4.1. Oocyte Collection

The patients were subjected to controlled ovarian stimulation following a short antagonist protocol. Pituitary suppression was performed by administration of (150–300 IU/day) rec-FSH (Gonal F 1050; Merck and Co, Madrid, Spain) and GnRH (Orgalutran^®; MSD and Co., Hoddesdon, UK). When the growth of at least three follicles >16 mm was observed, ovulation was induced by the administration of 250 μg of rec-hCG (Ovitrelle, Merck, London, UK). Oocyte retrieval was performed 36 h after hCG administration via ultrasound-guided transvaginal puncture-aspiration. Cumulus-corona-oocyte (CCO) complexes denudation was carried out using hyaluronidase SynVitro^® Hyadase; Origio^® Solution, Màlov, Denmark) for a maximum of 30 sec with a denudation pipette (Denudation pipette Flexipet^®, Cook^® Medical, Bloomington, IN, USA). Removal of cumulus-corona cells is required to evaluate and classify the oocyte nuclear maturation state. Despite coming from stimulated cycles, a total of 199 GV oocytes showed an intracytoplasmic nucleus called the germinal vesicle, characteristic of the prophase of the first meiotic division. After the IVM rescue, we included in this study all the oocytes that showed an approximately circular shape and a variable size between 120–140 μm; homogeneous or slightly heterogeneous cytoplasm with no granularity such as inclusions or refractile bodies. Four oocytes (2%) were excluded from the study for being giant, presenting dimorphisms in their zona pellucida, or showing large vacuoles or signs of atresia/degeneration in their ooplasm.