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Twelve nulliparous New Zealand White female rabbits, and eight nulliparous Hereford heifers were used for profile analysis of the oviductal layers after in vivo and in vitro fixation of the oviducts. The unstraightened oviducts were cut into eight segments of about 1 cm starting from the uterotubal junction. Using a projection microscope, the areas of the lumen, mucosa + submucosa, and musculature of cross sections from each segment were measured planimetrically. The lumen of segments 5 and 6 (from the fimbriae) had the smallest surface area at all reproductive stages in both species. At about 72 and 168 hours after ovulation, the surface area of the lumen at these two segments increased by two-fold compared to the estrous stage. The ampullary-isthmic junction (AIJ) is probably located at 6/10 to 7/10 of the oviductal length (segment 5 or 6) in both species. The widening of the lumen at the AIJ, 60-70 hours post ovulation, is sufficient to allow the trapped eggs to pass rapidly to the uterus. The whole isthmus and particularly the AIJ seems to function as a sphincter, the closure of which may be coordinated by neuro-endocrine mechanisms.

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