Definition of Thymus

Thymus: A lymphoid organ situated in the center of the upper chest just behind the sternum (breastbone). It is in the thymus that lymphocytes mature, multiply, and become T cells. (That is why they are called T cells. The T is for thymus.)

The thymus develops in embryonic life as an outgrowth of a structure known as the third branchial pouch which is invaded by lymphoid stem cells. They arrive through the blood and enter the outer cortex of the thymus, where they proliferate. They move through the inner cortex, mature and acquire T cell surface markers. And then in the medulla (center) of the thymus, they become fully mature T cells and enter the bloodstream. The process of T cell maturation is regulated by hormones produced by the thymus, including thymopoietin and thymosin.

The thymus reaches its greatest size at puberty. It then begins to involute and much of the lymphoid tissue is replaced by fibrous tissue and fat. Its function accordingly declines after puberty.

Congenital absence of the thymus (congenital athymia) or loss of the thymus in the newborn period (neonatal thymectomy) results in complete lack of functional T cells and immune deficiency.

The name thymus comes from a supposed resemblance to the bud of the herb thyme (in Latin, thymus). The thymus is also called the thymus gland.