Effects of homologous oviductal epithelial cell co-culture on in vitro embryo development in livestock species: A systematic review and meta-analysis protocol

Purpose: The objective of this protocol is to outline the approach for a systematic review and meta-analysis aimed at evaluating the effects of homologous oviductal epithelial cell (OEC) co-culture on in vitro embryo developmental outcomes, including cleavage, blastocyst, expanded blastocyst, and hatched blastocyst development, as well as blastocyst quality assessed by total blastocyst cell number, in livestock species. Methods: The protocol is developed following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols (PRISMA-P). The protocol has been registered in PROSPERO (CRD420261405630). The approach is based on the Population, Intervention, Comparator, Outcome and Study design (PICOS) framework to answer the following research questions: 1) Does homologous oviductal epithelial cell (OEC) co-culture with oocytes, presumptive zygotes or embryos improve in vitro embryo developmental outcomes compared with culture systems without OEC co-culture in sheep, cattle, pigs, and goats? 2) What are the factors influencing the effectiveness of the OEC co-culture systems, such as the stage of OEC inclusion in the embryo production system, i.e., during oocyte maturation, fertilization, embryo culture, or combined stages? The process will include: 1) development of the search strategy and inclusion criteria; 2) data extraction; 3) risk of bias assessment; and 4) data analysis. The risk of bias of the included in vitro studies will be evaluated using the National Toxicology Program-Office of Health Assessment and Translation (NTP-OHAT) Risk of Bias Tool. Given the anticipated heterogeneity in study designs, species, OEC models, and culture conditions, meta-analyses will be conducted using a random-effects model, with pooled effect estimates calculated using the inverse-variance method. Species will be assessed as a main effect, whereas subgroup analyses will be performed according to the oviductal cell co-culture systems and/or the stage at which OEC co-culture is introduced within the in vitro embryo production system. For studies where numerical data are insufficient for meta-analysis or cannot be extracted, findings will be synthesized descriptively. Publication bias will be assessed using funnel plots and, where appropriate, Egger’s regression test, while sensitivity analyses will be conducted using the leave-one-out method. Conclusions: This systematic review and meta-analysis will provide a comprehensive synthesis of the evidence on the effect of OEC-oocyte/embryo co-culture systems. The findings will help clarify the current state of knowledge, identify key research gaps, provide a foundation for future studies aimed at optimizing in vitro embryo production systems, and improve our understanding of potential sources of heterogeneity across studies.