thymus

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Related to Thymus glands: immune system, Thymus hormones, Thyroid glands

thymus

 [thi´mus]
a ductless gland lying in the upper mediastinum beneath the sternum; it reaches its maximum development during puberty and continues to play an immunologic role throughout life, even though its function declines with age. During the last stages of fetal life and the early neonatal period, the reticular structure of the thymus entraps immature stem cells arising from the bone marrow and circulating in the blood. The thymus preprocesses these cells, causing them to become sensitized and therefore capable of maturing into a specific differentiated type of lymphocyte. After sensitization by the thymus, the cells reenter the blood and are transported to developing lymphoid tissue, where they seed the cells that eventually become T lymphocytes, a type essential to the development of cell-mediated immunity. If the thymus is removed or becomes nonfunctional during fetal life, the lymphoid tissue fails to become seeded with the sensitized lymphocytes and the body's cell-mediated arm of immunity fails to develop. It is this arm of immunity that is mainly responsible for rejection of organ transplants and resistance to intracellular microbial infection, and perhaps plays a role in natural resistance to cancer.
Thymus.

thy·mus

, pl.

thy·mi

,

thy·mus·es

(thī'mŭs, thī'mī, thī'mus-ez), [TA]
[TA] A primary lymphoid organ, located in the superior and anterior mediastinum and lower part of the neck, that is necessary in early life for the normal development of immunologic function. It reaches its greatest relative weight shortly after birth and its greatest absolute weight at puberty; it then begins to involute, and much of the lymphoid tissue is replaced by fat. The thymus consists of two irregularly shaped parts united by a connective tissue capsule. Each part is partially subdivided by connective tissue septa into lobules, 0.5-2 mm in diameter, which consist of an inner medullary portion, continuous with the medullae of adjacent lobules, and an outer cortical portion. It is supplied by the inferior thyroid and internal thoracic arteries, and its nerves are derived from the vagus and sympathetic nerves.
Synonym(s): thymus gland
[G. thymos, excrescence, sweetbread]

thymus

/thy·mus/ (thi´mus) a bilaterally symmetrical lymphoid organ consisting of two pyramidal lobules situated in the anterior superior mediastinum, each lobule consisting of an outer cortex, rich in lymphocytes (thymocytes) and an inner medulla, rich in epithelial cells. The thymus is the site of production of T lymphocytes: precursor cells migrate to the outer cortex, where they proliferate, then move through the inner cortex, where T-cell surface markers are acquired, and finally into the medulla, where they become mature T cells; maturation is controlled by hormones produced by the thymus, including thymopoietin and thymosin. The thymus reaches maximal development at about puberty and then undergoes gradual involution.

thymus

(thī′məs)
n. pl. thy·muses
A small glandular organ that is situated behind the top of the breastbone, consisting mainly of lymphatic tissue and serving as the site of T cell differentiation. The thymus increases gradually in size and activity until puberty, after which it begins to atrophy.

thymus

[thī′məs] pl. thymuses, thymi
Etymology: Gk, thymos, thyme, flowers
a single unpaired lymphoid organ that is located in the mediastinum, extending superiorly into the neck to the lower edge of the thyroid gland and inferiorly as far as the fourth costal cartilage. The thymus is the primary central gland of the lymphatic system. The endocrine activity of the thymus is believed to depend on the hormone thymosin, which is composed of biologically active peptides critical to the maturation and the development of the immune system. The T cells of the cell-mediated immune response develop in this gland before migrating to the lymph nodes and spleen. The gland consists of two lateral lobes closely bound by connective tissue, which also encloses the entire organ in a capsule. Superficial to the gland is the sternum. Lying deep to the thymus are the great vessels and the cranial part of the pericardium. The two lobes of the gland differ in size, and in many individuals the right lobe overlaps the left lobe. The thymus is about 5 cm long, 4 cm wide, and 6 mm thick. The lobes are composed of numerous lobules, which are separated by delicate connective tissue. Each lobule is composed of a dense cellular cortex and an inner, less dense medulla. The thymus develops in the embryo from the third branchial pouch and increases in size until attaining a weight of 12 to 14 g before birth. The size of the organ relative to the rest of the body is largest when the individual is about 2 years of age. The thymus usually attains its greatest absolute size at puberty, when it weighs about 35 g. After puberty the organ undergoes involution. With aging the gland may change from pinkish-gray to yellow and in the elderly individual may appear as small islands of thymic tissue covered with fat and surrounded by the yellowish capsule. The normal involution of the thymus may be superseded by rapid accidental involution caused by starvation or by acute disease. Compare spleen.
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Thymus

thy·mus

, pl. thymi, pl. thymuses (thī'mŭs, -mī, -mŭs-ĕz) [TA]
A primary lymphoid organ, located in the superior mediastinum and lower part of the neck, which is necessary in early life for the normal development of immunologic function. It reaches its greatest relative weight shortly after birth and its greatest absolute weight at puberty; it then begins to involute, and much of the lymphoid tissue is replaced by fat. The thymus consists of two irregularly shaped parts united by a connective tissue capsule. Each part is partially subdivided by connective tissue septa into lobules, which consist of an inner medullary portion, continuous with the medullae of adjacent lobules, and an outer cortical portion.
Synonym(s): thymus gland.
[G. thymos, excrescence, sweetbread]

thymus

(thi'mus) [Gr. thymos]
Enlarge picture
THYMUS
A primary lymphoid organ located in the mediastinal cavity anterior to and above the heart, where it lies over the superior vena cava, aortic arch, and trachea. The thymus comprises two fused lobes, the right larger than the left. The lobes are partially divided into lobules, each of which has an outer cortex packed with immature and developing T lymphocytes (thymocytes) and an inner medulla containing a looser arrangement of mature T lymphocytes. See: illustration

The thymus is the primary site for T-lymphocyte differentiation; here, T lymphocytes acquire their range of antigen receptors. During the prenatal period, lymphoid stem cells migrate from the bone marrow to the thymus, filling the cortex of the lobules. Developing thymocytes acquire their characteristic CD surface antigens and their binding receptors. As the thymocytes then move from the cortex into the medulla of the lobules, some are protected but many undergo cell death in a process that culls out those reactive to autoantigens. Less than 5 % of the thymocytes mature into T cells that pass out of the lobules and migrate to the spleen, lymph nodes, and other lymphoid tissue, where they control cell-mediated immune responses. The thymus produces at least four hormones: thymopoietin, thymulin, thymus humoral factor, and the thymosins.

At birth, the thymus weighs 10-15 g; by puberty, it weighs about 20 g. After this, the cortical regions of the thymus shrink and become replaced by adipose tissue, although the thymus continues to produce hormones and some thymocytes into old age.

Pathology

Lack of a thymus or thymus hypoplasia is one component of DiGeorge syndrome, which is marked by severe lack of cell-mediated immunity; removal of the thymus of an adult is less catastrophic but leads to a less effective response to new antegens. Thymic hyperplasia results from the growth of lymph follicles containing both B lymphocytes and dendritic cells. It is found in myasthenia gravis and, occasionally, in other autoimmune diseases (e.g., Graves' disease, rheumatoid arthritis, and systemic lupus erythematosus). Thymomas involve only the thymic epithelial cells. Other tumors, including those associated with Hodgkin's disease and lymphomas, involve thymocytes.

accessory thymus

A lobule isolated from the mass of the thymus gland. It is also called a supernumerary thymus.
Synonym: supernumerary thymus

persistent hyperplastic thymus

Thymus persisting into adulthood, sometimes hypertrophying.

supernumerary thymus

Accessory thymus.

thymus

A small flat organ of the lymphatic system situated immediately behind the breastbone, that is apparent in children but inconspicuous after puberty. The thymus processes primitive LYMPHOCYTES so that they differentiate into the T cells of the immune system. It also differentiates T cells into T1 and T2 classes, a process that is influenced by the early environment of the individual.

thymus

an endocrine gland situated in the neck region of most vertebrates, but close to the heart in mammals. It produces LYMPHOCYTES which then move to lymph nodes. The thymus produces a hormone called thymosin which causes the lymphocytes to form ANTIBODY-producing plasma cells immediately after birth, but regresses in adult animals.

Thymus

An organ near the base of the neck that produces cells that fight infection. It is at its largest at puberty, then declines in size and function during adult life.

thymus

lymphoid organ; located in lower neck/upper chest; essential for normal immunological development during childhood; regresses from puberty onwards, but enlarged in adults with myasthenia gravis

thy·mus

, pl. thymi, pl. thymuses (thī'mŭs, -mī, -mŭs-ĕz) [TA]
A primary lymphoid organ, located in the superior mediastinum and lower part of the neck, which is necessary in early life for the normal development of immunologic function.
[G. thymos, excrescence, sweetbread]

thymus (thī´məs),

n a single unpaired gland located in the mediastinum that is the primary central gland of the lymphatic system. The T cells of the cell-mediated immune response develop in this gland before migrating to the lymph nodes and spleen.

thymus

a primary lymphoid organ lying in the cranial mediastinum or in the neck or throat, (depending on the species), which reaches its maximum development during puberty and continues to play an immunological role throughout life, even though its function declines with age. Called also sweetbread.
During the last stages of fetal life and the early neonatal period, the reticular structure of the thymus entraps immature 'stem' cells arising from the bone marrow and circulating in the blood. The thymus preprocesses these cells, causing them to become antigen-specific and therefore capable of maturing into a type of lymphocyte that is essential to the regulation of immune responses generally and the development of cell-mediated immunity. More than 90% of T lymphocytes produced in the thymus are destroyed there in a process sometimes referred to as clonal purging, which is conceptually associated with the removal of self-reactive cells, i.e. only nonself-reactive cells leave the thymus. After development in the thymus, these lymphocytes re-enter the blood and are transported to developing secondary lymphoid tissues, such as lymph nodes and spleen, where they seed the cells that eventually become thymus-dependent or T lymphocytes. If the thymus is removed or becomes nonfunctional during fetal life, the secondary lymphoid tissue and blood fail to become seeded with the T lymphocytes and the body's cell-mediated arm of immunity fails to develop. It is this arm of immunity that is mainly responsible for rejection of organ transplants and resistance to microbial infection, and plays a role in the elimination of cells potentially able to give rise to cancer.

thymus atrophy
leads to failure of the cell-mediated arm of the body's immunity.
References in periodicals archive ?
That all changed in September of 2014, when an international research group reported their development of a fully intact and fully functional thymus gland from cells that were successfully transplanted into an animal host.
A transplantable thymus would be a tremendous breakthrough in human medicine--not only because of its implications for the field of regenerative medicine, but also because of the important role the thymus gland plays in maintaining a healthy immune system.
In September 2014, the journal Nature Cell Biology published a study by an international research group reporting their development of an intact, fully functioning thymus gland that was successfully transplanted into an animal host.
The thymus gland begins to decline in late adolescence, and has almost disappeared by the age of 40.
Yorkhill Sick Children's Hospital in Glasgow yesterday admitted removing thymus glands from children during operations and using them for research.
A hospital spokeswoman said: "In some cases, to carry out complex heart surgery, it is necessary to remove the thymus gland.
All recipient pigs had their thymus glands removed 3 weeks prior to transplantation to rule-out the possibility of tolerance induction by usual host mechanisms.
women who as infants received x-ray treatments to shrink enlarged thymus glands confirms this grim view, revealing a 3.