Blood formation (synonym hemopoiesis) is the process of formation, development and maturation of the blood corpuscles: erythrocytes (erythropoiesis), leukocytes (leukopoiesis), platelets (thrombopoiesis). In the embryo, blood formation begins in the yolk sac; from the 2nd month, this function is assumed by the liver , and from the 4th month, bone marrow hematopoiesis occurs, which by the time of birth completely displaces the hepatic. Erythrocytes, granulocytes and platelets are formed in the liver and in the bone marrow. Lymphocytes appear only on the 4th month, when lymph nodes are formed; the spleen begins to produce them only after birth. The red blood cells of the fetus during the first 3 months - megaloblasts (large nuclear cells, which, upon maturation, turn into large erythrocytes - megalocytes) are gradually replaced by normoblasts giving rise to normal erythrocytes. In extrauterine life, the megaloblastic (embryonic) type of hematopoiesis occurs in pernicious anemia and similar diseases. By the time of birth of the fetus, the final nature of blood formation is established. In a child, in principle, it does not differ from the blood formation of an adult. Up to 4 years of age, lymphopoiesis is more active than granulopoiesis, then the usual ratio for adults occurs.Go
The ancestor of all blood elements is the primary blood cell - hemocytoblast (see color table). From it, proeritroblasts develop in the bone marrow, giving rise to the formation of erythrocytes, myeloblasts, from which all granulocytes are formed (neutrophils, eosinophils, basophils), monoblasts - the founders of monocytes and megakaryoblasts, which are the source of platelet formation. In the lymphoid organs, the hemocytoblast is transformed into a lymphoblast - the founder of lymphocytes. There is also another theory that suggests the existence of a so-called stem cell, functioning as the parent cell of the blood. This concept is functional. A stem cell can be an element that is potent to blood formation: reticular, lymphoid-reticular cell, lymphocyte; hemocytoblast is considered as one of the phases of cell development from stem to mature blood cells. From the cells of the reticular stroma of the bone marrow plasma cells are formed - cells, along with lymphocytes, which produce gamma globulin and play an important role in protecting the body from infections. Maturation of cells occurs at the site of blood formation; normally, only mature cells enter the peripheral blood. The cellular composition of the blood and blood- forming organs is a system that is in dynamic equilibrium in a healthy organism: the ongoing destruction of the formed elements is balanced by the corresponding blood formation. Such an equilibrium is maintained by a complex of regulatory mechanisms. Hematopoiesis is influenced by the central and vegetative nervous system , a number of hormones, vitamins and special hematopoietic factors (see Casla factors, Cyanocobalamin). In pathological conditions, some factors (blood loss, hemolysis, lack of oxygen in the blood, toxins of some microbes) stimulate blood formation, others (lack of iron, Castle factors, hypersplenism, radiation injuries, toxins of a number of viruses) inhibit it.
The scheme of development of blood cells in extrauterine life