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
Lymph	tracheobronchial , parasternal
Precursor	third branchial pouch
MeSH	Thymus+gland
Dorlands/Elsevier	t_10/12807749

In human anatomy, the thymus is an organ located in the upper anterior portion of the chest cavity just behind the sternum. Human anatomy, which with physiology and biochemistry is a complementary basic medical science is primarily the scientific study of the morphology of the adult Human body In fields of Anatomy, anatomical terms of location are descriptive terms to help identify relative positions or directions within a species The thoracic cavity (or chest cavity) is the chamber of the human body (and other animal bodies that is protected by the Thoracic wall ( Thoracic cage The sternum (from Greek στέρνον sternon, "chest" or breastbone) is a long flat Bone located in the center of the thorax (chest Hormones produced by this organ stimulate the production of certain infection-fighting cells. It is of central importance in the maturation of T cells. T cells belong to a group of White blood cells known as Lymphocytes, and play a central role in Cell-mediated immunity.

Contents 1 History 2 Function 2.1 Phases of thymocyte maturation 3 Anatomy 4 Development 4.1 Embryology 5 Structure 5.1 Cortex 5.2 Medulla 5.3 Vasculature 6 Cancer 7 Other animals and second thymus 8 References 9 External links 10 Additional images

History

The thymus was known to the Ancient Greeks. Galen was the first to note that the size of the organ changed over the duration of a person's life. Galen ( Greek: Γαληνός Galēnos; Latin: Claudius Galenus, Aelius Galenus, Claudius Aelius Galenus, or ^[1]

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 in 1961 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. An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor Jacques Francis Albert Pierre Miller FRS (born April 2 1931) is a distinguished research scientist ^[2][3] Recently advances in immunology have allowed the fine dissection of the function of the thymus in T cell maturation. Immunology is a broad branch of biomedical Science that covers the study of all aspects of the Immune system in all Organisms It deals with

Function

In the two thymic lobes, lymphocyte precursors from the bone-marrow become thymocytes, and subsequently mature into T cells. A lymphocyte is a type of White blood cell in the Vertebrate Immune system. Thymocytes are T cell precursors which develop in the Thymus. 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. Immune system|Passive immunity|Innate immune system The adaptive immune system is composed of highly specialized systemic cells and processes that eliminate or prevent Pathogenic Loss of the thymus at an early age through genetic mutation or surgical removal results in severe immunodeficiency and a high susceptibility to infection. ^[4]

The stock of T-lymphocytes is built up in early life, so the function of the thymus is diminished in adults. It is, therefore, largely degenerated in elderly adults and is barely identifiable, consisting mostly of fatty tissue. ^[5]

The ability of T cells to recognize foreign antigens is mediated by the T cell receptor. The T cell receptor or TCR is a molecule found on the surface of T lymphocytes (or T cells that is in general responsible for recognizing Antigens bound 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 T cell receptor or TCR is a molecule found on the surface of T lymphocytes (or T cells that is in general responsible for recognizing Antigens bound Thymocytes are T cell precursors which develop in the Thymus. Peptides (from the Greek πεπτίδια, "small digestibles" are short Polymers formed from the linking in a defined order of α- Amino The major histocompatibility complex ( MHC) is a large genomic region or Gene family found in most Vertebrates It is the most gene-dense region The random nature of the genetic rearrangement results in a requirement of central tolerance mechanisms to remove or inactivate those T cells which bear a T cell receptor with the ability to recognise self-peptides. Central tolerance is the mechanism by which newly developing T cells and B cells are rendered non-reactive to self The T cell receptor or TCR is a molecule found on the surface of T lymphocytes (or T cells that is in general responsible for recognizing Antigens bound

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:

1. 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^[6]. Thymocytes are T cell precursors which develop in the Thymus.
2. 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. ^[7]. Growth factors include thymopoietin and thymosin. The term growth factor refers to a naturally occurring Protein capable of stimulating cellular growth proliferation and Cellular differentiation. Thymopoietin, also known as TMPO, is a human Gene. Thymopoietin is a protein involved in the induction of CD90 in the Thymus. Thymosin is an actin-binding protein in cells The predominant form of thymosin thymosin β4 is a member of a highly conserved family of Actin monomer-sequestering
3. 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[8]. 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. The major histocompatibility complex ( MHC) is a large genomic region or Gene family found in most Vertebrates It is the most gene-dense region Peptides (from the Greek πεπτίδια, "small digestibles" are short Polymers formed from the linking in a defined order of α- Amino )	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. Regulatory T cells (sometimes known as suppressor T cells) are a specialized subpopulation of T cells that act to suppress activation of the Immune system If these peripheral tolerance mechanisms also fail, autoimmunity may arise. Immune or immunological tolerance is the process by which the Immune system does not attack an Antigen. Autoimmunity is the failure of an organism to recognize its own constituent parts 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. This is an article about the rock music band "Circulatory System"

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. The thorax is a division of an Animal 's body that lies between the head and the Abdomen. The neck is the part of the Body on many limbed Vertebrates that distinguishes the head from the Torso or trunk 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 The thyroid is one of the largest Endocrine glands in the body

It is covered by the sternum, and by the origins of the sternohyoidei and sternothyreoidei. The sternum (from Greek στέρνον sternon, "chest" or breastbone) is a long flat Bone located in the center of the thorax (chest The sternohyoid muscle is a thin narrow muscle attaching the Hyoid bone to the Sternum, one of the paired Strap muscles of the Infrahyoid muscles 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. Fascia (făsh'ē-ə pl fas·ci·ae (făsh'ē-ē adj fascial (făsh'ē-əl (from Latin: a band is the Soft tissue component of

In the neck, it lies on the front and sides of the trachea, behind the sternohyoidei and sternothyreoidei. The neck is the part of the Body on many limbed Vertebrates that distinguishes the head from the Torso or trunk The traceartes, or windpipe, is a tube that has an inner diameter of about 20-25 mm and a length of about 10-16 cm in humans

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 In the development of vertebrate animals, Pharyngeal or branchial pouches form on the endodermal side between the Branchial arches and pharyngeal A germ layer is a collection of cells formed during animal Embryogenesis. The neural crest, a transient component of the Ectoderm, is located in between the Neural tube and the epidermis (or the free margins of the Neural folds Mesenchyme refers to loosely organized connective tissue present in the embryo regardless of origin

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. The pharynx (plural pharynges) is the part of the Neck and Throat situated immediately Posterior to (behind the Mouth and Nasal 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. In the development of vertebrate animals, Pharyngeal or branchial pouches form on the endodermal side between the Branchial arches and pharyngeal ^[9]

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. Haematopoiesis (from Ancient Greek haima blood poiesis to make (or hematopoiesis in the United States sometimes also haemopoiesis or A Lymph node ( lɪmf noʊd is an organ consisting of many types of cells and is a part of the Lymphatic system.

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. Atrophy is the partial or complete Wasting away of a part of the Body. 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"). Fats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water 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. Sex steroids, also known as gonadal steroids, are Steroid hormones that interact with Vertebrate Androgen or Estrogen receptors ^[10] Patients with the autoimmune disease Myasthenia gravis commonly (70%) are found to have thymic hyperplasia or malignancy. Autoimmune diseases arise from an overactive Immune response of the body against substances and tissues normally present in the body Myasthenia gravis (literally "serious muscle-weakness" from Greek μύς "muscle" "weakness" and Latin gravis Hyperplasia (or "hypergenesis" is a general term referring to the proliferation of cells within an organ or tissue beyond that which is ordinarily seen in e ^[11] The reason or order of these cirumstances has yet to be determined by medical scientists.

Age	Grams
birth	about 15 grams;
puberty	about 35 grams
twenty-five years	25 grams
sixty years	less than 15 grams
seventy years	about 0 grams

Structure

Each lateral lobe is composed of numerous lobules held together by delicate areolar tissue; the entire organ being enclosed in an investing capsule^[12] of a similar but denser structure. In Anatomy, a lobe is a clear anatomical division or extension that can be determined without the use of a Microscope (at the Gross anatomy level Areolar tissue (a type of "loose connective tissue" exhibits loosely organized fibers abundant blood vessels and a lot of seemingly empty space 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. A Lymph node ( lɪmf noʊd is an organ consisting of many types of cells and is a part of the Lymphatic system.

The follicles are irregular in shape and are more or less fused together, especially toward the interior of the organ. Each follicle is from 1 to 2 mm in diameter and consists of a medullary and a cortical portion^[13], and these differ in many essential particulars from each other. In Anatomy and Zoology the cortex ( Latin: "bark" "rind" "shell" or "husk" is the outermost (or "superficial"

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. The lymphatic system in Vertebrates is a network of conduits that carry a clear fluid called Lymph. Epithelial reticular cells (or epithelioreticular cells) are a structure in both the cortex and Medulla of the Thymus. This network forms an adventitia to the blood vessels. Adventitia is the outermost Connective tissue covering of any organ, vessel, or other structure

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 a molecule found on the surface of T lymphocytes (or T cells that is in general responsible for recognizing Antigens bound

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. Reticulum (rɨˈtɪkjələm Reticle) is one of the minor southern Constellations It was introduced in the early 17th century by the astronomer Isaac Habrecht Hassall's corpuscles (or thymic corpuscles) are structures found in the medulla of the human Thymus, formed from type VI Epithelial Reticular ^[14] These concentric corpuscles are composed of a central mass, consisting of one or more granular cells, and of a capsule formed of epithelioid cells. An epithelioid cell is a Mesenchymal cell whose morphology resembles that of Epithelial cells They are the remains of the epithelial tubes, which grow out from the third branchial pouches of the embryo to form the thymus. In the development of vertebrate animals, Pharyngeal or branchial pouches form on the endodermal side between the Branchial arches and pharyngeal 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. The blood vessels are part of the Circulatory system and function to transport Blood throughout the body A plexus is a network It has more specific meanings in multiple fields In the center of the medullary portion there are very few vessels, and they are of minute size.

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 T cell receptor or TCR is a molecule found on the surface of T lymphocytes (or T cells that is in general responsible for recognizing Antigens bound 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 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. Arteries are Blood vessels that carry blood away from the Heart. In Human anatomy, the internal thoracic artery ( ITA) previously known as the internal mammary artery (a name still common among Surgeons, is

The veins end in the left innominate vein, and in the thyroid veins. In the Circulatory system, a vein is a Blood vessel that carries Blood back toward the Heart (as opposed to Artery, a blood vessel The left and right brachiocephalic veins (or Innominate veins) in the upper chest are formed by the union of each corresponding Internal jugular vein and

The nerves are exceedingly minute; they are derived from the vagi and sympathetic nervous system. A nerve is an enclosed cable-like bundle of peripheral Axons (the long slender projections of Neurons. The vagus nerve (ˈveɪˌgəs (VĀ-gəs (also called pneumogastric nerve or cranial nerve X) is the tenth of twelve paired Cranial nerves, and is the The Sympathetic Nervous System ( SNS) is a branch of the Autonomic nervous system along with the Enteric nervous system and Parasympathetic nervous Branches from the descendens hypoglossi and phrenic reach the investing capsule, but do not penetrate into the substance of the organ. The hypoglossal nerve is the twelfth Cranial nerve (XII leading to the tongue The phrenic nerve arises from the third fourth and fifth cervical Spinal nerves (C3-C5 in Humans.

Cancer

Two primary forms of tumours originate in the thymus. See also Cancer A tumor or tumour is the name for a swelling or lesion formed by an abnormal growth of cells (termed neoplastic

Tumours originating from the thymic epithelial cells are called thymomas, and are found in about 25-50% of patients with myasthenia gravis. In Medicine ( Oncology) thymoma is a Tumor originating from the Thymus. Myasthenia gravis (literally "serious muscle-weakness" from Greek μύς "muscle" "weakness" and Latin gravis Symptoms are sometimes confused with bronchitis or a strong cough because the tumor presses on the recurrent laryngeal nerve. Bronchitis is an Inflammation of the Bronchi. More specifically it may refer to Acute bronchitis, caused by viruses or bacteria and lasting 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. Tumours originating from the thymocytes are called thymic lymphomas. Lymphoma a type of Neoplasm that originates in Lymphocytes (a type of White blood cell in the vertebrate Immune system)

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 ^[15] and also occurs in 5 out of 6 human fetuses. A mouse (plural mice) is a small Animal that belongs to one ^[16] As in humans, the Guinea pig's thymus naturally atrophies as the animal reaches adulthood, but in the hairless "Skinny pig" breed (which arose from a spontaneous laboratory mutation) it often possesses no thymic tissue whatsoever, and the organ cavity is replaced with cystic spaces. The Guinea pig (also commonly called the cavy after its scientific name Cavia porcellus) is a species of Rodent belonging to the family Caviidae Alopecia universalis or alopecia areata universalis is a Medical condition involving rapid loss of all hair including Eyebrows and Eyelashes The Skinny is an almost Hairless breed of Guinea pig. Skinny pigs typically have hair on their muzzles feet and legs but are nude over the remainder of A cyst is a closed sac having a distinct membrane and Division on the nearby tissue.

When animal thymic tissue is sold in a butcher shop or at a meat counter, thymus is known as sweetbread. Sweetbreads are the thymus glands of lamb, Beef, or Pork. There are two different connected Glands one set in the neck and the

References

External links

• Virtual Slidebox at Univ. Iowa Slide 287

Additional images

Endocrine system (thymus is #4)

Lymphatic system

Scheme showing development of branchial epithelial bodies. The University of Iowa, is a major teaching service and Research university located on a campus in Iowa City Iowa, on the banks of the Iowa River I, II, III, IV. Branchial pouches.

This article incorporates text or images from the public domain Gray's Anatomy, Lea & Febiger, 1917 edition. The public domain is a range of abstract materials &ndash commonly referred to as Intellectual property &ndash which are not owned or controlled by anyone Henry Gray's Anatomy of the Human Body (or Gray's Anatomy as it has commonly been shortened is an English-language Human anatomy Textbook

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