Epigenetics of oogenesis.

Epigenetic changes include all modifications affecting the expression of genes without changing the nucleotide sequence of the genome. Most studied epigenetic changes include DNA methylation, histone alterations and non-coding RNAs. DNA methylation is an important epigenetic mark, protecting the genome during gametogenesis and early embryo development. Demethylation process is a genome-wide event, taking place in two distinct waves during gametogenesis. The first event helps restore naïve pluripotency of the zygote, while the second event aids in the loss of parental epigenetic memory and facilitates specification of gametes. Histone modifications were recognized in murine and human primordial germ cells where their subsets condense chromatin, protecting it from dynamic changes taking place during gamete maturation. Deacetylation of histones was recognized as an important prerequisite of chromosomal segregation during metaphase II. Germline-specific ncRNAs and piRNAs are important in inhibiting transposon activity during gametogenesis, protecting overall genome stability. All epigenetic changes are prone to disruption, especially by exogenous factors. In recent years, with the increase in infertility, the association between assisted reproductive technology (ART) and its effects on epigenome remodeling of gametes have gained importance. The aim of this review is to summarize the epigenetic modifications crucial for oocyte development, while highlighting their role in reproductive disorders and ART.