Histology. Hematopoiesis

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Histology. Hematopoiesis

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Topic 11. Bleeding

Hematopoiesis (hemocytopoiesis) is the process of formation of blood cells.

There are two types of hematopoiesis:

1) myeloid;

2) lymphoid.

In turn, myeloid hematopoiesis is divided into:

1) erythrocytopoiesis;

2) granulocytopoiesis;

3) thrombopoiesis;

4) monocytopoiesis.

Lymphoid hematopoiesis is divided into:

1) T-lymphocytopoiesis;

2) B-lymphocytopoiesis.

In addition, hematopoiesis is divided into two periods:

1) embryonic;

2) postembryonic.

The embryonic period leads to the formation of blood as a tissue and therefore represents the histogenesis of blood. Postembryonic hematopoiesis is the process of physiological regeneration of blood as a tissue.

Embryonic period of hematopoiesis

It is carried out in embryogenesis in stages, replacing different organs of hematopoiesis. Accordingly, there are three stages:

1) yolk;

2) hepatothymusolienal;

3) medullothymus-lymphoid.

1. The yolk stage is carried out in the mesenchyme of the yolk sac starting from the 2nd - 3rd week of embryogenesis, from the 4th it decreases and completely stops by the end of the 3rd month.

First, in the yolk sac, as a result of the proliferation of mesenchymal cells, the so-called blood islands are formed, which are focal accumulations of process cells.

The most important moments of the yolk stage are:

1) the formation of blood stem cells;

2) the formation of primary blood vessels.

Somewhat later (on the 3rd week), vessels begin to form in the mesenchyme of the body of the embryo, but they are empty slit-like formations. Pretty soon, the vessels of the yolk sac are connected to the vessels of the body of the embryo, and the yolk circle of blood circulation is established. From the yolk sac through these vessels, stem cells migrate into the body of the embryo and populate the anlage of future hematopoietic organs (primarily the liver), in which hematopoiesis is then carried out.

2. Hepatotimusolienal stage) of hematopoiesis is carried out first in the liver, a little later in the thymus (thymus gland), and then in the spleen. In the liver, mainly myeloid hematopoiesis occurs (only extravascularly) from the 5th week until the end of the 5th month, and then gradually decreases and completely stops by the end of embryogenesis. The thymus is laid down on the 7th - 8th week, and a little later T-lymphocytopoiesis begins in it, which continues until the end of embryogenesis, and then in the postnatal period until its involution (at 25 - 30 years). The spleen is laid on the 4th week, from the 7th - 8th week it is populated with stem cells, and universal hematopoiesis begins in it, i.e. both myelo- and lymphopoiesis. Hematopoiesis is especially active in the spleen from the 5th to the 7th months, and then myeloid hematopoiesis is gradually suppressed, and by the end of embryogenesis (in humans) it completely stops.

3. Medullothymus-lymphoid stage of hematopoiesis. The laying of the red bone marrow begins from the 2nd month, hematopoiesis begins in it from the 4th month, and from the 6th month it is the main organ of myeloid and partially lymphoid hematopoiesis, that is, it is a universal hematopoietic organ. At the same time, lymphoid hematopoiesis is carried out in the thymus, spleen and lymph nodes.

As a result of the successive change of hematopoietic organs and the improvement of the process of hematopoiesis, blood is formed as a tissue, which in newborns has significant differences from the blood of adults.

Post-embryonic period of hematopoiesis

It is carried out in the red bone marrow and lymphoid organs (thymus, spleen, lymph nodes, tonsils, lymphoid follicles).

The essence of the process of hematopoiesis lies in the proliferation and gradual differentiation of stem cells into mature blood cells.

In the scheme of hematopoiesis, two series of hematopoiesis are presented:

1) myeloid;

2) lymphoid.

Each type of hematopoiesis is subdivided into varieties (or series) of hematopoiesis.

Myelopoiesis:

1) erythrocytopoiesis (or erythrocyte series);

2) granulocytopoiesis (or granulocyte series);

3) monocytopoiesis (or monocytic series);

4) thrombocytopenia (or platelet series).

Lymphopoiesis:

1) T-lymphocytopoiesis (or T-lymphocytic series;

2) B-lymphocytopoiesis;

3) plasmacytopoiesis.

In the process of gradual differentiation of stem cells into mature blood cells, intermediate cell types are formed in each row of hematopoiesis, which form classes of cells in the hematopoiesis scheme.

In total, six classes of cells are distinguished in the hematopoietic scheme.

I class - stem cells. By morphology, the cells of this class correspond to a small lymphocyte. These cells are pluripotent, that is, they are able to differentiate into any blood cell. The direction of differentiation depends on the content of formed elements in the blood, as well as on the influence of the microenvironment of stem cells - inductive influences of stromal cells of the bone marrow or other hematopoietic organ. Maintaining the stem cell population is carried out as follows. After mitosis of a stem cell, two are formed: one enters the path of differentiation to a blood cell, and the other takes on the morphology of a small lymphocyte, remains in the bone marrow, and is a stem cell. The division of stem cells occurs very rarely, their interphase is 1–2 years, while 80% of stem cells are at rest and only 20% are in mitosis and subsequent differentiation. Stem cells are also called collin-forming units because each stem cell produces a group (or clone) of cells.

Class II - semi-stem cells. These cells are limitedly pluripotent. There are two groups of cells - the precursors of myelopoiesis and lymphopoiesis. Morphologically similar to a small lymphocyte. Each of these cells gives rise to a clone of the myeloid or lymphoid series. The division occurs every 3-4 weeks. The maintenance of the population is carried out similarly to pluripotent cells: after mitosis, one cell enters further differentiation, and the second remains semi-stem.

Class III - unipotent cells. This class of cells is poetinsensitive - the precursors of their hematopoietic series. In morphology, they also correspond to a small lymphocyte and are capable of differentiation into only one blood cell. The frequency of division of these cells depends on the content of poetin in the blood - a biologically active substance specific for each series of hematopoiesis - erythropoietin, thrombopoietin. After mitosis of cells of this class, one cell enters into further differentiation to a uniform element, and the second maintains a population of cells.

Cells of the first three classes are combined into a class of morphologically unidentifiable cells, since all of them resemble a small lymphocyte in morphology, but their developmental abilities are different.

Class IV - blast cells. Cells of this class differ in morphology from all others. They are large, have a large loose nucleus (euchromatin) with 2-4 nucleoli, the cytoplasm is basophilic due to the large number of free ribosomes. These cells often divide, and all daughter cells enter into further differentiation. Blasts of various hematopoietic lines can be identified by their cytochemical properties.

Class V - maturing cells. This class is characteristic of its hematopoietic series. In this class, there may be several varieties of transitional cells from one (prolymphocyte, promonocyte) to five in the erythrocyte series. Some maturing cells can enter the peripheral circulation in small numbers, such as reticulocytes or stab leukocytes.

VI class - mature shaped elements. These classes include erythrocytes, platelets and segmented granulocytes. Monocytes are not terminally differentiated cells. They then leave the bloodstream and differentiate into the final class, the macrophages. Lymphocytes differentiate into a final class upon encountering antigens, whereby they turn into blasts and divide again.

The set of cells that make up the line of stem cell differentiation into a certain uniform element forms a differon (or histogenetic series). For example, erythrocyte differon is:

1) stem cell (class I);

2) semi-stem cell - the precursor of myelopoiesis (class II);

3) unipotent erythropoietin-sensitive cell (class III);

4) erythroblast (class IV);

5) maturing cell - pronormocyte, basophilic normocyte, polychromatophilic normocyte, oxyphilic normocyte, reticulocyte (class V);

6) erythrocyte (class VI).

In the process of maturation of erythrocytes in class V, the synthesis and accumulation of hemoglobin, the reduction of organelles and the cell nucleus occur. Normally, the replenishment of erythrocytes is carried out due to the division and differentiation of maturing cells - pronormocytes, basophilic and polychromatophilic normocytes. This type of hematopoiesis is called homoplastic. With severe blood loss, the replenishment of erythrocytes is carried out not only by the strengthening of maturing cells, but also by cells of IV, III, II and even class I - a heteroplastic type of hematopoiesis occurs.

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

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