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© 2009 Sinauer Associates, Inc. UNCORRECTED PAGE PROOFS
CELL DEATH AND CELL RENEWAL Stem cell
renewal Proliferation Differentiation Differentiated cells FIGURE 17.17 Stem cell proliferation Stem cells divide to form one daughter
cell that remains a stem cell and a second that proliferates and then differentiates. 100 billion blood cells are lost every day in humans, and must be continually produced from hematopoietic stem cells in the bone marrow (Figure
17.18). Descendants of the hematopoietic stem cell continue to proliferate
and undergo several rounds of division as they become committed to specific differentiation pathways that are determined by growth factors that
channel precursor cells along specific pathways of blood cell differentiation.
Once they become fully differentiated, blood cells cease proliferation, so the
maintenance of differentiated blood cell populations is dependent on continual division of the self-renewing hematopoietic stem cell.
The intestine provides an excellent example of stem cells in the selfrenewal of an epithelial tissue. The inte...
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