Thymus (thymus gland)

Thymus (thymus, or, as it used to be called this organ, thymus gland, thymus gland) is, like the bone marrow, the central organ of immunogenesis. Stem cells penetrating the thymus from the bone marrow with blood flow, after passing a series of intermediate stages, are transformed into T-lymphocytes responsible for the reactions of cellular immunity. Subsequently, T-lymphocytes enter the bloodstream, leave the thymus and colonize the thymus-dependent zones of the peripheral organs of immunogenesis. Reticuloepithelial cells of the thymus secrete biologically active substances, called thymic (humoral) factor. These substances affect the function of T-lymphocytes.

Thymus consists of two asymmetric parts: the lobus dexter and the lobus sinister. Both shares can be spliced or closely in contact with each other at the middle level. The lower part of each lobe is dilated, and the upper part is narrowed. Often the upper parts protrude in the neck area in the form of a two-pronged fork (hence the name "thymus gland"). The left share of the thymus is about half the time longer than the right one. In the period of its maximum development (10-15 years), the thymus weight reaches an average of 37.5 g, and the length is 7.5-16.0 cm.

The topography of the thymus (thymus gland)

The thymus is located in the anterior part of the superior mediastinum, between the right and left mediastinal pleura. The position of the thymus corresponds to the upper interpleural field when the pleural boundary is projected onto the anterior thoracic wall. The upper part of the thymus often enters the lower parts of the pre-tracheal interfascial gap and lies behind the sternum-sublingual and sternum-thyroid muscles. The anterior surface of the thymus is convex, is attached to the posterior surface of the handle and the body of the sternum (up to level IV of the costal cartilage). Behind the thymus there are the upper part of the pericardium, covering the anterior and anterior pulmonary trunks in the front, the aortic arch with large vessels leaving it, the left brachiocephalic and the superior vena cava.

The structure of the thymus (thymus gland)

The thymus has a tender thin connective tissue capsule (capsula thymi) from which interlobular septa septa corticales divide the thymus substance into lobuli thymi into its cortical substance. The parenchyma of the thymus consists of a darker cortex (cortex thymi) and a lighter medulla thymi, which occupies the central part of the lobules.

The stroma of the thymus is represented by the reticular tissue and the stellate form by multiflorous epithelial cells - the epitheliourethilocytes of the thymus.

In the loops of the network formed by the reticular cells and the reticular fibers, as well as epitheliourethilocytes, thymus lymphocytes (thymocytes) are located.

In the brain, there are dense corpuscles of the thymus (corpuscula thymici, the body of Hassala), formed by concentrically located, strongly flattened epithelial cells.

Development and age-related features of the thymus

The epithelial component of the thymus develops as a paired organ from the epithelium of the head in all vertebrates. In humans, the thymus is laid in the form of a paired protrusion of the epithelium of the 3rd and 4th gill pockets at the end of the first and beginning of the second month of intrauterine life. Subsequently, the epithelial part of the thymus develops from the epithelium of only the third gill pockets, and the bookmark from the fourth pockets is early reduced or preserved as rudimentary formations (islets) located near the thyroid gland or inside it. In the epithelial lining of the thymus, the lymphoid elements of this organ (thymocytes) develop from the stem cells that enter the bone marrow. The rudiments of the thymus grow in the caudal direction, elongate, thicken, approach each other. The elongated thin upper (proximal) part of the thymus embryo, called "ductus thymopharyngeus", gradually disappears, and the lower thickened part forms the thymus share. At the 5th month of intrauterine development, the thymus has a lobed structure, cortical and brain substance is clearly visible in it.

Thymus is formed before other organs of the immune system and at the time of birth has a significant mass - an average of 13.3 g (from 7.7 to 34 g). After birth for the first 3 years of a child's life, the thymus grows most intensively. In the period from 3 to 20 years the mass of the thymus is quite stable (on the average 25.7-29.4 g). After 20 years, the weight of the thymus gradually decreases due to age-related involution. In the elderly and the elderly the mass of the thymus is 13-15 g. With age, the microscopic structure of the thymus changes. After birth (up to about 10 years), the cortex predominates in the thymus. The parenchyma of the thymus occupies up to 90% of the body volume. By the age of 10, the size of the cortical and medulla is approximately equal. Later the zone of cortical substance becomes thinner, the number of thymocytes decreases. In the body, fatty tissue grows with the connective tissue. In people over 50 years of age, it is 90% of the body volume. The parenchyma of the thymus does not completely disappear in the process of age involution, but is preserved in the form of islets surrounded by a fatty tissue lying behind the sternum.

[1], [2], [3], [4], [5], [6], [7]

Blood supply and innervation of the thymus

To the thymus from the inner thoracic artery, the aortic arch and the brachiocephalic trunk, thymus branches (rr.thymici) leave. In inter-coronal septa, they are divided into smaller branches that penetrate into the lobules, where they branch out to the capillaries. Thymus veins flow into the brachiocephalic veins, as well as into the internal thoracic veins.

The nerves of the thymus are branches of the right and left vagus nerves, and also come from the cervicothoracic (stellate) and superior thoracic nodes of the sympathetic trunk.

[8], [9], [10], [11], [12]