Hemopoiesis (hematopoiesis) includes both erythropoiesis and leukopoiesis (development of red and white blood cells, respectively), as well as thrombopoiesis (development of platelets; Fig. 10.16). Blood cells have a limited life span; they are continuously produced and destroyed. The ultimate objective of hemopoiesis is to maintain a constant level of the different cell types found in the peripheral blood. Both the human erythrocyte (life span of 120 days) and the platelet (life span of 10 days) spend their entire life in the circulating blood. Leukocytes, however, migrate out of the circulation shortly after entering it from the bone marrow and spend most of their variable life spans (and perform all of their functions) in the tissues.
In the adult, erythrocytes, granulocytes, monocytes, and platelets are formed in the red bone marrow; lymphocytes are also formed in the red bone marrow and in the lymphatic tissues. To study the stages of blood cell formation, a sample of bone marrow is prepared as a stained smear in a manner similar to that described on page 270 for the preparation of a smear of blood.
Hemopoiesis is initiated in early embryonic development.
During fetal life, both erythrocytes and leukocytes are formed in several organs before the differentiation of the bone marrow. The first or yolk-sac phase of hemopoiesis begins in the third week of gestation and is characterized by the formation of “blood islands” in the wall of the yolk sac of the embryo. In the second, or hepatic phase, early in fetal development, hemopoietic centers appear in the liver (Fig. 10.17). Blood cell formation in these sites is largely limited to erythroid cells although some leukopoiesis occurs in the liver. The liver is the major blood-forming organ in the fetus during the second trimester. The third or bone marrow phase of fetal hemopoiesis and leukopoiesis involves the bone marrow (and other lymphatic tissues) and begins during the second trimester of pregnancy. After birth, hemopoiesis takes place only in the red bone marrow and lymphatic tissues, as in the adult (Fig. 10.18). The precursors of both the blood cells and germ cells arise in the yolk sac.
Monophyletic Theory of Hemopoiesis
According to the monophyletic theory of hemopoiesis, blood cells are derived from a common hemopoietic stem cell.
Considerable circumstantial evidence has for many years supported the monophyletic theory of hemopoiesis in which all blood cells arise from a common stem cell. Decisive evidence for the validity of the monophyletic theory has come with the isolation and demonstration of the hemopoietic stem cell (HSC). The hemopoietic stem cell, also known as pluripotential stem cell (PPSC), is capable not only of differentiating into all the blood cell lineages but also of self-renewal ( i.e., the pool progenitor (CLP, CFU-L)