Children's organ that performs the immune function and hematopoietic - thymus. Why is it called childish? What happens to him in old age? And what is its clinical significance? You will find answers to these and many other questions in this article.
The role of the thymus in the human body
Thymus performs a hematopoietic function. What does it mean? He deals with the differentiation and training (immunological) of T-lymphocytes. It is also important that the "memory" of lymphocytes is very long, and therefore a child who has been ill with the same chickenpox will not get sick again in 99% of cases. This is called permanent immunity. In addition to the proliferation and differentiation of T-lymphocytes, the thymus is involved in the cloning of immune cells. By the way, I would like to note that the decrease in immunity to the thymus is directly related. A decrease in T-lymphocytes entails a whole cascade of reactions that lower immunity. And this explains a lot in pediatrics, when, for example, against the background of some banal disease, a secondary infection or secondary disease occurs.
Besides this thymusproduces a variety of hormones. These include: thymus humoral factor, thymalin, thymosin, and thymopoietin. These hormones also perform an immune function.
Thymus: histology, structure, functions
Thymus is a typical parenchymal organ (stroma and parenchyma are isolated in it). If you look at the appearance of the histological structure of the thymus, it can be noted that the organ is lobulated.
Each lobule has a dark and a light zone. In scientific terms, this is the cortex and medulla. As already mentioned, the thymus performs an immune function. Therefore, it can rightly be called a stronghold of the children's immune system. So that this stronghold does not fall from the first foreign protein-antigen that comes across, you need to create some kind of protective function for it. And nature created this protective function, calling it the blood-thymus barrier.
Summary of the histology of the thymus barrier
This barrier is represented by a network of sinusoidal capillaries and subcapsular epithelium. This barrier includes capillary epithelial cells. That is, the antigens that are produced by pathogenic organisms immediately enter the bloodstream, from there they spread throughout the human body. The thymus is no exception, where these antigens can end up. How will they get there? They can get there through the microvasculature, that is, through the capillaries. The photo below shows the histology of the preparation from the thymus, the vessels in the stroma are clearly visible.
Inside the capillary is lined with endothelial cells. They are covered by the basement membrane of the capillary. Between this basement membrane and the outer one is the perivascular space. Macrophages are present in this space, which are able to phagocytize (absorb) pathogenic microorganisms, antigens, and so on. Behind the outer membrane are hundreds of lymphocytes and reticuloepithelial cells that protect the thymus microvasculature from antigens and pathogens.
Thymus cortex
The cortical substance consists of a number of structures, for example, these are cells of the lymphoid series, macrophage, epithelial, supporting, "Nanny", stellate. Now let's take a closer look at these cells.
- Stellate cells - secrete thymic peptide hormones - thymosin or thymopoietin, regulate the process of growth, maturation and differentiation of T-cells.
- Lymphoid cells - these include those T-lymphocytes that have not yet matured.
- Support cells - necessary to create a kind of frame. Most supporting cells are involved in maintaining the blood-thymus barrier.
- Nanka's cells - have depressions (invaginations) in their structure, in which T-lymphocytes develop.
- Epithelial cells are the bulk of the cells of the thymus cortex.
- Cells of the macrophage series are typical macrophages that have the function of phagocytosis. They are also participants in the blood-thymus barrier.
Development of T-lymphocytes on a histological preparation
Iflook at the preparation from the periphery, then here you can find T-lymphoblasts that are dividing. They are located directly under the thymus capsule itself. If you go from the capsule in the direction of the medulla, you can see already maturing, as well as fully mature T-lymphocytes. The entire development cycle of T-lymphocytes takes approximately 20 days. As they develop, they develop a T-cell receptor.
After the lymphocytes have matured, they interact with the epithelial cells. Here there is a selection according to the principle: suitable or unsuitable. Further differentiation of lymphocytes occurs. Some will become T-helpers, while others will become T-killers.
What is it for? Each T-lymphocyte interacts with different antigens.
Approaching the medulla, already mature T-lymphocytes that have undergone differentiation are checked according to the principle of danger. What does it mean? Can this lymphocyte harm the human body? If this lymphocyte is dangerous, then apoptosis occurs with it. That is, the destruction of the lymphocyte. In the medulla there are already matured or maturing T-lymphocytes. These T cells then enter the bloodstream, where they disperse throughout the body.
The medulla of the thymus gland is represented by protective cells, macrophage and epithelial structures. In addition, there are lymphatic vessels, blood vessels and Hassall's corpuscles.
Development
The histology of thymus development is very interesting. Both diverticula originate from the 3rd gill arch. And both of these strands grow into the mediastinum, most often the anterior. Very rarelythe thymus stroma is formed by additional strands of 4 pairs of gill arches. From blood stem cells, lymphocytes are formed, which will later migrate from the liver to the bloodstream, and then to the fetal thymus. This process occurs early in fetal development.
Analysis of a histological specimen
A brief histology of the thymus is as follows: since it is a classic parenchymal organ, the laboratory assistant first examines the stroma (skeleton of the organ), and then the parenchyma. Inspection of the preparation is first done at a high magnification in order to examine and orient in the organ. Then they switch to a large increase to examine the tissues. The preparation is most often stained with hematoxylin-eosin.
Thymus stroma
Outside the organ is a connective tissue capsule. It covers the body from all sides, giving shape. Connective tissue partitions pass inside the organ from the connective tissue capsule, they are also called septa, which divide the organ into lobules. It is worth noting that both the connective tissue capsule and the connective tissue septa consist of dense, formed connective tissue.
The inflow or outflow of blood to the body is carried out through the vessels. These vessels also pass through the elements of the stroma. Distinguishing an artery from a vein is very easy. Firstly, the easiest way is to do it according to the thickness of the muscle layer. An artery has a much thicker layer of muscle tissue than a vein. Secondly, the choroid of a vein is much thinner than that of an artery. Below in the photo, the histology of the thymus can be seen on the preparation.
To see the elements of the stroma inside the lobule, you need to switch to a large magnification. So the laboratory assistant can see reticular epitheliocytes. By their nature, these cells are epithelial, have processes that communicate with each other. Thus, the cells hold the thymus frame from the inside, as they are tightly connected to the elements of the parenchyma.
The laboratory assistant most often does not see the cells of the reticuloepithelial tissue themselves, since they are hidden by numerous layers of parenchyma. Thymocytes are so tightly adjacent to each other that they overlap the cells of the stroma. But in a single order, one can still see oxyphilic-stained cells between thymocytes in the light gaps. These cells have large nuclei that are arranged in a chaotic manner.
Thymus parenchyma
Thymus parenchyma should be considered in a single slice. Therefore, the laboratory assistant, after examining the stroma, returns to a small increase. When the laboratory assistant returned to his original position, he sees a sharp contrast. This contrast indicates that each lobule is composed of a cortex and a medulla.
Cortex
It is worth noting that the thymus parenchyma is represented by lymphocytes. In the cortex, which stains purple on the preparation (basophilic stain), lymphocytes are closely spaced relative to each other. In addition to elements of the stroma and lymphocytes, the laboratory assistant will not see anything else in the cortical substance.
Marrow
Oxyphilic coloration prevails in the medulla, andnot basophilic as in cortical. This is explained by the fact that the number of lymphocytes decreases sharply, and they are less often located relative to each other. Among the lymphocytes in the medulla, thymic bodies can be seen. These structures are often referred to in textbooks as Hassall bodies.
Hassal's corpuscles on the preparation are formed by twisted structures. In fact, these are ordinary dead, keratinizing fragments of the stroma - the same epithelioreticulocytes. Gassall's corpuscles are oxyphilic-stained elements of the thymus medulla.
Very often, students differentiate the thymus preparation in histology by Hassal's bodies. They are a characteristic feature of the drug, always located exclusively in the medulla. The photo below shows these thymus bodies.
If there are no swirling red structures in the bodies, Hassall's bodies look just like white spots. Sometimes they are compared to voids (artifacts) of the drug, which are often formed during its preparation. In addition to their resemblance to artifacts, thymic bodies are similar to vessels. In this case, the laboratory assistant looks at the presence of the muscle layer and the presence of red blood cells (if the latter are absent, then this is the thymus body).
Thymus involution
As mentioned at the beginning of the article, the thymus is a child's gland. Of course, this is not entirely true, but the presence of an organ does not always mean that it is functioning.
When a child reaches the age of one, then at this moment there comes a peak in the production of lymphocytes, respectively, and the work of the gland. After gradually thymusreplaced by adipose tissue. By the age of twenty, half of the thymus consists of adipose and lymphoid tissue. And by the age of fifty, almost the entire organ is represented by adipose tissue. This involution is due to the fact that T-lymphocytes have a lifelong memory that accompanies the human body throughout its life. As there are enough T-lymphocytes in the blood, the thymus simply remains the organ that "maintains" the constancy of T-lymphocytes in the blood.
Thymus histology involution can occur much faster due to precipitating factors. These factors can be acute infectious diseases, chronic diseases, radiation, etc. Due to these factors, the level of cortisone and hormones of a steroid nature significantly increases in the blood, they destroy immature T-lymphocytes, thereby destroying the thymocytes themselves, replacing them with adipose tissue.
