Extended In Vitro Maturation Enhances Oocyte Developmental Competence but Alters Gene Expression in Bovine Embryos Derived From Oocytes With Slow-Predicted Nuclear Maturation Speed.

To identify the optimal in vitro maturation (IVM) duration for bovine oocytes with different nuclear maturation speeds (NMS), this study assessed how varying IVM durations (24, 28, and 32 h) affect developmental competence and embryo quality in oocytes with fast- or slow-predicted NMS classified via machine learning. Developmental competence was evaluated through cleavage rates, first cleavage timing and patterns, and blastocyst formation under individual culture. Embryo quality was assessed via differential staining of inner cell mass and trophectoderm and expression analysis of quality-related genes in formed blastocysts. For oocytes with slow-predicted NMS, extending IVM to 28 h increased cleavage rates and accelerated first cleavage timing (p < 0.01). The lower blastocyst formation rates of oocytes with slow-predicted NMS matured for 24 h improved when IVM reached 28 h, becoming comparable to fast-predicted NMS oocytes. However, extended IVM decreased expression of pluripotency-related genes (e.g., NANOG and OCT4; p < 0.01) regardless of predicted NMS. In conclusion, extending IVM duration to 28 h improved developmental competence of slow-predicted NMS oocytes, highlighting the importance of fertilization timing relative to nuclear maturation completion, though it reduced expression of key pluripotency genes. Individualized IVM protocols based on predicted NMS can enhance bovine embryo production efficiency.