What is Stem Cell ?
The definition of stem cells must be on a functional basis. Even as the identification of structural attributes of stem cells at the morphological or molecular levels becomes possible [current candidates include high levels of expression of the multidrug resistance gene and certain combinations of integrin expression], it will always be the seductive function of stem cells that will be their defining feature.
Functionally, stem cells are the multipotential, self-renewing cells that sit at the top of the lineage hierarchy and proliferate to make differentiated cell types of a given tissue in vivo.
It is important to restrict this definition to single cells that, once developed, self-renew for the lifetime of the organism in order to distinguish stem cells from the many types of more transient progenitor cells (with limited self-renewal life-spans) that are present, especially in complex organisms. This may be more than semantic classification, because stem and progenitor cells defined by self-renewal ability may constitute different classes of cells under different molecular regulation across tissue types. In vivo in adult organisms, stem cells can divide repeatedly to replenish a tissue or may be more quiescent, as in the mammalian brain. Rather than considering stem cells as undifferentiated cells, it may be more productive to think of them as appropriately differentiated for their specific tissue niches, with perhaps the ability to display more potential phenotypes in alternate niches. Stem cells can divide symmetrically during development to expand their numbers and asymmetrically to self-renew and give rise to a more differentiated progeny. Indeed, as suggested for mammalian hematopoietic stem cells, the differentiation of specific blood progenitors from the asymmetric division of stem cells may be stochastic, with only the rate of proliferation of the stem cells under specific regulation.