The thymus is a lymphoepithelial organ located in the superior mediastinum

The thymus is a bilobed organ located in the superior mediastinum, anterior to the heart and great vessels. It develops bilaterally from the third (and sometimes also the fourth) branchial (oropharyngeal) pouch. During development, the epithelium invaginates, and the thymic rudiment grows caudally as a tubular projection of the endodermal epithelium into the mediastinum of the chest. The advancing tip proliferates and ultimately becomes disconnected from the branchial epithelium. Multipotential lymphoid stem cells (CFU-Ls) from the bone marrow that are destined to develop into immunocompetent T cells invade the epithelial rudiment and occupy spaces between the epithelial cells so that the thymus develops into a lymphoepithelial organ.

The thymus is fully formed and functional at birth. It persists as a large organ until about the time of puberty, when T cell differentiation and proliferation are reduced and most of the lymphatic tissue is replaced by adipose tissue (involution). The organ can be restimulated under conditions that demand rapid T cell proliferation.

Connective tissue surrounds the thymus and subdivides it into thymic lobules

The thymus possesses a thin connective tissue capsule from which trabeculae extend into the parenchyma of the organ. The capsule and trabeculae contain blood vessels, efferent (but not afferent) lymphatic vessels, and nerves. In addition to collagen fibers and fibroblasts, the connective tissue of the thymus contains variable numbers of plasma cells, granulocytes, lymphocytes, mast cells, adipose cells, and macrophages.

The trabeculae establish domains in the thymus called thymic lobules. They are not true lobules, but cortical caps over portions of the highly convoluted but continuous inner medullary tissue (Fig. 13.24). In some planes of section, the "lobular" arrangement of the cortical cap and medullary tissue superficially resembles a lymphatic nodule with a germinal center, which often confuses students. Other morphologic characteristics (described below) allow positive identification of the thymus in histologic sections.

The thymic parenchyma contains developing T cells in an extensive meshwork formed by epithelioreticular cells

The outer portion of the parenchyma, the thymic cortex, is markedly basophilic in H&E preparations because of the closely packed developing T lymphocytes with their intensely staining nuclei. These T lymphocytes, also called thymocytes, occupy spaces within an extensive meshwork of epithelioreticular cells (Fig. 13.25). Macrophages are also dispersed among the cortical cells. The developing T cells arise from CFU-Ls, which originate in bone marrow. As development proceeds in the thymus, the cells derived from CFU-Ls pass through a series of developmental stages that are reflected by their expression of different CD molecules.

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 tis sue 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, i.e., capsule, trabeculae, and perivascular connective tissue.

• Type II epithelioreticular cells are located within the cortex. The transmission electron microscope (TEM) reveals maculae adherentes (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

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