Factors influencing sperm-zona pellucida binding in vitro using the intact zona model

The zona pellucida binding assay assesses the ability of spermatozoa to bind to the zona pellucida. The present study investigated the influence of: (i) prior oocyte exposure to spermatozoa on subsequent sperm-zona pellucida binding in vitro; and (ii) cryopreservation of oocytes. Only oocytes obtained from fertile donors were used and the binding capacity of non-inseminated, cryopreserved oocytes was compared with both inseminated/unfertilized, cryopreserved oocytes and inseminated/unfertilized, non-cryopreserved oocytes recovered from in-vitro fertilization cultures on sperm-zona pellucida binding using an intact zona model. There was no statistically significant difference in sperm-zona binding between non-inseminated, cryopreserved oocytes (range 9.6-23.2), inseminated/unfertilized, cryopreserved oocytes (range 15.0-16.0) and inseminated/ unfertilized, non-cryopreserved oocytes (range 3.3-23.0). The coefficient of variation for sperm binding to all oocyte groups was very large (range 37-121%). We conclude that neither prior exposure of human oocytes to human spermatozoa nor cryopreservation of human oocytes influences the subsequent binding of a different population of spermatozoa to the zona pellucida. However, large oocyte-to-oocyte variation of sperm-zona binding may diminish the usefulness of this assay in clinical practice and as a research tool.