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thy·mus·es(thī'mŭs, thī'mī, thī'mus-ez), [TA]
thy·mus, pl. thymi, pl. thymuses (thī'mŭs, -mī, -mŭs-ĕz) [TA]
Synonym(s): thymus gland.
thymus(thi'mus) [Gr. thymos]
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.
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.