Sperm Motility

Principles

Spermatozoa, after passage through the epididymis, are motile cells. Sperm motility becomes critical at the time of fertilization because it allows or at least facilitates passage of the sperm through the zona pellucida. Without technologic intervention, a non-motile or abnormally-motile sperm is not going to fertilize. Hence, assessing the fraction of a sperm population that is motile is perhaps the most widely-used measure of semen quality.

In evaluating motility with most species, sperm are classified as non-motile, progressively motile or non-progressively motile. A progressively motile sperm swims forward in an essentially straight line, whereas a non-progressively motile sperm swims, but with an abnormal path, such as in tight circles.

Another term that is sometimes used is "total motility" which refers to the fraction of sperm that display any type of movement. This concept is rarely used for evaluating animal semen, but is the norm for describing human sperm motility.

There is considerable variability among species in what is considered normal or acceptable motility. For example, to pass a breeding soundness examination, it is recommended that bulls have greater than 30% progressively motile sperm, stallions greater than 60% and dogs greater than 70%. Normal humans typically have total sperm motility of greater than 50%.

When analyzing motility, it is important to know whether the semen sample has been abused in any way. Exposure to heat, cold, any kind of residue on collection equipment, or the wrong pH or osmolality of an extender can adversely affect motility. Motility is also affected by periods of sexual inactivity - males that have not ejaculated for prolonged periods often have poor motility on the first ejaculate, but much better motility for a second ejaculate collected soon thereafter.

Techniques for Evaluation of Sperm Motility

There are several means for evaluating motility. Which technique to use depends on experience of the operator, the desire for precision and repeatability, and availability of equipment. In almost all cases, motility is reported as the percentage of sperm that manifest motility (usually progressive motility).

Manual motility estimates (wet mount): This commonly-used technique involves placing a sample of diluted semen on a microscope slide, examining it with a microscope and estimating the fraction of the population that is motile.

More specifically, a sample of semen is diluted in warm extender or buffered saline, and about 10 to 20 microliters of this sample is pipetted onto a clean, prewarmed microscope slide. A coverslip is lowered onto the sample, avoiding formation of air bubbles if possible, and the slide is examined using a microscope with a 20X objective. At least ten widely-spaced fields are examined to provide an estimate of the percentage of motile cells. If estimating motility is a commonly required technique, a microscope warm stage may be worth purchasing.

A bright field microscope can be used for evaluating motility if the field diaphragm is closed to enhance contrast and ability to visualize sperm. A much better choice is a phase contrast microscope or a microscope equipped for differential interference contrast (DIC). Examine the images above to appreciate the difference in these three types of microscopy - clearly, unstained sperm are difficult to observe using bright field.

Manual motility estimates are easy to perform and require minimal equipment. In the hands of an experienced evaluator, manual estimates generally provide good estimates of motility. The chief limitation of this technique is its subjective nature. Samples with outstanding motility tend to be scored lower than they should be ("it can't be that good ..") and poor samples tend to be scored higher than they should ("it can't be that bad ..").

Track motility estimates: A wet mount of diluted semen is prepared as described for a manual estimate, and sperm are photographed using an exposure time long enough for motile sperm to leave tracks across the image (roughly 0.2 seconds). Using this technique, non-motile sperm appear .. well .. non-motile, and sperm with non-progressive motility leave tracks that are circular or otherwise abnormal. If a digital camera is used, the image can be displayed on a computer monitor for convenient tabulation.

The images below depict the track motility technique. On the left, three progressively motile sperm (two are marked) are represented by three straight tracks. On the right, a circular track is seen, representing a non-progressively motile sperm. Non-motile sperm are seen in both images.

Track motility estimates require more time to perform that manual estimates, but they are objective and repeatable. Additionally, one can save the images for a permanent record.

Computer-aided motility analysis : A number of systems have been developed that allow motility to be analyzed with the aid of a computer. These systems acquire an image in essentially the same manner as described for the track motility technique, and image processing software detects and tabulates the tracks and immotile cells. Other information is readily obtained, including sperm velocity and other motion characteristics. Additional information on these tools is provided under Computer-Aided Semen Analysis.