Stem cells are multipotential in nature. They can develop into any type of tissue (differentiate) and thereby create various tissues and eventually ‘‘build’’ organs such as muscle, liver, or blood. The differentiated cells function in specialized ways, and most of them stop growing. Stem cells fail to stop dividing. Tumors contain immature cells that exhibit differentiation failures. Such a rare defectively differentiated subset of stem cells in tissues has been proposed as the origin of tumor cells. An example is acute promyelocytic leukemia, where stem cells have been blocked from achieving differentiation.
Normal cells can stop growing permanently, a process of arrest that is designated cell senescence. Cells survive for varying lengths of times. At one extreme, brain cellsmight last a lifetime, butwhite cells in blood survive for only about twomonths. Cancer cells, in contrast, are immortalized and have an unlimited potential to proliferate. This indefinite proliferation requires activity of telomerase, an enzyme that at each cycle of the cell adds back DNA sequences of telomeres to the ends of chromosomes. Telomerase is active inmalignant cancer cells but not in normal cells
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