Assessment of sperm quality: a flow cytometric approach.

For many years, scientists have sought to develop laboratory assays that accurately predict the fertilizing capacity of a semen sample. This goal, however, has proven elusive and will most likely be very difficult to achieve, due to the complex nature of the problem. Part of the problem results from the many attributes that a spermatozoon must possess to fertilize an egg, and how laboratory assays can evaluate all of these attributes simultaneously. The percentage of motile sperm in a sample is most commonly used to evaluate semen quality. This assay, however, is not highly correlated with the fertilizing capacity of semen samples. One reason motion assays do not correlate well with fertility is that we are evaluating only one of many attributes that a sperm must possess to fertilize an oocyte. One of the problems of measuring multiple sperm attributes is the time and cost required. Using flow cytometric assays, multiple sperm attributes, including cell viability, acrosomal integrity, and mitochondrial function, can be measured simultaneously in sperm cells. In addition, the ability of sperm to undergo capacitation and the acrosome reaction, as well as the chromosomal integrity of sperm can be measured using flow cytometry. Flow cytometry permits us to evaluate 50,000 sperm in less then 1 min and at reasonable cost. Although flow cytometry is a powerful tool for evaluating many sperm attributes, it cannot evaluate all of the attributes a sperm cell requires to fertilize an oocyte. Therefore, laboratory assays are also being developed to evaluate the ability of sperm: (1) to bind to the oocyte, by evaluating the ability of sperm to bind to the perivitelline membrane of the hen egg in vitro; (2) to undergo an acrosome reaction in vitro, after treatment with membrane destabilizing compounds; and (3) to penetrate oocytes in vitro. When data from multiple sperm assays are used, higher correlations with the fertilizing potential of a semen sample is achieved. For example, in a study conducted utilizing five stallions, the percentage of motile sperm in semen samples correlated poorly with fertility (r(2)=0.22), however, when data for sperm motility, viability and penetration rates into zona-free hamster oocytes were utilized together, these data explained 72% of the differences in the fertility of the stallions (r=0.849; [Theriogenology 46 (1996) 559]). Armed with a battery of tests, which evaluate many different sperm attributes, researchers should be able to more accurately estimate the fertilizing potential of semen samples.