Characterization of the oviductal sperm reservoir in cattle.

A reservoir for sperm has been found in the oviductal isthmus in several species. Sperm are apparently trapped in the reservoir by binding to the oviductal epithelium, although other factors may be involved. We hypothesized that binding sites for bovine sperm are limited to the isthmus and are regulated by the hormonal state of the cow. Ipsilateral oviducts were obtained from heifers that were preovulatory (in estrus), had ovulated recently (within 12 h), or were in diestrus (Day 10). The isthmic and the ampullar epithelium were milked out and incubated separately in serum-free (SFRE-199-2) medium, at 39 degrees C in 5% CO2. Frozen-thawed sperm from bulls were added to the epithelium and coincubated for 15 min. The number of spermatozoa that bound to explants was not affected by stage of cycle or by anatomic origin of the explants (p > 0.05). In an additional experiment, oviducts were infused with sperm in vivo and then prepared for scanning electron microscopy, which revealed that sperm were associated with ciliated epithelium in both the isthmus and ampulla. Thus, bovine sperm may form a reservoir in the isthmic end of the oviduct because it is the first oviductal region that they encounter.