Thymus histology. Thymus (thymus gland)

Date: 04.03.2020

Thymus- the central organ of lymphoid hematopoiesis and immune defense organism. In the thymus, there is an antigen-independent differentiation of bone marrow precursors of T-lymphocytes into immunocompetent cells - T-lymphocytes. The latter carry out reactions of cellular immunity and are involved in the regulation of humoral immunity, which occurs, however, not in the thymus, but in the peripheral organs of hematopoiesis and immune defense. In addition, thymus extracts have been found to contain more than 20 biologically active substances, including distant action, which allows the thymus to be attributed to the glands of the endocrine system.

Thymus development... The thymus is laid in the 2nd month of embryogenesis in the form of small protrusions of the walls of the 3rd and 4th pairs of branchial pockets. At the 6th week, the primordium of the gland has a distinctly expressed epithelial character. At the 7th week, it loses its connection with the wall of the head intestine. The epithelium of the anlage of the gland, forming outgrowths in the mesenchyme, acquires a network-like structure. Initially, the dense epithelial anlage of the gland is loosened due to its colonization with lymphocytes. Their number increases rapidly, and the gland acquires the structure of the lymphoepithelial organ.

The ingrowing mesenchyme with blood vessels subdivides thymus into slices. In each lobule, the cortical and medulla are distinguished. In the histogenesis of the thymus in the medulla of the lobules, layered epithelial formations are formed - epithelial pearls, or Hassal's little bodies. In their composition, dense epithelial cells are determined, concentrically layering on top of each other.

Thymus structure... Outside thymus covered with a connective tissue capsule. The septa extending from it - septa - divide the thymus into lobules. The basis of the lobule is formed by process epithelial cells - epithelioreticulocytes, in the reticular skeleton of which there are thymic lymphocytes (thymocytes). The source of the development of T-lymphocytes are bone marrow hematopoietic stem cells. Further, the precursors of T-lymphocytes (pretimocytes) enter the thymus with blood and turn into lymphoblasts here.

10- Thymus the gland (thymus) produces hormones:
1- Thymosin
2- Tmopoietin
3- Connection with Anahata chakra
4- Connection of the Anahata chakra with the Soul body
5- Sections controlling the upper blood pressure
6- Areas controlling lower blood pressure
7- Areas that control the heart rate

STROMA

dense stroma:

· soft stroma: reticuloepithelial tissue; in the cortex there are special types of cells of the reticuloepithelial stroma - epithelial cells-nurses, dendritic epithelial cells of the cortical layer; in the medulla there are also special types of cells of the reticuloepithelial stroma - interdigital dendritic cells, epithelial cells medulla, Hassal's little body

FUNCTIONS OF RETICULOEPITELIAL STROMA CELLS- participation in the differentiation of T-lymphocytes, which is provided by contact interactions with lymphocytes and by the production of thymic hormones (thymosin, thymalin, thymopoietin)

PARENCHYMA structural element parenchyma is lobule of thymus consisting of cortex and medulla

cortical substance: formed by precursor cells of T-lymphocytes, T-lymphoblasts, T-lymphocytes at different levels of differentiation, dying T-lymphocytes, macrophages lying in the cells of the reticuloepithelial stroma, due to the presence of a large number of cells, it stains intensely and looks darker in comparison with the brain substance
functions: antigen-independent differentiation of T-lymphocytes, recognition and destruction of T-lymphocytes, aimed at interacting with autoantigens (censor function)
medulla: formed by T-lymphocytes, macrophages, sometimes plasma cells
functions: the exact functions are unknown, possibly some stages of antigen-independent differentiation of T-lymphocytes

FEATURES OF BLOOD SUPPLY

the cortex and medulla are supplied separately
blood from the cortex without entering the medulla immediately flows out of the thymus
in the cortical substance there is hematothymic barrier; the structure of its wall:
1. (blood ->) capillary endothelium -> 2. basement membrane of the capillary, may be pericytes and adventitious cells -> 3. pericapillary space -> 4. basement membrane of reticuloepithelial cells -> 5. reticuloepithelial cells -> (parenchyma)

INVOLUTION OF THYMUS
during life, the thymus undergoes reverse development - this age-related involution; under stress and under the influence of glucocorticoid hormones occurs fast or accidental involution thymus; both types of involution consist in the death of lymphoid cells, a decrease in the mass of the organ and the replacement of the parenchyma with connective tissue

SOURCES OF DEVELOPMENT

mesenchyme- capsule and septa

· epithelium of the 3rd and 4th branchial pockets- reticuloepithelial stroma

Bone marrow- parenchyma (lymphoid cells, macrophages)

89)SPLEEN

STROMA

· dense stroma: capsule and septa (septa in the spleen are called trabeculae) are formed by dense fibrous connective tissue, where there are many elastic fibers, SMCs are found

soft stroma: reticular tissue; in the white pulp - in the lymphoid follicles - there are special types of cells of the reticular stroma - dendritic cells and interdigital cells; dendritic cells are located in the center of proliferation of the lymphoid follicle, participate in the differentiation of B-lymphocytes; interdigital cells are located in the periarterial zone of the follicle, participate in the differentiation of T-lymphocytes

PARENCHYMA (PULP) formed by white and red pulp

white pulp: presented lymphoid follicles, the following zones are distinguished in them:

breeding center- there are mainly B-lymphocytes at different levels of differentiation, dendritic cells of the reticular stroma; antigen-dependent differentiation of B-lymphocytes (B-zone) occurs in this area
periarterial zone- there are mainly T-lymphocytes at different levels of differentiation, interdigital cells of the reticular stroma; antigen-independent differentiation of T-lymphocytes (T-zone) occurs SUCH ZONE is only in the spleen follicles
- there is an interaction of T- and B-lymphocytes, which is necessary for their differentiation

functions: antigen-dependent differentiation of T- and B-lymphocytes

red pulp: represented by blood, which is in sinusoids and perisinusoidal spaces
functions:

death of old erythrocytes - old erythrocytes have a reduced osmotic resistance (resistance to a decrease osmotic pressure plasma), and the osmotic pressure of the plasma may decrease in the sinusoids of the spleen, old erythrocytes cannot withstand such changes in osmotic pressure and undergo hemolysis, after which their remnants are phagocytized by macrophages; in addition, old erythrocytes have few sialic acids in the cytomembrane glycocalyx, they are recognized by macrophages and phagocytosed
death of old platelets, which are recognized and phagocytosed by macrophages
blood depot - due to the presence of arterial and venous sphincters, blood can be deposited in the red pulp, this is facilitated by the extensibility of the capsule and trabeculae of the spleen
the final stages of antigen-dependent differentiation of lymphocytes (plasmacytopoiesis)

BLOOD SUPPLY

splenic artery
segmental arteries
trabecular artery
pulp artery
central artery - the part of the pulp artery passing through the lymphoid follicle is called the central artery
brush arterioles (there are precapillary sphincters)
short capillaries
FURTHER BLOOD MAY FLOW IN TWO WAYS
venous sinusoidal capillary
OR
blood flows directly into the pulp, into the perisinusoid space
pulp venule (there are sphincters)
trabecular vein
segmental veins
splenic veins

the structure of the wall of the venous sinusoidal capillary of the spleen:

· Fenestrated endothelium, to which a huge number of macrophages are attached;

fenestrated basement membrane
reticular fibers

SOURCES OF DEVELOPMENT

mesenchyme- stroma (capsule, trabeculae, reticular tissue)

red bone marrow- cells of red and white pulp

90) THE LYMPH NODES

STROMA

dense stroma: capsule and septa are formed by RVST
soft stroma: reticular tissue; in the cortical substance - in lymphoid follicles there is a special type of cells of the reticular stroma - dendritic cells that are involved in the differentiation of B-lymphocytes; v paracortical zone there are special types of cells of the reticular stroma - interdigital cells that are involved in the differentiation of T-lymphocytes

PARENCHYMA formed korkovym, medulla and paracorticasal area

cortical substance: represented by lymphoid follicles; in the follicle are distinguished:

breeding center where antigen-dependent differentiation of B-lymphocytes occurs
mantle layer, marginal layer- in these layers, there is an interaction of T- and B-lymphocytes, which is necessary for their differentiation

in the lymphoid follicles, mainly antigen-dependent differentiation of B-lymphocytes occurs, therefore this part is called the B-zone of the lymph node

paracortical zone: formed by accumulations of lymphoid tissue on interior surfaces follicles; antigen-dependent differentiation of T-lymphocytes occurs here, therefore this area is called the T-zone
medulla: formed from accumulations of lymphoid tissue in the inner parts of the lymph node; they are called cords; the final stages of differentiation of T- and B-lymphocytes can occur in the medulla

LYMPH NODE SINUSES- channels through which lymph flows inside the lymph node

the following sinuses are distinguished: subcapsular, cortical, cerebral, portal

structure of the sinus wall:

fenestrated endothelium to which many macrophages are attached
fenestrated basement membrane (sometimes absent)
reticular fibers, reticular cells(there may be a small number of SMCs in the portal sinus)

SOURCES OF DEVELOPMENT

mesenchyme- stroma (capsule, septa, reticular tissue)

red bone marrow- parenchyma

91) Human Respiratory System- a set of organs that provide the function of external respiration (gas exchange between the inhaled atmospheric air and the blood circulating in the pulmonary circulation).

Gas exchange is carried out in the alveoli of the lungs, and is normally aimed at capturing oxygen from the inhaled air and releasing it into external environment carbon dioxide formed in the body.
Respiratory system

Part one. Overall plan buildings, development; the structure of the airways.

The respiratory system is a collection of organs that provide in the body external respiration, as well as a number of important not respiratory function.
(Internal respiration is a complex of intracellular redox processes).

The respiratory system includes various bodies that perform air-conducting and respiratory (i.e. gas exchange) functions: the nasal cavity, nasopharynx, larynx, trachea, bronchi and lungs. Thus, in the respiratory system, one can distinguish:

extrapulmonary airways;
and the lungs, which in turn include:

- intrapulmonary airways (the so-called bronchial tree);
- actually the respiratory section of the lungs (alveoli).

The main function of the respiratory system is external respiration, i.e. the absorption of oxygen from the inhaled air and the supply of blood with it, as well as the removal of carbon dioxide from the body. This gas exchange is carried out by the lungs.

Among the non-respiratory functions of the respiratory system, the following are very important:

thermoregulation,
deposition of blood in a well-developed vascular system lungs,
participation in the regulation of blood coagulation due to the production of thromboplastin and its antagonist - heparin,
participation in the synthesis of some hormones, as well as inactivation of hormones;
participation in water-salt and lipid metabolism;
participation in voice formation, sense of smell and immune defense.

The lungs are actively involved in the metabolism of serotonin, which is destroyed by monoamine oxidase (MAO). MAO is detected in macrophages, in the mast cells of the lungs.>

In the respiratory system, bradykinin is inactivated, lysozyme, interferon, pyrogen, etc. are synthesized. In case of metabolic disorders and the development of pathological processes, some volatile substances are released (acetone, ammonia, ethanol, etc.).

The protective filtering role of the lungs consists not only in the retention of dust particles and microorganisms in the airways, but also in the capture of cells (tumor, small blood clots) by the vessels of the lungs ("traps").

Development

The respiratory system develops from the endoderm.

The larynx, trachea and lungs develop from one common primordium, which appears at 3-4 weeks by protrusion of the ventral wall of the anterior intestine. The larynx and trachea are laid at the 3rd week from the upper part of the unpaired saccular protrusion of the ventral wall of the anterior intestine. In the lower part, this unpaired primordium is divided along the midline into two sacs, giving the primordia of the right and left lungs. These sacs, in turn, are later subdivided into many interconnected smaller protrusions, between which the mesenchyme grows. At the 8th week, the rudiments of the bronchi appear in the form of short, even tubes, and at the 10-12th week, their walls become folded, lined with cylindrical epithelial cells (a tree-like branched bronchial system - the bronchial tree is formed). At this stage of development, the lungs resemble a gland (glandular stage). At the 5-6th month of embryogenesis, the development of terminal (terminal) and respiratory bronchioles, as well as alveolar passages, surrounded by a network of blood capillaries and growing nerve fibers (tubular stage), occurs.

From the mesenchyme surrounding the growing bronchial tree, smooth muscle, cartilage tissue, fibrous connective tissue of the bronchi, elastic, collagen elements of the alveoli, as well as layers connective tissue sprouting between the lobules of the lung. From the end of the 6th - the beginning of the 7th month and before birth, part of the alveoli and the alveolocytes of the 1st and 2nd types lining them differentiate (alveolar stage).

During the entire embryonic period, the alveoli look like collapsed vesicles with an insignificant lumen. At this time, the visceral and parietal pleura are formed from the visceral and parietal sheets of the splanchnotome. At the first inhalation of a newborn, the alveoli of the lungs expand, as a result of which their cavities sharply increase and the thickness of the alveolar walls decreases. This promotes the exchange of oxygen and carbon dioxide between the blood flowing through the capillaries and the air of the alveoli.

Airways

These include nasal cavity, nasopharynx, larynx, trachea and bronchi. In the airways, as the air moves, it is purified, humidified, warmed, received by gas, temperature and mechanical stimuli, as well as the volume of inhaled air is regulated.

The wall of the airways (in typical cases - in the trachea, bronchi) consists of four membranes:

mucous membrane;
submucosa;
fibrocartilaginous membrane;
adventitious shell.

In this case, the submucosa is often considered as part of the mucous membrane, and they speak of the presence of three membranes in the airway wall (mucosa, fibrocartilaginous and adventitious).

Thymus- the central organ of lymphoid hematopoiesis and the body's immune defense. In the thymus, there is an antigen-independent differentiation of bone marrow precursors of T-lymphocytes into immunocompetent cells - T-lymphocytes. The latter carry out reactions of cellular immunity and are involved in the regulation of humoral immunity, which occurs, however, not in the thymus, but in the peripheral organs of hematopoiesis and immune defense. In addition, more than 20 biologically active substances, including those of distant action, were found in thymus extracts, which allows the thymus to be attributed to the glands of the endocrine system.

Thymus structure... Outside, the thymus gland is covered with a connective tissue capsule. The septa extending from it - septa - divide the thymus into lobules. The basis of the lobule is formed by process epithelial cells - epithelioreticulocytes, in the reticular skeleton of which there are thymic lymphocytes (thymocytes). The source of the development of T-lymphocytes are bone marrow hematopoietic stem cells. Further, the precursors of T-lymphocytes (pretimocytes) enter the thymus with blood and turn into lymphoblasts here.

In the cortex of the thymus some of them under the influence of emitted epithelial cells peptide hormones- thymosin, thymopoietin, etc., as well as macrophages are converted into antigen-reactive T-lymphocytes - they acquire receptors for strictly defined antigens. They leave the thymus without entering the medulla, and populate the thymus-dependent zones. lymph nodes and spleen. Here, in the peripheral organs of immunogenesis, they further mature into T-killers (cytotoxic), T-helpers, after which they are capable of recirculation, cloning (proliferation), and the formation of memory cells.

Other T-lymphoblasts turn into autoimmune competent cells reactive to autoantigens. They are destroyed by apoptosis (approximately 95% of cells) and are phagocytosed by macrophages.

T-lymphocyte specialization process in the cortical substance of the thymus lobules occurs under conditions that protect against excessive action of antigens on them. This is achieved through the formation of a hematothymic histion (barrier), consisting of the endothelium of the hemocapillaries with a distinctly continuous basement membrane, the pericapillary space with the intercellular substance and macrophages, as well as epithelium-reticulocytes with their basement membrane. The cortical substance of the thymus has an independent microvasculature. On the contrary, in the medulla of the thymus, the capillary network does not play a barrier role, and through the endothelium of these capillaries, mature lymphocytes can leave and return to the thymus, i.e., recirculate.

In the thymus these lymphocytes are found in the medulla. Mature lymphocytes leave the thymus through the wall of the postcapillary venule.

Thymus medulla has a lighter color, since there are fewer lymphocytes than in the cortex. The epithelial skeleton stands out more clearly, and the epithelioreticulocytes are larger and more numerous. In the middle part of the medulla, the layered epithelial corpuscles of Gassal are found. With age, their number and size increase. In the thymus stroma, in addition to epithelioreticulocytes, there are macrophages, dendritic cells of bone marrow origin, neuroendocrine cells - derivatives of the neural crest, as well as myoid cells.

Age-related changes and thymus reactivity.

Age-related involution occurs after 20 years thymus... This is accompanied by a decrease in the number of lymphocytes and the development of adipose tissue. In cases where the thymus gland does not undergo reverse development, there is a reduced body resistance to infections and intoxication.

Fabrics thymus are highly reactive structures. Under the action of damaging factors (intoxication, trauma, etc.), the release of T-lymphocytes into the blood and their mass death are observed, mainly in the cortex. The involution of the thymus arising from such stress-influences is called temporary, fast-passing, or accidental. The observed phenomena of lymphocytolysis and phagocytosis by macrophages of dying lymphocytes are considered as possible ways release of growth factors and DNA necessary for tissue repair processes. The selection of T-lymphoblasts is also associated with the death of lymphocytes under these conditions.

Reactive changes in the thymus are in close correlation with functional changes in the adrenal cortex and the level of glucocorticoids in the blood. In the thymus, adrenergic and cholinergic nerve fibers were identified, which enter the organ along with the vessels.

The thymus (thymus gland) develops from the epithelium of the branchial pockets and mesenchyme. It reaches its greatest development by the age of puberty, and later undergoes age-related involution, in which the organ parenchyma is gradually replaced by adipose and connective tissue.

The thymus gland has paired cervical lobes running along the trachea and the thoracic part located in the pericardial mediastinum.

The thymus is built on the principle of a compact parenchymal organ - it contains elements of the stroma and parenchyma. The stroma is represented by a capsule of dense loose connective tissue that covers it from the outside, and an interlayer of loose connective tissue separating the parenchyma into lobules. Blood vessels and nerves pass through the layers.

The thymus parenchyma is formed by epithelial and lymphoid tissues. Epithelial cells have processes that give them a similarity to cells of reticular tissue, and therefore they are called reticuloepithelial cells. These cells provide support, nutrition and protection for developing T-lymphocytes, and also produce a number of hormones that regulate their development and the processes of immunogenesis.

In each lobule, the cortical and medulla are distinguished. The cortical substance is distinguished by a dark purple color and is an accumulation of T-lymphocytes, or thymocytes, differentiating from semi-stem cells. The medulla is less saturated with lymphocytes, it is colored lighter - in pink-violet color. In it, one can distinguish between the epithelial base of the parenchyma and pink thymic bodies (Gassal's little bodies), consisting of a concentric layer of dying reticuloepithelial cells. In the process of antigen-independent differentiation, T-lymphocytes acquire immunoglobulin receptors, which allow them to distinguish their substances and cells from foreign ones.

Primarily differentiated T-lymphocytes through postcapillary venules at the border of the cortex and medulla enter the bloodstream and populate the peripheral organs immune system... There, after contact with antigens, they turn into blast forms, multiply and, differentiating again, form effector subpopulations of lymphocytes, which ensure the formation of cellular and humoral immunity.

Question 20. The structure and function of the lymph nodes.

Lymph nodes are found in mammals and waterfowl. They develop along the lymphatic vessels from the seals of the mesenchyme.

Lymph node functions:

cleansing the lymph flowing through the lymph nodes;

proliferation and antigen-dependent differentiation of T- and B-lymphocytes;

an immune response to antigens involving T and B lymphocytes;

enrichment of lymph with lymphocytes, plasma cells, antibodies that detoxify antigens throughout the body.

Structure:

The lymph nodes are covered with a capsule of fibrous connective tissue, from which trabeculae, which form the stroma of the organ, extend into the parenchyma.

Outside the capsule on the convex side of the node, blood vessels are found in the adipose tissue, and in the capsule itself - bringing lymphatic vessels, on the concave side of the node, in its gate - outgoing lymphatic and feeding blood vessels.

The parenchyma of the organ is formed by the reticular and lymphoid tissues, the cortex and medulla are revealed in it. On the periphery of the cortical substance are lymphoid (cortical) nodules, or follicles. B-lymphocytes multiply and differentiate in them. The central parts of the follicles are characterized by a pink-purple color - light centers, their peripheral zones form a crown of nodules.

The inner, paracortical, cortical zone is formed by diffusely scattered T-lymphocytes.

From follicles, B-lymphocytes, mature and transforming into plasmocytes, move to the medulla, forming dark-violet cerebral cords (pulp cords).

The light areas of the parenchyma with a small number of lymphocytes represent the marginal, intermediate and central sinuses, through which lymph slowly seeps. They are limited to reticuloendothelial cells. Sinus macrophages free the lymph from foreign particles.

Thymus is a lymphoepithelial organ located in the mediastinum, it reaches its maximum development in youth. While other lymphoid organs develop exclusively from the mesenchyme (mesoderm), the thymus is of dual embryonic origin. Its lymphocytes develop in the bone marrow from cells of mesenchymal origin; they invade the epithelial anlage, which has developed from the endoderm of the third and fourth pharyngeal pockets.

Thymus is covered with a connective tissue capsule, which is embedded in the parenchyma and divides it into incomplete lobules, so the cortical and medulla of adjacent lobules are connected to each other. Each lobule contains a dark zone located on its periphery - the cortical substance and a centrally lying light-colored zone - the medulla.

Cortical substance consists of a large population of T-lymphocyte progenitor cells (known as thymocytes), epithelioreticular cells that form a network, and macrophages. Since the cortex contains more small lymphocytes than the medulla, it stains more dark color... Epithelioreticular cells have a stellate shape and light-colored oval nuclei. They are usually associated with similar neighboring cells through desmosomes.

On the epithelial origin of these cells indicate bundles of intermediate keratin filaments (tonofibrils) in their cytoplasm. A subpopulation of epithelioreticular cells located in the cortex is represented by thymus nursing cells, which contain numerous (20-100) maturing lymphocytes in their cytoplasm.

Brain matter contains epithelium reticular cells, numerous differentiated T-lymphocytes and thymic bodies, or Gassal's little bodies - structures with unknown function, characteristic of this part of the organ. These little bodies are composed of concentrically arranged flattened epithelial-reticular cells that are filled with keratin filaments. Sometimes they undergo calcification.

Blood supply to the thymus

Arterioles and capillaries in the thymus are surrounded by processes of epithelioreticular cells. The capillaries of the thymus are formed by non-fenestrated endothelium and contain a very thick basal lamina, which makes these blood vessels especially impermeable to proteins. As a result, most of the antigens circulating in the blood do not enter the cortex of the thymus, since this is impeded by the so-called hematothymic barrier.

Thymus site. The cortex can be recognized by its dark color, the medulla by its light color and the presence of Gassal's bodies, which are found only in the medulla. Color: pararosanilin - toluidine blue.

V thymus there are no supplying lymphatic vessels, and, unlike the lymph nodes, it is not a filter for lymph. The few lymphatic vessels that are found in the thymus are all efferent; they are located in the walls blood vessels and in the connective tissue forming the septa (septa) and capsule.

Role of the thymus in T cell differentiation

V thymus terminal differentiation and selection of T-lymphocytes occur. Thymus mass relative to body weight is maximal immediately after birth; he reaches his largest sizes at puberty, after which it undergoes involution, nevertheless, it continues to produce lymphocytes until old age.

Committed T cell precursors, giving rise to T-lymphocytes, do not contain T-cell receptors on their surface and have a CD4 and CD8 phenotype. They first appear in the liver of the embryo at the early stages of fetal development, and then migrate from the bone marrow to the thymus in both the fetus and the adult. Having penetrated the thymus, the precursors of T cells colonize the cortex, where they divide by mitosis.

V cortical substance, they recognize autoantigens associated with MHC molecules of classes I and II, which are present on the surface of epithelial cells, macrophages and dendritic cells. Maturation and selection of T-lymphocytes in the thymus are very complex processes which include positive and negative selection of T cells. Some of these processes are believed to take place inside the nanny cells. In short, thymocytes, whose T-cell receptors are unable to bind or, conversely, bind autoantigens too strongly (about 95% of them the total), undergo induced death by the apoptosis mechanism and are removed by macrophages. The remaining T cells survive and migrate to the medulla.

Migration depends on the influence of chemokines and on the interaction of thymocytes with the intercellular substance of the thymus. Mature CD4 or CD8 T cells, which contain T cell receptors on their surface, leave the thymus, enter the bloodstream, pass through the veins of the medulla, and are distributed throughout the body.

Secretion processes in the thymus

Thymus produces several proteins that act as growth factors that stimulate the proliferation and differentiation of T-lymphocytes. Apparently, they are paracrine factors affecting the thymus. At least four hormones have been identified: thymosin-a, thymopoietin, thymulin, and thymic humoral factor.

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Thymus

Thymus, she is the thymus, is an important organ responsible for the quality of the immune system of humans or animals. It is laid in the body of the embryo at 7 weeks, and is the first organ of the endocrine and lymphoid system.

Iron got its name for appearance resembling a fork with two prongs. Consists of two parts, divided into shares. Parts of the gland can be fused, but they can simply be tightly pressed. They are not always symmetrical; one part of the gland may be larger. The gland is covered with connective tissue.

Everything you need to know about the thymus

It is located in the chest, in its upper part, and is divided into the cortex (outer layer) and the medulla. The cortical layer consists of epithelial and hematopoietic cells. In the epithelial cells, a number of hormones, supporting cells, and cells are produced, due to which the maturation of lymphocytes occurs. Hematopoietic cells are also responsible for the growth of T-lymphocytes and macrophages.

Both parts of the gland contain a large number of T - lymphocytes. The cells of this group are responsible for recognizing foreign organisms and eliminating them. Immature bone marrow cells also enter the thymus, which precede the formation of T-lymphocytes. During maturation, some of the T-lymphocytes are able to overcome not only viral cells, but also healthy ones. To prevent this from happening, in the medulla of the thymus, this part of the lymphocytes dies. The rest of the T-lymphocytes capable of recognizing the virus are sent through the bloodstream to the site of inflammation.

The gland has a bright pink color in a newborn, but after puberty it turns yellow. The uniqueness of this gland lies in the fact that it normally weighs 15 g in an infant, then active growth begins during childhood and adolescence. After 18 years, the gland gradually decreases in size, and by old age it completely disappears, leaving behind only connective tissue.

The functions of the gland are to learn, form and move immune T cells... During the first year of a child's life, the thymus gland takes over all the body's defense functions. Gradually, with the development and growth of other organs, some of the tasks of the thymus gland are distributed to them.

The thymus gland produces a number of hormones necessary for normal work organism. These include thymalin, thymosin, IGF-1, thymopoietin. Thymosin is responsible for skeletal growth, for maintaining high level immunity, participates in the work of the hypothalamus and pituitary gland.

Until now, there are disputes about which systems include the thymus gland, and what exactly is its main task. For last years it belongs to the endocrine or lymphoid system. To track the function of the thymus gland, experiments were carried out to remove it from animals. The result was always the same - the animals were susceptible to infections, developmental delay was noted bone tissue, deformation of the skeleton.

Disorders of the thymus gland in early age lead to a loss of resistance to bacteria and viruses. Such a child is constantly ill, susceptible to viral infections. Protective functions the body decreases with an increase in the thymus gland. You can make such a diagnosis by making an X-ray of the chest area. The enlarged gland looks like a dark spot against the background of the lungs. In case of serious lesions of the gland, it is removed. But more often doctors advise to strengthen the immune system with a medication.

thymus

Functions

It produces hormones: thymosin, thymalin, thymopoietin, insulin-like growth factor-1 (IGF-1), thymic humoral factor - all of them are proteins (polypeptides). With hypofunction of the thymus, immunity decreases, since the number of T-lymphocytes in the blood decreases.

Development

The size of the thymus is maximum in childhood, but after the onset of puberty, the thymus undergoes significant atrophy and involution. An additional decrease in the size of the thymus occurs with aging of the body, which is partly associated with a decrease in immunity in the elderly.

Regulation

Diseases of the thymus

MEDAC syndrome
Di Giorgi syndrome
Myasthenia gravis - maybe independent disease but is often associated with thymoma

Tumors

Thymoma - from epithelial cells of the thymus
T cell lymphoma - from lymphocytes and their precursors
Pre-T-lymphoblastic tumors in some cases have primary localization in the thymus and are detected as massive infiltration in the mediastinum, followed by rapid transformation into leukemia.
neuroendocrine tumors
more rare tumors (of vascular and nervous origin)

Thymus tumors can be a manifestation of type I multiple endocrine neoplasia syndrome.

Human thymus

Anatomy

Appearance

The thymus gland is a small organ of pinkish-gray color, soft consistency, its surface is lobular. In newborns, its dimensions are on average 5 cm in length, 4 cm in width and 6 cm in thickness, and its weight is about 15 grams. The growth of the organ continues until the onset of puberty (at this time, its dimensions are maximum - up to 7.5-16 cm in length, and the mass reaches 20-37 grams). With age, the thymus undergoes atrophy and in old age hardly distinguishable from the surrounding adipose tissue of the mediastinum; at 75 years old, the average weight of the thymus is only 6 grams. As it involutions, it loses White color and due to an increase in the proportion of stroma and fat cells in it, it becomes more yellow.

Topography

The thymus is located at the top chest, just behind the sternum (upper mediastinum). In front of it, the handle and the body of the sternum are adjacent to the level IV of the costal cartilage; behind - top part pericardium, covering the initial sections of the aorta and pulmonary trunk, aortic arch, left brachiocephalic vein; from the sides - the mediastinal pleura.

Structure

In humans, the thymus consists of two lobes that can be spliced or simply snug against each other. The lower part of each lobe is wide and the upper part is narrow; thus, the top pole may resemble a two-pronged fork (hence the name).

The organ is covered with a capsule of dense connective tissue, from which the bridges extend in depth, dividing it into lobules.

In animals (the thymus gland) is developed in fetuses and young animals. It consists of unpaired thoracic lying in front of the heart, and paired cervical passing in the form of outgrowths on the sides of the trachea. With age, the iron begins to dissolve, and then disappears.

Thymus of the newborn: topography. Illustration from Gray's Anatomy

Blood supply, lymph drainage and innervation

The blood supply to the thymus comes from the thymic, or thymic branches of the internal thoracic artery, ( rami thymici arteriae thoracicae internae), thymic branches of the aortic arch and brachiocephalic trunk and branches of the superior and inferior thyroid arteries. Venous outflow is carried out along the branches of the internal thoracic and brachiocephalic veins.

Lymph from the organ flows into the tracheobronchial and parasternal lymph nodes.

The thymus gland is innervated by the branches of the right and left vagus nerves, as well as sympathetic nerves originating from the superior thoracic and stellate nodes of the sympathetic trunk, which are part of nerve plexuses that surround the vessels feeding the organ.

Histology

Microscopic structure of the thymus gland

The thymic stroma is of epithelial origin, originating from the epithelium of the anterior part of the primary intestine. Two strands (diverticula) originate from the third branchial arch and grow into the anterior mediastinum. Sometimes the thymus stroma is also formed by additional strands from the fourth pair branchial arches... Lymphocytes originate from blood stem cells migrating to the thymus from the liver on early stages intrauterine development... Initially, various blood cells proliferate in the thymus tissue, but soon its function is reduced to the formation of T-lymphocytes. The thymus gland has a lobular structure; in the tissue of the lobules, cortical and medulla are distinguished. The cortical substance is located on the periphery of the lobule and looks dark in the histological micropreparation (it contains many lymphocytes - cells with large nuclei). In the cortex are arterioles and blood capillaries that have a hemato-thymic barrier that prevents the drift of antigens from the blood.

The cortical substance contains cells:

epithelial origin:
- supporting cells: form a "frame" of tissue, form a blood-thymic barrier;
- stellate cells: secrete soluble thymic (or thymus) hormones - thymopoietin, thymosin and others, which regulate the growth, maturation and differentiation of T cells and functional activity mature cells the immune system.
- nurse cells: have invaginations in which lymphocytes develop;
hematopoietic cells:
- lymphoid series: maturing T-lymphocytes;
- macrophage series: typical macrophages, dendritic and interdigitating cells.

Directly under the capsule in cellular composition dividing T-lymphoblasts predominate. Deeper are the maturing T-lymphocytes, gradually migrating to the medulla. The ripening process takes about 20 days. During their maturation, gene rearrangement and the formation of a gene encoding TCR (T-cell receptor) occur.

Then they undergo positive selection: in interaction with epithelial cells, "functionally suitable" lymphocytes are selected that are able to interact with HLA; during development, the lymphocyte differentiates into a helper or killer, that is, either CD4 or CD8 remains on its surface. Further, in contact with the epithelial cells of the stroma, cells are selected that are capable of functional interaction: CD8 + lymphocytes capable of receiving HLA I, and CD4 + lymphocytes capable of receiving HLA II.

The next stage - negative selection of lymphocytes - occurs at the border with the medulla. Dendritic and interdigitating cells - cells of monocytic origin - select lymphocytes capable of interacting with antigens of their own body and trigger their apoptosis.

The medulla mainly contains ripening T-lymphocytes. From here they migrate into the bloodstream of venules with high endothelium and spread throughout the body. The presence of mature recirculating T-lymphocytes here is also assumed.

The cellular composition of the medulla is represented by supporting epithelial cells, stellate cells, macrophages. There are also efferent lymphatic vessels and Gassal's little bodies.

RESTORATION OF THYMUS

Scientists from the University of Connecticut Health Center (USA) have developed a method for in vitro directed differentiation of mouse embryonic stem cells (ESCs) into thymic epithelial progenitor cells (PET), which differentiated in vivo into thymic cells and restored its normal structure.