The presence of viscous mucus in some mammalian oviducts has been related to the arrest of spermatozoa on their ascent to the site of fertilization. This study describes the distribution of intraluminal mucus in the porcine oviductal sperm reservoir during estrus, using histochemistry and electron microscopy, including high resolution cryo-scanning electron microscopy (Cryo-SEM) of frozen tissue samples. The luminal contents and apical epithelium stained with Alcian blue (AB) and periodic-acid Schiff (PAS), were most conspicuous in the lower isthmus of pre-ovulatory females. The SEM-observations of glutaraldehyde/AB-fixed specimens confirmed the presence of a conspicuous intraluminal material, most prominent in the sperm reservoir of pre- and peri-ovulatory animals. Such intraluminal material was consistently undetectable in glutaraldehyde-fixed, conventionally processed materials. In Cryo-SEM, a homogenous mass obliterated the lumen, especially in the pre-ovulatory UTJ/lower isthmus, diminishing towards the ampulla. Post-ovulation, the isthmic lumen appeared wider, showing only patches of amorphous material. If water sublimation was allowed during Cryo-SEM, the homogeneity of the luminal contents disappeared, leaving fibrillar remnants associated with the cilia and microvilli. The results clearly demonstrate the presence of luminal mucus in restricted areas of the pig oviduct, the pre-ovulatory sperm reservoir in particular, diminishing after ovulation. Its implication in the arrest and release of viable spermatozoa for fertilisation is discussed.
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