THYMUS

pyramid-shaped lymphoid organ that, in humans, is immediately beneath the breastbone at the level of the heart. The organ is called thymus because its shape resembles that of a thyme leaf. Unlike most other lymphoid structures, the thymus grows rapidly and attains its greatest size relative to the rest of the body during fetal life and the first years after birth. Thereafter, it continues to grow, but more slowly than the other organs. At the onset of puberty, the thymus begins a slow process of shrinking. This gradual diminution in size continues for the rest of the individual's life. The thymus is divided into two lobes, lying on either side of the midline of the body, and into smaller subdivisions called lobules. It is covered by a dense connective-tissue capsule, which sends fibres into the body of the thymus for support. The thymus tissue is distinguishable into an outer zone, the cortex, and an inner zone, the medulla. The organ is composed principally of two types of cells, called, respectively, lymphocytes (see lymphocyte) and reticular cells. The reticular cells form a loose meshwork, as in a lymph node, while the spaces between them are packed with lymphocytes. The cortex, characterized by its heavy lymphocyte concentration, is the site of much lymphocytic proliferation. Proliferation of lymphocytes in the thymus is distributed evenly throughout the cortex, instead of in germinal centres, as occurs in other lymphoid tissue. Some of the daughter cellscalled T (thymus-derived) cellsthat are produced in the cortex migrate to the medulla, where they enter the bloodstream through the medullary veins, adding to the lymphocytes seen in the peripheral blood and the lymphoid organs. During the involution, or shrinking, of the thymus the cortex becomes thin. Lymphocytes disappear and are replaced by fat tissue from the partitions between the lobules. The process of involution is never complete, and the bits of thymus tissue that remain are probably sufficient to maintain its function. The functions of the thymus that have so far been observed relate chiefly to the newborn. Removal of the organ in the adult has little effect, but when the thymus is removed in the newborn, T cells in the blood and lymphoid tissue are depleted, and failure of the immune system causes a gradual, fatal wasting disease. The animal whose thymus has been removed at birth is less able to reject foreign-tissue grafts or to make antibodies to certain antigens. Moreover, certain parts of the white pulp of the spleen and lymph nodes are much reduced in size. These results demonstrate that the T cells produced in the thymus and transported to the lymphoid tissues are crucial elements in the development of immunity. It is known that most of the lymphocytes that are produced in the thymic cortex die without leaving the organ. Since those T cells that do leave the thymus are equipped to react against foreign antigens, it is assumed that the thymus destroys lymphocytes that would engage in an autoimmune reactionthat is, would react against the individual's own tissues. The thymus differs structurally from other lymphoid organs in that it does not have lymphatic vessels draining into it. It is not a filter like the lymph nodes, which are situated so that microorganisms and other antigens are exposed to their cells. The thymic lymphocytes are sealed off from the rest of the body by a continuous layer of epithelial (covering) cells that entirely surround the organ. While thus sequestered, the lymphocytes differentiate, or acquire the capabilities to perform specialized tasks. (It has been suggested that hormonal functions of the thymus aid in this differentiation.) Of these specialized lymphocytes, helper T cells work synergistically with the thymus-independent lymphocytes (B cells) to produce antibodies. Cytotoxic T cells directly attack invading microorganisms and foreign tissue, such as organ transplants.