Zona‐free sperm penetration assay and inducers of the acrosome reaction: A model for sperm microinjection under the zona pellucida

Abstract

In order to minimize the percentage of false-negative results in the zona-free sperm penetration assay (SPA), a wide range of substances and/or physical agents capable of inducing the acrosome reaction (AR) have been incorporated in the incubation medium. These agents can also be used for treatment of severe male infertility using the technique of sperm microinjection under the zona pellucida (SMUZ). In the present review, the percentages of acrosome-reacted spermatozoa induced by several physiological, biochemical or physical agents published in the literature are compared in order to find the most efficient method(s) of inducing the AR in human sperm as a previous requirement of optimizing the technique of SMUZ. A working estimate of the level of efficiency of a given AR inducer is calculated by adding up its range score in each of three different arrangements from the highest to the lowest value of percentages of AR and differences in percentages of AR and penetration indexes between treated and control groups in SPA. The agents able to induce the AR by nonphysiological (electropermeabilization, lysophosphatidyl choline, and freezing-thawing) have better positions in this hierarchical system than those ones which require the active participation of sperm membrane receptors or second messenger systems (progesterone, zona pellucida, and stimulators of protein kinase A). Electropermeabilization appears to be the most efficient AR inducer. However, more possibilities need to be explored to enhance the relatively low percentages of acrosome-reacted spermatozoa shown by infertile men.