SPERM PENETRATION ASSAY

BACKGROUND

Purpose of the Sperm Penetration Assay

In the past, the routine semen analysis was the best method of diagnosing a male's fertilizing potential. However, is has become clear that men with poor semen analysis can be fertile and, conversely, men with good semen analysis may not be fertile. This suggests that a measure of sperm function would allow the physician to more accurately predict if a male could truly fertilize human ova. The Sperm Penetration Assay (SPA) measures the ability of sperm to fuse and penetrate the ovum membrane by setting up a biological model in a culture dish.

Human sperm are exposed to hamster ova that have the zona pellucida removed. The more sperm that penetrate the hamster ova the more likely the sperm are to penetrate a human ova. This information has become more important since the cost of a failure in In Vitro Fertilization (IVF) is so expensive in dollars and emotional stress. Fortunately, the SPA provides new information as to the true functional state of human sperm and has become an important tool in monitoring male fertility.

INTERPRETATION OF SPERM-HAMSTER OVUM PENETRATION ASSAY (SPA)

The spermatozoa of patients with proven fertility penetrate all hamster eggs (100%) with a range of 6-20 sperm penetrations/egg (P/E). Values less than these are suggestive of relative degrees of impaired penetrating capacity. For example, a semen specimen that results in penetration of all eggs (100%) but with an average penetrations/egg of 1.0 has good, but not optimal, penetrating capacity; whereas a specimen that results in penetration of only 10% of the eggs with a mean penetration of 0.1 sperm/egg is indicative of much poorer penetrating capacity. These results indicate that the former patient (1 sperm penetration/egg) has a lower probability of achieving a pregnancy than a pregnancy-proven donor (6-20 P/E); conversely, this same patient should have a much better chance of achieving a pregnancy than the patient with a mean penetration of 0.1 sperm/egg.

It should be remembered that the SPA only provides information on penetrability and is not directly related to bulk parameters of semen quality, i.e., sperm concentration and percent motile sperm in the ejaculate. For the purposes of the assay, motile sperm only are collected and the number of sperm used in each assay is approximately the same. Thus, the assay provides a comparison between patients based on equal numbers of motile sperm.

It is possible that a patient having 200 eggs penetrated and sperm penetrations/egg as a patient having only 1 million sperm with 40% motility. Clearly, in spite of their identical performance in the sperm-ovum penetration assay, these patients would not have the same fertility potential.