Epigenetic Mechanisms in Early Mammalian Development

In one sense, development of multicellular organisms begins well before fertilization because the molecules and controlling mechanisms that direct early development are put in place during oogeneisis. The informational content of a mammalian egg at fertilization is not only restricted to its DNA sequence, but also to various DNA and chromatin modifications, specific macromolecules (RNAs and proteins), and possibly the characteristic architecture of the cytoplasm and plasma membrane. These sources of stored, necessary information can be viewed as epigenetic controlling mechanisms (Fig. 1). In the ensuing text we will briefly touch on these subjects, delineating what is presently known about them and, more important, what still remains to be eclucidated. The literature on these subjects is vast and space restrictions prevent us from citing each relevant paper, for which we apologize