Oocyte Morphometric Assessment and Gene Expression Profiling of Oocytes and Cumulus Cells as Biomarkers of Oocyte Competence in Sheep

SIMPLE SUMMARY: In vitro production (IVP) of embryos is an essential element of many reproductive biotechnologies. Although some remarkable achievements have been obtained for ovine IVP, the overall efficiency is still low. A limiting factor of the technique is the great variability of the oocyte quality used in IVP. The current selection criterion of oocytes is based on a morphological factor, which is insufficient to determine oocytes’ competence. Therefore, the identification of biomarkers for the selection of oocytes of high developmental competence is a major research goal. The purpose of this study was to evaluate the morphometric parameters of oocytes and the relative gene expression of oocytes and cumulus cells (CCs) as biomarkers of oocyte quality after individually culturing them in order to increase the final efficiency of the IVP technique in sheep. We observed that higher blastocyst rates were obtained from oocytes classified as intermediate and large according to total diameter, oocyte diameter, and zona pellucida thickness. Moreover, the expression of genes related to oocyte quality was higher in oocytes classified as large and in CCs from oocytes classified as large and thus able to reach the blastocyst stage. Oocyte morphometric assessments and gene expression in CCs may be used as biomarkers of oocyte quality. ABSTRACT: Oocyte quality is crucial for subsequent embryo development and so it is a major challenge in assisted reproductive technologies. The aim of the present work was to evaluate the morphometric parameters of oocytes (experiment 1) and the relative gene expression of oocytes and cumulus cells (CCs) (experiment 2) as biomarkers of oocyte quality after individually culturing them (one oocyte or embryo/drop). In experiment 1, individually matured oocytes were measured and classified into small, intermediate, and large oocytes after a cluster analysis, based on total diameter (with zona pellucida, ZP), oocyte diameter (without ZP), and ZP thickness. These oocytes were individually fertilized in vitro and cultured. The embryo development was evaluated up to the blastocyst stage. According to the total diameter, oocyte diameter, and ZP thickness, the blastocyst rate decreased in the small oocytes group (3.1 ± 3.1, 14.1 ± 9.4, and 26.7 ± 3.9, respectively) compared to the intermediate (29.4 ± 5.2, 30.5 ± 10.1, and 28.6 ± 9.6, respectively) and large oocytes groups (54.2 ± 13.5, 44.4 ± 3.9, and 67.6 ± 12.4, respectively). In addition, the probability of reaching the blastocyst stage was positively related to the total diameter (p < 0.001), oocyte diameter (p < 0.05), and ZP thickness (p < 0.001). Furthermore, the relative gene expression of BAX, BCL2, GDF9, and GJA1 was lower in oocytes classified as large. In experiment 2, the mRNA transcript relative abundance pattern of genes in CCs was evaluated according to oocyte total diameter and developmental stage reached. CCs from oocytes classified as large and oocytes capable of developing to the blastocyst stage had a lower relative expression of BAX, STAR, and PTGS2, while a higher expression of HAS2 and SDC2 transcript was observed for those oocytes. In conclusion, oocyte morphometric parameters and gene expression analysis in oocytes and CCs provide methods for the identification of the most competent oocytes for assisted reproductive technologies in sheep.