Lecture 31 – Blood Formation

The various types of blood cells are produced in the bone marrow. (10 to the 11th power of them each day in an adult human). Blood cells arise from a single type of cell called a hemopoietic stem cell – an adult multi-potent stem cell.
Stem cells:
are very rare; about one in every 10,000 bone marrow cells,
are likely attached by adherens junctions to osteoblasts lining the inner surface of bone cavities,
express a cell-surface protein designated CD34,
produce by mitosis, two kinds of progeny – more stem cells and cells that begin to differentiate along the paths leading to the various kinds of blood cells. (For instance, a mouse that has had all of his stem cells killed by radiation can besaved by a single dose of a stem cell.)

Which path is taken is regulating the need for more of that type of blood cell which is, in turn, controlled by appropriate cytokines and/or hormones.
Interleukin-7 is the major cytokine in stimulating bone marrow stem cells. These stem cells start down the ‘lymphoid’ path leading to the various lymphocytes (mostly B cells and T cells).
Some of the cytokines that drive the differentiation of the “myeloid” leukocytes are:
Erythropoietin (EPO), produced by the kidneys, enhances the production of red blood corpuscles (RBC’s).
Thrombopoietin (TPO), assisted by Interleukin -11 (IL-11), stimulates production of megakaryocytes. Their fragmentation produces platelets.
Granulocyte-macrophage colony-stimulating factor (GM-CSF), as its name suggests, sends cells down the path leading to both those cell types. One path or the other is taken.
Under the influence of the granulococyte colony-stimulating factor (G-CSF), they differentiate into neutrophils.
Further stimulated by interleukin-5 (IL-5) they develop into eosinophils.
Interleukin-3 (IL-3) participates in the differentiation of most of the white blood cells but plays a prominent role in the formation of basophils, which are responsible for some allergies.
Stimulated by macrophage colony-stimulating factor (M-CSF) the granulocyte/macrophage progenitor cells differentiate into monocytes, macrophages, and dendritic cells. (DC’s).

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