Thymus

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Thymus

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

{|cellpadding="0" cellspacing="0" style="float:right;" 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. It is most active before puberty, after which it shrinks in size and activity in most individuals and is replaced with fat.

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 function of the thymus in the immune system was determined by Jacques Miller. In the two thymic lobes, lymphocytes precursors mature into T cells (where T stands for “thymus”)Miller JF. The discovery of thymus function and of thymus-derived lymphocytes. Immunol Rev 185:7-14, 2002. full text. The key function of the thymus is the selection of the T cell repertoire that the immune system uses to combat infections. This involves selection of T cells that are functional (positive selection), and elimination of T cells that are autoreactive (negative selection). Positively-selected cells will be taken care of by specialized Nurse Cells. In order to be positively-selected, pre-T cells will have to interact with several cell surface molecules, MHC/HLA, to ensure reactivity and specificity. 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 peripheral exist to silence these cells such as anergy, deletion, cross-tolerance and regulatory T cells. If these mechanisms also fail, autoimmunity may arise.

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. It is covered by the sternum, and by the origins of the Sternohyoidei and Sternothyreoidei. Below, it rests upon the pericardium, being separated from the aortic arch and great vessels by a layer of fascia. In the neck, it lies on the front and sides of the trachea, behind the Sternohyoidei and Sternothyreoidei. 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. The thymus is of a pinkish-gray color, soft, and lobulated on its surfaces. It is about 5 cm in length, 4 cm in breadth below, and about 6 mm in thickness. At birth, it weighs about 15 grams; at puberty, it weighs about 35 grams; after this it gradually decreases to 25 grams at twenty-five years, less than 15 grams at sixty, and about 6 grams at seventy years.

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. 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. In the latter, numerous lymphoid cells make their appearance, and are aggregated to form lymphoid follicles. These lymphoid cells are probably derivatives of the entodermal cells which lined the original diverticula and their subdivisions. Additional portions of thymus tissue are sometimes developed from the fourth branchial pouches. Thymus continues to grow until the time of puberty and then begins to atrophy.

Structure

Each lateral lobe is composed of numerous lobules held together by delicate areolar tissue; the entire gland being enclosed in an investing capsule 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, which 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, and these differ in many essential particulars from each other. 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. This network forms an adventitia to the blood vessels. 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. 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.

The arteries supplying the thymus are derived from the internal mammary, and from the superior and inferior thyroids. The veins end in the left innominate vein, and in the thyroid veins. The nerves are exceedingly minute; they are derived from the vagi and sympathetic. Branches from the descendens hypoglossi and phrenic reach the investing capsule, but do not penetrate into the substance of the gland.

Cancer

Tumors of the thymus are found in about 10% of patients with myasthenia gravis. Thymus tumors tend to develop undetected since they find enough space for growths. Symptoms are confused with a bronchitis or a strong cough because the tumor presses on the cough nerve. Spreading of the tumor cells is likely.

Other animals and second thymus

Almost all vertebrates have a thymus in the chest, with similar structure and function as the human thymus. Some animals have a smaller second functional thymus in the neck; for mice this was reported in 2006Terszowski G et al. (2006) Evidence for a Functional Second Thymus in Mice. Science. 2 March 2006. PMID 16513945. Up to 5 out of 6 human fetuses also have a second thymus in the neck.Surprise organ discovered in mice, Nature News, 2 March 2006

References

Thorax