A lymphoid gland of vertebrates, which in mammals lies in the upper part of the chest close to the great vessels and the heart, its shape and size being determined by the surrounding structures. In humans, its size shows great individual variation at any given age; it is present at birth (average weight 13 g/0·5 oz), and continues to grow until puberty (average weight in the adolescent 37 g/1·3 oz), after which it gradually regresses. The thymus appears to have a special relation to cells of the immune system. Its presence is essential in the newborn for the development of lymphoid tissue and immunological competence. In adults it is concerned with lymphocyte production, of which only a few are released into the circulation, the remainder being destroyed within the gland itself. In early life, precursor cells migrate from bone marrow into the thymus, where they mature to become T-lymphocytes (T-cells) responsible for cellular immunity.
In human anatomy, the thymus is an organ located in the upper anterior portion of the chest cavity. The thymus plays an important role in the development of the immune system in early life, and its cells form a part of the body's normal immune system.
The thymus is also present in many other animals. When animal thymus tissue is sold in a butcher shop or at a meat counter, thymus is known as sweetbread.
Function
The importance of the thymus in the immune system was discovered by Jacques Miller. In the two thymic lobes, lymphocyte precursors mature into T cells (where T stands for “thymus”).
The thymus is critically required for the production of the vast majority of T cells. Once made, T cells leave the thymus and patrol the body. They protect against foreign invaders by making
immune responses, that are initiated via T cell receptors expressed by these T cells. Each T cell has a different T cell receptor, allowing the immune system to recognize many distinct foreign
invaders by generating many T cells. The generation of T cells expressing distinct T cell receptors occurs within the thymus, and can be conceptually divided into three phases:
A rare population of hematopoietic progenitors enters the thymus from the blood, and expands by cell division to generate a large population of immature thymocytes.Immature thymocytes each make distinct T cell receptors by a process of gene rearrangement. This process is error-prone, and some thymocytes fail to make functional T cell receptors, whereas other thymocytes make T cell receptors that are autoreactive. .
Immature thymocytes undergo a process of selection, based on the specificity of their T cell receptors. This involves selection of T cells that are functional (positive selection), and elimination of T cells that are autoreactive (negative selection). In order to be positively-selected, thymocytes will have to interact with several cell surface molecules, MHC/HLA, to ensure reactivity and specificity. Postive selection eliminates (apoptosis) weak binding cells and only takes high medium binding cells. Cells that pass both levels of selection are released into the bloodstream to perform vital immune functions.
Negative selection is not 100% complete. Some autoreactive T cells escape thymic censorship, and are released into the circulation. Additional mechanisms of tolerance active in the periphery exist to silence these cells such as anergy, deletion, and regulatory T cells.
Anatomy
The thymus will, if examined when its growth is most active, be found to consist of two lateral lobes placed in close contact along the middle line, situated partly in the thorax, partly in the neck, and extending from the fourth costal cartilage upward, as high as the lower border of the thyroid gland.
Development
The thymus appears in the form of two flask-shape endodermal diverticula, which arise, one on either side, from the third branchial pouch, and extend lateralward and backward into the surrounding mesoderm and neural crest-derived mesenchyme in front of the ventral aorta. By further proliferation of the cells lining the flask, buds of cells are formed, which become surrounded and isolated by the invading mesoderm. In the latter, numerous lymphoid cells make their appearance, and are aggregated to form lymphoid follicles. These lymphoid cells originate from hematopoietic precursors.
Structure
Each lateral lobe is composed of numerous lobules held together by delicate areolar tissue; The cortical portion is mainly composed of lymphoid cells, supported by a network of finely-branched epithelial cells, which is continuous with a similar network in the medullary portion. In the medullary portion, the reticulum is coarser than in the cortex, the lymphoid cells are relatively fewer in number, and there are found peculiar nest-like bodies, the concentric corpuscles of Hassall. These concentric corpuscles are composed of a central mass, consisting of one or more granular cells, and of a capsule formed of epithelioid cells.
The arteries supplying the thymus are derived from the internal mammary, and from the superior and inferior thyroids.
Cancer
Tumors of the thymus (thymomas) are found in about 25-50% of patients with myasthenia gravis.
Other animals and second thymus
Almost all vertebrates have a thymus in the chest, with similar structure and function as the human thymus.
Additional images
|
Endocrine system (thymus is #4) |
Lymphatic system |
Intrathymic T Cell Differentiation |
User Comments Add a comment…