Thymus



The thymus is a central (primary) lymphatic organ located in the thorax. Here, T-lymphocytes acquire immunological immunocompetence and autotolerance under the influence of reticular epithelial cell differentiation factors, which form the stroma of the whole organ. The embryonic origin of the thymus is twofold - the entodermal epithelial base arises from the 3rd and 4th-gill glands, while the lymphocytes are formed by lymphopoiesis from the mesenchyme.

Position and shape
The thymus lies in the anterior upper mediastinum. The size of the thyroid gland changes with age, reaching its greatest development in childhood, and since puberty, it has been slowly disappearing and being replaced by adipose tissue. In the child, it practically extends from the thyroid gland to the pericardium, in the adult it is located only in a small section behind the sternum manubri.

Syntopy


The thymus is located in close proximity to the sternum, followed by a layer of veins, then a layer of arteries. In childhood, when it reaches the neck, it is covered by infrahyoid muscles. From the right and left, it is formed by the organs of the mediastinum, especially the pleura (or lungs).

Function
The main function is the differentiation of lymphocytes into immunocompetent T-lymphocytes, i.e. lymphocytes capable of defending the body. In the newborn, it affects the development of lymphatic tissue in other organs and lymph nodes. Another function is the control of lymphopoiesis, i.e. proliferation, maturation, differentiation and acquisition of immunocompetence of lymphocytes within the thymus itself, but also in other lymphatic organs.

Macroscopic structure
The thymus consists of two lobes of unequal size (lobus dexter et sinister) connected by a ligament, the left one is usually larger. The surface of the organ is soft and flexible and is covered by a thin fibrous membrane (capsule) that penetrates into the thymus. The fibrous protrusions form septa, which divide the lobes into smaller lobes. The division into lobes is visible on the surface. The lobes are further divided into secondary and primary lobes with a size of 0.2–5 mm.

Microscopic structure


The thymus is the central lymphatic organ, so its stromal is the reticular epithelium. In the meshes of the reticulated epithelium, travelled lymphocytes that do not form here are captured. Each lobule has 2 parts:


 * cortex - at the edges of the lobe, darker;
 * medulla - inside the lobe, lighter.

Thymus cortex
The dark coloration of the thymus cortex conditions a large number of T-lymphocytes, which differentiate and multiply here. Integral membrane proteins are built into their membrane, enabling the identification and function of the T-lymphocyte - immunocompetence and autotolerance. This process is controlled by cytokines and chemokines produced by thymic epithelial cells. Only properly functioning lymphocytes leave the cortex, non-functional lymphocytes are eliminated by macrophages, and some of them are also eliminated by apoptosis. Only somatic-type blood capillaries with a continuous lining penetrate into the cortex, the endothelial wall of which is part of the hematothymic barrier which protects developing T-lymphocytes from antigens. With age, there is a physiological involution of the thymus in the cortex, during which the proliferation and differentiation of T-lymphocytes slow down, the cortex grows through adipose tissue, and white spots are visible in the light microscope after fixation of the extracted adipocytes.

Thymus medulla
Compared to the cortex, the thymus medulla is weaker in color, which is due to the lower number of T-lymphocytes present. There are only immunocompetent T cells. The special structures of the thymus marrow are Hassal's bodies, which are formed by keratinization of reticular epithelial cells (which takes place by the accumulation of keratohyalin granules in these cells). The number and size of bodies increase with age, they can calcify over time. Histological specimens may also occur in the child's thymus, but these are pathological changes. The medulla of all lobes is connected by a central strip ("tractus centralis"), in which larger thyroid vessels run. High endothelial postcapillary venules occur at the border between the cortex and the medulla (HEC - high endothelial cells); through their wall, T-lymphocytes penetrate into the bloodstream and colonize the thymodependent areas of peripheral lymphatic organs.

Related articles

 * Lymphatic organs