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Regulation of attachment of equine spermatozoa to homologous oviduct epithelium was investigated by co-culture of spermatozoa with oviductal epithelial cell explants. Stallion spermatozoa were incubated with explants derived from the isthmus and ampulla of follicular, postovulatory, and diestrous mares. Steroid treatments (estradiol, progesterone, or control) were applied across all explant groups. Estimates of motility and total numbers of attached spermatozoa were made 0.5, 24, and 48 h after initiation of co-culture. Equine spermatozoa attached by their rostral acrosomal region to both ciliated and nonciliated oviduct epithelial cells. Steroid treatment had no effect on either motility or total number of attached spermatozoa. Motility of spermatozoa attached to ampullar and isthmic explants did not differ. However, at both 24 h and 48 h, motility of spermatozoa attached to follicular-stage explants exceeded that of spermatozoa attached to postovulatory or diestrous-stage explants (p < 0.05). The number of spermatozoa that bound to explants was affected by stage of cycle, anatomic origin of explant, and time in co-culture (p < 0.001), as well as the interaction of cycle stage, anatomic origin, and time in co-culture (p < 0.001). More spermatozoa bound to explants of isthmic than ampullar origin, and more spermatozoa bound to follicular and postovulatory explants than to diestrous explants (p < 0.05). These data support the existence of a spermatozoal reservoir in the oviductal isthmus of the mare and suggest that there may be cycle stage-specific regulation of both motility and the number of spermatozoa attached to oviductal epithelium.

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