Histology. Organs of hematopoiesis and immunological defense

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Histology. Organs of hematopoiesis and immunological defense

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Topic 30

The organs of hematopoiesis and immunological protection include red bone marrow, thymus gland (thymus), lymph nodes, spleen, as well as lymphatic follicles of the digestive tract (tonsils, lymphatic follicles of the intestine) and other organs. They form a single system with blood.

They are divided into central and peripheral organs of hematopoiesis and immunological protection.

The central organs include the red bone marrow, the thymus gland, and an analogue of the Bag of Fabricius, which is still unknown in mammals. In the red bone marrow, stem cells produce erythrocytes, granulocytes, platelets (platelets), B-lymphocytes, and precursors of T-lymphocytes. In the thymus, T-lymphocyte precursors are converted into T-lymphocytes. In the central organs, antigen-independent reproduction of lymphocytes occurs.

In the peripheral hematopoietic organs (lymph nodes, hemolymph nodes, spleen), T- and B-lymphocytes brought here from the central organs multiply and differentiate under the influence of antigens into effector cells that provide immunological protection. In addition, there is a culling of dying blood cells.

The hematopoietic organs function in a friendly manner and ensure the maintenance of the morphological composition of the blood and immunological homeostasis in the body.

Despite differences in the specialization of hematopoietic organs, they all have similar structural and functional features. They are based on reticular connective, and sometimes epithelial tissue (in the thymus gland), which, together with fibroblasts and macrophages, forms the stroma of organs and plays the role of a specific microenvironment for developing cells. In these organs, the reproduction of hematopoietic cells, the temporary deposition of blood or lymph occurs. Hematopoietic organs, due to the presence in them of special phagocytic and immunocompetent cells, also carry out a protective function and are able to purify the blood or lymph from foreign particles, bacteria and the remains of dead cells.

Bone marrow

The bone marrow is the central hematopoietic organ, where a self-sustaining population of stem cells is located, where both myeloid and lymphoid cells are formed.

Structure. In the adult human body, red and yellow bone marrow are distinguished.

Red bone marrow is the hematopoietic part of the bone marrow. It fills the spongy substance of flat bones and epiphyses of tubular bones, and in an adult organism it averages about 4-5% of the total body weight. Red bone marrow is dark red in color and has a semi-liquid consistency, making it easy to prepare thin smears on glass.

The reticular tissue of the structural basis of the bone marrow has a low proliferative activity. The stroma is penetrated by many blood vessels of the microvasculature, between which are located hematopoietic cells: stem cells, semi-stem cells (morphologically unidentifiable), various stages of maturation of erythroblasts and myelocytes, megakaryoblasts, megakaryocytes, lymphoblasts, B-lymphocytes, macrophages and mature blood cells. Lymphocytes and macrophages take part in the protective reactions of the body. The most intense hematopoiesis occurs near the endosteum, where the concentration of stem hematopoietic cells is approximately 3 times greater than in the center of the bone marrow cavity.

Hematopoietic cells are arranged in islets. In the process of maturation, erythroblasts surround a macrophage containing iron of phagocytosed erythrocytes, and receive from it a molecule of this metal to build the heme part of hemoglobin. Macrophages serve as a kind of feeder for erythroblasts, which are gradually enriched with iron at their expense. Macrophages phagocytize cell debris and defective cells. Immature erythroid cells are surrounded by glycoproteins. As cells mature, the amount of these biopolymers decreases.

Granulocytopoietic cells are also located in the form of islands, but are not associated with macrophages. Immature cells of the granulocytic series are surrounded by protein glycans. In the process of maturation, granulocytes are deposited in the red bone marrow, where they are approximately 3 times more than erythrocytes, and 20 times more than granulocytes in peripheral blood.

Megakaryoblasts and megakaryocytes are located in close contact with the sinuses so that the peripheral part of their cytoplasm penetrates into the lumen of the vessel through the pores. The separation of fragments of the cytoplasm in the form of platelets occurs directly into the bloodstream.

Among the islets of myeloid cells there are small clusters of bone marrow lymphocytes (null lymphocytes, B-lymphocytes) and monocytes, which usually surround the blood vessel in dense rings. Experiments with the transplantation of bone marrow lymphocytes into the spleen of animals irradiated with a lethal dose showed the presence among them of stem, semi-stem and unipotent hematopoietic cells.

During the differentiation of B-lymphocytes, the structural and regulatory genes of immunoglobulins are depressed, the synthesis of immunoglobulins inside the cell and their appearance on the membrane of B-lymphocytes in the form of antigen-recognizing receptors.

Under normal physiological conditions, only mature blood cells penetrate through the wall of the sinuses of the bone marrow. Myelocytes and normoblasts enter the blood only in pathological conditions of the body. The reasons for such selective permeability of the sinus wall remain insufficiently clear, but the fact of the penetration of immature cells into the bloodstream is always a sure sign of a disorder in bone marrow hematopoiesis.

The cells released into the bloodstream perform their functions either in the vessels of the microvasculature (erythrocytes, platelets), or when they enter the connective tissue (lymphocytes, leukocytes) and peripheral lymphoid organs (lymphocytes). In particular, lymphocyte precursors (null lymphocytes) and mature B-lymphocytes migrate to the thymus-independent zones of the spleen, where they are cloned into immunological memory cells and cells that directly differentiate into antibody-producing cells (plasma cells) already during the primary immune response.

Yellow bone marrow in adults is located in the diaphysis of tubular bones. It is a regenerated reticular tissue, the cells of which contain fatty inclusions. Due to the presence of pigments such as lipochromes in fat cells, the bone marrow in the diaphysis has a yellow color, which determines its name. Under normal conditions, the yellow bone marrow does not carry out a hematopoietic function, but in the case of large blood loss or in case of toxic poisoning of the body, foci of myelopoiesis appear in it due to the differentiation of stem and semi-stem cells brought here with blood.

There is no sharp boundary between yellow and red bone marrow. A small number of fat cells are constantly found in the red bone marrow. The ratio of yellow and red bone marrow may vary depending on age, nutritional conditions, nervous, endocrine and other factors.

Vascularization. The bone marrow is supplied with blood through vessels that penetrate through the periosteum into special openings in the compact substance of the bone. Entering the bone marrow, the arteries branch into ascending and descending branches, from which arterioles depart radially, which first pass into narrow capillaries (2–4 microns), and then in the endosteal region continue into wide thin-walled sinusoidal capillaries (or sinuses) with slit-like pores. with a diameter of 10 - 14 microns. Blood is collected from the sinuses into the central venule.

Thymus (or thymus) gland (thymus)

The thymus gland is the central organ of lymphocytopoiesis and immunogenesis. From the bone marrow precursors of T-lymphocytes, antigen-independent differentiation occurs in it into T-lymphocytes, varieties of which carry out cellular immunity reactions and regulate humoral immunity reactions.

The thymus gland is an unpaired organ, not completely divided into lobules, which is based on a process epithelial tissue that has invaginated during development so that the basal layer of the epithelium with the basement membrane faces outward and borders on the surrounding connective tissue, which forms a connective tissue capsule. Partitions extend from it inside, dividing the gland into lobules. In each lobule, a cortex and a medulla are distinguished.

The cortical substance of the lobules is infiltrated with T-lymphocytes, which densely fill the gaps of the reticular epithelial backbone, giving this part of the lobule a characteristic appearance and a dark color on the preparations. In the subcapsular zone of the cortical substance there are large lymphoid cells - lymphoblasts, which, under the influence of hematopoietic factors (thymosin), secreted by stromal epithelial cells, proliferate. These T cell precursors migrate here from the red bone marrow. New generations of lymphocytes appear in the thymus gland every 6-9 hours. T-lymphocytes of the cortical substance migrate into the bloodstream without entering the medulla. These lymphocytes differ in the composition of markers and receptors from the T-lymphocytes of the medulla. With the blood flow, they enter the peripheral organs of lymphocytopoiesis - the lymph nodes and the spleen.

The cells of the cortical substance are in a certain way delimited from the blood by a hematotissue barrier that protects the differentiating lymphocytes of the cortical substance from an excess of antigens. It consists of endothelial cells of hemocapillaries with a basement membrane, a pericapillary space with single lymphocytes, macrophages and intercellular substance, as well as epithelial cells with their basement membrane.

The medulla of the lobule on the preparations has a lighter color, since it contains a smaller number of lymphocytes compared to the cortical substance. The lymphocytes of this zone represent the recirculating pool of T-lymphocytes and can enter and exit the bloodstream through postcapillary venules and lymphatic vessels. A feature of the ultramicroscopic structure of process epithelial cells is the presence in the cytoplasm of grape-shaped vacuoles and intracellular tubules, the surface of which forms microoutgrowths. The basement membrane is reduced.

Vascularization. Inside the organ, the arteries branch into interlobular and intralobular, which form arcuate branches. From them, almost at a right angle, blood capillaries depart, forming a dense network, especially in the cortical zone. The capillaries of the cortical substance are surrounded by a continuous basement membrane and a layer of epithelial cells that delimit the pericapillary space (barrier). In the pericapillary space filled with liquid contents, lymphocytes and macrophages are found. Most of the cortical capillaries pass directly into the subcapsular venules.

Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu.

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