Reproduction in Domestic Animals

Factors Affecting Oocyte Quality 395physiological status of the donor cow also affects oocytequality. For example, oocytes from small follicles aremore competent during follicular growth phase ascompared with oocyte collected during the dominantphase (Machatkova et al. 2004). In addition, individualdifferences between donor cows in the developmentalcompetence of oocytes collected by ovum pick up mayindicate genetic control of oocyte competence (Tamassiaet al. 2003).In mammals, atresia is observed at any stage offollicular growth (Cahill et al. 1979). One of the earliestsigns of follicular atresia is the drop of estradiolproduction by granulosa cells. We measured aromataseactivity in granulosa cells of dissected bovine follicleswhile the corresponding oocytes were submitted individuallyto IVP and we observed that oocytes able todevelop to the blastocyst stage were originating fromfollicles displaying high aromatase activity (Mermillodet al. 1999). Interestingly, when atresia was induced byprolonged follicular phase under gonadotropin suppressionin superovulated heifers, oocyte quality was notaffected (Oussaid et al. 2000). This contrast may indicatethat growing follicles containing low-quality oocyteare induced into atresia, whereas artificially inducingatresia of large follicles does not affect oocyte quality.Therefore, atresia appears a consequence of oocyte lowquality rather than a cause of oocyte degradation.Modifying hormonal environment by exogenous FSHinjection allows more follicles to reach ovulation. Thisprocedure is widely used in multiple ovulation andembryo transfer schemes in cattle (Lonergan 2007) andsmall ruminant (Cognie et al. 2003) species. Optimizedtreatment involving FSH stimulation followed by a 48-hcoasting period even allowed to produce quite homogeneouspopulations of competent oocytes, reaching up to80% of development to the blastocyst stage (Blondinet al. 2002). Modifying the hormonal environmentreduces the selection pressure on follicles, resulting ina higher proportion of follicles progressing to the preovulatorystage. However, some ultrastructure abnormalitieshave been reported in sheep superovulatedoocytes (O’Callaghan et al. 2000). This may be the resultof artificially driving to ovulation follicles containingpoorly fitted oocytes.Oocyte Differentiation In vitroBased on the hypothesis that developmental competenceis progressively acquired by an increasing proportion ofoocytes during follicular growth, it would appearinteresting to mimic this differentiation in vitro to allowmore oocytes to become competent. In vitro continuationof oocyte differentiation implies that culturedoocytes are maintained at the germinal vesicle stage toallow ongoing regulation of expression of relevant genesduring a prematuration period. Several methods havebeen proposed to maintain oocytes in meiotic arrestin vitro, including the use of chemicals aimed atmaintaining high cAMP level in oocytes such asforskolin, isobutyl methylxanthine or cAMP stableanalogues (Sirard et al. 1998). Other drugs inhibitingprotein synthesis (cycloheximide) or phosphorylation (6-dimethyl aminopurine) were also tested (Lonergan et al.1997). All of these drugs were able to maintain a more orless reversible inhibition of meiotic resumption incultured cattle oocytes. However, no improvement ofoocyte quality was obtained after such inhibitions.More recently, drugs directly acting by inhibition ofenzymatic activity of M-phase promoting factor (MPF),the key element of G2 ⁄ M transition in eukaryotic cellcycle, were used to maintain oocytes at the germinalvesicle stage. MPF is a heterodimer of CDK1 catalyticsubunit and cyclin B regulatory subunit. Interestingresults have been obtained by using puric moleculescompeting with ATP for binding the catalytic pocket ofMPF heterodimer. Butyrolactone I (Lonergan et al.2000) and roscovitine (Mermillod et al. 2000) weresuccessfully used in this view and provided an efficientand reversible way to maintain cattle oocytes at thegerminal vesicle stage in culture for 24–48 h. However,as shown in Fig. 2, 24 h culture of germinal vesicleoocyte did not result in improvement of developmentpotential even under different stimulation (Ponderatoet al. 2002). However, although these molecules allowedan efficient block of meiotic progression, several meiosisrelated cytoplasmic events did occur under inhibition,such as modifications of protein synthesis andphosphorylation patterns, indicating the initiation ofcytoplasmic maturation (Vigneron et al. 2004a). Interestingly,this approach allowed to point out severalMPF-independent signalling pathways (Akt, JNK andAurora A) activated during meiotic resumption (Vigneronet al. 2004b) and probably involved in the regulationof some aspects of cytoplasmic maturation.Amongst these pathways, Aurora A, a serine ⁄ threonineprotein kinase, seems to be a major player in thephosphorylation cascade leading to MPF activation,meiotic resumption and post-transcriptional control ofgene expression (Uzbekova et al. 2008). Failing toincrease the quality of immature oocytes by allowingthem to continue their late differentiation during aprematuration culture under chemical meiotic inhibitionmay be attributed to the diversity of the signallingmechanisms involved in the regulation of differentmaturation aspects. Alternatively, it may be due to anearlier determination of oocyte quality, by the time ofhigher transcriptional activity in growing oocytes.Oocyte Gene ExpressionThe current hypothesis to explain differential ability ofoocytes to develop after fertilization is that the oocyteshould store the appropriate set of mRNA and proteinsthat will be required during the period of genomicsilencing from GVBD to major zygotic transition(MZT) and for the onset of MZT. Transcriptionalactivity is high in growing oocytes up to early antralstages of follicular growth and decreases to a basallevel thereafter, as follicular diameter reaches 3 mmand the diameter of the oocyte becomes 110 lm inbovine (Fair et al. 1997). A transcription burst issuspected just before GVBD (Rodriguez and Farin2004a,b). Genes expressed at that time are supposed tobe involved in meiotic resumption and progression butmay also have a role during early development(Rodriguez et al. 2006).Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag