Biochemical and morphological markers of oocyte quality in female dogs: the role of cumulus cells and reactive oxygen species

Abstract The in vitro maturation (IVM) rate of canine oocytes is generally considered low, with less than 20 % achieving meiotic resumption, and oocyte quality being a determining factor. This study was performed to evaluate the oocyte quality of healthy bitches using cumulus cell (CC) viability, reactive oxygen species (ROS) analysis, and oocyte morphometry. Twelve bitches, with a mean age of 2.3 ± 1.3 years and mean weight of 9.4 ± 5.2 kg, underwent ovariosalpingohysterectomy for oocyte collection. Morphological classification and viability testing of the cumulus-oophorus complex (COCs) with Brilliant Cresyl Blue were subsequently performed. Viable COCs were divided into groups according to their morphological classification (GI, GII or GIII). An H2DCFDA probe was used to evaluate ROS levels in the oocytes, while Hoechst 33342 and propidium iodide probes were used to assess viable and apoptotic CCs, respectively. For morphometry, the total oocyte diameter, cytoplasmic diameter, zona pellucida thickness, radius, and perimeter were measured. Data were analysed using analysis of variance. The GI group had the highest percentage of viable oocytes, the greatest mean values for morphometric parameters, and the lowest levels of intracellular ROS. No difference was observed between the GI and GII groups in terms of the percentage of viable CCs. In conclusion, further investigation is required to establish acceptable ROS levels for selecting oocytes intended for IVM in bitches.