Stem sense: a proposal for the classification of stem cells.
Stem cells, while difficult to define, hold great promise as tools for understanding development and as therapy. However, this difficulty in defining stem cells has led to a multiplicity of stem cells that may or may not be distinct. The lack of common standards or definitions, the absence of a common forum for discussion, and the range in the ability to manipulate his/her favorite system of stem cells has led to further fragmentation of a field bedeviled by controversy. I suggest that stratification and classification of stem cells on the basis of their function, characteristics, and capabilities would be of enormous benefit to the community. This absence of uniform nomenclature and classification has led to many contradictory claims as to the abilities of stem cells and has made it very difficult to generalize across systems and cell types. I illustrate the problem by providing two examples of how the lack of uniform definitions has slowed progress. I suggest that the effort to establish a consensus on what constitutes a tissue-specific stem cell (definition) and how one would stratify cells (classification) would greatly facilitate progress and perhaps help resolve some of the outstanding controversies.