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FORMATION OF BLOOD CELLS
The primary source of blood cells is the mesenchyme connective tissue in the embryo. Three phases of embryonic hematopoiesis merge, resulting in the formation of blood (see figure 1-2).
Figure 1-2. Embryonic hematophoiesis.
During the first two months of embryonic development, the mesoblastic phase occurs. The blood cells are formed in the blood islands of the yolk sac. Immature blood cells develop, having large nuclei containing a vesicular chromatin meshwork. The nuclei are surrounded by a thin rim of cytoplasm. The cells gradually develop hemoglobin to become a nucleated red blood cell. Mitosis occurs and daughter cells contain more hemoglobin. Finally the nucleus is lost and the nature erythrocyte is produced.
At two months, the hepatic phase begins. Blood cell development shifts to the body of the fetus as the organs of the reticuloendothelial system (liver, spleen, thymus, etc.) are formed. During this phase, red cells begin their normal development. Granulocytes and thrombocytes are formed in the liver while lymphocytes and monocytes are formed in the spleen and thymus.The myeloid phase begins during the fifth month of gestation. Blood cells are formed in the bone marrow and lymphatic system that at the time of birth constitute the total sources of hematopoiesis. The bone marrow is the principle source of production of erythrocytes, granulocytes, and thrombocytes. The lymph nodes are the primary sites of production of lymphocytes, monocytes, and plasmacytes.
Blood formation at birth is confined primarily to the bone marrow (central medullary structure of the bone). Blood cells multiply by mitosis and then mature to a specific cell type. The mature cells lose the ability to reproduce and develop a definite life span. Regeneration of blood cells after birth involves multiplication of precursor cells, evolution of the definitive characteristics of each type, and release of mature cells.
Myelopoiesis is the production of blood cells and bone marrow by the bone marrow (medullary site of production). The red bone marrow is the principle source of production of red cells and white cells of the granulocytic series. At birth, the central medullary structure of bones is red bone marrow and it is actively engaged in hematopoiesis. At about 5 years of age non-hematopoietic type of marrow (fatty yellow bone marrow) that is a reserve potential tends to replace most of the red bone marrow. This partial replacement of red marrow is complete when the individual reaches maturity (about 18 years of age) at which time, active hematopoietic centers in bone tissue are limited almost exclusively to the sternum, pelvic area, vertebrae, skull, ribs, clavicle, scapuli, and the epiphyses of the long bones.Extramedullary hematopoiesis is blood production that occurs in sites other than the bone marrow. Active sites are the spleen, thymus, lymph nodes, and other lymphoid tissues. Cell production is largely limited to lymphopoiesis. The lymph nodes are the primary source of lymphocytes and plasmacytes (see figure 1-3).
Figure 1-3. Hematopoietic system in an adult 18-20 years of age.
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