hematopoiesis (redirected from fetal hematopoiesis)

hematopoiesis [‚he·məd·ō·pȯi′ē·səs]

(physiology)

The process by which the cellular elements of the blood are formed. Also known as hemopoiesis.

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Hematopoiesis

The process by which the cellular elements of the blood are formed. The three main types of cells are the red cells (erythrocytes), which serve to carry oxygen, the white cells (leukocytes), which function in the prevention of and recovery from disease, and the thrombocytes, which function in blood clotting. The formation of these cells is one of the most active and important processes in the body. Most of the circulating cells live only for a short time and must be replaced in order to maintain life. For instance, in the human adult a red blood cell has a life of 120 days; 250 billion new red cells have to be produced daily to replace those that are destroyed.

Blood cells originate in the reticuloendothelial tissue, which is a loose, fibrous, highly vascularized mesh of fibers, endothelial cells, and macrophages. Within the spaces of the tissue are found the precursor (blast) cells of the definitive adult types. For the sake of convenience, the reticuloendothelial tissue is divided into two general but imprecise types: lymphoid and myeloid tissue. Lymphoid tissue is primarily localized in the lymph nodes of the lymphatic system and is also in the spleen, thymus, and bone marrow. Several classes of white cells are produced, including the lymphocytes, macrophages, and monocytes. See Cellular immunology, Lymphatic system

Myeloid tissue is normally limited in humans to the red bone marrow of the ribs, sternum, vertebrae, and proximal ends of the long bones of the body. It is concerned with the production of the erythrocytes and certain types of leukocytes. The latter are the granular leukocytes (called eosinophils, basophils, and neutrophils on the basis of the affinity of granules in their cytoplasm for certain dyes) and megakaryocytes. Fragments of megakaryocytes form the blood platelets (thrombocytes), which are necessary for blood clotting. See Blood