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Thymic involution is the shrinking of the thymus with age, resulting in changes in the architecture of the thymus and a decrease in tissue mass. [1] Thymus involution is one of the major characteristics of vertebrate immunology, and occurs in almost all vertebrates, from birds, teleosts, amphibians to reptiles, though the thymi of a few species of sharks are known not to involute.
During involution, the thymus decreases in size and activity. [4] Fat cells are present at birth, but increase in size and number markedly after puberty, invading the gland from the walls between the lobules first, then into the cortex and medulla. [4]
Involution is the shrinking or return of an organ to a former size. At a cellular level, involution is characterized by the process of proteolysis of the basement membrane (basal lamina), leading to epithelial regression and apoptosis, with accompanying stromal fibrosis. The consequent reduction in cell number and reorganization of stromal ...
Thymus involution is probably the most relevant factor responsible for immunosenescence. Thymic involution is common in most mammals; in humans it begins after puberty, as the immunological defense against most novel antigens is necessary mainly during infancy and childhood. [11]
Majority of human naive T cells are produced very early in life when infant's thymus is large and functional. Decrease in naive T cell production due to involution of the thymus with age is compensated by so called "peripheral proliferation" or "homeostatic proliferation" of naive T cells which have emigrated from the thymus earlier in life.
Hassall's corpuscles (also known as thymic bodies) are structures found in the medulla of the human thymus, formed from eosinophilic type VI thymic epithelial cells arranged concentrically. These concentric corpuscles are composed of a central mass, consisting of one or more granular cells, and of a capsule formed of epithelioid cells.
In 1989, two scientific groups came up with the hypothesis that the thymus expresses genes which are in the periphery, strictly expressed by specific tissues (e.g.: Insulin produced by β cells of the pancreas) to subsequently present these so-called "tissue-restricted antigens" (TRAs) from almost all parts of the body to developing T cells in order to test which TCRs recognize self-tissues ...
Ectopic thymus is a condition where thymus tissue is found in an abnormal location . It usually does not cause symptoms, but may leads to a mass in the neck that may compress the trachea and the esophagus. It is thought to be the result of either a failure of descent or a failure of involution of normal thymus tissue.