Dr. Mary Scott discusses assessing stallion fertility.

Subfertility in valuable broodmares and stallions causes significant economic loss to owners and presents a diagnostic and/or therapeutic challenge to equine veterinary practitioners due to the variety of possible causes. UC Davis equine reproduction specialists are investigating reproductive function at the cellular level -- specifically, sperm function in the mare's reproductive tract and how it relates to equine subfertility.

Sperm transport and function in the female tract is a fundamental part of mammalian reproduction and both male and female components are essential to its success. Sperm must be capable of moving through the tract and must undergo capacitation (the maturational changes that are needed for a sperm to fertilize an egg). On the other hand, the female reproductive tract must provide a suitable environment for sperm survival, maturation, storage, and transit. A failure of normal sperm transport and function in the female tract may cause fertility problems, whether caused by sperm abnormalities or problems with the female reproductive tract.

UCD researchers compared sperm transport patterns in fertile and subfertile mares and identified some significant differences. In fertile mares, 1) the sperm were regionally distributed throughout the oviduct with more sperm in the lower (caudal) isthmus than in the upper regions of the oviduct, 2) the oviducts contained highly motile sperm located exclusively in the caudal isthmus, 3) sperm were seen in intimate contact with the surface of the uterotubal junction (entrance to the oviduct), and 4) the surfaces (epithelium) of the caudal isthmus tissues appeared healthy.

Image from a scanning electron microscope of sperm at the entrance to  the oviduct 18 hours after insemination.

In contrast, in subfertile mares, 1) the sperm were not regionally distributed throughout the oviduct, and, more importantly, there were significantly fewer sperm in the caudal isthmus than the fertile mares, 2) the oviducts contained very few motile sperm, 3) few sperm were seen in the uterotubal junction, and 4) the surfaces (epithelium) of the caudal isthmus tissues appeared damaged and abnormal. It is possible that the significantly lower number and poorer quality of sperm in the caudal isthmus of the subfertile mares may be due to abnormalities in the epithelium in that region.

UCD researchers also compared sperm transport patterns in sperm from fertile and subfertile stallions. Theyt determined that 1) significantly fewer sperm from the subfertile stallions reached the oviducts at four hours after insemination compared to fertile stallions, 2) subfertile stallions had significantly fewer structurally normal sperm in the oviduct than fertile stallions, 3) motile sperm from the fertile stallion were recovered from more than 90 percent of the oviducts, whereas only 25 percent of the oviducts had motile sperm from the subfertile stallions, and 4) for both subfertile and fertile stallions, the oviductal sperm had a significantly higher percentage of structurally normal sperm in comparison to sperm in the semen. This last observation suggests the occurrence of sperm selection during transport to the oviducts, and that selection is related to sperm structure. It appears that structurally abnormal sperm may be excluded from the oviduct because they do not interact normally with the tissues lining the mare's reproductive tract and cannot cross the uterotubal junction to enter the oviduct.

To explore this selection process, the researchers evaluated the uterotubal junction of mares following insemination with sperm containing high percentages of abnormal sperm. Using the scanning electron microscope, the researchers found that sperm were present in the uterotubal junction of all the mares evaluated, and were found in pockets and folds along the surface. They were often so intimately associated with the folds that in some instances, only the tails were visible. Strikingly, although structurally abnormal sperm were abundant (66-85 percent) in the inseminates of these mares, more than 90 percent of the sperm found in the uterotubal junction were structurally normal, and all appeared to have normal head structure. These results demonstrate that equine sperm with normal structure develop specific and intimate associations with the lining tissues of the mare's reproductive tract during transport to the oviducts. Furthermore, this study provides compelling evidence of a mechanism of sperm selection during sperm transport. Despite being motile, structurally abnormal sperm may not move through the uterus to reach the uterotubal junction following insemination. Or, alternatively, abnormal sperm may not undergo normal association with the surface of the uterotubal junction which may be necessary to enter the oviducts. These results strongly suggest that normal structure as well as progressive motility of sperm should be considered when calculating an insemination dose, particularly for stallions known to be subfertile.

In summary, these studies identified new information about normal sperm transport to the oviducts. We determined that 1) with subfertile mares and stallions, fewer sperm reached the oviducts at four hours after insemination, 2) the sperm population in the oviduct was of poorer quality when compared to fertile animals, 3) in mares that are susceptible to chronic uterine infection, abnormal uterine contractility and damage to the lining of the caudal isthmus, are potential causes of the abnormal sperm transport, and 4) in subfertile stallions, low sperm numbers in the oviduct may be caused by a high percentage of structurally abnormal sperm in the semen.

To gain more information about sperm transport in the mare, we are now examining tissues of mares at a later time point: 18 hours after insemination. Preliminary results show that sperm can be found in the uterotubal junction at this time. These results provide the first experimental evidence that the uterotubal junction may be a site of sperm storage prior to ovulation. In a collaborative study with researchers at Texas A&M University, researchers are comparing sperm transport for fertile and subfertile mares also at 18 hours after insemination. In addition to expanding our knowledge of sperm transport in the mare, this research can be used to develop more effective breeding management programs for subfertile mares and stallions. 

This research was funded by the Grayson-Jockey Club Research Foundation, the American Association of Equine Practitioners, and the UC Davis Center for Equine Health.

Please visit The UC Davis Center For Equine Health website for addtional information at http://www.vetmed.ucdavis.edu/ceh The Horse Report: Writer/Editor: Laurie FioEditor/WriterCenter for Equine HealthSchool of Veterinary MedicineUniversity of CaliforniaOne Shields AvenueDavis, CA 95616-8589phone: (530) 752-4434fax: (530) 752-9379e-mail: ltfio@ucdavis.edu