In children the marrow of most bones produces blood cells. By adulthood, however, only the bones of the chest, the base of the skull, and the upper portions of the limbs remain active. The bone marrow in an adult weighs almost as much as the liver, and it produces cells at an enormous rate.
All blood cells are descended from a single population of bone marrow cells called pluripotent hematopoietic stem cells, which are undifferentiated cells capable of giving rise to precursors (progenitors) of any of the different blood cells. When a pluripotent stem cell divides, its two daughter cells either remain pluripotent stem cells or become committed to a particular developmental pathway; what governs this "decision" is not known. The first branching yields either lymphoid stem cells, which give rise to the lymphocytes, or so-called myeloid stem cells, the progenitors of all the other varieties (Figure 14-6). At some point, the proliferating offspring of the myeloid stem cells become committed to differentiate along only one path, for example into erythrocytes.
Proliferation and differentiation of the various progenitor cells is stimulated, at multiple points, by a large number of protein hormones and paracrine agents collectively termed hematopoietic growth factors (HGFs). Thus, erythropoietin, the hormone described earlier, is an HGF. Others are listed for reference in Table 14-4. (Nomenclature can be confusing in this area since the HGFs belong to a still larger general family of messengers termed "cytokines," which are described in Chapter 20.)
The physiology of the HGFs is very complex because (1) there are so many of them, (2) any given HGF is often produced by a variety of cell types throughout the body, and (3) HGFs often exert other actions in addition to stimulating blood-cell production. There are, moreover, many interactions of the HGFs on particular bone marrow cells and processes. For example, although erythropoietin is the major stimulator of erythropoiesis, at least 10 other HGFs cooperate in the
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.