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

Abstract

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. This was a prospective study designed in a blinded fashion. Academic research environment approved by the Institutional Review Board. 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. 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. Tight binding of sperm to the zona pellucida under HZA conditions was evaluated after 4 hours of gametes coincubation. 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. Full meiotic competence of human oocytes is associated with an increased zona pellucida-binding potential.