Postnatal period of hematopoiesis. Features of the hematopoietic organs in children. The formation of hematopoiesis in the antenatal and postnatal periods. Features of the hemogram and coagulogram of a newborn child

During the intrauterine life of the fetus, 3 periods of hematopoiesis are distinguished. However, its various stages are not strictly delimited, but gradually replace each other.

For the first time, hematopoiesis (its first stage) is found in the 19-day-old embryo in the blood islands of the yolk sac.

The initial primitive cells containing hemoglobin and the nucleus, megaloblasts, appear. This first short period of hematopoiesis, predominantly erythropoiesis, is called outside embryonic hematopoiesis.

The second (hepato-splenic) period begins after 6 weeks. and reaches a maximum by the 5th month prenatal development person. First, hematopoiesis occurs in the liver and of all the processes of hematopoiesis, erythropoiesis is most pronounced and much weaker - leukopoiesis and thrombocytopoiesis. Megaloblasts are gradually replaced by erythroblasts. At the 3-4th month of intrauterine life, the spleen is included in hematopoiesis. It functions most actively as a hematopoietic organ from the 5th to the 7th month of development. It carries out erythrocyto-, granulocyto- and megakaryocytopoiesis. Active lymphocytopoiesis occurs in the spleen later - from the end of the 7th month of intrauterine development.

At the 4-5th month of intrauterine development, the third (bone marrow) period of hematopoiesis begins, which gradually becomes decisive in the production shaped elements blood.

By the time the child is born, hematopoiesis stops in the liver, and the spleen loses the function of forming red cells, granulocytes, megakaryocytes, while retaining the function of forming lymphocytes. Hematopoiesis occurs almost exclusively in the bone marrow.

Respectively different periods hematopoiesis (embryonic, fetal splenic-hepatic and bone marrow) there are three different types hemoglobin: embryonic (HbP), fetal (HBF) and adult hemoglobin (HbA). Embryonic hemoglobin (HHP) is found only on the most early stages embryo development. Already at the 8-10th week of pregnancy in the fetus, 90-95% is HBF, and in the same period HbA begins to appear (5-10%). At birth, the amount of fetal hemoglobin varies from 45 to 90%. Gradually, HBF is replaced by HNA. By the year, only 15% of HBF remains in the composition total hemoglobin erythrocytes, and by the age of 3, its amount should not exceed 2%. Types of normal hemoglobin differ in amino acid composition and affinity for oxygen.

There are also numerous anomalous types hemoglobins, which are inherited. General characteristic diseases associated with a genetically predetermined abnormality of hemoglobin is the tendency of red blood cells carrying pathological hemoglobin to hemolysis. In this case, hemolytic anemia develops.

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Hematopoiesis in the embryo and fetus

First blood formation in the embryo occurs in yolk sac from mesenchymal cells simultaneously with the development of blood vessels. This is the first, so-called angioblastic period of hematopoiesis. Blood islands surround the developing embryo from all sides.

As it was found out, in the mesenchyme of the embryo, as well as in the extraembryonic mesenchyme in higher vertebrates and in humans, the rudiments of blood tissue are isolated from mobile mesenchymal cells very early (obviously, due to the fact that the mesenchyme takes part in metabolism before all other tissues), or blood histioblasts (mesoblasts) and hemocytoblasts. In the blood islands of the mesenchyme, cells, rounding off or being released from the syncytial connection, are converted into primary blood cells. The cells that limit the blood islands become flat plates and, connecting like epithelial cells, form the wall of the future vessel. These flattened cells are called endothelial cells.

Also found in the blood islands are precursors of platelets, megakaryocytes, which also originate from mesoblasts.

After the formation of the first blood vessels the mesenchyme already consists of two parts: the bloodstream with liquid content, in which free blood cells are suspended, and the surrounding mesenchyme of the syncytial structure, which also contains mobile cells.

Primary hemohistioblasts (mesoblasts), which differentiate in blood islands, are fairly large cells round shape with basophilic cytoplasm and a nucleus in which large clumps of chromatin are clearly visible. These cells make amoeboid movements. Primary blood cells multiply intensively mitotically, and the vast majority of them turn into primary erythroblasts - megaloblasts.

The number of primary erythroblasts that continue to multiply mitotically increases all the time, but simultaneously with reproduction, the pictonization of the nucleus increases and primary erythroblasts, losing the nucleus, turn into primary large erythrocytes - megalocytes.

However, some of the primary cells remain in an undifferentiated state and give rise to hemocytoblasts, the parent elements of all subsequent blood cells.

Secondary (final) erythroblasts develop from hemocytoblasts in the vessels of the yolk field, which subsequently synthesize hemoglobin and become the final, or secondary, normoblasts. Vascular channels form in the blood islands, eventually uniting into a network of blood vessels. This network of primitive blood vessels on early stages contains primary erythroblasts and hemocytoblasts, and later mature erythroblasts and erythrocytes.

The development of erythrocytes in the early embryonic period is characterized by the fact that it proceeds inside the formed vessels. Granulocytes are formed from hemoblasts located around the vessels. This ends angioblastic hematopoiesis period. The yolk sac undergoes atrophy on the 4th - 5th week and the hematopoietic function of the vessels gradually stops.

From this time begins embryonic hematopoiesis: the place of formation of erythrocytes and leukocytes are the liver, bone marrow, lymph nodes.

In a maturing embryo and in further postnatal life, the development of hemocytoblasts and erythroblasts from the vascular endothelium no longer occurs. Blood formation takes place in the reticular adventitia, where histiocytes turn into erythroblasts.

Embryonic mesenchyme. additional role in early embryonic hematopoiesis, primary mesenchymal cells play directly in the body cavity, especially in the region of the anterior precordial mesenchyme. A small proportion of mesenchymal cells develop into erythroblasts, megakaryocytes, granulocytes, and phagocytic cells similar to the corresponding adult cells. The number of these cells is small, and large growths of blood cells, similar to the hematopoietic islets of the yolk sac, do not form in the mesenchyme of the body cavity. Stem cells located among these hematopoietic cells (outside the yolk sac) probably play leading role in the generation of subsequent generations of hematopoietic cells in the fetus and in the postnatal period, although the relative contribution of primary stem cells located in the yolk sac and outside it to later hematopoiesis is not yet clear.

Hematopoiesis in the liver. In the embryo (approximately 3 - 4 weeks of life), the liver is laid down by absorption of the glandular epithelium duodenum into mesenchymal tissue.

In humans, starting at about the 12 mm embryo stage (6 weeks of age), hematopoiesis gradually moves to the liver. The liver soon becomes the main site of hematopoiesis and is active in this regard until birth. As the liver endothermal bands form into transverse septa, they collide with wandering mesenchymal cells with lymphocyte morphology. These small round lymphoid cells, called lymphocytoid vagus cells, are subsequently trapped between the primary hepatic endothermal cords and the endothelial cells of the ingrowing capillaries. They form hemocytoblasts similar to those in the yolk sac. These hemocytoblasts soon form foci of hematopoiesis, similar to the blood islands of the yolk sac, where secondary erythroblasts form in large numbers. Secondary erythroblasts subsequently divide and differentiate into mature erythrocytes, with activation of hemoglobin synthesis and loss of the cell nucleus. Although mature erythrocytes are found in the liver of the embryo already at the age of 6 weeks, they appear in the circulation in significant quantities much later. Thus, by the fourth month of fetal life, the majority of circulating erythrocytes are represented by secondary mature forms. Megakaryocytes are also probably formed from hemocytoblasts in the liver of the embryo and fetus. In the embryonic liver, granulocytic cells are found, but they apparently develop not from hemocytoblasts, but directly from wandering lymphocytoid cells.

In humans, hematopoiesis in the liver usually stops by the end of the intrauterine period, and then the bone marrow remains the only organ where erythropoiesis and myelopoiesis occurs. At the 5th month of intrauterine life, due to the accumulation in the fetal liver of hematopoietic substances coming from maternal organism, megaloblastic hematopoiesis is finally replaced by normoblastic.

Hematopoiesis in the bone marrow. At the end of the 3rd month of the embryo's life, the bone marrow and spleen are laid simultaneously.

Embryonic bone marrow and myelopoiesis. Various bones the embryo does not form at the same time. Before others - the long bones of the additional skeleton. Initially, a cartilaginous model of each bone is formed. The central nucleus of the diaphysis subsequently ossifies, and soon after the ingrowth of mesenchymal cells, an area of ​​bone resorption develops from the periosteum. The process of movement of mesenchymal cells is accompanied by ingrowth into capillaries. The number of mesenchymal cells continues to increase due to the continuous influx of new cells, as well as the division of those that are already inside the newly formed medullary cavity. They produce non-cellular material, or matrix, that fills the developing bone cavity. From these early bone marrow mesenchymal cells, cells are formed that are morphologically similar to hemocytoblasts of the liver and yolk sac. Like the latter, they give rise to megakaryocytes and erythroid cells, as well as myeloid cells, including neutrophils, basophils, and eosinophils. The embryonic bone marrow differs markedly from the centers of the earlier development of hematopoiesis in that the formation of myeloid cells is especially vigorous here and dominates in hematopoiesis. The process of early myeloid cell formation, or myelopoiesis, begins in the central part of the medullary cavity and spreads from there to eventually invade the entire bone cavity. Erythropoiesis in the embryonic bone marrow develops a little later and is mainly mixed with the process of myelopoiesis, so that among the majority of maturing cells of the myeloid line, small foci of erythropoiesis can be observed. After birth in humans, hematopoiesis ceases in the liver, but continues in the bone marrow for the rest of life.

Lymphopoiesis. Lymphoid elements in the body of vertebrate embryos appear later than erythrocytes and granulocytes. The first rudiments of lymph nodes appear in the region of the cervical lymphatic sacs. In the very early period(in a human fetus about 3 months), the formation of lymphocytes occurs as follows. In the mesenchyme of the walls of the lymphatic sac, mobile hemohistoblasts begin to separate directly from the mesenchymal syncytium. The latter is converted into reticular blood, in the loops of which various free elements accumulate: hemohistoblasts, hemocytoblasts, macrophages and lymphocytes.

In the early stages of development of the rudiments of the lymph nodes, the presence of erythroblasts and myeloid elements is observed in them, however, the reproduction of these forms is quickly suppressed by the formation of lymphocytes.

The embryonic thymus develops as a derivative of the third gill pocket. The thymic epithelium is filled with wandering mesenchymal cells, which begin to multiply rapidly and differentiate into dimphocytes. At the same time, a small number of erythroid and myeloid cells are formed in the thymus, but the process of lymphopoiesis predominates. Lymphocytes formed in this organ are a special class of lymphocytes with a special function - participation in cellular immunity.

Spleen. Pulp loops contain large cells of reticular origin. Venous sinuses with active endothelium pass between the loops of the reticular tissue of the pulp. The development of lymphatic foci in the spleen occurs later: around the small arteries from the adventitious tissue and perivascular mesenchyme, reticular adenoid tissue develops with large quantity lymphocytes in its loops (rudiments of lymphatic follicles).

Bone marrow. Red bone marrow is 50% total weight of the entire bone marrow substance, including fatty bone marrow, and in terms of its entire weight it corresponds approximately to the weight of the largest human organ - the liver (1300 - 2000 g).

In children, red bone marrow predominates in the bones; starting from the age of 7, fatty bone marrow appears in the diaphysis of long bones. From the age of 20, the hematopoietic red bone marrow is limited to the epiphyses of long bones, short and spongy bones. In old age, due to the development of age-related osteosclerosis, the red bone marrow is replaced in some places by the yellow (fatty) bone marrow.

Bone tissue. Bone marrow tissue is a gently looped network consisting of branching reticular cells, anastomosing with each other with the help of the thinnest collagen fibrils; the loops of this network contain bone marrow elements, as well as fat cells. The reticular network (stroma bone marrow) is more pronounced in the fatty bone marrow; it is especially noticeable when pathological conditions accompanied by atrophy of the hematopoietic tissue and proliferation of blood elements.

very rich circulatory system The bone marrow is closed in the sense that there is no direct flushing of the hematopoietic parenchyma with blood. This, under normal conditions, prevents the release of immature cellular elements into peripheral blood.

Among the reticular elements of the bone marrow, the following forms are distinguished.

1. Undifferentiated cell, small lymphoid-reticular cell, which has a characteristic pear-shaped, caudate or fusiform shape, breaking away from the reticular syncytium, is morphologically difficult to distinguish from narrow protoplasmic lymphocytes.

2. Large lymphoid-reticular cell- a young, functionally active cell, found for the most part during regenerative processes.

3. Phagocytic large reticular cell- macrophage. This cell is irregular in shape, with a wide light blue cytoplasm and a small, round, eccentrically located nucleus. It contains azurophilic grains, phagocytosed nuclei, erythrocytes (erythrophage) and pigment lumps (pigmentophage), fat droplets (lipophage), etc.

4. Bone marrow fat cell. fat cell, originating from the reticular cell, when it loses fat, it can return to its original state and again receive the potency characteristic of the reticular cell, in particular, the ability to produce blood elements. Clinical Observations confirm the fact that the bone marrow, which is very poor in myeloid elements, but rich in fat cells, retains the ability to physiological regeneration.

5. Plasma cell, plasma cell. Plasma cells are found in normal bone marrow punctate in small quantities, amounting, according to different authors, from 0.1 to 3%.

Plasma cells will be discussed below in subsequent lectures.

Thus, identical processes occur in all hematopoietic organs of the embryo and fetus. Circulating primary hematopoietic stem cells settle in a specific tissue niche in a manner that is not yet fully understood. There they differentiate into cells recognizable as hematopoietic progenitors. These embryonic hematopoietic progenitors are likely capable of multilineage differentiation, but at each specific site, the process of hematopoiesis may be targeted to form a specific cell lineage, possibly under the influence of the local microenvironment. Different foci of embryonic hematopoiesis are active only at the corresponding stages of development. This activation is followed by programmed involution. The exception is the bone marrow, which is preserved as the main center of hematopoiesis in adults. Lymph nodes, spleen, thymus and other lymphoid tissues continue to perform a lymphopoietic function in an adult.

"I approve"

head department of pediatrics,

MD, Professor

A.I. Kuselman

/_____________________/

"_____" __________ 2007

For teachers of the 3rd year of the pediatric faculty on the topic:

ANATOMO-PHYSIOLOGICAL FEATURES

OF HEMATOPOISING ORGANS IN CHILDREN AND ADOLESCENTS.

LESSON DURATION - 2 HOURS.

MAIN QUESTIONS OF THE TOPIC:

Stages of embryonic hematopoiesis and their role in understanding the occurrence of foci of extramedullary hematopoiesis in pathology hematopoietic organs in children and adolescents.

Polypotent stem cell and stages of its differentiation.

Patterns of changes in the leukocyte formula with the age of children.

Erythrocyte germ and its changes in the postnatal period.

Granular system of hematopoiesis.

Lymphoid system of hematopoiesis.

Hemostasis system in children and adolescents

PURPOSE OF THE LESSON:

To study the anatomical and physiological features of the hematopoietic system in children.

The student must know.

Features of hematopoiesis in the fetus.

The modern scheme of hematopoiesis.

Changes in the erythrocyte germ of hematopoiesis after birth.

Changes in the leukocyte formula with the age of the child.

Age features of hemostasis in children and adolescents.

The student must be able to.

To master the technique of studying the hematopoietic organs in children and adolescents.

Evaluate blood tests in children and adolescents.

Questions for independent study by students.

The modern scheme of hematopoiesis.

Examination of the patient, evaluation of data from the study of peripheral blood in a patient with a norm.

LESSON EQUIPMENT: tables, diagrams, case histories.

TIME ALLOCATION:

5 minutes - organizational moment

30 min - survey

10 min - break

15 min - demonstration of the patient by the teacher

25 min - independent work of students.

METHODOLOGICAL INSTRUCTIONS.

Blood is one of the most labile fluid systems of the body, constantly coming into contact with organs and tissues, providing them with oxygen and nutrients, carrying waste products of metabolism to the excretory organs, participating in the regulatory processes of maintaining homeostasis.

The blood system includes organs of hematopoiesis and blood destruction (red bone marrow, liver, spleen, lymph nodes, other lymphoid formations) and peripheral blood, neurohumoral and physico-chemical regulatory factors.

The constituents of blood are formed elements (erythrocytes, leukocytes, platelets) and liquid part- plasma.

The total amount of blood in the body of an adult is 7% of body weight and is equal to 5 liters, or 70 ml per 1 kg of body weight. The amount of blood in a newborn is 14% of body weight or 93-147 ml per 1 kg of body weight, in children of the first three years of life - 8%, 4-7 years - 7-8%, 12-14 years 7-9% of body weight .

Embryonic hematopoiesis.

Hematopoiesis in the prenatal period of development begins early. As the embryo and fetus grow, the localization of hematopoiesis consistently changes in various organs.

Tab. 1. Development of the human hematopoietic system (according to N.S. Kislyak, R.V. Lenskaya, 1978).

Hematopoiesis begins in the yolk sac at the 3rd week of development of the human embryo. In the beginning, it comes down mainly to erythropoiesis. The formation of primary erythroblasts (megaloblasts) occurs inside the vessels of the yolk sac.

On the 4th week, hematopoiesis appears in the organs of the embryo. From the yolk sac, hematopoiesis moves to the liver, which by the 5th week of gestation becomes the center of hematopoiesis. Since that time, along with erythroid cells, the first granulocytes and megakaryocytes begin to form, while the megaloblastic type of hematopoiesis is replaced by normoblastic. By the 18-20th week of development of the human fetus, hematopoietic activity in the liver is sharply reduced, and by the end of intrauterine life, as a rule, it completely stops.

In the spleen, hematopoiesis begins from the 12th week, erythrocytes, granulocytes, megakaryocytes are formed. From the 20th week, myelopoiesis in the spleen is replaced by intense lymphopoiesis.

The first lymphoid elements appear at 9-10 weeks in the stroma of the thymus; in the process of their differentiation, immunocompetent cells, T-lymphocytes, are formed. By the 20th week, the thymus in terms of the ratio of small and medium lymphocytes is similar to the thymus of a full-term baby; by this time, immunoglobulins M and G begin to be detected in the fetal blood serum.

The bone marrow is formed at the end of the 3rd month of embryonic development due to mesenchymal perivascular elements penetrating together with blood vessels from the periosteum into the medullary cavity. Hematopoietic foci in the bone marrow appear from 13-14 weeks of fetal development in the diaphysis of the femur and humerus. By the 15th week, these loci show an abundance of young forms of granulo-, erythro-, and megakaryocytes. Bone marrow hematopoiesis becomes the main by the end of fetal development and throughout the postnatal period. Bone marrow in the prenatal period is red. Its volume increases by 2.5 times with the age of the fetus and by birth is about 40 ml. and it is present in all bones. By the end of gestation, fat cells begin to appear in the bone marrow of the extremities. After birth, during the growth of a child, the mass of the bone marrow increases and by the age of 20 it averages 3000 g, but the share of red bone marrow will be about 1200 g, and it will be localized mainly in flat bones and vertebral bodies, the rest will be replaced by yellow bone marrow.

The main difference in the composition of the formed elements of the fetal blood is a constant increase in the number of red blood cells, hemoglobin content, and the number of leukocytes. If in the first half of fetal development (up to 6 months) many immature elements (erythroblasts, myeloblasts, promyelocytes and myelocytes) are found in the blood, then in the following months, predominantly mature elements are contained in the peripheral blood of the fetus.

The composition of hemoglobin also changes. Initially (9-12 weeks) in megaloblasts there is primitive hemoglobin (HbP), which will be replaced by fetal hemoglobin (HbF). It becomes the main form in the prenatal period. Although erythrocytes with adult-type hemoglobin (HbA) begin to appear from the 10th week, its proportion before the 30th week is only 10%. By the birth of a child, fetal hemoglobin is approximately 60%, and an adult - 40% of the total hemoglobin of peripheral blood erythrocytes. An important physiological property of primitive and fetal hemoglobins is their higher affinity for oxygen, which is important in the prenatal period for providing the fetus with oxygen, when the oxygenation of the fetal blood in the placenta is relatively limited compared to the oxygenation of the blood after birth due to the establishment of pulmonary respiration.

The modern concept of hematopoiesis.

The modern understanding of hematopoiesis is based on the molecular genetic theory, according to which the molecular basis of the hematopoietic system is the genome of a single hematopoietic stem cell and its relationship with the elements of the cytoplasm, which ensures the transmission of information coming from the genome microenvironment. Neurohumoral regulation of hematopoiesis different stages development of the organism is not the same, however, in principle, its essence lies in the repression or depression of the corresponding sections of the DNA of the genome of hematopoietic cells.

In the scheme of hematopoiesis, stem cells make up 1 class pluripotent precursor cells. Further Grade 2 represent precursor cells of myelopoiesis and lymphopoiesis. These are the so-called lymphoid, morphologically undifferentiated cells, giving rise to the myeloid and lymphoid series. Next 3rd grade- poetin-sensitive cells, among which the proportion of proliferating is 60-100%, morphologically they also do not differ from lymphocytes. These cells respond to the humoral regulation of hematopoiesis in accordance with the specific needs of the body. Erythropoietin-sensitive cells form an erythroid lineage, leukopoietin-sensitive cells form a series of granulocytes and monocytes, and thrombopoietin-sensitive cells form a series that forms platelets.

The next stage of differentiation is 4th grade morphologically recognizable cells. The vast majority of them are in the proliferation stage. These are blast cells: plasmablast, lymphoblast, monoblast, myeloblast, erythroblast, megakaryoblast.

Further differentiation of cells is associated with specific rows of hematopoiesis. Elements called ripening make up 5th grade: proplasmocyte, prolymphocyte T, prolymphocyte B, promonocyte; further basophilic, neutrophilic and eosinophilic promyelocytes, myelocytes, metamyelocytes, stab. Next row: pronormocyte, normocyte (basophilic, polychromatophilic and oxyphilic), reticulocyte. And the last row - promegakaryocyte, megakaryocyte.

Completes the hematopoietic system 6th grade mature blood cells: plasmocytes, lymphocytes (T and B), monocytes, segmented basophils, neutrophils and eosinophils, erythrocytes, platelets. A class of macrophage cells is formed from a monocyte (histiocyte connective tissue, Kupffer cells of the liver, alveolar macrophage, spleen macrophage, bone marrow macrophage, lymph node macrophage, peritoneal macrophage, pleural macrophage, osteoclast, microglial cells of the nervous system).

Peripheral blood composition after birth.

Immediately after birth, the red blood of a newborn is characterized by an increased content of hemoglobin and a large number of red blood cells. On average, immediately after birth, the hemoglobin content is 210 g / l (fluctuations 180-240 g / l) and erythrocytes - 6 * 10 12 / l (fluctuations 7.2 * 10 12 / l - 5.38 * 10 12 / l) . From the end of the first, the beginning of the second day of life, there is a decrease in the content of hemoglobin (the largest - by the 10th day of life), erythrocytes (the largest by the 5-7th day).

The red blood of newborns differs from the blood of older children not only quantitatively, but also qualitatively; for the blood of a newborn, first of all, a distinct anisocytosis is characteristic, noted within 5-7 days, and macrocytosis, that is, somewhat larger in the first days life red blood cell diameter than later in life.

During the first hours of life, the number of reticulocytes - the precursors of erythrocytes - ranges from 8-13 0/00 to 42 0/00. But the curve of reticulocytosis, giving a maximum rise in the first 24-48 hours of life, then begins to decline rapidly and between the 5th and 7th days of life they reach the minimum figures.

Availability a large number erythrocytes, an increased amount of hemoglobin, the presence of a large number of young immature forms of erythrocytes in peripheral blood in the first days of life indicate intense erythropoiesis as a reaction to a lack of oxygen supply to the fetus during fetal development and during childbirth. After birth, in connection with the establishment of external respiration, hypoxia is replaced by hyperoxia. This causes a decrease in the production of erythropoietins, erythropoiesis is largely suppressed, and a decrease in the number of erythrocytes and hemoglobin begins.

There are also differences in the number of leukocytes. In the peripheral blood in the first days of life after birth, the number of leukocytes up to the 5th day of life exceeds 18-20*10 9 /l, and neutrophils make up 60-70% of all white blood cells. The leukocyte formula is shifted to the left due to the high content of stab and, to a lesser extent, metamyelocytes (young). Solitary myelocytes may also be seen.

The leukocyte formula undergoes significant changes, which is expressed in a decrease in the number of neutrophils and an increase in the number of lymphocytes. On the 5th day of life, their number is compared (the so-called first crossover), amounting to about 40-44% in the white blood formula. Then there is a further increase in the number of lymphocytes (up to 55-60% by the 10th day) against the background of a decrease in the number of neutrophils (approximately 30%). The shift of the blood formula to the left gradually disappears. At the same time, myelocytes completely disappear from the blood, the number of metamyelocytes decreases to 1% and stab to 3%.

In the process of a child's growth, the leukocyte formula continues to undergo its changes, and among the uniform elements, changes in the number of neutrophils and lymphocytes are especially significant. After a year, the number of neutrophils increases again, and the number of lymphocytes gradually decreases. At the age of 4-5 years, a crossover occurs again in the leukocyte formula, when the number of neutrophils and lymphocytes is again compared. In the future, there is an increase in the number of neutrophils with a decrease in the number of lymphocytes. From the age of 12, the leukocyte formula is not much different from that of an adult.

Along with the relative content of cells included in the concept of "leukocyte formula", their absolute content in the blood is of interest.

As can be seen from Table No. 1, the absolute number of neutrophils is greatest in newborns, in the first year of life their number becomes the smallest, and then increases again, exceeding 4 * 10 9 / l in peripheral blood. The absolute number of lymphocytes during the first 5 years of life is high (5 * 10 9 / l or more), after 5 years their number gradually decreases and by the age of 12 does not exceed 3 * 10 9 / l. Similarly to lymphocytes, changes occur in monocytes. Probably, such parallelism of changes in lymphocytes and monocytes is explained by the commonality of their functional properties, which play a role in immunity. The absolute number of eosinophils and basophils practically does not undergo significant changes in the process of child development.

Table No. 1. The absolute number (n * 10 9 / l) of white blood cells in children.

erythrocyte system.

A mature erythrocyte (normocyte) is a biconvex disk with a thickened peripheral part. Due to their elasticity, erythrocytes pass through capillaries that are smaller in diameter. The diameter of most of them is 7.8 microns, normally fluctuations from 5.5 to 9.5 microns are possible. In children of the first 2 weeks, there is a shift towards macrocytes (more than 7.7 microns), by 4 months of life, the number of macrocytes in the peripheral blood decreases. Erythrocytometric parameters in healthy children of different ages are presented in Table 2.

Due to the content of hemoglobin in red blood cells, they carry oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. In the 1st month of life, there is still a lot of “fetal hemoglobin” in the blood of a newborn, which has a high affinity for oxygen. By 3-4 months, there is normally no “fetal hemoglobin” in the child’s blood, which by this time has been completely replaced by hemoglobin “A” - “adult type”.

Blood baby compared with the blood of newborns, as well as older children, it is characterized by more low scores hemoglobin and erythrocytes. The amount of hemoglobin sharply decreases during the first months of life, decreasing in most cases by 2-3 months to 116-130 g/l, and sometimes to 108 g/l. Then, due to the increase in the production of erythropoietins, the content of the number of erythrocytes and hemoglobin increases slightly. The number of erythrocytes exceeds 4 - 4.5 * 10 12 / l, and the hemoglobin content begins to exceed 110-120 g / l, and already quantitatively throughout all periods of childhood differ little from its level in an adult.

Table number 2. Hematocrit value and erythrocytometric parameters in healthy children of different ages. (according to A.F. Tur, N.P. Shabalov, 1970).

The ratio of the diameter and thickness of the erythrocyte (D / T) is normally 3.4 - 3.9, the D / T ratio below 3.4 means a tendency to spherocytosis, above 3.9 - a tendency to planocytosis. Spherocytosis with microcytosis is characteristic of congenital hemolytic anemia, on the contrary, macroplanocytosis is often observed in liver diseases and in some forms of acquired hemolytic anemia.

In addition to transporting oxygen and carbon dioxide, erythrocytes transport amino acids, lipids, enzymes, hormones, immune bodies, metabolic products, and other substances. Their surface can also adsorb heterogeneous substances (antigens, toxins, medicinal and other substances).

Erythrocytes have antigenic properties that determine the blood group affiliation. They have two kinds of antigens (agglutinogens) "A" and "B". Accordingly, the blood serum contains two types of agglutinins "alpha" and "beta". Depending on the content of antigens in erythrocytes, 4 blood groups are distinguished: the first - 0 (1), the second - A (11), the third - B (111), the fourth - AB (1U). In cases where erythrocytes of group "A" enter the blood serum with agglutinin "alpha" or erythrocytes with antigen "B" in the blood serum with agglutinin "beta", an agglutination reaction occurs (gluing of erythrocytes). Group 0(1) erythrocytes in the body of any recipient do not undergo "gluing" and hemolysis, but continue to perform their function. The introduction of erythrocytes containing antigen A or B into the body of a child with blood type 0 (1) leads to their hemolysis, since the plasma contains agglutinins "alpha" and "beta". There may be other antigens in erythrocytes. For pediatric practice great importance omet determination of Rh blood belonging. Knowledge of its antigenic composition according to the AB0 systems and the Rh factor is important for solving the issues of compatibility and blood transfusion, understanding the pathogenesis, prevention and treatment of hemolytic disease of the newborn.

The resistance of erythrocytes is determined by their osmotic resistance to hypotonic sodium chloride solutions of various concentrations. With minimal resistance, the first signs of hemolysis are observed. Normally, it is 0.44 - 0.48% sodium chloride solution. At maximum resistance, complete hemolysis is observed. Normally, it is 0.32 - 0.36% sodium chloride solution. In the blood of newborns there are erythrocytes, both with reduced and increased osmotic resistance. This figure increases with blood loss.

The erythrocyte sedimentation rate (ESR) depends on many chemical and physical properties of blood. In newborns, when determined in the Panchenkov apparatus, it is 2 mm / hour, in infants - 4-8, in older children - 4-10, in adults - 5-8 mm / hour. The slower erythrocyte sedimentation in newborns is explained by the low content of fibrinogen and cholesterol in the blood, as well as by the thickening of the blood, which is especially pronounced in the first hours after birth.

The lifespan of erythrocytes, established by radiological methods, is equal in children older than a year and in adults 80-120 days.

granulocytic system.

The total number of granulocytes in the body of an adult is 2 * 10 10 cells. Of this amount, only 1% of granulocytes are in peripheral blood, 1% - in small vessels, the remaining 98% - on the bone marrow and tissues.

The lifetime of granulocytes is from 4 to 16 days, on average 14 days, of which 5-6 days are for maturation, 1 day for circulation in the peripheral blood and 6-7 days for staying in tissues.

Consequently, three periods of granulocyte life activity are mainly distinguished: bone marrow, stay in peripheral blood, stay in tissues.

Granulocytes of the bone marrow reserve are divided into two groups. The first is a mitotic, dividing pool. It includes myeloblasts, promyelocytes, myelocytes. The second group is a maturing, non-fissile pool. It includes metamyelocytes, stab and segmented neutrophils. The last group of cells is constantly updated due to the influx of cells from the mitotic pool. The non-dividing pool is the so-called granulocytic reserve of the bone marrow. Normally, the granulocytic reserve of the brain is completely replaced every 6 days. The number of granulocytes in the bone marrow reserve exceeds the number of granulocytes circulating in the blood by 20-70 times. Normally, despite the constant migration of neutrophils into tissues, their number in the bloodstream remains constant due to the washing out of leukocytes from the granulocytic reserve of the bone marrow. The non-dividing pool is also the main reserve of granulocytes mobilized on demand (infection, aseptic inflammation, pyrogens, etc.).

In the vascular bed, some of the neutrophils circulate in suspension, and some are located near the wall. Circulating and parietal blood cells constantly interact. The presence of neutrophils in the peripheral blood is short-term and ranges from 2 to 30 hours. Then neutrophils are deposited in the capillary network of various organs: in the lungs, liver, spleen.

Depending on the needs of the body, deposited neutrophils easily pass into the peripheral channel or are redistributed in the capillary network of other organs and tissues. From the capillary network, neutrophils migrate to tissues, where their main functions (phagocytosis, trophism, immunological and allergic processes) are manifested. The possibility of recycling granulocytes has not been proven.

lymphoid system.

The lymphoid system consists of the thymus gland, spleen, lymph nodes, circulating lymphocytes. In addition, in various areas of the body there are accumulations of lymphoid cells, especially significant in the tonsils, granules of the pharynx and group lymphatic follicles (Peyer's patches) of the ileum.

The thymus gland is one of the primary lymphoid organs. Here, T-cells multiply and mature from lymphoid stem cells.

The thymus gland is laid on the 6th week of intrauterine development. Thymocytes begin to form from the 7-8th week and by the 14th week are located mainly in the cortical layer thymus. Subsequently, the mass of the thymus gland rapidly increases, and its growth continues in the postnatal period.

Table No. 3. Mass of the thymus gland at different periods of life.

I.B. Alakaeva, N.V. Nepokulchitskaya, G.A. Samsygina, T.A. Vysotskaya

PECULIARITIES OF HEMOPOIESIS IN THE INTRAUTERINE PERIOD AND INFLUENCE OF CONGENITAL INFECTIONS ON IT

GOU VPO RSMU Roszdrav, Moscow

Embryonic hematopoiesis is characterized by a change in localization in a number of extra-embryonic and germinal organs. According to the leading role of this or that organ, three are distinguished, according to other authors - four periods: mesoblastic, hepatic, splenic, medullary.

The mesoblastic type of hematopoiesis occurs in the yolk sac, allantois, chorion, chorion stalk approximately by the end of the 2nd - the beginning of the 3rd week after fertilization. By this time, dense accumulations of mesenchymal cells - blood islands - appear under the endoderm. By the end of the 3rd week, the central cells of the islets round off and turn into hematopoietic cells. Peripheral cells flatten and become endotheliocytes of the resulting blood vessels. The first blood cells appear both outside the vessels and inside them. But as the vascular network grows, intravascular hematopoiesis becomes the leading one. Among the blood cells formed during this period, large primary erythropoietic cells containing nuclei predominate. There are large blasts with basophilic cytoplasm, proerythroblasts with polychromatophilic cytoplasm, erythroblasts, orthochromic with an eccentric nucleus and non-nuclear erythroblasts. All erythroblasts of this period are called megaloblasts, and the process is called megaloblastic hematopoiesis. Germ-type hemoglobin is highly oxygen-binding and occurs before 12 weeks of development. At the 7th-8th week of embryo development, megalocytes (hypochromic erythrocytes), normoblasts and normocytes appear, the number of which increases sharply by the 12th week (up to 74%), and megaloblasts practically disappear. Although during the mesoblastic period of hematopoiesis, predominantly erythropoiesis is noted, nevertheless, during this period, precursor cells of all hematopoietic sprouts can be found. Granulocytes are found in the blood of embryos at the 4th-5th week, lymphocytes - at the 6th week, and monocytes and activated macrophages - at the 8th week. Cells of granulocytic, monocytic, lymphocytic

leg and megakaryocytic rows are few. Hematopoiesis in extra-embryonic organs stops by the 9th week.

Hepatic stage hematopoiesis occurs from the 5th week of gestation. Within 3-6 months, the liver becomes the main organ of hematopoiesis, and the liver is also the site of erythropoietin formation. The source of hematopoiesis in the liver is a pluripotent hematopoietic stem cell. During the laying of the liver on the 3rd-4th week of embryogenesis, stem cells of the first generation are brought into the vascular system of the laying. Inside the vessels of the liver, megaloblasts are first formed. On the 4th-5th week, progenitor cells with basophilic cytoplasm and an eccentric nucleus, lymphoid cells, erythroblasts and macrophages appear between hepatocytes. From the 7th week, the number of primitive erythroblasts decreases and normocytes become predominant. At the 9th-15th weeks, definitive erythrocytes make up 95% of all hematopoietic liver cells. Hemoglobin of the embryonic type is replaced by fetal. Extravascular hematopoiesis becomes the leader. During the first 15 weeks, the level of granulocytopoiesis is low. From the 21st week, an increase in the number of granulocytes begins with localization in the connective tissue of the portal zones of the liver. Megakaryocytes are determined in the liver from the 5th week, lymphocytes - from the 7th week. The content of lymphocytes increases as the gestation period increases and by the 22nd-27th week they make up 10%. The liver contains stem and committed precursor cells of the myeloid and lymphoid series. In the liver, the formation of B-lymphocytes begins. Pre-B-lymphocytes are determined by the content of cytoplasmic immunoglobulins (Ig), B-lymphocytes - by membrane B-lymphocytes are detected in the liver of a human embryo at the 8th-9th week. Macrophages appear in significant quantities from the very beginning of hematopoiesis in the liver, but from the 6th week their number decreases. Most high quantity myeloid progenitor cells are observed at the 9th and 21st weeks of gestation. In the first rise (9th week), myelopoiesis wears monocy-

to-macrophage character, the activity of erythropoiesis precursor cells is also observed. At the 21st week - the second rise - myeloblasts and promyelocytes predominate, sometimes mature granulocytes. Spontaneous erythropoiesis is absent. By the time the child is born, hematopoiesis in the liver stops, although during the 1st week of the child's postnatal life, single hematopoietic elements can be detected in the child's liver.

The spleen is laid on the 5th-6th week of embryogenesis, hematopoiesis in the spleen begins from the 11th-12th week of gestation. Initially, granular, erythro- and megakaryocytopoiesis are determined in the spleen. Lymphocytes appear at the 11th week, and at 13 weeks B-lymphocytes with ^ receptors are detected. From the 12th week, the size of the spleen increases, differentiation of reticular cells occurs in the pulp, argyrophilic fibers and foci of myeloid hematopoiesis appear. White pulp is formed on the 15th week. Hemopoiesis in the spleen lasts up to 6 months of embryogenesis, on the 7th month myelopoiesis fades and lymphocytopoiesis intensifies. Some authors believe that the spleen plays a significant role not only as an organ of fetal hematopoiesis, but as a site of cell sequestration and destruction.

Formation of hematopoiesis in the bone marrow. The formation of the bone marrow is associated with the formation of bones. It appears on the 7th-8th week of embryogenesis in the clavicle, then on the 9th-10th week - in the tubular bones, on the 18th-19th week - in the ribs, vertebral bodies and sternum. In the fetus of the 11th-14th weeks of gestation, immature hematopoietic cells and erythrocytes are determined in the ilium, at the 23rd-27th week of gestation, elements of all three hematopoietic sprouts are found at all stages of development. In the diaphysis of the humerus and femur, among the bone marrow elements, cells of the myeloid and megakaryocytic series are determined. By the 22nd week of gestation, the amount of hematopoietic stem cells in the bone marrow is 1.6%. Embryonic bone marrow differs from other types of hematopoiesis in that myelopoiesis dominates here. Erythropoiesis in the embryonic bone marrow develops later and is mostly mixed with the process of myelopoiesis. Various foci of embryonic hematopoiesis are active at the corresponding stages of development. This activation is followed by a programmed involution. The exception is the bone marrow, which is preserved as the main center of hematopoiesis in adults.

There is a hypothesis about the qualitative difference of stem cells in different periods of a person's life. According to this hypothesis, the change in the places of the main hematopoiesis in embryogenesis is not a movement of the same stem

cells from one organ to another, but the proliferation of a different stem group of cells. In this connection, we see morphofunctional differences in fetal, newborn and adult erythrocytes, as well as a variety of leukemias in the form and age of patients.

The composition of the blood of the fetus reflects the dynamics of hematopoiesis in the organs of hematopoiesis. Up to 12 weeks, megaloblastic erythropoiesis occurs in the vascular bed, monocytes and macrophages circulate in it, phagocytizing individual erythroid cells and their nuclei. From the 13th week, the number of nucleated erythroid cells decreases and an increase in definitive erythroid cells begins. Most content nucleated erythroid cells are observed at 24-25 weeks. During the first 7 days of postnatal life, nucleated erythroid cells disappear. The first granulocytes and their precursors are determined in the blood of the embryo at 4-5 weeks. Up to 20 weeks, they make up 4-7% of all cells in the myelogram. At 21-23 weeks, granulocytopoiesis is activated in the bone marrow and a decrease in granulocyte precursor cells is noted in the blood and the number of mature granulocytes increases. At 6 weeks, lymphocytes are determined in the blood, by the 21-23rd week they make up 56-60% of all leukocytes. During this period, there is activity in the development of lymphoid organs. At the 24-25th week, the number of lymphocytes decreases to 27% and rises again at the 28-30th week to 43-48%. By the time of birth, the number of lymphocytes again decreases to 33-35%. From the 8th week, large granular lymphocytes - MK cells - appear. They make up 2-13% of all lymphocytes. T- and B-lymphocytes are detected in the blood from the 13th week. The content of T-lymphocytes from the 13th to the 40th week increases from 13 to 60%. The concentration of B-lymphocytes reaches maximum value(28%) at 21-23 weeks and 28-30 weeks.

The blood of a newborn has some features of the hemogram and leukocyte formula. Characteristically increased content erythrocytes - up to 6-7 million / μl. By the 10-14th day, the number of erythrocytes approaches the number of erythrocytes in adults, then by 3-6 months it decreases, from 5-6 months to 1 year it gradually increases. Newborns are characterized by anisocytosis, the presence of macrocytes and reticulocytes. Average duration the life of erythrocytes in children under 1 year is less than in adults. In the blood of a newborn, an increased content of hemoglobin and on the first day after birth averages 200 g / l. From the 2nd day, the hemoglobin level gradually decreases to 140-150 g/l by 1 month. The decrease in hemoglobin continues during the first six months of life, remains low up to 1 year, and only then begins to gradually increase. By 1 year of age

Pediatrics/2009/Volume 87/№4

fetal hemoglobin is replaced by adult-type hemoglobin. The level of platelets in the blood of a newborn is the same as in adults, fluctuations in the content during the first year of life are insignificant. Presence of young forms of thrombocytes is characteristic. The number of leukocytes on the first day after birth increased to 11.4-22.0 thousand / μl, starting from the 2nd day, the number of leukocytes decreases and reaches 7.6-12.4 thousand / μl by 1 month. During the first year of life, the white blood cell count remains relatively stable. AT leukocyte formula neutrophils predominate (60-65%), often with a shift to the left, monocytes make up 8-14%, eosinophils - 0.5-3%, basophils - up to 1%, lymphocytes - 20-30%. On the 4th day, the first physiological decussation- the number of neutrophils and lymphocytes is equalized. At the age of 1-2 years, lymphocytes make up 65%, neutrophils - 25%. At the age of 4, the second physiological crossover occurs - the number of lymphocytes and neutrophils again becomes the same, and the neutrophil profile is established by the age of 14-15.

An analysis of the literature data of the last 15 years showed that the problem of congenital infections(VI) due to the high teratogenic effect of various pathogens, as well as their effect on the hematopoiesis of the newborn.

According to many authors, hematological changes (anemia, neutropenia, thrombocytopenia) are more common in HI caused by a combination of the virus herpes simplex(HSV) with cytomegalovirus (CMV). Other authors described hematological changes in the presence of only herpes infection, while leukopenia and leukocytosis were equally noted, thrombocytopenia and anemia were less common. All authors believe that of the hematological manifestations in congenital CMVI, thrombocytopenia is more common (76%). Some authors associate the causes of thrombocytopenia and hemorrhagic syndrome with the reproduction of CMV in bone marrow megakaryocytes, others with disseminated intravascular coagulation. Bleeding, observed in 40-50% of cases of generalized herpes infection, is caused by disseminated intravascular coagulation. Bleeding is associated with thrombocytopenia and variable deficiency of fibrinogen and factors V and VIII.

In a number of observations, the hemorrhagic syndrome was characterized not only by subcutaneous hemorrhages and petechiae, but also by pulmonary and gastrointestinal bleeding. According to Shabaldin A.V. et al. , moderate anemia was detected in all children with CMVI, and the hemolytic nature of anemia occurred in one

th child, the rest had anemia mixed genesis(infectious and anemia of prematurity). Some authors noted in the peripheral blood leukocytosis with a shift to the left in the neutrophil series (50%). Cases of cytopenia have been described in combination with CMVI and HSV.

For the first time, the possibility of direct damage to HSV in the bone marrow, spleen and thymus (in situ hybridization method) has been proven. In addition, the immunosuppressive activity of HSV against T-lymphocytes and neutrophilic granulocytes was revealed.

At morphological study dead fetuses and newborns with generalized CMVI in the bone marrow showed rejuvenation of cells with a picture of reactive erythroblastosis and proliferation of immature cellular elements of the myeloid and erythroid series. Foci of extramedullary hematopoiesis were noted.

With chlamydial infection from the peripheral blood, according to the literature, anemia and monocytosis are more often observed, and eosinophilia may develop by the end of the 1st-2nd week. Other authors note that in 50% of cases there is a leukocytosis with a shift to the left in the neutrophilic series.

Severe thrombocytopenia, hemorrhagic rash on the skin are characteristic of acute toxoplasmosis.

According to the literature, all newborns with mycoplasma infection have normochromic anemia, eosinophilia, monocytosis, less often leukocytosis, neutrophilia.

Congenital rubella is characterized by the development of thrombocytopenic purpura. Most authors describe only peripheral blood thrombocytopenia.

Parvovirus B19 lytically multiplies in erythroblasts in the liver, spleen, bone marrow and leads to inhibition of erythropoiesis. There is a reduction in the life span of erythrocytes to 45-70 days, a sharp decline level of reticulocytes, up to their complete disappearance. Perhaps a temporary decrease in the level of lymphocytes, granulocytes, platelets.

An analysis of the literature data showed the presence of multidirectional studies related to fetal and newborn hemopoiesis. These studies are carried out at different periods of life of the fetus and children in the first months of life, are not systemic in nature and are mainly determined by those hematological changes that appear as a result of exposure to various pathogens on hematopoiesis.

Thus, the information obtained allows us to conclude that it is necessary to conduct research and identify changes in the hematopoiesis of the fetus and newborn as a result of exposure to this system of various infectious agents.

LITERATURE

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Lecture

Hematopoiesis.

Organization of the stem department hematopoietic system

The structure and functions of blood cells.

Hematopoiesis (hematopoiesis) - a multi-stage process of cell differentiation, as a result of which mature leukocytes, erythrocytes and platelets are released into the blood.

Hematopoiesis during fetal development.

The development of the hematopoietic system in humans begins early, proceeds with varying intensity, with a change in the predominant localization of hematopoiesis at different gestational periods. During prenatal development, topographically, 4 stages of hematopoiesis can be distinguished: mesoblastic, hepatic, splenic and bone marrow.

mesoblastic stage hematopoiesis occurs in the yolk sac by the end of the 2nd to the beginning of the 3rd week of gestation. Vessels are formed from the peripheral cells of the yolk sac, and hematopoietic cells are formed from the central ones. The latter are oval in shape, large in size, have a basophilic cytoplasm, a nucleus of a delicate mesh structure containing nucleoli. These cells gradually accumulate hemoglobin. By appearance they are similar to megaloblasts, they are called primitive erythroblasts. Although erythropoiesis is predominant during this period, progenitor cells of all hematopoietic lineages, including pluripotent stem cells, can be detected at this stage.

Starting from the 8th week of gestation, the hematopoietic islets in the yolk sac begin to regress, and by the 12-15th week megaloblasts disappear from the blood.

Hepatic stage hematopoiesis occurs from the 5th week of gestation, and in the next 3-6 months the liver is the main hematopoietic organ. The liver is also the site of erythropoietin formation. Initially, intense erythropoiesis occurs in the liver. By the 22-27th week, the number of erythroid elements decreases, and megaloblastic cells account for 1.3%. During the period of 6-7 weeks of gestation, neutrophilic cells are found in the liver, represented mainly by promyelocytes and myelocytes, eosinophils, basophils, monocytes, macrophages, megakaryocytes. The content of these cells (with the exception of macrophages and megakaryocytes) increases with increasing gestational age. Starting from 8-9 weeks, lymphocytes are detected, the content of which by 22-27 weeks is 10%.

During the period of hepatic hematopoiesis (weeks 6-27), 3-5% of undefined blasts are determined.

Starting from the 18-20th week, the hematopoietic activity of the liver gradually decreases and by the time of the birth of the child it stops, although during the 1st week of postnatal life, single hematopoietic elements may be detected.

Hematopoiesis in spleen occurs from the 12th week of gestation. Initially, granulo-, erythro-, megacaricytopoiesis is determined in the spleen. From the 15th week, B-lymphocytes appear. By 18-24 weeks, 80% are monocytomacrophage colonies. Hematopoiesis in the spleen reaches its maximum by the 4th month of gestation, and then declines and stops at the age of 6.5 months of intrauterine development.

The reduction of the foothold of extramedullary hematopoiesis coincides with the appearance of the first signs bone marrow hematopoiesis. It occurs approximately from the 4th month of gestation, reaching a maximum by the 30th week. Initially, CM occurs in the vertebral bodies, then in the ilium, diaphyses of the humerus and femur. Among the bone marrow elements, cells of the myeloid and megakaryocytic series are determined. At 12-20 weeks, pre-B cells predominate among the lymphoid elements in the fetus. After 30 weeks, CM is represented by all hematopoietic cells, it becomes the main source of blood cell formation. From 32 weeks of age all intervals bone tissue filled with hematopoietic tissue, because CM volume is equal to the volume of hematopoietic cells. By the time a child is born, hematopoiesis is almost completely limited to the bone marrow.

The development of lymphoid tissue and the thymus occurs relatively early (6-7th week of gestation). By 11-12 weeks, T-antigens appear in thymocytes. The first lymph nodes appear on the 10th week of gestation, and the lymphoid apparatus of the intestine - on the 14-16th week. Initially in lymph nodes myelopoiesis is noted, which is soon replaced by lymphocytopoiesis.

Thus, at different periods of gestation, hematopoiesis has different organ localization, and in some periods, hematopoiesis occurs simultaneously in different organs.

At the time of the birth of the child, the entire CM is red, i.e. hematopoietic.