thymus

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.

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thy·mus (thms)

n. pl. thy·mus·es

1. A lymphoid organ that is located in the superior mediastinum and lower part of the neck and is necessary in early life for the normal development of immunological function.

2. The thymus of a calf or lamb.

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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.

Mentioned in: Ataxia-Telangiectasia, Lymph Node Biopsy, Mediastinoscopy

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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 (thī´ms),

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.

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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.

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thymus atrophy

leads to failure of the cell-mediated arm of the body's immunity.