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A study of the time course of egg retention activity in the rabbit oviduct

Summary. Changes in egg retention activity of the oviduct were assessed during the first 5 days after HCG-induced ovulation in rabbits. Dextran microspheres were used as ovum surrogates. They were injected into the peritoneal cavity at 0, 10, 32, 41, 50, 64, 72 and 80 h following HCG. The distribution of ova and surrogates in the genital tract was assessed 24 h following surrogate injection in all groups and at 8 h following surrogate injection in animals injected 50 h after HCG. The distribution of microspheres injected up to 50 h after HCG was similar to that of eggs. Surrogates were not retained at the ampullary–isthmic junction (AIJ) shortly after passage of ova through this region. Moreover, spheres injected 38 h after ovulation reached the eggs at the proximal isthmus in 8 h or less. Surrogates injected at 64 h following HCG or later were retained at the AIJ, indicating resumption of retaining activity at this level. It is suggested that egg passage through the AIJ is associated with a temporary reduction of the retaining activity lasting for approximately 16 h. Retaining activity at the AIJ is regained before eggs pass from the proximal isthmus into the uterus.