Follicular atresia, follicular fluid hormones, and circulating hormones during the midluteal phase of the estrous cycle in pigs

The incidence of atresia, concentrations of follicular fluid steroids, and circulating concentrations of estradiol, FSH, and progesterone were investigated to determine whether there was any evidence for a wave of follicular activity or atresia between Days 7 and 15 of the estrous cycle in pigs. Cyclic pigs (gilts) were slaughtered on Days 7, 9, 11, 13, and 15 (4, 4, 4, 4, and 3 pigs per day), and 287 follicles > 2 mm were dissected from ovaries to recover follicular fluid and granulosa cells. Apoptotic (A(0)) granulosa cells were those containing sub-diploid DNA fluorescence as determined by flow cytometry. Follicles with greater than or equal to 10% A(0) granulosa cells were classified as atretic, and those with < 10% A(0) granulosa cells were designated nonatretic. The percentage of atretic follicles (AF) per pig was 35.5 +/- 4.7 (mean +/- SEM), and while the percentage of AF ranged from 12% to 73%, means did not differ significantly among days. Mean follicular estradiol concentration was higher (p < 0.05) in nonatretic follicles (NAF) than in AF; in NAF, the concentration decreased (p < 0.05) from 15 ng/ml on Day 7 to 2.5 ng/ml on Day 11 and then increased (p < 0.05) to 10 ng/ml on Day 15 of the cycle. Mean follicular progesterone concentration was higher (p < 0.05) in AF than in NAF, while follicular androstenedione concentration (20 +/- 1.5 ng/ml, overall mean +/- SEM) did not differ between AF and NAF. Mean plasma estradiol, FSH, and progesterone concentrations did not differ among days of the cycle or differ significantly during the last 42 h before slaughter. Plasma estradiol and FSH concentrations were not significantly correlated with percentage of AF or with follicular steroid concentrations. These results support the notion of continuous growth and atresia of ovarian follicles in pigs independent of changes in plasma FSH and follicular steroid concentration, without evidence for a dominant cohort of follicles during the luteal phase of the estrous cycle.