Epigenetic regulation in stem cell development, cell fate conversion, and reprogramming.

Stem cells are identified classically by an in vivo transplantation assay plus additional characterization, such as marker analysis, linage-tracing and in vitro/ex vivo differentiation assays. Stem cell lines have been derived, in vitro, from adult tissues, the inner cell mass (ICM), epiblast, and male germ stem cells, providing intriguing insight into stem cell biology, plasticity, heterogeneity, metastable state, and the pivotal point at which stem cells irreversibly differentiate to non-stem cells. During the past decade, strategies for manipulating cell fate have revolutionized our understanding about the basic concept of cell differentiation: stem cell lines can be established by introducing transcription factors, as with the case for iPSCs, revealing some of the molecular interplay of key factors during the course of phenotypic changes. In addition to de-differentiation approaches for establishing stem cells, another method has been developed whereby induced expression of certain transcription factors and/or micro RNAs artificially converts differentiated cells from one committed lineage to another; notably, these cells need not transit through a stem/progenitor state. The molecular cues guiding such cell fate conversion and reprogramming remain largely unknown. As differentiation and de-differentiation are directly linked to epigenetic changes, we overview cell fate decisions, and associated gene and epigenetic regulations.