ABSTRACT
The growth and maturation of the oocyte is a complex and highly regulated process. An important aspect of oocyte maturation is the establishment of the correct epigenetic status. ‘Epigenetics’ refers to processes such as DNA methylation or histone modifications that regulate gene activity without affecting the actual DNA sequence, but are heritable through cell division. The epigenetic state of the male and female germ cells is not equivalent; this was first discovered as a result of ingenious nuclear transplantation experiments carried out in the mid 1980s demonstrating that uniparental embryos are not viable1,2. Subsequently, a subset of mammalian genes was found to be subject to genomic imprinting. Importantly, a number of imprinted genes are essential for fetal growth and development, including the functioning of the placenta. These genes are expressed in a parent-of-origin specific manner, as a result of the different epigenetic profiles acquired by imprinted genes during male and female gametogenesis. The best characterized epigenetic modification is the methylation of cytosine residues in DNA, which is involved in establishing genomic imprints in the germ line. This establishment occurs during the growth phase
of oocyte development, and is beginning to be elucidated in greater detail. In recent years, increased concern has been focused on the potential for epigenetic dysregulation as a result of early embryo culture and assisted reproductive technologies3,4.
