Hematopoeisis means formation of the cells of the blood. All types of blood cells originate from hematopoetic stem cells, mostly found in the medulla of the bone marrow in adult dogs. Hematopoeitic stem cells are self-renewing – when they divide, some of the “daughter” cells will remain as stem cells to keep the stock levels up and other “daughter” cells will evolve into whichever type of blood cell is required by the body. In cases of bone marrow failure, it is possible for the liver, thymus, and spleen to resume their haematopoietic function (see extramedullary hematopoeisis). What type of blood cell these stem cells eventually become is determined by stimulating factors. For example, erthyropoeitin will stimulate red cell production (erythrocytes), thrombopoeitin will stimulate platelet (thrombocytes) production. Other types of blood cells have a far more complicated stimulation process. Incredibly, even after total irradiation of the bone marrow, stem stells will usually re-colonise.
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