While a large part of microscopic semen evaluation centers around motility (directional sperm movement) and morphology (sperm structure), some reproductive specialists think there's more to the story. In an attempt to better understand stallion fertility, Steven P. Brinsko, DVM, MS, PhD, Dipl. ACT, associate professor of theriogenology at Texas A&M University, presented the results of research investigating the relationships between sperm membrane integrity, motility, and morphology.

"Motility is important and many still look at it heavily, including me, but in a majority of studies there has been a poor correlation between motility and fertility," he said. "Similarly, morphology doesn't have as good a correlation with fertility as we might think.

"Integrity of the plasma membrane is essential for the proper sperm functions that lead to capacitation (a change in sperm that gives it the ability to acrosome react), the acrosome reaction (fusion of the outer acrosome membrane with the plasma membrane, which facilitates penetration of the egg), and fertilization," he went on. "Assessment of sperm membrane integrity has been reported to be a more accurate predictor of fertility than sperm motility. Although some sperm may seem morphologically normal and motile, membrane damage could render them incapable of fertilization."

The first hypothesis of the study was that motility, morphology, and membrane integrity are related, Brinsko said. The second was that relationships differ between the first and second ejaculates (i.e., sexually rested and sexually active samples). One hundred sperm per ejaculate were evaluated at random