithelioreticular cells exhibit certain features characteristic of epithelium such as intercellular junctions and intermediate filaments.
Six types of epithelioreticular cells are recognized on the basis of function: Three types in the cortex and three types in the medulla. Each type is designated by roman numerals. In the cortex, the following cell types are recognized.
 Type I epithelioreticular cells are located at the boundary of the cortex and the connective tissue capsule as well as between the cortical parenchyma and the trabeculae. They also surround the adventitia of the cortical blood vessels. In essence, type I epithelioreticular cells serve to separate the thymic parenchyma from the connective tissue of the organ. The occluding junctions between these cells reflect their function as a barrier that isolates developing T cells from the connective tissue of the organ—that is, capsule, trabeculae, and perivascular connective tissue.
 Type II epithelioreticular cells are located within the cortex. The transmission electron microscope (TEM) reveals maculae adherents (desmosomes) that join long cytoplasmic processes of adjacent cells. The cell body and cytoplasmic processes contain abundant intermediate filaments. Because of their processes, these cells are stellate. They have a large nucleus that stains lightly with H&E because of its abundant euchromatin. This nuclear feature allows the cell to be easily identified in the light microscope. Type II cells compartmentalize the cortex into isolated areas for the developing T cells. Unlike type I cells, type II cells express MHC I and MHC II molecules, which are involved in thymic cell education.
 Type III epithelioreticular cells are located at the boundary of the cortex and medulla. The TEM reveals occluding junctions between sheetlike cytoplasmic processes of adjacent cells. Like type I cells, type III epithelioreticular cells create a functional barrier—in this case, between the cortex and medulla. Like type II cells, they possess MHC I and MHC II molecules.
 Macrophages reside within the thymic cortex and are responsible for phagocytosis of T cells that do not fulfill thymic education requirements. These T cells are programmed to die before leaving the cortex. Approximately 98% of the T cells undergo this apoptosis and are then phagocytosed by the macrophages. The macrophages in the cortex are difficult to identify in H&E preparations. However, the periodic acid– Schiff (PAS) reaction readily defines them because of the
FIGURE 14.25 • Photomicrograph of a human thymus. a. The cortex contains a dense population of small, maturing T cells that creates the dark staining of this region of the thymus. The medulla, in contrast, appears lighter. The medulla also contains the thymic corpuscles that stain with eosin and give it a further distinction. 120. b. This higher-magnification photomicrograph shows the medulla with a thymic corpuscle (left) and surrounding cells. Thymic corpuscles are isolated masses of closely packed, concentrically arranged type VI epithelioreticular cells; these cells exhibit flattened nuclei. The more central mass of the corpuscle contains fully keratinized cells. In addition to numerous lymphocytes, the micrograph also shows type V epithelioreticular cells (arrows), with their eosinophilic cytoplasm and large, pale-staining nuclei. 600.
staining of their numerous large lysosomes. Accordingly, these macrophages are called PAS cells.