Effect of capacitation on bull sperm binding to homologous oviductal epithelium.

Sperm binding to oviductal epithelium is thought to be an important mechanism regulating sperm reservoir formation in the oviduct. On the basis of evidence in the hamster, we hypothesized that capacitation affects release of bovine sperm, allowing them to fertilize. Oviducts were obtained from the ovulatory side of estrous Holstein heifers. The isthmic and ampullar epithelia were milked out and reduced to fragments, which formed everted vesicles (explants). Explants were placed in tissue culture wells in TALP medium and incubated at 39 degrees C in 5% CO2. Frozen-thawed sperm were prepared by swim-up in TALP and capacitated by incubation for 4 h in TALP with 20 micrograms/ml heparin (without glucose). Uncapacitated sperm were used immediately after dilution into capacitation medium. Within 2 h of surgery, sperm were added to the explants and incubated with them for 15 min. Sperm and explants were videotaped, and the tapes were analyzed to determine the numbers of sperm bound per surface area. ANOVA did not show a difference between the number of sperm bound/0.1 mm2 in the isthmus and ampulla (p > 0.05); however, an effect of capacitation was detected (p = 0.0015). Also, the percentage of capacitated sperm, determined by chlortetracycline labeling, was greater in sperm that remained free-swimming in the presence of explants than in the absence of explants (p = 0.001). In conclusion, capacitation appears to be involved in the release of bovine sperm from oviductal epithelium and therefore could enable sperm to leave the reservoir and fertilize oocytes.