Structural, biochemical and functional aspects of sperm-oocyte interactions in pigs

Polyspermic fertilization is still a major issue in porcine IVF systems. New information is available to characterize the zona pellucida (ZP) at different developmental stages by scanning electron microscopy (SEM) and by confocal microscopy to show the distribution of ZP glycoproteins. SEM images indicated no differences between in vivo and in vitro matured oocytes; however a change in the surface structure between immature and matured oocytes, as well as between mature oocytes and preimplantation embryos was obvious. In addition, spermatozoa were more tightly fixed in the ZP of in vivo produced compared to the ZP of in vitro produced embryos. The ZP undergoes biochemical changes during maturation prior to fertilization. The acidity of the ZP increases during maturation as indicated by a shift of 1.3 pl units for ZPB/ZPC and 0.8 pl units for ZPA in 2D gel electrophoresis, which is based on increasing sulfation of the oligosaccharides during maturation. Mass spectrometry in combination with in-gel deglycosylation allowed the mapping of new glycosylation sites. Functionality of the ZP also depends on its maturation status. Induction of the acrosome reaction was delayed when capacitated spermatozoa were exposed to immature oocytes.