Sperm in the oviduct

Abstract

In past studies, University of California, Davis researchers found that the sperm population that reaches the oviducts has a signifcanfly greater percentage of structurally normal sperm than the sperm population in the semen. This finding suggests that selection for normal sperm occurs during sperm transport to the oviduct. As they move through the female tract, sperm intimately contact the epithelium. Researchers hypothesized that structurally abnormal sperm do not interact normally with the epithelium and are excluded from the oviduct because they cannot cross the complex entrance to the oviduct at the uterotubal junction (UTJ). Recently, Drs. Irwin K. Liu and Mary A. Scott have conducted related studies designed to describe the structural morphology of equine sperm present at the UTJ of normal mares, four hours after insemination. They examined the relationship between morphological quality and tissue association of these sperm. The studies allowed them to describe the tissue surface characteristics in regions of sperm accumulation. They used scanning electron microscopy to examine equine sperm at the UTJ in 12 tissues collected from six young maiden mares. The mares' oviducts were flushed for sperm recovery four hours after insemination, and the UTJ tissues were fixed in formalin and reserved for future study. These fixed tissues offered a unique opportunity to evaluate sperm-epithelium association for normal and abnormal equine spermatozoa because: 1) each mare was inseminated with semen containing a high percentage of morphologically abnormal sperm (ranging from 66 85 percent abnormal), and 2) previously, sperm were observed at the UTJ using this technique, and thus, researchers anticipated that similar observations could be made with these tissues. They identified sperm in all 12 UTJ tissues, indicating that they are consistently present at this location and time. They found sperm (singly or in clusters) in intimate contact with the epithelium, and in some instances, only the tails of sperm could be seen at the upper edge of a deep furrow or pocket. Some sperm appeared to have been fixed in place while moving along the surface; others were suspended above the epithelium. It was noted that often cilia on the epithelial surface would engulf the head or tail of a sperm, indicating a specific interaction between the epithelium and the sperm. They determined that greater than 90 percent of the sperm at the UTJ were structurally normal. These results demonstrate that, during transport to the oviducts, normal equine sperm develop specific and intimate associations with the tissues lining the mare's reproductive tract. Further, this study provides compelling evidence of a sperm selection mechanism during sperm transport. Despite the presence of large numbers of abnormal sperm in the semen, few abnormal sperm were seen at the UTJ. It is possible that, despite being motile, structurally abnormal sperm may not move through the uterus to reach the UTJ following insemination. Or, alternatively, abnormal sperm may not undergo normal association with the UTJ's surface which may be necessary to enter the oviducts. Current guidelines for artificial insemination suggest that a standard insemination dose consist of 500 million progressively motile sperm. This study's results strongly suggest that normal structure as well as progressive motility should be considered when calculating an insemination dose, particularly for stallions known to be subfexfile. Research has shown that, in normal mares, a single insemination up to six days prior to ovulation can result in pregnancy. However, the location of a sperm reservoir in the mare's reproductive tract has not been identified. In this study, researchers identified sperm in UTJ tissues collected at 18 hours after preovulatory insemination. The sperm's intimate contact with the tissue surface suggests an active interaction. These preliminary results provide evidence of a possible location for sperm storage in the mare's reproductive tract. UC Davis researchers have also reported that sperm can be found at the UTJ at four hours after insemination in normal mares, but are less likely to be there in subfertile mares that have abnormal sperm lxansport to the oviducts. If differences also exist at 18 hours after insemination, this study would supply additional evidence of abnormal sperm function that could lead to subfertility in mares.