Figure 1. The concepts of developmental programming and reprogramming.

During developmental differentiation or programming, different cell types acquire distinct epigenetic profiles mediated by transcription factors (TF) and epigenetic modifications. In this figure, the epigenetic code is reflected by modifications of histones associated with gene activation (Ac, acetylation; H3K9Ac, acetylation of histone H3 at lysine 9; H3K4Me, methylation of histone 3 at lysine 4) or repression (H3K9Me and H3K27Me, methylation of histone H3 at lysine 9 and 27, respectively) and by methylation of CpG dinucleotides. Developmental programming results in somatic cells that exhibit decreased pluripotency. Reprogramming, achieved by exogenous expression of transcription factors, allows for a cell to switch from an epigenome of usually reduced potency to one of pluripotency. Open circles represent unmethylated CpGs; filled circles represent methylated CpGs.