AGE AND HORMONAL DEPENDENCE OF ACQUISITION OF OOCYTE COMPETENCE FOR EMBRYOGENESIS IN PREPUBERTAL CALVES

This research was designed to improve our understanding of oocyte matu ration and acquisition of developmental competence of oocytes using pr epubertal heifers as a model. Oocytes were collected by ultrasound-gui ded transvaginal oocyte retrieval from 20 age-matched calves at 5, 7, 9, and 11 mo of age that had or had not received gonadotropin stimulat ion. 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 Ii-mo-old calves, respectively). Corresponding numbers for the gonadotropin-stimulated heifers were 18 +/- 2, 16, 2, 13 +/- 1, and 1 5 +/- 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 f or in vitro maturation and fertilization. A higher proportion of grade A and B oocytes was found for the stimulated vs. unstimulated prepube rtal 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 stimula tion was not significant for peri- and postpubertal calves at 9 (53% v s. 38%, p > 0.05) and 11 (53% vs. 38%, p > 0.05) mo of age. Embryo dev elopment 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 f or 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 m o of age, embryo development to morula and blastocyst stages was simil ar with and without gonadotropin stimulation (48% vs, 40%, p > 0.05) a nd was comparable to that of adult cow oocytes (45%, p > 0.05). The da ta suggest that the acquisition of oocyte competence for normal embryo development in prepubertal calves is influenced by animal age and hor monal treatment.