Reproductive cycle and in vitro maturation of canine oocyte: A meta-analysis approach

Different outcomes of canine in vitro oocyte maturation depend on the phases of the donor's estrus cycle. This study aimed to examine this topic using a meta-analysis approach. From 1765 identified records in three major scientific databases, 145 were selected after the screening, 27 were evaluated for eligibility, and, finally, by removing 3 studies with high heterogeneity, 11 studies were used for the meta-analysis. The anestrus (ANE) phase was the reference for evaluating the luteal (LUT) and follicular (FOL) phases. The effect size data were selected as dichotomous types, and publication bias and heterogeneity were used for the quality assessments. The results showed a higher risk ratio (RR) of meiosis resumption in oocytes from the germinal vesicle stage in the LUT and FOL phases (∼30% more than ANE oocytes). The RR of being oocytes at the germinal vesicle breakdown (GVBD) stage was similar in all three phases. When compared to ANE, the RR of oocytes reaching the metaphase I (MI) stage from GVBD were higher in the FOL and LUT phases (30% and 14%, respectively). Moreover, the rate of oocytes reaching metaphase II (MII) was higher in the FOL and LUT phases (85% and 30%, respectively) than in the ANE phase. Being at an active phase (FOL or LUT phases) increased the rates of transition from the GVBD to MI stages (18%) and the MI to MII stages (48%) as compared with the ANE phase. It was assumed that oocytes in the LUT phase can reach GVBD and MI, whereas oocytes in the FOL phase are more capable of reaching MII than oocytes in anestrus.