|precursor||third pharyngeal arch|
|system||lymphatic system, part of the immune system|
|function||support the development of functional t cells|
the thymus is a specialized primary lymphoid organ of the immune system. within the thymus, t cells mature. t cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders. the thymus is composed of two identical lobes and is located in the anterior superior mediastinum, in front of the heart and behind the sternum. each lobe of the thymus can be divided into a central medulla and a peripheral cortex which is surrounded by an outer capsule.
the cortex and medulla play different roles in the development of t cells. cells in the thymus can be divided into thymic stromal cells and cells of hematopoietic origin (derived from bone marrow resident hematopoietic stem cells). developing t cells are referred to as thymocytes and are of hematopoietic origin. stromal cells include epithelial cells of the thymic cortex and medulla, and dendritic cells. the thymus provides an environment for development of t cells from precursor cells. the cells of the thymus provide for development of t cells that are functional and self-tolerant. therefore, one of the most important roles of the thymus is the induction of central tolerance.
the thymus is largest and most active during the neonatal and pre-adolescent periods. by the early teens, the thymus begins to decrease in size and activity and the tissue of the thymus is gradually replaced by adipose tissue (fat). nevertheless, residual t lymphopoiesis continues throughout adult life.