Reproduction in Domestic Animals

396 P Mermillod, R Dalbie` s-Tran, S Uzbekova, A The´lie, J-M Traverso, C Perreau, P Papillier and P MongetEGF (n = 195)Rescovitine (n = 202)Rescovitine = EGF (n = 208)Fig. 2. Cleavage and rate of blastocystsat 7, 8 and 9 days post-inseminationfor oocytes matured in199 medium supplemented with10 ng ⁄ ml of epidermal growthfactor (EGF) either immediatelyafter collection (EGF) or after aprematuration treatment underroscovitine inhibition in mediumalone (roscovitine) or in the presenceof EGF. Means of three replicates.Inhibition of meioticresumption did not affect embryodevelopment, even under EGF stimulationNevertheless, if oocyte quality relies on mRNAstorage, these mRNAs could be stored during maintranscriptional activity in oocytes from pre-antral folliclesand kept in a stable form for future use duringmaturation or after fertilization. Indeed, whereas insomatic tissues, mRNA content directly reflects contemporaryprotein expression pattern, post-transcriptionalgene expression regulation is slightly different inoocyte where messengers could be masked and maintainedfor long periods of time and then be processed fortranslation or destruction through polyadenylation regulation(Bettegowda and Smith 2007) or RNA interferencemechanisms (Murchison et al. 2007).However, if it is admitted that oocyte developmentalcompetence may be acquired during late folliculargrowth, one should admit that determinant messengersmay be produced in oocytes from late antral follicles.This transcription activity could be quantitatively lowbut functionally determinant. Much research has beendevoted towards identification of these critical mRNAduring the past 10 years.The development of highly sensitive molecular biologymethods has allowed to identify a panel of genesthat are preferentially expressed in oocytes of mammals(Cui and Kim 2007). The list of these genes is currentlyactively expanding and will probably continue to growduring the forthcoming years, because of active researchwork and increasing sensitivity of molecular techniques.In particular, subtractive suppressive hybridization(SSH) approach allowed to evidence hundreds of genesoverexpressed in cattle oocytes as compared withsomatic tissues (Pennetier et al. 2005; Vallee et al. 2005).In mouse oocytes, knockout experiments allowed topoint out several genes, which should be expressed inoocyte to allow embryo development after fertilization.These genes are referred to as ‘maternal effect genes’such as maternal antigen that embryo require (Mater) orzygotic arrest-1 (Zar-1). These genes may be involved inthe onset of transcriptional activity in zygote nucleusduring MZT occurring at the two-cell stage in mice(Minami et al. 2007) and at the eight-cell stage inruminant species.Some orthologous genes of mouse maternal-effectgenes have been cloned in domestic species. For example,we cloned bovine Mater and Zar-1 orthologousgenes in cattle (Pennetier et al. 2004; Uzbekova et al.2006). However, the study of bovine Mater expressionprofile (Pennetier et al. 2006) indicated that this genewas transcribed and translated at early stages offollicular growth, long before the suspected time ofdevelopmental competence acquisition. Although nomaternal effect gene has been shown to be translatedspecifically during late folliculogenesis, it could beproposed that some determinant genes are progressivelyaccumulated during this period and that full developmentalcompetence is acquired as a threshold of mRNAor protein from these genes are reached in the oocyte.However, given that a significant population offollicles degenerate by atresia at different stages of theirgrowth, another explanation may be that folliclescontaining competent oocytes are more able to survivethe increasingly challenging hormonal environmentthrough a better coordination of follicular somatic cellsproliferation and differentiation.Oocyte Regulation of Follicular GrowthThe role of somatic follicular cells in the establishmentof oocyte competence has been well established. Cumuluscells are participating to oocyte meiotic arrest andresumption and oocyte metabolism (glutathione storage,glucose metabolism), they also participate to oocyteÓ 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag