Different cell types in an organism are defined by their specific pattern of highly regulated gene expression, thus only expressing a subset of all genes that consitute the genome. Cell differentiation therefore describes the change from one gene expression pattern to another which is highly regulated by a gene regulatory network. The switch from one pattern to another commonly occurs due to cell signaling induced by e.g. growth factors. After binding of the ligand, secreted by one cell to the receptor of the other cell, the recptor activates cellular proteins, starting a cascade of reactions and finally activating a dormant transcription factor or cytoskeletal protein, thus activating the differentiation process.
Epigenetic processes, which determine if a stem, progenitor or mature cell fate is adopted, play a crucial role in the maintainance of the specific expression pattern of cell types through a number of cell divisions. The three transcription factors Oct4, Sox2 and Nanog are essential for the maintainance of pluripotency, thus being highly expressed in undifferentiated cells such as embryonic stem cells. It seems that these factors regulate gene expression in pluripotent cells by the modification of histones and DNA methylation which affects the accessibility of the target genes.
Extrinsic signaling might lead to epigenetic remodeling since extrinsic signaling also leads modifications in the gene expression pattern by activation or repression of transcription factors. However only little data is available.