Although the epithelioreticular cells of the thymic cortex play an important role in the development of immunocompetent T cells, recent evidence shows that T cells at the different stages of differentiation control the microarchitecture of the thymic epithelioreticular cells, a phenomenon called crosstalk. The developing lymphocytes and epithelioreticular cells thus influence each other during T-cell development.
Thymic or Hassall’s corpuscles (derived from type VI epithelioreticular cells) are a distinguishing feature of the thymic medulla.
The thymic medulla, the inner portion of the parenchyma, contains a large number of epithelioreticular cells and loosely packed T cells (Fig. 14.25). The medulla stains less intensely than the cortex because, like the germinal centers of lymph nodules, it contains mostly large lymphocytes. These lymphocytes have pale-staining nuclei and quantitatively more cytoplasm than small lymphocytes. Like the cortex, the medulla also contains three types of epithelioreticular cells:  Type IV epithelioreticular cells are located between the cortex and the medulla close to type III cells. They possess sheetlike processes with occluding junctions between adjacent cells as well as between them and type III cells. In cooperation with type III cells, they create the barrier at the corticomedullary junction.
 Type V epithelioreticular cells are located throughout the medulla. Like the type II cells located in the cortex, processes of adjacent cells are joined by desmosomes to provide the cellular framework of the medulla and to compartmentalize groups of lymphocytes. These nuclei contrast markedly with the densely staining lymphocyte nuclei.
 Type VI epithelioreticular cells form the most characteristic feature of the thymic medulla, the thymic (Hassall’s) corpuscles (Fig. 14.26 and Plate 41, page 487). Thymic corpuscles are isolated masses of closely packed, concentrically arranged type VI epithelioreticular cells that exhibit flattened nuclei. TEM studies of these cells reveal keratohyalin granules, bundles of cytoplasmic intermediate filaments, and lipid droplets. The cells are joined by desmosomes. The center of a thymic corpuscle
FIGURE 14.26 • Electron micrograph of a thymic (Hassall’s) corpuscle. This relatively low-magnification electron micrograph shows some of the nuclei (N) and cytoplasm of the concentrically arranged epithelioreticular cells of a thymic (Hassall’s) corpuscle. Bundles of intermediate filaments, keratohyalin granules, and lipid droplets are also evident within the cytoplasm of the epithelioreticular cells. Fully keratinized cells (black layer) are present in the center of the thymic corpuscle. 5,000. (Courtesy of Dr. Johannes A. G. Rhodin.) may display evidence of keratinization, not a surprising feature for cells developed from oropharyngeal epithelium. Thymic corpuscles are unique, antigenically distinct, and functionally active multicellular components of the medulla. Although the function of thymic corpuscles is not fully understood, it is thought that thymic corpuscles produce interleukins (IL-4 and IL-7) that function in thymic differentiation and education of T lymphocytes.
Blood vessels pass from the trabeculae to enter the parenchyma of the thymus. Typically, the blood vessels enter the medulla from the deeper parts of the trabeculae and carry a sheath of connective tissue along with them. This perivascular connective tissue sheath varies in thickness. It is thicker around larger vessels and gradually becomes thinner around smaller vessels. Where it is thick, it contains reticular fibers, fibroblasts, macrophages, plasma cells, and other cells found in loose connective tissue; where it is thin, it may contain only reticular fibers and occasional fibroblasts.