Absence of block to polyspermy at the human oolemma

Abstract

Objective: To study the fusiogenic ability of human spermatozoa and oocytes. Design: Retrospective study of 3,027 oocytes included in a program of subzonal insemination (SUZI). Setting: Assisted fertilization program in an academic research environment. Patient(s): Couples with characterized male factor infertility or previous unexplained IVF failures. Intervention(s): Subzonal insemination (SUZI) was performed after study of the sperm pathology. The number of microinjected spermatozoa was controlled. Main Outcome Measure(s): Fertilization and polyspermia rates were analyzed according to the number of microinjected spermatozoa and to the indication of SUZI. Result(s): The fertilization rate increased linearly between one and three microinjected spermatozoa. Above this number, the rate plateaued around 25%. Polyspermia was correlated with the number of microinjected spermatozoa (r = 0.97). The fusiogenic ability of motile sperm cells was dependent on the semen characteristics and the sperm pathology. Observed diploid and polyspermia rates did not differ from the calculated probability of microinjecting only one or at least one fertilizing spermatozoon into the perivitelline space, respectively. Conclusion(s): Data support the hypothesis that a physiologic block at the human oolemma is absent. The post-SUZI fertilization rate also can be explained by the probability of finding one fertilizing spermatozoon among those that were microinjected and by the limited number of sperm heads allowed to decondense in the ooplasm.