Distribution of mucus and sperm in bovine oviducts after artificial insemination: the physical environment of the oviductal sperm reservoir. 
There is a sperm reservoir in the caudal oviduct of cattle and other mammals.  We had observed trapping of sperm by mucus produced by explants of bovine oviductal epithelium in vitro; therefore, we used techniques designed for preserving mucus and luminal dimensions to determine whether mucus is associated with the reservoir in vivo.  Heifers were synchronized by prostaglandin injections and inseminated during estrus.  Oviducts on the side of ovulation were surgically removed either 8-10 h after insemination (preovulatory) or 50-55 h after insemination (postovulatory).  Segments (1 cm) taken from the uterotubal junction (UTJ), caudal 3 cm of isthmus, and mid ampulla were snap frozen.  Frozen sections were coated with collodion and postfixed in phosphate-buffered formaldehyde containing cetyl-pyridinium chloride.  Sections were alternately stained with periodic acid-Schiff stain (PAS) or alcian blue/PAS.  Most of the oviductal lumen was highly branched with passages that measured only a few microns across and were filled with mucus.  In limited areas, the lumen opened to 100 microm across and was only lightly stained for mucus.  Overall, the lumen was much narrower than in sections prepared by standard fixation and paraffin embedding.  Sperm were found scattered throughout the lumen of the UTJ and isthmus, in both the narrow, deeply stained luminal areas and the wider, lightly stained areas.  The numbers tapered off cranially, especially prior to ovulation.  In conclusion, the combination of narrow passages and mucus would appear to impede sperm progress, contributing to the creation of a reservoir.