Human gamete fusion test: An experimental model for assessing the fusion potential of sperm from nonfertilizing normospermic patients

Objective: To develop an experimental model for assessing the ability of sperm to bind and subsequently fuse with the oolema and to use this test for evaluating the fusion potential of nonfertilizing human sperm. Design: Aged human oocytes (n = 108) were denuded of their zonae and loaded with DNA- specific bisbenzamidazole fluorochrome. Sibling oocytes were inseminated by semen samples from either normospermic patients (study group; n = 12) who have repeatedly failed to achieve fertilization in vitro or patients demonstrating high fertilization rates (control; n = 12). Gamete fusion was ascertained by fluorescent microscopy and validated by scanning electron microscopy. Test results were analyzed in relation to current partial zona dissection treatment outcome. Setting: Infertility and IVF Unit of an academic tertiary referral medical center and university-based basic research laboratory. Results: In samples achieving fertilization by partial zona dissection (7/12; 9% to 23% fertilization rate), the mean numbers of fused spermatozoa were similar to those of the control (2.1 compared with 2.6) and significantly higher than in the partial zona dissection-failed fertilization subgroup. In six of seven cases in which partial zona dissection yielded embryos, fluorescent spermatozoa were detected on the oolema, whereas in all but one partial zona dissection-failed cases gamete fusion was not observed. Conclusions: The suggested human gamete fusion test may help focus on specific sperm dysfunction in nonfertilizing patients. It correlates well with the outcome of partial zona dissection-assisted IVF trials and may provide useful information before this treatment is attempted.