The aim of this study is to examine the effect of bovine oocyte maturation, fertilization or culture in vivo or in vitro on the proportion of oocytes reaching the blastocyst stage, and on blastocyst quality as measured by survival following vitrification. In Experiment 1, 4 groups of oocytes were used: (1) immature oocytes from 2-6 mm follicles; (2) immature oocytes from > 6 mm follicles; (3) immature oocytes recovered in vivo just before the LH surge; and (4) in vivo matured oocytes. Significantly more blastocysts developed from oocytes matured in vivo than those recovered just before the LH surge or than oocytes from 2-6 mm follicles. Results from > 6 mm follicles were intermediate. All blastocysts had low survival following vitrification. In Experiment 2, in vivo matured oocytes were either (1) fertilized in vitro or (2) fertilized in vivo by artificial insemination and the resulting presumptive zygotes recovered on day 1. Both groups were then cultured in vitro. In vivo fertilized oocytes had a significantly higher blastocyst yield than those fertilized in vitro. Blastocyst quality was similar between the groups. Both groups had low survival following vitrification. In Experiment 3a, presumptive zygotes produced by in vitro maturation (IVM)/fertilization (IVF) were cultured either in vitro in synthetic oviduct fluid, or in vivo in the ewe oviduct. In Experiment 3b, in vivo matured/in vivo fertilized zygotes were either surgically recovered on day 1 and cultured in vitro in synthetic oviduct fluid, or were nonsurgically recovered on day 7. There was no difference in blastocyst yields between groups of zygotes originating from the same source (in vivo or in vitro fertilization) irrespective of whether culture took place in vivo or in vitro. However, there was a dramatic effect on blastocyst quality with those blastocysts produced following in vivo culture surviving vitrification at significantly higher rates than their in vitro cultured counterparts. Collectively, these results indicate that the intrinsic quality of the oocyte is the main factor affecting blastocyst yields, while the conditions of embryo culture have a crucial role in determining blastocyst quality.

Methods to predict numbers of healthy oocytes in the ovaries of young adults could have important diagnostic relevance in family planning and animal agriculture. We have observed that peak antral follicle count (AFC) determined by serial ovarian ultrasonography during follicular waves is very highly reproducible within individual young adult cattle, despite 7-fold variation among animals. Herein, we tested the hypothesis that AFC is positively associated with the number of morphologically healthy oocytes and follicles in ovaries and with serum concentrations of anti-Müllerian hormone (AMH), an indirect marker for number of healthy follicles and oocytes in ovaries. In the present study, age-matched young adult cattle (12-18 mo old) were subjected to serial ultrasonography to identify animals with a consistently high (> or =25 follicles that were > or =3 mm in diameter) or low (< or =15 follicles) AFC during follicular waves. Differences in serum AMH concentrations, ovary weight, and number of morphologically healthy and atretic follicles and oocytes were determined. The phenotypic classifications of cattle based on AFC during follicular waves or AMH concentrations both predict reliably the relative number of morphologically healthy follicles and oocytes in ovaries of age-matched young adult cattle.

BACKGROUND: The significance of the high variation in numbers of follicles produced during reproductive cycles in humans and cattle is unknown. METHODS: We selected beef heifers with high (25) or low (15) numbers of ovarian follicles and determined the association with alterations in FSH and estradiol concentrations, as well as responsiveness to superstimulation and embryo quality. The variation in follicle numbers was also compared with oocyte quality in natural cycles using IVF and abattoir sourced bovine ovaries. RESULTS: Results show that: (i) FSH was lower (P < 0.03) in animals with high compared with low follicle numbers per follicle wave; (ii) after superovulation, in the high versus low follicle number group, the number of oocytes/embryos recovered after insemination (10.6 + + + + + 2.7 versus 4.7 + + + + + 0.7) and the number of transferable embryos (5.4 + + + + + 1.3 versus 3.8 + + + + + 0.8) per animal were greater (P < 0.05), whereas the proportion of transferable embryos (50.7% versus 79.8%) was lower (P < 0.05); (iii) in unstimulated animals, the numbers of high-quality oocytes harvested and in-vitro fertilized oocytes developing into blastocysts were up to 4-fold greater (P < 0.05) for ovaries with high versus low numbers of follicles, but the proportions of oocytes developing into blastocysts were similar in the two groups. CONCLUSION: Phenotypic classification based on numbers of follicles may be useful to improve superovulation procedures. The lower proportion of transferable embryos following superovulation of ovaries with high numbers of follicles is probably not the result of differences in the quality of oocytes before superovulation.

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