Control of ovarian follicular growth and maturation by the corpus luteum and the placenta during pregnancy in sheep

Ovarian follicular growth and maturation and its control throughout pregnan cy have not been described fully in sheep. Experiment 1 characterized the s ize and maturation (steroid production in vitro and aromatase activity) of ovarian follicles obtained at days 20, 50, 80 and 110 of pregnancy compared with those obtained at day 12 of the oestrous cycle. There was no differen ce in the number of small follicles (< 3 mm in diameter) between cyclic and pregnant ewes, regardless of the stage of pregnancy. There was a marked re duction (P < 0.01) in the number of medium follicles (3-5 mm) starting at d ay 80 of pregnancy. Large follicles (> 5 mm) were not detected at day 110 o f pregnancy. In vitro testosterone output by follicles was constant through out pregnancy. Oestradiol output remained steady until day 80, but decrease d markedly at day 110 of pregnancy. This decrease was associated with a red uction in aromatase activity in follicles obtained at this stage. Experimen t 2 examined the effect of administration of high concentrations of progest erone between day 100 and day 120 after mating on resumption of follicular growth in ewes that underwent Caesarean section at day 99 of pregnancy. In ewes that underwent Caesarean section, progesterone supplementation was suc cessful in mimicking the profile found in pregnant ewes, but did not preven t re-initiation of follicular growth, as demonstrated by the presence of la rge follicles (> 5 mm) at day 120 after mating. Experiment 3 examined the e ffects of PGF(2 alpha)-induced regression of the corpus luteum of day 100 o f pregnancy on resumption of follicular growth. High concentrations of PGF( 2 alpha) (0.28 mg kg(-1) body weight) administrated at day 100 of pregnancy were required to initiate regression of the corpus luteum. At day 120 afte r mating, the mean (+/- SEM) diameter of the largest follicle in PGF(2 alph a)-treated ewes (3.40 +/- 0.47 mm) was significantly greater (P < 0.05) tha n that in control pregnant ewes (2.52 +/- 0.34 mm). Experiment 4 examined t he effect of removal of the fetus and of the corpus luteum at day 100 of pr egnancy on resumption of ovulation. Removal of the corpus luteum by PGF(2 a lpha) treatment at the time of removal of the fetus resulted in earlier occ urrence of short luteal phases (27.8 versus 40.6 days, PGF(2 alpha)-treated versus non treated) but did not alter the timing of the first normal lutea l phases (41 days). In conclusion, the results from these experiments indic ate that placental compounds play a major role in inhibiting follicular gro wth and maturation during late pregnancy in sheep.