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 are destined to develop into immunocompetent T cells that 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.
General Architecture of the Thymus
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
FIGURE 14.24 • Photomicrograph of an infant human thymus. This H&E preparation reveals multiple lobules separated by connective tissue trabeculae that extend into the organ from the surrounding capsule. Each lobule is composed of a dark-staining basophilic cortex and a lighter-staining and relatively eosinophilic medulla. The medulla is actually a continuous branching mass surrounded by the cortex. The cortex contains numerous densely packed lymphocytes, whereas the medulla contains fewer lymphocytes. Note that in some instances the medulla may bear a resemblance to germinal centers of lymphatic nodules (upper right and center left). Such isolated medullary profiles are continuous with the overall medullary tissue, but this continuity may not be seen within the plane of section. 25.
medullary tissue (Fig. 14.24 and Plate 41, page 487). 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 hematoxylin and eosin (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 mesh-work of epithelioreticular cells (Fig. 14.25). 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.