Assessment of cumulus expansion capacity based on morphological characteristics and its effect on bovine in...

The quality of oocytes is crucial for embryonic development. The morphology of cumulus-oocyte complexes (COCs) and cumulus expansion are widely used indicators of oocyte quality; however, quantitative evidence is limited. We investigated the potential of assessing the quality of COCs based on their objective morphological characteristics and the impact of cumulus expansion on bovine in vitro-produced embryo development. COCs were graded before in vitro maturation (IVM) according to the International Embryo Technology Society (IETS) standards, and their morphological characteristics (area, perimeter, circularity, and solidity) were measured. The cumulus expansion rate and nuclear maturation were then evaluated after IVM. Decision tree models were developed based on COC morphology to classify COC grades and predict the cumulus expansion rate. In another experiment, COCs were divided into two groups based on a 100% cumulus expansion threshold, and embryonic development was assessed. The results revealed that the morphological characteristics of COCs were associated with the IETS grades and cumulus expansion rate but not with nuclear maturation. The predictive models for cumulus expansion capacity achieved an accuracy of 75.5%. COCs with greater cumulus expansion exhibited a significantly higher proportion of ≥ eight-cell-stage embryos on day 2 (14.0 ± 5.9%, P < 0.05). However, the blastocyst formation rate did not differ according to the extent of cumulus expansion. These findings indicate that an objective morphological assessment can predict cumulus expansion capacity and may provide a non-invasive and reproducible approach for selecting high-quality COCs before IVM, potentially indicating an association with the speed of early embryonic development.