Embryonal hemopoiesis - Practical hematology of children's age

The hemopoietic fabric develops already at early stages of an embryogenesis of the person. The first centers of a hemopoiesis at an embryo are in vitellicle islands. Then there is a consecutive change of the centers of a hemopoiesis — a liver, a spleen, marrow (tab. 1).
The first period of a haemo cytopoiesis begins on 2 — 3 weeks of development of an embryo. Primitive blood cells are formed of the central cells of blood islands of an extra embryonal mesenchyma, and from located on the periphery vessels are formed. Blood cells are generally provided by primitive erythroblasts. These are the cells of rounded shape, the big sizes containing a kernel. On the first weeks of an embryogenesis the erythroidal cells coming to a blood channel contain a kernel, but already by the end of 6 weeks of pre-natal development in a circulatory bed there are primary erythrocytes which lost a kernel. The main feature of a hemopoiesis is that already at this stage the vitellicle of an embryo contains stem polipotentny cells, originative to all sprouts of a hemopoiesis. The unilateral orientation of a differentiation of stem cells towards an erythrocytopoiesis during blood formation in a vitellicle is probably caused by influence of a microenvironment and humoral influence.
On 4 — 5 weeks the vitellicle is exposed to an atrophy, and its hemopoietic function gradually stops. On it the first stage comes to an end and actually embryonal hemopoiesis begins. On 3 — 4 weeks of an embryogenesis there is laying of a liver which on 5 — 6 weeks becomes a hemopoiesis source — the hepatic period of a hemogenesis. The first elements produced by a liver are primary erythroblasts (megaloblasts) which turn into primary erythrocytes (megaloytes). For 2 — 3 months of pre-natal development secondary erythroblasts already begin to be formed, gradually substituting primitive elements. During hepatic blood formation the bulk is made by erythroidal cells, but already predecessors of a granulotsitarny row are defined on 8 — 10 weeks of pre-natal development.
Hemopoiesis in a liver reaches a maximum to 18 — 20 weeks of pre-natal development and further begins to decrease. As a rule, by the end of the pre-natal period a hemopoiesis in a liver stops.
At the end of 3 months of life of an embryo the spleen and marrow are almost at the same time stuffed up. From 12th week of pre-natal development the spleen joins in hemopoiesis process. The spleen role as universal body of a hemopoiesis during the prenatal period is limited. Within several weeks in a mesenchyma of a spleen there are colonies of the stem polipotentny cells providing an erythrocytopoiesis, a granulocytopoiesis, formation of megacaryocytes. Then in a spleen there is a development of the lymphatic centers and the intensive lymphopoiesis begins with the 20th week of an embryogenesis. The spleen loses functions of universal body of a hemopoiesis and begins to produce the V-lymphocytes bearing immunoglobulins. However the spleen, as well as lymph nodes, is secondary lymphoid education, it plays the main role during the postnataldy period. In the period of an embryogenesis, and also in the first years of post-natal development of the person the importance in a lymphopoiesis belongs to primary lymphoid bodies which at vertebrata the thymus gland is, and at birds — a bursa of Fabricius.

Table 1. Development of hemopoietic system of the person (N. S. Kislya to, R. V. Lenskaya, 1978)

The thymus gland is put on 5 — 6 weeks of pre-natal development, and about 9 — 10 weeks in its stroma the first lymphocytes are already found.
In an embryogenesis the thymus gland plays an active role as the body predetermining a differentiation of T lymphocytes. Gistofunktsionalny formation of a thymus gland as body happens quickly enough and from 20th week of pre-natal development thymocytes of an embryo are similar to the child's thymocytes. The thymus gland role in a lymphopoiesis remains during the early post-natal period, dies away only after formation of peripheral lymphatic system and movement of work of a lymphopoiesis.
Development of peripheral lymphatic fabric begins at the beginning of 4 months of an antenatal life. The first rudiments of lymph nodes appear in the field of cervical lymphatic bags. Lymphatic system reaches full development after the child's birth.
On 13 — the 14th week of pre-natal development in marrow the first centers of a hemopoiesis appear. Their localization gradually changes. During the early embryonal period they are concentrated in tubular bones (a diaphysis of femoral, humeral bones), in process of development of a bone skeleton they move to flat spongy bones.
At children of the first years of life in a diaphysis of tubular bones the hemopoiesis centers as 50% of all marrowy substance with * are stavlyat by red marrow remain. Since 6 flyings, there is substitution of red marrow fatty marrow, and the centers of a haemo cytopoiesis remain in flat spongy bones and an epiphysis of long tubular bones. Already at early stages of pre-natal development marrow contains polipotentny stem cells though generally has a myeloid focus of a hemopoiesis — eritro-, granulo-and megakariotsitopoez. Nevertheless marrow is of great importance and in development of V-cells though it is not similar to a bursa of Fabricius of birds. By the time of the child's birth the extramedullary centers of a miyelotsitopoez are almost completely liquidated and marrow, in the beginning tubular, and after flat spongy bones becomes the center of a hemopoiesis. The spleen and lymph nodes keep the value in processes of a lymphocytopoiesis.