DNA Methylation Reprogramming in Preimplantation Development

Shortly after fertilisation the embryonic cells pass through a transient state of totipotency followed by the formation of unipotent trophoblast and pluripotent embryonic stem cells, i.e. the first determined cell lineages which can be derived from the early blastocyst. The molecular processes leading to totipotency formation are initiated by molecules and enzymes provided by the egg cytoplasm. The first molecular changes that can be observed are extensive reconfigurations of the sperm chromatin accompanied by major changes in epigenetic marks of the DNA and chromatin. The epigenetic reprogramming starts in the paternal pronucleus of the zygote and eventually affects both parental chromosomes yielding strongly altered DNA and histone modifications at later developmental stages. In this chapter we will discuss the major molecular differences occurring during the first phase of epigenetic reprogramming with a focus on DNA modification dynamics in the mammalian zygote.