Hematopoiesis in animals
Hematopoiesis in animals is the processes of formation of erythrocytes, leukocytes and platelets in the body. Formation of erythrocytes and leukocytes respectively are known as erythropoiesis and leukopoiesis.
During embryonic state the blood islands of pander of the yolk sac functions as a site of hematopoiesis. The mesenchymal cells of the liver, spleen, bone marrow and lymph glands are the hemopoietic organs in early fetal life.
Thebone marrow is concerned with the production of erythrocytes, granulocytes and platelets during postnatal life, whereas the lymphocytes production occurs in thelymphoid tissues of lymph glands, Payer’s patches of intestine, spleenand thymus.
The lymphoid tissues of the bone marrow and also the spleen are the sites of production formonocytes. In ruminants, hemolymph nodes (hemal) functions as spleen. It takes part in the erythropoiesisduring the foetal period, while granulopoiesis, is more prevalent in postnatal life.
- The mesenchymal cells of the yolk sac produce primitive stem cells, which give rise to the pleuripotent stem cells (colony forming units – CFU). These stem cells give rise to five different blast cells, viz.
- Proerythroblast to form RBC
- Myeloblastto form neutrophils, eosinophils and basophils
- Monoblast to form monocyte
- Lymphoblast to form lymphocyte
- Megakaryoblastto form platelets. Depending on the microenvironment, i.e., the location of the stem cells and the growth factors, the stem cells differentiate into progenitor cells of different blood cells (Committed Stem Cells-CSC). A CSC that produces erythrocytes is called colony-forming unit-erythrocyte (CFU-E). Similarly, CFU that produce granulocytes and monocytes are designated as CFU-GM.
The stem cells continue to divide throughout the life of the animal and a part of the cells remains as pleuripotent stem cells and retained in the bone marrow to maintain supply of stem cells.
The pleuripotent stem cells differentiate to form the CSC. Several hemopoietic growth factors and differentiation factors stimulate the growth and differentiation of these stem cells into a particular progenitor cells.
Cytokinins are the growth factors that regulate the formation of blood cells. Two cytokinins that stimulate red cell and WBC formation are the colony stimulating factors and interleukins. Erythropoietin increases erythrocyte precursor formation.