General features of stem cells

Although still a contentious issue, the prevailing view is that stem cells have the capacity for unlimited or prolonged self-renewal and the ability to produce at least one type of highly differentiated descendent (9-12). It is also generally accepted that, between the stem cell and its terminally differentiated progeny, there is an intermediate population of committed progenitors with limited proliferative capacity and restricted differentiation potential, sometimes defined as transit-amplifying cells. The primary function of this transit population is to increase the number of differentiated cells produced by each stem cell division. Thus, a stem cell, which has high self-renewal capacity, may actually divide relatively infrequently.

Stem cells are thought to undergo asymmetric division to yield one stem cell daughter and one differentiated daughter (progenitor). This may be true in certain situations, but the population of stem cells can still be self-maintaining when some cell divisions yield two stem cell daughters and others yield two differentiating daughters (10). Symmetric divisions allow the size of the stem cell pool to be regulated by factors that control the probability of self-renewing vs differentiative divisions (3).

Another characteristic attributed to stem cells is that they divide slowly or rarely. This is thought to be true for stem cells in the skin (13) and bone marrow (14). Other kinds of stem cells, however, divide more rapidly. Stem cells in the mammalian intestinal crypt have been estimated to divide every 12 h (3).

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