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.

Semen from 10 stallions (presented for routine breeding soundness examination, with 10-80% per cycle pregnancy rates) was collected twice about an hour apart following at least a week of sexual rest, then again the next day. "Many (sperm) looked quite morphologically normal, but stained as having disrupted membranes," Brinsko reported. Conversely, he added, "I have often been amazed at how many sperm with poor morphology stained as membrane-intact."

The results? Between normal morphology and sperm membrane integrity, there was no association whatsoever. "There was a high percentage of morphologically normal sperm with membrane damage and vice versa (morphologically abnormal, but no membrane damage)," reported Brinsko.

As for the relationship between total sperm motility and membrane integrity, again no linear relationship was found. "Some stallions had very high progressive motility, but much lower membrane integrity," he noted. "Progressive motility and normal morphology had a much better correlation (positive linear relationship).

"Overall, the percentage of normal sperm and membrane-intact sperm was not different between the first and third ejaculates," he went on. "In the first ejaculates, morphologically normal sperm were just as likely to be membrane-damaged as they were to be membrane-intact. However, the percentage of normal sperm that were membrane-damaged was much lower in the third ejaculate, where morphologically normal sperm were almost twice as likely to be membrane-intact compared to the first ejaculate."

Brinsko stated the following observations based on the study:

  • Membrane-damaged sperm can be motile.
  • Morphologically normal is not equal to membrane-intact.
  • Stored epididymal sperm can deteriorate without concomitant alteration in morphology. Therefore, good sperm morphology should not be used alone to assume that semen quality is acceptable in ejaculates of sexually rested stallions.

He also offered the following recommendations for practitioners evaluating stallion fertility:

  • Examine multiple ejaculates.
  • Examine multiple sperm attributes to give the best indication of semen quality.

In conclusion, Brinsko said, "Although fertility was not assessed in the present study, the data presented here may help to explain results of earlier studies where correlations between fertility and motility, or fertility and morphology, were poor or non-existent. These findings also raise concerns over using only the number of progressively motile sperm or number of morphologically normal sperm in an ejaculate as an assessment of semen quality.

"This would be especially worrisome if these methods were employed on the first ejaculate obtained after sexual rest to determine its suitability for cooling or freezing," he went on. "Stresses associated with cooling or freezing could magnify membrane damage that existed before processing. When this occurs, fertility could be much lower than would be expected based on the morphologic and motion characteristics of the sample."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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