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Prolonged dominance of follicles and reduced viability of bovine oocytes

Prolonging the lifespan of bovine follicles is known to result in reduced fertility after ovulation and insemination. In this study, the effect of prolonged development of follicles on oocyte viability was examined. In Expt 1, six cows in the aspirated-prolonged-follicle group received a vaginal progesterone releasing device on day 4 of the oestrous cycle (day 1 = ovulation) and prostaglandin F₂ (40 mg) on day 6. Ultrasound-guided follicular aspiration was performed on day 13. Six cows in the aspirated-growing-follicle group received prostaglandin F₂ on day 6, and follicular aspiration on day 7. In Expt 2, all cows were stimulated with 36 mg FSH-P to develop multiple large follicles for study. Three cows in the prolonged-multiple-follicle group received the same treatment as did cows in Expt 1, but were ovariectomized on day 13. Three cows in the growing-multiple-follicle group received prostaglandin F₂ on day 6 and were ovariectomized on day 7. Oocytes recovered in both experiments were stained to reveal the stage of nuclear development. All oocytes from aspirated-prolonged and prolonged-multiple follicles showed expanded cumulus cells and condensed chromatin dispersed in the ooplasm, with possible germinal vesicle breakdown. Oocytes from aspirated-growing and growing-multiple follicles showed compact cumulus cells and an intact germinal vesicle. Plasma concentrations of oestradiol in both growing follicle groups increased until oocyte recovery. Oestradiol in the aspiratedprolonged-follicle group increased after luteolysis on day 6 and remained high until follicular aspiration. In contrast, in the FSH-stimulated prolonged-multiple-follicle group, oestradiol fell to trace amounts on day 8 and remained low. Oestradiol concentrations in follicular fluid were consistent with plasma concentrations for all groups. Bovine oocytes from prolonged dominant follicles undergo premature maturation in vivo, which perhaps accounts for the poor fertility observed in other studies when oestrous synchronization with progestins prolongs follicle lifespan.