Synonym
Sweetbread
Definition
The thymus is an unpaired lymphatic organ (part of the lymphatic system), which is located in the thorax in the front part of the mediastinum. It is located above the heart and behind the breastbone. Laterally, the thymus is covered by the pleura on both sides.
In most cases, it develops from the 3rd pharyngeal cavity. Since it serves the primary development of T cells, it is called the primary lymphatic organ, like the bone marrow (equivalent for B cells). The organ consists of two interconnected, asymmetrical lobes.
Connective tissue divides the lobes into further small lobes. The size of the thymus depends on age. In childhood it reaches its maximum size with an average weight of 30g.
Especially after puberty, involution (reduction) of the organ begins, in which the thymus tissue is gradually replaced by non-functional fatty tissue. In adulthood, therefore, there is only a residual thymus body with an average weight of 18g. This is often referred to as the retrosternal fat body.
Microscopically, the basic thymus tissue can be divided into a cell-rich cortex and a medulla further inside. The latter in particular contains the Hassall bodies typical of the thymus, which probably represent onion-like assemblies of epithelial cells (cells from surfaces). Their function has not yet been clarified. In the cortex, on the other hand, the thymocytes lie in a basic framework of epithelial cells. For this reason, the thymus is the only one of all lymphatic organs that is called a lymphoepithelial organ.
Function and task of the thymus
The task of the thymus is the development and differentiation of immune cells, the so-called T cells. The thymus is therefore a kind of training center for these cells: Immature cells, which are formed in the bone marrow, migrate into the thymus via blood vessels. There they are called thymocytes.
After several divisions, the thymocytes migrate from the cortex through the thymus lobe towards the medulla. On this way they mature and receive the T cell receptor, a protein anchored in the surface of T cells and capable of recognizing antigens. During maturation, a distinction is made between positive and negative selection, depending on whether the antigens are endogenous or foreign.
In the beginning, a positive selection takes place. Only those cells whose receptor recognizes peptides via so-called MHC molecules, but does not bind too strongly, are allowed to develop further. Subsequently, the negative selection takes place.
In order to ensure that the T-cell does not attack the body’s own structures, a selection of those cells that tolerate them (self-tolerance) takes place. All cells that do not meet these criteria die by programmed cell death (apoptosis). Only 5-10% of all T-cells survive maturation.
Only these cells enter the blood to subsequently colonize the secondary lymphatic organs. In addition to its function as an immune organ, the thymus also functions as a hormone gland. The produced messenger substances thymosin, thymopoetin and thymus factor influence the maturation of immune cells in the lymphatic organs.