The Origin of Blood Cells

Blood cells originate in bone marrow from hemocyto-blasts, a type of stem cell (fig. 14.3). A stem cell can differentiate into any of a number of specialized cell types. As hemocytoblasts divide, the new cells respond to different secreted growth factors, called colony-stimulating factors, that turn on some genes and turn off others. This exposure to growth factors ultimately sculpts the distinctive formed elements of blood, including the cellular components of the immune system. A protein called thrombopoietin (TPO) stimulates large cells called megakaryocytes to proliferate. These cells eventually come apart to yield platelets.

H What are the major components of blood? ^9 What factors affect blood volume? ^9 How is hematocrit determined? How do blood cells form?

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