Ability of sperm to bind to the zona pellucida varies from egg to egg, according to investigators.
The binding of the sperm to the zona pellucida varies from egg to egg within the same woman, even when the sperm source is unchanged, a study found.
“This is something that may be clinically significant and change the way some infertile couples are counseled, because to us it seems the numbers of the sperm binding to the zona may reflect the egg quality and maturity instead of the sperm quality,” said study investigator Shehua Shen, MD, assistant adjunct professor of obstetrics and gynecology and director of embryology at the University of California in San Francisco. The researchers studied 481 oocytes from 36 patients. All oocytes were individually graded with a sperm binding score at the time of fertilization assessment 16 to 18 hours after insemination. All the eggs were stripped using a pipette, leaving only the tightly bound sperm embedded in the zona pellucida. Using an inverted microscope, the researchers counted the number of tightly bound sperm for each individual egg at 200 x magnification.
Sperm binding within a single oocyte cohort was highly variable. The investigators theorize that this may be related to intrinsic factors of the egg, which could become better defined through their ongoing study.
“We are wondering if the degree of binding may be a good marker of oocyte maturity,” said Dr. Shen, who reported study findings at the annual meeting of the American Society for Reproductive Medicine in New Orleans. “Sperm binding to intact, viable oocytes in a culture dish should not be thought of in the same manner as the hemizona assay.
“Just because a sperm is capable of binding, does not guarantee that it will. The role of the egg should not be underestimated in the binding process.” The ongoing study now includes assessment according to follicle size, in addition to egg maturity and/or fertilization outcome.
The investigators are tracking fertilized eggs until the day of transfer back to the uterus to further elucidate if the binding score itself may be a predictive marker of embryo quality. Most recent data suggest a strong correlation be-tween the binding score and embryo fragmentation on day 3 of in vitro culture. These findings are preliminary and will need to be replicated in a larger number of subjects.