Overcoming Batch Variability: A Chemically Defined XRS-Based System for Ovine In Vitro Embryo Production

Conventional IVM and IVF systems for sheep embryos rely on fetal bovine serum (FBS) and estrous sheep serum (ESS), which present challenges of batch variability, ethical concerns, and supply limitations. This study evaluated XRS Standard serum replacer as a complete substitute for both FBS and ESS in sheep IVEP. A 2 × 2 factorial design generated all four possible IVM/IVF combinations-FBS/ESS, FBS/XRS, XRS/ESS, and XRS/XRS (each additive at 10% v/v in IVM and IVF media). The FBS/ESS combination served as the conventional control. Cleavage rates (Day 2) and blastocyst development (Day 7) were quantified. XRS/XRS achieved 22.45% blastocyst rates-a 2.0-fold improvement over controls (11.00%, p < 0.01). FBS/XRS showed intermediate efficacy (15.43%), whereas ESS-supplemented groups exhibited reduced competence (12.71% and 11.00%, respectively). Total blastocyst cell numbers (122.1 ± 22.3 vs. 126.0 ± 22.4, p > 0.05) remained comparable across groups, demonstrating preserved embryo quality. This protocol eliminates animal-derived components while maintaining developmental competence, aligning with the 3Rs (reduction, refinement, and replacement) and offering a scalable solution for commercial ovine embryo production.