Epigenetic reprogramming: enforcer or enabler of developmental fate?

A single fertilized egg is programmed to differentiate into a multitude of distinct cell types that comprise a multicellular organism. Epigenetic mechanisms such as DNA methylation and histone modifications are intricately involved in regulating developmental potential and cellular identity by establishing permissive or repressive chromatin states that are mitotically heritable. Here, we review the dynamics of major epigenetic marks during early mammalian development, and explore the question of whether DNA methylation and chromatin modifications enable or enforce changes that lead to the first cell fate decision.