Thymus
Thymus
The thymus of a full-term fetus, exposed in situ.
Gray's	subject #274 1273
Artery	derived from internal mammary artery, superior thyroid artery, and inferior thyroid artery
Nerve	vagus
Precursor	third branchial pouch
MeSH	Thymus+gland
Dorlands/Elsevier	t_10/12807749

This article discusses the bodily organ. For the herb genus Thymus, see Thyme.

In human anatomy, the thymus is an organ located in the upper anterior portion of the chest cavity. It is of central importance in the maturation of T cells. Image File history File links Illu_thymus. ... Image File history File links Gray1178. ... Section of an artery For other uses, see Artery (disambiguation). ... In human anatomy, the internal thoracic artery (ITA) (previously known as the internal mammary artery) is a vessel that supplies the chest wall and mamma, a term used for breast in anatomy. ... The superior thyroid artery arises from the external carotid artery just below the level of the greater cornu of the hyoid bone and ends in the thyroid gland. ... The inferior thyroid artery passes upward, in front of the vertebral artery and Longus colli; then turns medialward behind the carotid sheath and its contents, and also behind the sympathetic trunk, the middle cervical ganglion resting upon the vessel. ... Nerves (yellow) Nerves redirects here. ... The vagus nerve is tenth of twelve paired cranial nerves and is the only nerve that starts in the brainstem (somewhere in the medulla oblongata) and extends all the way down past the head, right down to the abdomen. ... This article or section is in need of attention from an expert on the subject. ... Pharyngeal or branchial pouches form on the endodermal side between the branchial arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches. ... Medical Subject Headings (MeSH) is a huge controlled vocabulary (or metadata system) for the purpose of indexing journal articles and books in the life sciences. ... Elseviers logo. ... Species About 350 species, including: Thymus adamovicii Thymus bracteosus Thymus broussonetii Thymus caespititius Thymus camphoratus Thymus capitatus Thymus capitellatus Thymus carnosus Thymus cephalotus Thymus cherlerioides Thymus ciliatus Thymus cilicicus Thymus cimicinus Thymus comosus Thymus comptus Thymus doerfleri Thymus glabrescens Thymus herba-barona Thymus hirsutus Thymus hyemalis Thymus integer Thymus lanuginosus... Human anatomy or anthropotomy is a special field within anatomy. ... In zootomy, several terms are used to describe the location of organs and other structures in the body of bilateral animals. ... The thoracic cavity is the chamber of the human body (and other animal bodies) that is enclosed by the ribcage and the diaphragm. ... T cells are a subset of lymphocytes that play a large role in the immune response. ...

History

Due to the large numbers of apoptotic lymphocytes, the thymus was originally dismissed as a "lymphocyte graveyard", without functional importance. The importance of the thymus in the immune system was discovered by Jacques Miller, by surgically removing the thymus from three day old mice, and observing the subsequent deficiency in a lymphocyte population, subsequently named T cells after the organ of their origin. ^[1] Recently advances in immunology have allowed the fine dissection of the function of the thymus in T cell maturation. A scanning electron microscope image of a single neutrophil (yellow), engulfing anthrax bacteria (orange). ... Jacques Francis Albert Pierre Miller is a distinguished research scientist. ... Immunology is a broad branch of biomedical science that covers the study of all aspects of the immune system in all organisms. ...

Function

In the two thymic lobes, lymphocyte precursors from the bone-marrow become thymocytes, and subsequently mature into T cells. Once mature, T cells emigrate from the thymus and constitute the peripheral T cell repertoire responsible for directing many facets of the adaptive immune system. Loss of the thymus at an early age through genetic mutation or surgical removal results in severe immunodeficiency and a high susceptibility to infection. ^[2]. The ability of T cells to recognize foreign antigens is mediated by the T cell receptor. The T cell receptor undergoes genetic rearrangement during thymocyte maturation, resulting in each T cell bearing a unique T cell receptor, specific to a limited set of peptide:MHC combinations. The random nature of the genetic rearrangement results in a requirement of central tolerance mechanisms to remove or inactive those T cells which bear a T cell receptor with the ability to recognise self-peptides. A lymphocyte is a type of white blood cell involved in the human bodys immune system. ... Thymocytes are T cell precursors which develop in the thymus. ... The immune system is the collection of organs and tissues involved in the adaptive defense of a body against foreign biological material. ... The T cell receptor or TCR is responsible for recognizing antigen bound to Major histocompatibility complex (MHC). ... The T cell receptor or TCR is responsible for recognizing antigen bound to Major histocompatibility complex (MHC). ... Thymocytes are T cell precursors which develop in the thymus. ... Peptides (from the Greek πεπτος, digestible), are the family of short molecules formed from the linking, in a defined order, of various α-amino acids. ... MHC I (1hsa) vs MHC II (1dlh) (more details. ... Central tolerance is a condition caused by tumor cells caused by tumor antigens inhibiting the immune system which has no reaction. ... The T cell receptor or TCR is responsible for recognizing antigen bound to Major histocompatibility complex (MHC). ...

Phases of thymocyte maturation

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^[3].

• 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. ^[4]. Growth factors include thymopoietin and thymosin.

• 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).

type:	functional (positive selection)	autoreactive (negative selection)
location:	cortex	medulla
	In order to be positively-selected, thymocytes will have to interact with several cell surface molecules, MHC/HLA, to ensure reactivity and specificity[5]. Thymocytes are T cell precursors which develop in the thymus. ... Growth factor is any of about twenty small proteins that attach to specific receptors on the surface of stem cells in bone marrow and promote differentiation and maturation of these cells into morphotic constituents of blood. ... Thymopoietin is a protein involved in the induction of CD90 in the thymus. ... Thymosin- Functions in the preprocessing of T cells and development of B cells to plasma cells to produce antibodies. ... Image File history File links Download high-resolution version (3117x4230, 1190 KB) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Thymus Cytotoxic T cell ... Positive selection eliminates (apoptosis) weak binding cells and only takes high medium binding cells. (Binding refers to the ability of the T-cell receptors to bind to either MHC class I/II or peptide molecules.) MHC I (1hsa) vs MHC II (1dlh) (more details. ... Peptides (from the Greek πεπτος, digestible), are the family of short molecules formed from the linking, in a defined order, of various α-amino acids. ...	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. Anergy is a theory in immunobiology in which there is a lack of reaction by the bodys defence mechanisms when foreign substances come into contact with the body. ... This article or section should be merged with suppressor T cell Regulatory T cells (also known as suppressor T cells) are characterised by virtue of their expression of the cell surface markers CD4 and CD25. ... If these central tolerance mechanisms also fail, autoimmunity may arise. Central tolerance is a condition caused by tumor cells caused by tumor antigens inhibiting the immune system which has no reaction. ... Autoimmunity is the failure of an organism to recognize its own constituent parts (down to the sub-molecular levels) as self, which results in an immune response against its own cells and tissues. ...

Cells that pass both levels of selection are released into the bloodstream to perform vital immune functions. Red blood cells (erythrocytes) are present in the blood and help carry oxygen to the rest of the cells in the body Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells, platelets). ...

Anatomy

The thymus is of a pinkish-gray color, soft, and lobulated on its surfaces. At birth it is about 5 cm in length, 4 cm in breadth, and about 6 mm in thickness. The organ enlarges during childhood, and atrophies at puberty.

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. Diagram of a tsetse fly, showing the head, thorax and abdomen The thorax is a division of an animals body that lies between the head and the abdomen. ... A human neck. ... The costal cartilages are bars of hyaline cartilage which serve to prolong the ribs forward and contribute very materially to the elasticity of the walls of the thorax. ... The thyroid gland and its relations In anatomy, the thyroid (IPA θaɪɹoɪd) is an endocrine gland. ...

It is covered by the sternum, and by the origins of the Sternohyoidei and Sternothyreoidei. The sternum or breastbone is a long, flat bone located in the center of the thorax (chest). ... The Sternohyoideus (Sternohyoid muscle) is a thin, narrow muscle, which arises from the posterior border of the medial end of the clavicle, the posterior sternoclavicular ligament, and the upper and posterior part of the manubrium sterni. ... The Sternothyreoideus (or Sternothyroid muscle) is shorter and wider than the Sternohyoideus, beneath which it is situated. ...

Below, it rests upon the pericardium, being separated from the aortic arch and great vessels by a layer of fascia. The pericardium is a double-walled sac that contains the heart and the roots of the great vessels. ... For the embryological structure, see Aortic arches. ... Fascia is specialized connective tissue layer which surrounds muscles, bones, and joints, providing support and protection and giving structure to the body. ...

In the neck, it lies on the front and sides of the trachea, behind the Sternohyoidei and Sternothyreoidei. A human neck. ... Look up trachea in Wiktionary, the free dictionary. ... The Sternohyoideus (Sternohyoid muscle) is a thin, narrow muscle, which arises from the posterior border of the medial end of the clavicle, the posterior sternoclavicular ligament, and the upper and posterior part of the manubrium sterni. ... The Sternothyreoideus (or Sternothyroid muscle) is shorter and wider than the Sternohyoideus, beneath which it is situated. ...

The two lobes generally differ in size; they are occasionally united, so as to form a single mass, and sometimes separated by an intermediate lobe.

Development

Embryology

The two main components of the thymus, the lymphoid thymocytes and the thymic epithelial cells, have distinct developmental origins. The thymic epithelium is the first to develop, and appears in the form of two flask-shape endodermal diverticula, which arise, one on either side, from the third branchial pouch (pharyngeal pouch), and extend lateralward and backward into the surrounding mesoderm and neural crest-derived mesenchyme in front of the ventral aorta. A diverticulum (plural: diverticula) is medical term for an outpouching of a hollow (or a fluid filled) structure in the body. ... Pharyngeal or branchial pouches form on the endodermal side between the branchial arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches. ... Organs derived from each germ layer. ... The neural crest, a component of the ectoderm, is one of several ridgelike clusters of cells found on either side of the neural tube in vertebrate embryos. ... Mesenchyme (also known as embryonic connective tissue) is the mass of tissue that develops mainly from the mesoderm (the middle layer of the trilaminar germ disc) of an embryo. ... The aorta (generally pronounced or ay-orta) is the largest artery in the human body, originating from the left ventricle of the heart and bringing oxygenated blood to all parts of the body in the systemic circulation. ...

Here they meet and become joined to one another by connective tissue, but there is never any fusion of the thymus tissue proper. The pharyngeal opening of each diverticulum is soon obliterated, but the neck of the flask persists for some time as a cellular cord. By further proliferation of the cells lining the flask, buds of cells are formed, which become surrounded and isolated by the invading mesoderm. Additional portions of thymus tissue are sometimes developed from the fourth branchial pouches. ^[6] The pharynx (plural: pharynges) is the part of the neck and throat situated immediately posterior to the mouth and nasal cavity, and cranial to the esophagus, larynx, and trachea. ... Pharyngeal or branchial pouches form on the endodermal side between the branchial arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches. ...

During the late stages of the development of the thymic epithelium, hematopoietic lymphoid cells from bone-marrow precursors immigrate into the thymus and are aggregated to form lymphoid follicles. Red blood cells (erythrocytes) are present in the blood and help carry oxygen to the rest of the cells in the body Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells, platelets). ... Lymph nodes are components of the lymphatic system. ...

Atrophy

The thymus continues to grow between birth and puberty and then begins to atrophy, a process directed by the high levels of circulating sex hormones. Proportional to thymic size, thymic activity (T cell output) is most active before puberty. Upon atrophy, the size and activity are dramatically reduced, and the organ is primarily replaced with fat (a phenomenon known as "involution"). The atrophy is due to the increased circulating level of sex hormones, and chemical or physical castration of an adult results in the thymus increasing in size and activity. ^[7] Atrophy is the partial or complete wasting away of a part of the body. ... Puberty refers to the process of physical changes by which a childs body becomes an adult body capable of reproduction. ... FAT may mean: DILL :]] Factory acceptance test (see Acceptance test), a software engineering concept Far Eastern Air Transport, an airline of the Republic of China on Taiwan File Allocation Table, a file system format used by Microsoft operating systems; and others Forces Armées Tchadiennes, the Chad armed forces of... Sex hormones are hormones that affect the reproductive system. ...

Puberty refers to the process of physical changes by which a childs body becomes an adult body capable of reproduction. ...

Structure

Histology

Minute structure of thymus.

Each lateral lobe is composed of numerous lobules held together by delicate areolar tissue; the entire gland being enclosed in an investing capsule^[8] of a similar but denser structure. The primary lobules vary in size from that of a pin's head to that of a small pea, and are made up of a number of small nodules or follicles. Image File history File links Thymus. ... Image File history File links Thymus. ... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... The term lobe generally refers to a projecting part of an object, but it can have more specific meanings. ... Types of connective tissue Section of the human esophagus. ... A gland is an organ in an animals body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). ... A capsule, in anatomy, is a cover or envelope partly or wholly surrounding a structure. ... A nodule describes an aggregation of similar cells or particles in a number of scientific fields: In medicine it refers to a small aggregation of cells. ... In botany, a follicle is a type of simple dry fruit produced by certain flowering plants. ...

The follicles are irregular in shape and are more or less fused together, especially toward the interior of the gland. Each follicle is from 1 to 2 mm in diameter and consists of a medullary and a cortical portion^[9], and these differ in many essential particulars from each other. In botany, a follicle is a type of simple dry fruit produced by certain flowering plants. ... Medulla in general means the inner part, and derives from the Latin word for marrow. In medicine it is contrasted to the cortex. ... The outermost layer of the brain, the cortex is rich in neurons and is the site of most sophisticated neural processing (See also: cerebral cortex). ...

Cortex

The cortical portion is mainly composed of lymphoid cells, supported by a network of finely-branched epithelial reticular cells, which is continuous with a similar network in the medullary portion. This network forms an adventitia to the blood vessels. Wikipedia does not have an article with this exact name. ... Epithelial reticular cells (or epithelioreticular cells) are a structure in both the cortex and medulla of the thymus. ... This article needs to be wikified. ...

The cortex is the location of the earliest events in thymocyte development, where T cell receptor gene rearrangement and positive selection takes place. Thymocytes are T cell precursors which develop in the thymus. ... The T cell receptor or TCR is responsible for recognizing antigen bound to Major histocompatibility complex (MHC). ...

Medulla

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.^[10] These concentric corpuscles are composed of a central mass, consisting of one or more granular cells, and of a capsule formed of epithelioid cells. They are the remains of the epithelial tubes, which grow out from the third branchial pouches of the embryo to form the thymus. Each follicle is surrounded by a vascular plexus, from which vessels pass into the interior, and radiate from the periphery toward the center, forming a second zone just within the margin of the medullary portion. In the center of the medullary portion there are very few vessels, and they are of minute size. Reticulum (Latin for reticle), is one of the minor southern (declination -60 degrees) constellations. ... Hassalls corpuscles (or thymic corpuscles) are structures in the human thymus, composed of epithelial reticular cells. ... Corpuscle is J.J. Thomsons term for a subatomic particle similar to the electron. ... If a cell has all of the other properties of epithelia, but has no free surface, it is called an epithelioid. ... Pharyngeal or branchial pouches form on the endodermal side between the branchial arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches. ... Vascular is an adjective for the word vessel and refers to tube-like structures. ... PLEXUS (Physics Learning EXperience Using Software) is a name of project that was started by Vibor Cipan, physics student form Croatia and it is based on utilization of usage of computer technology to enhance learning of physics. ...

The medulla is the location of the latter events in thymocyte development. Thymocytes that reach the medulla have already successfully undergone T cell receptor gene rearrangement and positive selection, and have been exposed to a limited degree of negative selection. The medulla is specialised to allow thymocytes to undergo additional rounds of negative selection to remove auto-reactive T cells from the mature repertoire. The gene AIRE is expressed in the medulla, and drives the transcription of organ-specific genes such as insulin to allow maturing thymocytes to be exposed to a more complex set of self-antigens than is present in the cortex. The T cell receptor or TCR is responsible for recognizing antigen bound to Major histocompatibility complex (MHC). ... The Autoimmune Regulator, abbreviated AIRE, is a human gene which is expressed in the thymus. ...

Vasculature

The arteries supplying the thymus are derived from the internal mammary, and from the superior thyroid and inferior thyroids. Section of an artery An artery or arterial is also a class of highway. ... In human anatomy, the internal thoracic artery (ITA) (previously known as the internal mammary artery) is an artery that supplies the anterior chest wall and the breasts. ... The superior thyroid artery arises from the external carotid artery just below the level of the greater cornu of the hyoid bone and ends in the thyroid gland. ... Inferior thyroid can refer to: Inferior thyroid veins Inferior thyroid artery Category: ...

The veins end in the left innominate vein, and in the thyroid veins. In biology, a vein is a blood vessel which carries blood toward the heart. ... The Brachiocephalic vein is also known as the innominate vein, the left and right brachiocephalic veins in the upper chest are formed by the union of each corresponding jugular vein and subclavian vein. ... Thyroid veins can refer to: Inferior thyroid veins Superior thyroid vein Category: ...

The nerves are exceedingly minute; they are derived from the vagi and sympathetic nervous system. Branches from the descendens hypoglossi and phrenic reach the investing capsule, but do not penetrate into the substance of the gland. A nerve is an enclosed, cable-like bundle of nerve fibers or axons, which includes the glia that ensheath the axons in myelin. ... The vagus nerve (also called pneumogastric nerve or cranial nerve X) is the tenth of twelve paired cranial nerves, and is the only nerve that starts in the brainstem (within the medulla oblongata) and extends, through the jugular foramen, down below the head, to the abdomen. ... Grays FIG. 838– The right sympathetic chain and its connections with the thoracic, abdominal, and pelvic plexuses. ... The hypoglossal nerve is the twelfth cranial nerve. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

Cancer

Two primary forms of Tumours originate in the thymus. Tumor (American English) or tumour (British English) originally means swelling, and is sometimes still used with that meaning. ...

Tumours originating from the thymic epithelial cells are called thymomas, and are found in about 25-50% of patients with myasthenia gravis. Symptoms are sometimes confused with bronchitis or a strong cough because the tumor presses on the cough nerve. All thymomas are potentially cancerous, but they can vary a great deal. Some grow very slowly. Others grow rapidly and can spread to surrounding tissues. Treatment of thymomas often requires surgery to remove the entire thymus gland. In medicine (oncology), thymoma is a neoplasm of the thymus. ... Myasthenia gravis (sometimes abbreviated MG; from the Greek myastheneia, lit. ... Bronchitis is inflammation of the bronchi (medium-size airways) in the lungs. ...

Tumours originating from the thymocytes are called thymic lymphomas. Lymphoma is a general term for cancers that develop in the lymphatic system. ...

Thymus in medicine

Since many decades isolated active ingredients from the thymus gland (thymosine alpha 1, thymosine beta 4, etc.) have also been used successfully in order to treat humans with a low immune defence system. For more information visit the following web site: www.thymus-therapie.org

Other animals and second thymus

The thymus is also present in most vertebrates, with similar structure and function as the human thymus. Some animals have multiple secondary (smaller) thymi in the neck, this phenomenon has been reported for mice ^[11] and also occurs in 5 out of 6 human fetuses.^[12] Mice may refer to: An abbreviation of Meetings, Incentives, Conferencing, Exhibitions. ...

When animal thymic tissue is sold in a butcher shop or at a meat counter, thymus is known as sweetbread. Sweetbread is the name of a dish made of the thymus (neck/throat/gullet sweetbread) or the pancreas (belly/stomach/heart sweetbread) of an animal younger than one year old. ...

References

1. ^ Miller JF. Events that led to the discovery of T-cell development and function--a personal recollection. Tissue Antigens. 2004 Jun;63(6):509-17. full text
2. ^ Miller JF. The discovery of thymus function and of thymus-derived lymphocytes. Immunol Rev 185:7-14, 2002. full text
3. ^ Schwarz BA, Bhandoola A. Trafficking from the bone marrow to the thymus: a prerequisite for thymopoiesis. Immunol Rev 209:47, 2006. full text
4. ^ Sleckman BP, Lymphocyte antigen receptor gene assembly: multiple layers of regulation. Immunol Res 32:153-8, 2005. full text
5. ^ Baldwin TA, Hogquist KA, Jameson SC, The fourth way? Harnessing aggressive tendencies in the thymus. “J Immunol.” 173:6515-20, 2004. http://www.jimmunol.org/cgi/content/full/173/11/6515]
6. ^ Swiss embryology (from UL, UB, and UF) qblood/lymphat03
7. ^ Sutherland JS. Activation of thymic regeneration in mice and humans following androgen blockade. J Immunol 2005 15;175(4):2741-53
8. ^ Histology at BU 07403loa
9. ^ Histology at BU 07401loa
10. ^ Histology at USC lymp/c_61
11. ^ Terszowski G et al. (2006) Evidence for a Functional Second Thymus in Mice. Science. 2 March 2006. PMID 16513945
12. ^ Surprise organ discovered in mice, Nature News, 2 March 2006

This article or section is in need of attention from an expert on the subject. ... University of Lausanne, museum and library The University of Lausanne (in French: Université de Lausanne) or UNIL in Lausanne, Switzerland was founded in 1537 as a school of theology, before being made a university in 1890. ... University of Berne The University of Berne is a university in the Swiss capital of Berne. ... The University of Fribourg (French: Université de Fribourg; German: Universität Freiburg) is a university in the city of Fribourg, Switzerland. ... For the unrelated Jesuit university in Chestnut Hill, see Boston College. ... For the unrelated Jesuit university in Chestnut Hill, see Boston College. ... The University of Southern California (commonly referred to as USC, SC, Southern California, and incorrectly as Southern Cal[1]), located in the University Park neighborhood in Los Angeles, California, USA, was founded in 1880, making it Californias oldest private research university. ... Science is the journal of the American Association for the Advancement of Science (AAAS). ...

External links

• [1] German Society for Thymus Therapy
• Virtual Slidebox at Univ. Iowa Slide 287

The University of Iowa, or Iowa for short, is a major national research university located on a 1,900-acre campus in Iowa City, Iowa, USA, on the Iowa River in East Central Iowa. ...

Additional images

Endocrine system (thymus is #4) Endocrine system Pineal gland Pituitary gland Thyroid gland Thymus Adrenal gland Pancreas Ovary Testis [1] File links The following pages link to this file: Endocrine system Categories: United States government images ...

Lymphatic system Image File history File linksMetadata Illu_lymphatic_system. ...

Scheme showing development of branchial epithelial bodies. I, II, III, IV. Branchial pouches. Image File history File links No higher resolution available. ...