Age and Hormonal Dependence of Acquisition of Oocyte Competence for Embryogenesis in Prepubertal Calves1

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Abstract
This research was designed to improve our understanding of oocyte maturation and acquisition of developmental competence of oocytes using prepubertal heifers as a model. Oocytes were collected by ultrasound-guided transvaginal oocyte retrieval from 20 age-matched calves at 5, 7, 9, and 11 mo of age that had or had not received gonadotropin stimulation. Numbers of oocytes recovered from unstimulated heifers decreased with age, being 15 ± 2, 12 ± 1, 7 ± 1, and 7 ± 1 (for 5-, 7-, 9-, and 11-mo-old calves, respectively). Corresponding numbers for the gonadotropin-stimulated heifers were 18 ± 2, 16 ± 2, 13 ± 1, and 15 ± 2. Cumulus-oocyte complexes were graded from A to D on the basis of their morphology, and the better-grade A and B oocytes were used for in vitro maturation and fertilization. A higher proportion of grade A and B oocytes was found for the stimulated vs. unstimulated prepubertal calves at 5 mo of age (92% vs. 49%, p < 0.05) and 7 mo of age (96% vs. 63%, p < 0.05), but the improvement of this parameter by stimulation was not significant for peri- and postpubertal calves at 9 (53% vs. 38%, p > 0.05) and 11 (53% vs. 38%, p > 0.05) mo of age. Embryo development to morula and blastocyst stages was poorer (0–11%, p < 0.05) for oocytes collected from unstimulated calves at 5–9 mo of age than for those from the age-matched but gonadotropin-stimulated groups (10%, 39%, and 31%, at 5, 7, and 9 mo of age, respectively). In calves 11 mo of age, embryo development to morula and blastocyst stages was similar with and without gonadotropin stimulation (48% vs. 40%, p > 0.05) and was comparable to that of adult cow oocytes (45%, p > 0.05). The data suggest that the acquisition of oocyte competence for normal embryo development in prepubertal calves is influenced by animal age and hormonal treatment.

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