Stem cells: shibboleths of development, part II: Toward a functional definition

Our previous discourse on stem cell characteristics led to the conclusion that the qualities deemed essential for a cell to be considered a "stem cell" are neither firmly established nor universally accepted, and this we accept as editorial policy. In that study, self-renewal, asymmetric division, phenotypic markers, and other attributes touted as being indicative of cells being stem cells were critically questioned as fundamental to the definition of a stem cell, leading us to seek a functional definition instead. Here, we offer further considerations, and elaborate on the characteristics that diverse investigators feel are essential for a cell to function as a stem cell, either in development or body maintenance. We hope that this discourse will promote further reflection, culminating with a definition that is widely accepted and universally applicable. We confess this goal has not been reached, neither here nor elsewhere. The outstanding goal of understanding what stem cells are, a prerequisite of characterizing what stem cells do and how they do it, is still outstanding.