Human preovulatory oocytes have a higher sperm-binding ability than immature oocytes under hemizona assay conditions: Evidence supporting the concept of 'zona maturation'

Objective: The purpose of this study was to investigate the sperm-binding potential of human oocytes at different stages of nuclear maturation under hemizona assay (HZA) conditions. Design: This was a prospective study designed in a blinded fashion. Setting: Academic research environment approved by the Institutional Review Board. Patients: Surplus oocytes, donated by patients undergoing in vitro fertilization therapy after gonadotropin stimulation, were analyzed. Semen from a fertile donor was used in all assays. Interventions: Five groups of oocytes were considered: (1) immature, prophase I; (2) metaphase I; (3) metaphase II; (4) inseminated, unfertilized metaphase II; and (5) immature, prophase I oocytes matured in vitro to metaphase II. Oocytes were stored in salt solution (pH 7.2) and microbisected before assay. Main Outcome Measure(s): Tight binding of sperm to the zona pellucida under HZA conditions was evaluated after 4 hours of gametes coincubation. Results: Metaphase II oocytes (groups 3 and 4) had significantly higher binding than other groups (P = 0.0001). The mean value of the difference between the two halves (hemizona) was not significant, thus showing a small intra-assay variation for all maturational stages. Conclusions: Full meiotic competence of human oocytes is associated with an increased zona pellucida-binding potential.