Species-Specific Mechanisms of Sperm Transport: Predominant Roles of Motility in Ewes and Uterine Contractility in Mares

The factors controlling sperm transport within the female genital tract vary among species. The aim of this study was to quantify the respective roles of sperm motility and uterine contractions in passage of sperm within the reproductive tract of the mare and ewe. Sperm imaging was performed in vivo within the female genital tract of mares and ewes using probe-based confocal laser endomicroscopy (pCLE) after intrauterine insemination during two stages of the cycle characterized by high (oestrus) or low (luteal phase) uterine contractility. Fluorescent microbeads (2 µm in diameter) were inseminated simultaneously with spermatozoa to assess non-motile particle displacement. In oestrous mares, spermatozoa exhibited a descending distribution gradient from the uterine body toward the upper uterine horn, with a pronounced selection between the mid- and upper-horn regions, whereas beads were recovered at similar levels throughout the tract, from the vagina to the tip of the horn. In contrast, in oestrous ewes, spermatozoa exhibited an ascending distribution gradient from the uterine body toward the upper uterine horn, while beads were largely expelled toward the vagina and failed to reach the horn. These distribution patterns were altered when uterine contractions were reduced during the luteal phase, as both mares and ewes then displayed a similar and homogeneous distribution of beads along the uterine horn. This study demonstrates that sperm motility is essential in the ewe, where strong uterine contractions prevail, whereas in the mare, sperm are mainly transported by moderate uterine contractions.