thymi

a ductless gland lying in the upper mediastinum beneath the sternum; it reaches its maximum development during puberty and continues to play an immunologic role throughout life, even though its function declines with age. During the last stages of fetal life and the early neonatal period, the reticular structure of the thymus entraps immature stem cells arising from the bone marrow and circulating in the blood. The thymus preprocesses these cells, causing them to become sensitized and therefore capable of maturing into a specific differentiated type of lymphocyte. After sensitization by the thymus, the cells reenter the blood and are transported to developing lymphoid tissue, where they seed the cells that eventually become T lymphocytes, a type essential to the development of cell-mediated immunity. If the thymus is removed or becomes nonfunctional during fetal life, the lymphoid tissue fails to become seeded with the sensitized lymphocytes and the body's cell-mediated arm of immunity fails to develop. It is this arm of immunity that is mainly responsible for rejection of organ transplants and resistance to intracellular microbial infection, and perhaps plays a role in natural resistance to cancer.

A primary lymphoid organ located in the mediastinal cavity anterior to and above the heart, where it lies over the superior vena cava, aortic arch, and trachea. The thymus comprises two fused lobes, the right larger than the left. The lobes are partially divided into lobules, each of which has an outer cortex packed with immature and developing T lymphocytes (thymocytes) and an inner medulla containing a looser arrangement of mature T lymphocytes. See: illustration

The thymus is the primary site for T-lymphocyte differentiation; here, T lymphocytes acquire their range of antigen receptors. During the prenatal period, lymphoid stem cells migrate from the bone marrow to the thymus, filling the cortex of the lobules. Developing thymocytes acquire their characteristic CD surface antigens and their binding receptors. As the thymocytes then move from the cortex into the medulla of the lobules, some are protected but many undergo cell death in a process that culls out those reactive to autoantigens. Less than 5 % of the thymocytes mature into T cells that pass out of the lobules and migrate to the spleen, lymph nodes, and other lymphoid tissue, where they control cell-mediated immune responses. The thymus produces at least four hormones: thymopoietin, thymulin, thymus humoral factor, and the thymosins.

At birth, the thymus weighs 10-15 g; by puberty, it weighs about 20 g. After this, the cortical regions of the thymus shrink and become replaced by adipose tissue, although the thymus continues to produce hormones and some thymocytes into old age.

Pathology

Lack of a thymus or thymus hypoplasia is one component of DiGeorge syndrome, which is marked by severe lack of cell-mediated immunity; removal of the thymus of an adult is less catastrophic but leads to a less effective response to new antegens. Thymic hyperplasia results from the growth of lymph follicles containing both B lymphocytes and dendritic cells. It is found in myasthenia gravis and, occasionally, in other autoimmune diseases (e.g., Graves' disease, rheumatoid arthritis, and systemic lupus erythematosus). Thymomas involve only the thymic epithelial cells. Other tumors, including those associated with Hodgkin's disease and lymphomas, involve thymocytes.

A lobule isolated from the mass of the thymus gland. It is also called a supernumerary thymus.

Thymus persisting into adulthood, sometimes hypertrophying.

pertaining to the thymus.