By Brigid L.M. Hogan
To produce a human being, a single fertilized egg must generate billions of cells and more than 250 different cell types. Fortunately, all is not over when the last cell is made and the last organ system assembled. Throughout life, most tissues continually generate new cells, either to replace those lost by wear and tear or to satisfy increased demand. For example, when athletes train at high altitudes the number of their circulating blood cells increases in response to increased need for oxygen delivery. This capacity for cell regeneration is particularly evident in adult tissues such as skin, hair, bone marrow and intestine, but it probably occurs to some extent in most organsincluding the brain, where, until recently, conventional wisdom held that neurons were irreplaceable.
The ability of a tissue to renew and repair itself depends on small groups of cells known as stem cells. These stem cells exist throughout life in close proximity with specialized "nurse" cells located in tiny niches in the body. Nurse cells provide growth factors and other signals that help maintain the unique properties of the stem cellstheir capacity to generate differentiated, or specialized, progeny with a limited life span while making more of themselves at the same time. Paradoxically, stem cells divide very little, while their descendants often multiply exuberantly en route to their final identity.
Stem cells, and the various strategies they use to maintain their numbers, have long fascinated biologists.1 But that interest has now reached fever pitch with the unexpected discovery that stem cells from some adult tissues can be reprogrammed, albeit at extremely low efficiency, to give rise to differentiated cells of other tissues.2 For example, it appears that under certain conditions a few blood stem cells can give rise to muscle, and neuronal stem cells to blood, in adult mice. These observations raise the possibility that we might someday be able to repair damaged organs starting with only a few residual stem cells taken from another tissue in the same body.
1 Readers interested in references and further reading about stem cells and the ethical debates surrounding them should consult the many review articles in Science, 287, pp. 1417-1442 and Cell 100, pp. 143-168.
2 Science, Vol. 287, Fig. 1, p. 1428, 25 February 2000, © AAAS.