The relationship between day of formation and functional life span of induced corpora lutea in the pig.

Corpora lutea induced in pigs on Day 6 regressed with the naturally formed corpora lutea towards the end of the cycle. Hormonal induction of corpora lutea on Days 8, 10 or 16 significantly prolonged the cycle despite involution of the original corpora lutea about 16 days after ovulation. The interval between the exogenous administration of hcg and the appearance of the next oestrus varied with the day on which the accessory corpora lutea were formed. The estimated functional life span of the corpora lutea induced on Days 6, 8, 10 and 16 was 12\m=.\5 14\m=.\5, 15\m=.\0and 19\m=.\3days respectively. That these induced corpora lutea were fully functional was shown by the luteal progesterone levels estimated using the protein-binding and u.v. spectrophotometric techniques. Hysterectomy resulted in both the spontaneous corpora lutea and those induced on Day 6 being maintained for at least 60 days. It is concluded that Days 6 to 8 may represent a critical phase in the relationship between the uterus and the corpora lutea in non-pregnant pigs.