Figure 1325

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. xl20. 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 epithe-lioreticular 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 epithe-lioreticular cells (arrows), with their eosinophilic cytoplasm and large, pale-staining nuclei. X600.

into isolated areas for the developing T cells. Unlike type 1 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 sheet-like 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 stain ing of their numerous large lysosomes. Accordingly, these macrophages are called PAS cells.

While 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. 13.25). 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 TV epithelioreticular cells are located between the cortex and the medulla close to type III cells. They possess sheet-like 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 or Hassall's corpuscles (Fig. 13.26). Thymic corpuscles are isolated masses of closely packed, concentrically arranged type VI epithelioreticular cells that exhibit flattened nuclei. TEM studies of these cells reveal kera-tohyalin granules, bundles of cytoplasmic intermediate filaments, and lipid droplets. The cells are joined by desmosomes. The center of a thymic corpuscle 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, histochemical studies show that they produce thymic hormones (e.g., thymosin and thymopoietin).

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.

The blood-thymus barrier protects developing lymphocytes in the thymus from exposure to antigens

Lymphocytes reaching the thymic cortex are prevented from contact with antigen by a physical barrier called the blood-thymus barrier (Fig. 13.27). The components that

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