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Unformatted text preview: entire liver within a few days. Proliferation of remaining cells Stem Cells Regeneration of liver Most fully differentiated cells in adult animals, however, are no longer capable of cell division. Nonetheless, they can be replaced by the proliferation of a subpopulation of less differentiated self-renewing cells called stem cells that are present in most adult tissues. Because they retain the capacity to proliferate and replace differentiated cells throughout the lifetime of an animal, stem cells play a critical role in the maintenance of most tissues and organs. The key property of stem cells is that they divide to produce one daughter cell that remains a stem cell and one that divides and differentiates (Figure 17.17). Because the division of stem cells produces new stem cells as well as differentiated daughter cells, stem cells are selfrenewing populations that can serve as a source for the production of differentiated cells throughout life. The role of stem cells is particularly evident in the case of several types of differentiated cells, including blood cells, sperm, epithelial cells of the skin, and epithelial cells lining the digestive tract--all of which have short life spans and must be replaced by continual cell proliferation in adult animals. In all of these cases, the fully differentiated cells do not themselves proliferate; instead, they are continually renewed by the proliferation of stem cells that then differentiate to maintain a stable number of differentiated cells. Stem cells have also been identified in a variety of other adult tissues, including skeletal muscle and the nervous system, where they function to replace damaged tissue. Stem cells were first identified in the hematopoietic (blood-forming) system by Ernest McCulloch and James Till in 1961 in experiments showing that single cells derived from mouse bone marrow could proliferate and give rise to multiple differentiated types of blood cells. Hematopoietic stem cells are well-characterized and the production of blood cells provides a good example of the role of stem cells in maintaining differentiated cell populations. There are several distinct types of blood cells with specialized functions: erythrocytes (red blood cells) that transport O2 and CO2; granulocytes and macrophages, which are phagocytic cells; platelets (which are fragments of megakaryocytes) that function in blood coagulation; and lymphocytes that are responsible for the immune response. All these cells have limited life spans ranging from less than a day to a few months, and all are derived from the same population of hematopoietic stem cells. More than
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CELL DEATH AND CELL RENEWAL 17 Stem cell Self 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 intestine is lined by a single layer of epithelial cells that are responsible for the digestion of food and absorption of nutrients. These intestinal epithelial cells are exposed to an extraordinarily harsh environment and have a lifetime of only a few days before they die by apoptosis and are shed into the digestive tract. Renewal of the intestinal epithelium is therefore a continual process throughout life. New cells This material cannot be copied, reproduced, manufactured, or disseminated in any form without express written permission from the publisher. 2009 Sinauer Associates, Inc. UNCORRECTED PAGE PROOFS
CHAPTER 17 Hematopoietic stem cell Myeloid Lymphoid Reticulocyte Megakaryocyte Monocyte Erythrocyte Platelets Neutrophil Macrophage Eosinophil Granulocytes Basophil B lymphocyte T lymphocyte FIGURE 17.18 Formation of blood cells All of the different types of blood cells develop from a hematopoietic stem cell in the bone marrow. The precursors of differentiated cells undergo several rounds of cell division before they differentiate. are derived from the continuous but slow division of stem cells at the bottom o...
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