What does it take to make a developmentally competent mammalian egg?

Abstract

Background: A limitation to our ability to distinguish between developmentally competent and incompetent eggs is our still only partial knowledge of the critical features that are needed to make a good egg and when during oogenesis these specific characteristics are acquired. The main objective of this review is to summarize the results of areas of investigation that are contributing to our still inadequate understanding of the molecular aspects of making developmentally competent eggs.

Methods: For each area discussed, a systematic search was made using PubMed. The search was without temporal limits but mainly yielded publications between 1982-1999 (23%) and 2000-2011 (77%).

Results: Taking an oocyte-centred view, we describe throughout folliculogenesis: (i) the factors that regulate oocyte growth; (ii) the role of oocyte-cumulus cell dialogue; (iii) the epigenetic organization of the oocyte genome and (iv) the storage and regulation of maternal RNAs.

Conclusions: The multifaceted complex of factors involved in oocyte growth constitutes the backbone on which oocyte developmental competence is built up. Operating behind the expression of these factors is a specific epigenetic signature established during oogenesis, but our knowledge is only approximate and major efforts will be required for more accurate analyses at specific gene loci. The growing research on small silencing RNAs during oogenesis and early oocyte development is revealing these molecules' critical role in mRNA degradation. Our next challenge will be to dissect the complex interactions among the different molecular players identified and to establish the presence of functional links among these factors.