Epigenetic states in stem cells.

Whereas embryonic stem cells can differentiate into all cell types of the body, stem cells found in somatic tissues display more restrictive differentiation capacity. The extent of multi-lineage differentiation ability of stem cells is believed to be associated with the potential for expression of developmentally- and differentiation-regulated genes. Growing evidence suggests that this potential for gene expression in undifferentiated cells is regulated by epigenetic processes on DNA and chromatin in regulatory and coding regions. Genome-wide mapping of DNA methylation profiles and of post-translational histone modifications in stem cells and differentiated cells has led to the establishment of chromatin states primarily on promoters of active, repressed and potentially active genes. These maps contribute to unveiling regulatory mechanisms by which genes are poised for transcription in undifferentiated cells. We summarize here the current view of how specific combinations of epigenetic marks may define the pluripotent state.