Thymus (thymus gland).

The thymus (or, as this organ was previously called, the thymus gland, the goiter gland) is, like the bone marrow, the central organ of immunogenesis. Stem cells penetrating the thymus from the bone marrow with the bloodstream, after passing a number of intermediate stages, turn into T-lymphocytes responsible for cellular immunity reactions. Subsequently, T-lymphocytes enter the blood, leave the thymus and populate thymus-dependent zones of the peripheral organs of immunogenesis. Reticuloepitheliocytes of the thymus secrete biologically active substances called thymic (humoral) factors. These substances affect the functions of T-lymphocytes.

The thymus consists of two asymmetrical lobes: the right lobe (lobus dexter) and the left lobe (lobus sinister). Both lobes may be fused or closely adjacent to each other at the middle. The lower part of each lobe is widened, 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 lobe of the thymus is longer than the right in about half the cases. During the period of its maximum development (10-15 years), the thymus mass reaches an average of 37.5 g, and the length is 7.5-16.0 cm.

Topography of the 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 superior interpleural field with the projection of the pleural boundaries onto the anterior chest wall. The superior part of the thymus often extends into the inferior parts of the pretracheal interfascial space and lies behind the sternohyoid and sternothyroid muscles. The anterior surface of the thymus is convex, adjacent to the posterior surface of the manubrium and body of the sternum (up to the level of the IV costal cartilage). Behind the thymus are the superior part of the pericardium, which covers the initial parts of the aorta and pulmonary trunk in front, the aortic arch with the large vessels extending from it, the left brachiocephalic and superior vena cava.

Structure of the thymus gland

The thymus has a delicate, thin connective tissue capsule (capsula thymi), from which interlobular septa (septa corticales) extend inside the organ into its cortex, dividing the thymus into lobules (lobuli thymi). The thymus parenchyma consists of a darker cortex (cortex thymi) and a lighter medulla (medulla thymi), occupying the central part of the lobules.

The thymus stroma is represented by reticular tissue and star-shaped multi-branched epithelial cells - thymus epithelioreticulocytes.

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

In the medulla there are dense thymus bodies (corpuscula thymici, Hassall's bodies), formed by concentrically located, highly 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 cephalic intestine in all vertebrates. In humans, the thymus is laid down as a paired protrusion of the epithelium of the III and IV gill pockets at the end of the 1st - beginning of the 2nd month of intrauterine life. Subsequently, the epithelial part of the thymus develops from the epithelium of only the third gill pockets, and the anlage of the fourth pockets is early reduced or preserved as rudimentary formations (islets) located near the thyroid gland or inside it. In the epithelial anlage of the thymus, lymphoid elements of this organ (thymocytes) develop from stem cells coming here from the bone marrow. The rudiments of the thymus grow in the caudal direction, lengthen, thicken, and converge with each other. The elongated thin upper (proximal) part of the thymus rudiment, called the "ductus thymopharyngeus", gradually disappears, and the lower thickened part forms the thymus lobe. In the 5th month of intrauterine development, the thymus has a lobular structure, in which the cortex and medulla are clearly visible.

The thymus is formed earlier than other organs of the immune system and by the time of birth has a significant mass - on average 13.3 g (from 7.7 to 34 g). After birth, during the first 3 years of a child's life, the thymus grows most intensively. In the period from 3 to 20 years, the thymus mass is quite stable (on average 25.7-29.4 g). After 20 years, the thymus mass gradually decreases due to age-related involution. In elderly and old people, the thymus mass is 13-15 g. With age, the microscopic structure of the thymus changes. After birth (approximately up to 10 years), the thymus is dominated by the cortex. The thymus parenchyma occupies up to 90% of the organ's volume. By the age of 10, the sizes of the cortex and medulla are approximately equal. Subsequently, the cortex zone becomes thinner, the number of thymocytes decreases. Fatty tissue grows in the organ along with connective tissue. In people over 50 years of age, it makes up 90% of the organ's volume. The thymus parenchyma does not completely disappear during age-related involution, but remains in the form of islets surrounded by fatty tissue lying behind the sternum.

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Blood supply and innervation of the thymus

Thymic branches (rr.thymici) extend to the thymus from the internal thoracic artery, the aortic arch, and the brachiocephalic trunk. In the interlobular septa, they divide into smaller branches that penetrate into the lobules, where they branch out to the capillaries. The 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 originate from the cervicothoracic (stellate) and superior thoracic ganglia of the sympathetic trunk.

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