Series Introduction: Stem cells near the century mark

The concept of a stem cell, now nearing its hundredth year as one of the organizing principles of developmental biology, shows no sign of losing its youthful luster. At a time when the transfer of biological concepts to clinical practice drives much of science, the properties of stem cells in various tissues are attracting increasing levels of interest. This attention is not restricted to the germ line and blood, the traditional domain of stem cells as defined by early 20th-century studies on animal development, but has been extended to tissues not typically thought to turn over. As novel sites, properties, and functions have been identified for these cells, the definition of a stem cell has shifted repeatedly. Like some stem cells, this concept has expanded greatly and has displayed a remarkable degree of plasticity.