Hematopoiesis definition - Pluripotency, Differentiation, and Proliferation

The process through which the body generates new BLOOD cells. In the adult, the red BONE MARROW and the LYMPH tissues (primarily the lymph nodes and the SPLEEN) manufacture the blood cells the body needs, with extramedullary resources for ERYTHROCYTE production available as reserves from the LIVER, spleen (erythrocytes), and yellow BONE marrow. Researchers do not fully understand the mechanisms of hematopoiesis though know complex interactions of hormones, proteins, and chemicals regulate the processes by which the body makes new blood cells. There are two major divisions of hematopoiesis: erythropoiesis (production of erythrocytes) and leukopoiesis (production of leukocytes).

Pluripotency, Differentiation, and Proliferation

As best researchers understand the mechanisms of hematopoiesis, all blood cells arise from pluripotent BLOOD STEM CELLS that have the ability to develop into any of the blood cell types. The first level of hematopoiesis occurs when a blood stem cell either proliferates, extending the volume of pluripotent cells, or differentiates into one of two committed lineages, myeloid or lymphoid. The lymphoid lineage will produce lymphocytes and monocytes, and the myeloid lineage will produce erythrocytes, granulocytes (basophils, eosinophils, and neutrophils), and platelets. Each lineage generates a number of differentiations or stages of development. The length of time it takes for a pluripotent cell to produce a mature blood cell varies with the type of blood cell and other physiologic factors, ranging from 6 days for an erythrocyte to 14 days for a neutrophil.

Erythropoiesis

Erythropoiesis begins with committed myeloid cells that differentiate into myeloblasts or proerythrocytes. Myeloblasts will become granulocytes, the majority of which will be neutrophils. Proerythrocytes will become erythrocytes (more than 99 percent) or platelets (less than 1 percent). Numerous substances influence and regulate erythropoiesis. Among them are

• ERYTHROPOIETIN (EPO), a HORMONE the KIDNEYS secrete that stimulates the bone marrow to increase differentiation of proerythrocytes and thus increase erythrocyte production
• intrinsic factor, or erythrocyte-maturing factor, which the STOMACH secretes to facilitate erythrocyte maturation
• vitamin B12, also called extrinsic factor, which interacts with intrinsic factor
• iron, which is an essential component of HEMOGLOBIN (the protein complex within erythrocytes that binds with oxygen)

An erythrocyte goes through several stages of development before reaching a mature enough stage, that of reticulocyte, to leave the bone marrow. After 24 hours in circulation in the blood, the reticulocyte evolves to its final stage of maturity and becomes a fully functional erythrocyte. Erythrocytes circulate in the blood for about 120 days. The red bone marrow releases 2 million reticulocytes per minute into the blood circulation; the spleen extracts a comparable number of old erythrocytes from the circulation to maintain the correct proportion of erythrocytes in the blood.

Platelets arise from proerythrocytes that differentiate to become megakaryoblasts and then megakaryocytes. The megakaryocytes release fragments of their cytoplasm, which become platelets. While megakaryocytes are the largest cells in the bone marrow, platelets are the smallest particles in the blood. The spleen retains about 30 percent of the platelets the bone marrow produces, releasing them when a COAGULATION cascade sends chemical signals summoning platelets to the site of clot formation.

Leukopoiesis

Leukopoiesis, the production of white blood cells, takes place in both the bone marrow (granulocytes) and the lymph tissues (monocytes and lymphocytes). In general, all three types of leukocytes make up less than 1 percent of the blood cells in circulation. Many factors influence leukopoiesis, including immune status and whether an INFECTION is present in the body. Leukocytes also undergo a series of developmental evolutions before reaching maturity. Lymphocytes the lymph tissues release are immature and migrate to the THYMUS (T-cell lymphocytes) or the bone marrow (B-cell lymphocytes) to mature.

See also CELL STRUCTURE AND FUNCTION; HEMOLYSIS.

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