Almost all adult tissues require periodic replenishments and damage repairs. A group of slowdividing cells, called stem cells, nestled within each tissue carries out thejob throughout the lifespan of an organism.
Stem cells have a unique capabilityof undergoing regulated asymmetric divisions resulting in self-renewal and generationof a progeny cell with predetermined cell-fates (Friedenstein et al., 1976, 1974; Gilbert, 2000). Often, the stem cell progenyundergoes a few rounds of mitoses before differentiation. The capacity todivide and differentiate varies with the type of stem cell (reviewed in Weissman, 2000). They are broadly classified into two categories on the basis of the developmentalstate of the derived tissue.
Stem cells derived from an early embryo(inner cell mass of the blastocyst), known as embryonic stem cells,possess potential to generate almost all cell types (except extra-embryonictissue) and hence termed as pluripotent (Smith, 2001). Stem cellsmaintaining adult tissues, known as adult stem cells, have reduced potency andcan give rise to limited cell types. Specificadult stem cells have been described as multipotent (multiple cell types),e.
g., hematopoietic stem cells which generate a large variety of blood andassociated tissue (Hoang,2004);oligopotent (only few cell types), e.g. neuronal stem cells which generatedifferent types of neurons and glia (Reynoldset al., 1992),and unipotent (single cell type), e.g. germline stem cells (GSCs) which produceeither egg or a sperm (Yuanand Yamashita, 2010).
Unraveling the molecular mechanisms regulating (orgoverning) the stem cell self-renewal has been of great interest indevelopmental biology (Guoet al., 2011; Hirai et al., 2011; Simons and Clevers, 2011; Staal et al., 2011;Yun et al.
, 2010; Zhao et al., 2011). Some of the criticalunresolved issues are how the cell fates are defined and what regulates theextent of divisions of the stem cell progeny.
Deciphering the underlyingsignaling molecules involved in these processes are expected to play essential roles in developing regenerativemedicine and therapies.