The thymus is a lymphatic organ that exhibits certain unique structural features. The supporting reticular stroma arises from endodermal ep-ithelium and produces a cellular reticulum. There are no reticular fibers associated with these cells; instead, the cells, designated epithe-lioreticular cells, serve as the stroma. Lymphocytes come to lie in the interstices of the cellular reticulum, and these two cellular elements, the lymphocytes and the epithelioreticular cells, comprise the bulk of the organ. The stem lymphocytes that migrate into the endodermal rudiment in the embryo derive from the yolk sac and, later, from the red bone marrow. These lymphocytes proliferate and become immunologically com-petent in the thymus, differentiating into the thymus-dependent lymphocytes (i.e., T lymphocytes). Some of these lymphocytes migrate to other tissues to populate the thymus-dependent portions of lymph nodes and spleen as well as to reside in the loose connective tissue. Many lymphocytes die or are destroyed in the thymus because in the random process by which they acquire the ability to recognize and react to antigens they become programmed against “self” antigens. Numerous macrophages are present to phagocytize these destroyed lympho-cytes. A blood-thymus barrier is formed by the sheathing of the perivascular connective tissue of the thymus by the epithelioreticular cells. In addition, there are no afferent lymphatic vessels to the thymus. Thus, it cannot react to circulating antigens. The thymus involutes during ado-lescence and is often difficult to recognize in the adult. A connective tissue capsule (Cap) surrounds each lobe of the two lobes of the thymus and sends trabeculae (T) into the parenchyma to form lobules. The lobules are not completely separate units; rather, they interconnect because of the discontinuous nature of the trabeculae.
Thymus, human, H&E ×40. whereas the medulla contains fewer lymphocytes and is consequently less densely packed.
Examination of the thymus at low magnification reveals the lobules (L) composed of a dark-staining basophilic cortex (C) and a lighter-staining and relatively eosinophilic medulla (M). The cortex contains numerous densely packed lymphocytes,
Thymus, human, H&E ×140.
It is the relative difference in the lymphocyte population (per unit area) and, in particular, the staining of their nuclei with hematoxylin that creates the difference in appearance between cortex (C) and medulla (M). Note that some of the medullary areas bear a resemblance to germinal centers of other lymphatic organs because of the medulla appearing as isolated circular areas (upper left of top figure). The medullary component, however, is actually a continuous branching mass surrounded by cortical tissue. Thus, the “isolated” medullary profiles are actually united with one another, although not within the plane of section. A suggestion of such continuity can be seen on the right in top figure where the medulla appears to extend across several lobules.
The main cellular constituents of the thymus are lymphocytes (thymocytes), with characteristic small, round, dark-staining nuclei, and epithelioreticular supporting cells, with large pale-staining nuclei. Both of the cell types can be distinguished in figure on right, which provides a high-magnification view of the medulla. Because there are fewer lymphocytes in the medulla, it is the area of choice to examine the epithelioreticular cells. The thymus also contains macrophages; however, they are difficult to distinguish from the epithelioreticular cells.
Medulla, thymus, human, H&E ×600.