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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.
The thymus continues to grow after birth reaching the relative maximum size by puberty. [2] It is most active in fetal and neonatal life. [9] It increases to a mass of 20 to 50 grams by puberty. [3] It then begins to decrease in size and activity in a process called thymic involution. [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 ...
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.
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.
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A thymocyte is an immune cell present in the thymus, before it undergoes transformation into a T cell. [1] Thymocytes are produced as stem cells in the bone marrow and reach the thymus via the blood. Thymopoiesis describes the process which