Definition
The thymus is a single pair of lymphoid organs located in the front part of the mediastinum. Thus, it is located above the heart and behind the sternum. The thymus is on both sides of the pleura. Usually, it develops from the third pharyngeal pouch. It is called the primary lymphoid organ because it is responsible for the primary development of T cells.
Structure and function
This organ has two asymmetric flaps, which are connected through connective tissue. The size of the thymus is dependent on age. It reaches its maximum size in childhood and weighs 30 g. The involution begins after puberty, when it is replaced by fat tissue. In adults, the thymus has an average weight of 18 g. This is often referred to as the retrosternal fat pad.
The thymus is situated in a cell-rich crust and can be viewed through a microscope. There are the typical Hassall's corpuscles of the thymus, which are an onion-like enrichment of epithelial cells (surface cells). Their function is not yet clear. In contrast, the cortex contains thymocytes. Therefore, the thymus is the only lymphoid organ that is also called a lymphoepithelial organ.
The thymus plays an important role in the development and differentiation of immune cells, called T cells. It is a kind of training ground for these cells. Immature cells that are formed in the bone marrow migrate into the thymus through the blood vessels. There they turn into thymocytes. Then they migrate through the cortex after several divisions of the thymic mark. This way they mature and develop T-cell receptors, which allow them to recognize antigens. There can be a negative selection and a positive one in the maturation process, depending on whether the antigens involved are endogenous or exogenous. First there is a positive selection. In this case, only those cells develop that have a so-called peptides receptor, but do not strongly bind to the MHC molecules. Then there is a negative selection, which ensures that the T-cell structures do not attack the body's own cells (self-tolerance). All cells that do not meet criteria die by programmed cell death (apoptosis). Just 5-10% of all T cells survive the maturation process. Only these cells reach the lymphoid organs and then colonize. Besides functioning as an immune organ, the thymus also acts as a hormone. The mediators it produces - thymosin, thymic and thymopoetin - influence the maturation of immune cells in the lymphoid organs.
Thymectomy
In children, the surgical removal of the thymus (thymectomy) causes impairment in the development of the immune system. However, this is the best way to eliminate the symptoms of myasthenia gravis. This is an autoimmune reaction against receptors on the motor plate of the muscles that leads to progressive muscle weakness. About half of patients with myasthenia gravis present an enlargement of the thymus, which is usually a thymoma (usually a benign tumor). Therefore, the autoimmune reaction is amplified as tumor cells have antigens similar to the muscle receptors. Therefore, patients can be relieved of the essential complaints through the surgical removal of the thymus.
The DiGeorge syndrome
The DiGeorge syndrome refers to an impaired development of the third and fourth pharyngeal pouches, which are normally formed during the embryonic development. The thymus is formed mainly from the third pharyngeal pouch, so an under-development of this organ can even cause the absence of the thymus. Other structures can also be affected, such as the parathyroid glands, teeth or throat muscles. The reduced or lack of training for T-cell patients leads to more or less severe immune defects. The most frequently encountered are heart defects, short stature and a general developmental disorder. The goal of therapy is a reduced exposure of patients to potentially immunostimulatory microorganisms. Therefore, hand disinfection and even face masks are important measures, as well as a well-developed vaccination schedule supplemented by antibiotic therapy. In principle, patients can also have a T-cell transfer, in which case the compatibility of HLA molecules is crucial (endogenous antigens on the white blood cells), otherwise the body may reject them. Therefore, normally, doctors use the cells of siblings with an identical HLA structure to perform this kind of procedures.
More information on anatomy lessons is available on the internet.
No comments:
Post a Comment