Fertilization abnormalities and pronucleus size asynchrony after intracytoplasmic sperm injection are related to oocyte postmaturity

Objective: To study the effect of delayed intracytoplasmic sperm injection (ICSI) on the fertilization and cleavage of human in vitro matured (NM) ooc ytes. Design: Prospective experimental study. Setting: Academic hospital-based fertility center. Patient(s): The experimental group consisted of 73 spare germinal vesicle-s tage oocytes from 25 patients. The control group consisted of sibling in vi vo matured oocytes from the same patients that were subjected to ICSI in th e clinical program. Intervention(s): Equal numbers of sibling IVM oocytes were subjected to ICS I either soon after maturation (30 hours, group 1) or after a 6-hour delay (36 hours, group 2). In a subsequent set of experiments, spermatozoa were l abeled with a fluorescent mitochondria-specific vital dye and injected into 17 IVM oocytes that were intentionally aged by 6-8 hours. The resultant zy gotes were fixed. Main Outcome Measure(s): Incidence of fertilization and cleavage, numbers a nd mean diameters of pronuclei, and incidence of zygotes with significant p ronucleus size asynchrony. Identification of the male pronucleus by its pro ximity to the fluorescent sperm midpiece remnant. Result(s): Group 2 had significantly lower rates of normal fertilization (6 0%) than the control group (82.9%) and significantly lower cleavage rates ( 46.7%) than both group 1 (85%) and the control group (98.1%). The incidence of oocytes that developed one pronucleus and pronucleus size asynchrony wa s significantly higher in group 2 (32% and 40%, respectively) than in group 1 (4% and 5%, respectively) and in the control group (4.1% and 4.4%, respe ctively). All the zygotes with significant pronucleus size asynchrony that developed after delayed ICSI with labeled spermatozoa showed proximity of t he fluorescent sperm midpiece remnant to the smaller pronucleus. Conclusion(s): For IVM oocytes, the incidence of one pronucleus, pronucleus size asynchrony (possibly related to a smaller male pronucleus), and cleav age failure increase when ICSI is delayed after maturation. Thus, the timin g of ICSI is critical for optimum fertilization of IVM oocytes. (C) 1999 by American Society for Reproductive Medicine.