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

ABSTRACT using cattle as a model. This was accomplished through repeated oocyte collections by transvaginal oocyte retrievalThis research was designed to improve our understanding of (TVOR) in the same groups of animals at various ages. oocyte maturation and acquisition of developmental compe- Successful approaches to obtaining oocytes by TVOR for tence of oocytes using prepubertal heifers as a model. Oocytes embryo production from adult cattle have been widely doc-were collected by ultrasound-guided transvaginal oocyte retriev-al from 20 age-matched calves at 5, 7, 9, and 11 mo of age that umented the decade [6-13]. Numerous studies have had or had not received gonadotropin stimulation. Numbers of investigated the developmental competence of calf oocytes, oocytes recovered from unstimulated heifers decreased with but results are controversial. Utilization of the potentially age, being 15 2, 12 1, 7 1, and 7 1 (for 5-, 7-, 9-, and valuable large reservoir of oocytes in prepubertal calves 11-mo-old calves, respectively). Corresponding numbers for the would be possible once the biology of maturation to full gonadotropin-stimulated heifers were 18 + 2, 16 + 2, 13 1, competence is understood. and 15 2. Cumulus-oocyte complexes were graded from A to Follicular recruitment occurs early in fetal development, D on the basis of their morphology, and the better-grade A and and antral follicles 3-5 mm in diameter are already visible B oocytes were used for in vitro maturation and fertilization. A in the ovaries of calves a few days after birth, reaching a higher proportion of grade A and B oocytes was found for the maximum number when the animals are 2 mo old [14]