Use of mouse oocytes to evaluate the ability of human sperm to activate oocytes after failure of activation by intracytoplasmic sperm injection.

The objective of the present study was to investigate the nuclei of human sperms that failed to fertilize human oocytes after intracytoplasmic sperm injection (ICSI). The sperms were injected into mouse oocytes by a piezo-micromanipulator, and some of these oocytes were artificially activated with strontium chloride (SrCl2) after ICSI. The oocytes were fixed, stained, and subjected to chromosomal analysis. The survival rate of mouse oocytes injected with infertile human sperms was 92.0% (46/50), while that of the control mouse oocytes injected with fertile human sperms was 73.6% (81/110). The rate of two pronuclei (2PN) formation was 0 (0/46) by the infertile sperms and 81.5% (66/81) by the fertile ones, a significant difference (p < 0.01). Sperm chromosomes in non-activated oocytes were present as premature chromosome condensation (PCC). Artificial activation after ICSI increased the 2PN formation rate in the infertile group to 90.3% (28/31). The results of the present study suggest that infertile sperms have a low potential to spontaneously activate oocytes and to form pronuclei. Thus, artificial activation after ICSI may rescue oocytes fertilized with infertile human sperms that do not produce 2PN. The present study proved the usefulness of mouse oocytes as specimens in evaluating the oocyte-activating capacity of objective human sperms prior to ICSI treatment.