Relationship between sperm transport and penetration of eggs in the rabbit oviduct

Abstract

Unfertilized eggs recovered from donor rabbits shortly after ovulation induced by injection of HCG were transferred to the oviducts of mated recipients or recipients inseminated with 200 million spermatozoa and injected with HCG at known times. It was found that although only 40% of such eggs were penetrated in 1.5 h following transfer to mated recipients at 9-9.75 h after mating, 58 and 66% were penetrated during the same period if transfers were delayed until 13.75-14.25 and 19.5-20.0 h after mating, respectively. By 24 h after mating the percentage of eggs penetrated had fallen again to 43%. It was shown by ligation of the oviduct at the uterotubal junction that adequate sperm was in the oviduct by 9.25-9.75 h after insemination to ensure this enhanced fertilization following transfer at 14 or 17 h but not 20 h after insemination. The results implied that although sperm continued to migrate from the uterus into the oviduct, more sperm in the oviduct at later times was not responsible for this phenomenon. Further experiments in which ligation was done at the ampullar isthmic junction or the middle of the isthmus showed that sperm was retained in the proximal isthmus just above the uterotubal junction for several hours. When whole oviducts were fixed and serially sectioned and the number and distribution of sperm determined, it was found that at 9 h after insemination little sperm was at the site of fertilization. In the oviducts of animals killed at 14 or 20 h after insemination, the sperm in the oviduct (irrespective of whether the uterotubal junction had been ligated or not) had become redistributed with many more found at the site of fertilization. It is concluded that more eggs are penetrated in a short period of time (1.5 h) following late transfers because there is the immediate possibility of egg and sperm meeting.