Follicle dynamics and aromatase activity in high-ovulating Meishan sows and in Large-White hybrid contemporaries

This study compared the dynamics of follicle development during the follicular phase in Meishan sows (n = 10) with a high ovulation rate, with those in contemporary Large-White hybrid sows (n = 12) (ovulation rates: 27.7 and 17.6, respectively; p < 0.01). Ovaries were recovered on Day 16 or 19 of the cycle, and all follicles > or = 1 mm in diameter were dissected. Fluid was aspirated from all those > or = 2 mm, and the granulosa and theca cells were incubated to estimate aromatase activity and estradiol production. The total number of follicles dissected was higher in Meishan than in Large-White hybrid animals (p < 0.001), and whereas the number decreased (p < 0.01) between Days 16 and 19 in Large- White hybrid sows, there was no significant change in Meishan animals. The mean diameter of Meishan follicles recovered on each of Days 16 and 19 was smaller (p < 0.001) than that of those recovered from LargeWhite hybrids. Total estradiol production in vitro was higher (p < 0.01) by follicles > or = 2 mm from Meishan sows. There was an interaction (p < 0.01) between day of the cycle, breed, and follicle diameter in aromatase activity in both granulosa cells and theca tissue, and in follicular fluid estradiol concentrations in follicles > or = 2-< 6 mm. The largest three size categories of follicles recovered on Day 19 from each breed were classified as preovulatory (29.0 and 17.7 for Meishan and Large-White hybrid, respectively), and the mean diameter was smaller (p < 0.001) in Meishan animals. Follicular fluid estradiol concentrations and total estradiol synthesis by granulosa and theca combined were higher (p < 0.05) in Meishan than in Large-White hybrid sows. In conclusion, high-ovulating Meishans maintained a higher number of follicles during the follicular phase, which overall synthesized more estradiol in vitro, than did Large-White hybrid animals.