Hematopoiesis (gemopoéz) - Is the process of formation and development of blood cells. There are embryonic hematopoiesis, beginning in the early stages of embryonic development and lead to the formation of blood as a tissue, and postembryonic, which can be seen as a process of physiological regeneration of blood. The number of blood cells in an adult is relatively constant, despite, a large number of them die every day. Dead cells are replaced by new, formed in the blood-forming organs. This is done because of blood stem cells, that throughout the life of the body is constantly replenished proliferative pool myeloid and lymphoid tissues.

In the formation and development of blood cells play an important role stroma microenvironment and blood-forming organs. Stromal cells histogenetic independent of hematopoietic cells. They are responsible for the transfer of the factors of the microenvironment, as well as for directing the differentiation of hematopoietic stem cells.

The constancy of the composition of the blood cells and bone marrow provided regulatory mechanisms, by which the processes of proliferation and differentiation of cells connected to each other. The essence of the mechanism of regulation of hematopoiesis is the repression or depression corresponding to regions of the genome of hematopoietic stem cells at different stages of development of the organism. The processes of proliferation and differentiation can be of different intensities. During embryonic hematopoiesis are more intense, than in the post-embryonic period. These processes are enhanced after significant blood loss, massive hemolysis, under hypoxic conditions, etc.. P.

Blood formation in the embryonic period

In the embryonic period hematopoiesis occurs first in the yolk sac, and then sequentially in the liver, spleen, thymus, lymph nodes, and bone marrow.

According to some researchers the yolk sac is a source of pluripotent stem cell, then populating other blood-forming organs and triggering there hematopoiesis. Thus, during fetal development hematopoiesis performed in various organs with unequal nature of the microenvironment. The yolk sac is created endodermal epithelial cells, among which were found the primitive accumulation of blood cells.

First blood cells, formed in the wall of the yolk sac, called primitive erythroblasts. The term "megaloblasts"Only recommended to refer to cells, appearing with pernicious anemia and some other kinds of postnatal pathology. Despite the morphological similarity of these cells, they differ on a number of biochemical indices, first and foremost on the type of synthesized hemoglobin.

In the early period of hematopoiesis in the yolk sac, and in blood tsiruliruyuschey no morphologically recognizable cells granular series, what, apparently, due to the peculiarities of the microenvironment, limiting the differentiation of stem cells in a certain direction. Along with this set, that in the yolk sac cells formed mice, which can differentiate into immunocompetent.

Embryonic hematopoiesis in the liver

Liver It lays approximately 3-4 weeks of human embryonic, And in the fifth week, she becomes the center of hematopoiesis. Along with erythrokaryocytes, which appear first, liver occurs granulocytes, and megakaryocytes. Besides, This period begins hematopoiesis in liver cells and appear lymphocytopoiesis, involved in the cellular immune response.

In the second half of fetal development in the liver begins to grow into the connective tissue, which divides the body into segments. In parallel, there are changes the cellular composition of the microenvironment. Hematopoiesis in the liver continues to the end of the antenatal period, but along with it begin to function and other hematopoietic organs.

Embryonic hematopoiesis in the spleen

At 5-6 weeks of fetal development is laid spleen. By the end of the third - the fourth month it found the first islands of hematopoietic tissue. By the fifth month of fetal development is a universal spleen hematopoietic organ, in which all blood cells develop.

Stem cells in the spleen It is surrounded by reticular tissue of mesenchymal origin. Colony-forming stem cells develop into myeloid first, lymphoblastic and megakaryocytic directions, but as the spleen, and changing the composition of the surrounding tissue is lost the ability to differentiate into myeloid direction. Shortly after the birth of the child becomes an organ of the spleen lymphocytopoiesis. The reason for this transformation is considered settling spleen lymphocytes or their precursors from the thymus and other lymphoid organs.

Role of lymph nodes in hematopoiesis

Bookmark lymph nodes in the human embryo is not at the same time. At the end of the second month of fetal development there are lymph nodes. In the second half of pregnancy has been developing most of the lymph nodes, although their final formation occurs in the postnatal period. Blood formation in the lymph nodes is marked by 7-8 weeks of fetal development and the first is universal, However, starting from 12-14 weeks of becoming lymphoid, due to changes in the microenvironment of hematopoietic cells- predecessors.

Hematopoiesis in the thymus begins with the tenth week of development and flows in the direction of the lymphoid.

Human bone marrow is laid on the second month of its development. At the same time, it found the first signs of hematopoiesis, and on the sixth month of pregnancy, he becomes the central authority of hematopoiesis. Initially, bone marrow cells predominate erythrogenesis, but as the growth and development of the fetus increases the percentage of cells of granulocytic and megakaryocytic series.

Hematopoiesis in the post-embryonic period

In the post-embryonic period of the formation of blood is carried out mainly in the bone marrow, spleen, lymph nodes. Red blood cells, granulocytes and platelets develop in the bone marrow. Lymphocytes are formed in lymph nodes, and other lymphoid organs. Monocytes, for the modern theory of hematopoiesis, are bone marrow-derived cells.

Driving hematopoiesis

According to modern ideas, in addition to hematopoietic tissue morphologically recognizable cells have progenitor cells of various classes. This position is reflected in the scheme of hematopoiesis, according to which there are five classes of hematopoietic cells.

The first class of stem cells It contains pluripotent hematopoietic cells, the second - Pluripotent cells and bipotent, third - Unipotentnymn cells, fourth - Blast cells - progenitors of series, fifth - Maturing cells.

The cells of monocytic series, According to available data, capable of phagocytosis, pinocytosis and adhere to glass. This allowed the authors to combine hematopoiesis scheme monocytes with different types of macrophages in the mononuclear phagocyte system. This system includes only phagocytic cells, have receptors for immunoglobulins and complement and strengthens immunoglobulins capable of phagocytosis.

Expected, that all cells, included in the system of mononuclear phagocytes, including monocytes, have a bone marrow-derived.