Table 15-11 presents a menu of regulatory factors that are involved in the process of hematopoiesis. A simplified scheme of hematopoietic cell differentiation is presented in Figure 15-21, including an indication of the site of action of the hormones and cytokines. The process of hematopoiesis has its origin in a single small population of self-renewing pluripotent stem cells that can be stimulated to differentiate along one of four general pathways; these include (a) the production of the antibody-generating cells the B and T lymphocytes, which are stimulated by interleukins 1-7; (b) the production of the white blood cell line, including monocytes, granulocytes, and macrophages whose collective production is largely stimulated by the family of colony-stimulating factors; (c) the production of platelets, which is promoted by several cytokines and hormones including erythropoietin; and (d) the production of the red blood cells, which is largely stimulated by erythropoietin.
The stem cells appear to require exposure to multiple cytokines to initiate the cell-differentiative process to create committed stem cells. It should be appreciated that although Figure 15-22 and Table 15-11 imply a linear cell differentiation process along precise pathways with precise locations for effects of the cytokines and growth factors, the in vivo reality is believed to be considerably more complex; e.g., there are no examples where one cytokine or regulatory action is confined to cells of a single hematopoietic lineage.
A detailed consideration of the complexities of the hematopoietic process is beyond the scope of this presentation; however, a brief discussion of the various classes of growth factors and hormones is presented in the following.
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