Following attempted in vitro fertilization (IVF), 13 nonfertilized oocytes were examined by scanning electron microscope (SEM), in order to determine the cause of failure. Fifty-eight IVF treatments were performed on 32 infertile couples. Semen analysis and the hamster test were normal in all patients. Each hasband fertilized most of his wife's oocytes and the mean fertilization rate was 93.2±2.6%: of 114 mature oocytes used 101 (88.6%) were fertilized. Nonfertilized oocytes were first examined by light microscope. They were then fixed, and processed for SEM. No oocytes had spermatozoa in the perivi telline space. The outer surface of the zona pellucida displayed a typical porous, net-like appearance in most cases. Oocytes with a smooth surface of the zona were also seen. Many spermatozoa in early stage of penetration were observed. The spermatozoa fixed on the zona had reacted acrosomes. The surface architecture and the sperm-zona interaction observed on the nonfertilized oocytes were not different from those of fertilized ova. It is clear that fertilization failure in human IVF usually oocur at the level of zona pellucida. The subsurface area of the zona seems to be primarily responsible for penetration block.
