Does zona pellucida thickness influence the fertilization rate?

In human in-vitro fertilization (IVF), the cumulus oophorus is routinely removed to assess fertilization and hence the thickness of the zona pellucida is measurable. This study aimed to measure the thickness of the zona pellucida and to assess its influence on fertilization rate in IVF programmes. The zona pellucida thickness varies from 10 to 31 microns with a mean of 17.5 microns. One-way analysis of variance revealed that in IVF trials performed with normal semen, the zona pellucida of fertilized oocytes (16.6 +/- 3.2 microns) was significantly thinner than the zona pellucida of unfertilized oocytes (18.9 +/- 4.0 microns; P < 0.001). As measured on micro-injected oocytes, the zona pellucida thickness did not change between ovulation and 16-20 h after fertilization. Zona pellucida thickness was not related to ooplasm diameter. In conclusion, zona pellucida thickness appears to be an additional factor that should be taken into account when interpreting the fertilization rate. Zona pellucida thickness influences sperm penetration, even when the spermatozoa are considered normal. From a clinical point of view, a thick zona pellucida (> or = 22 microns) could be an indicator for the use of micro-injection procedures.