Structural Aspects of the Zona Pellucida of In Vitro-Produced Bovine Embryos: A Scanning Electron and Confocal Laser Scanning Microscopic Study1

Structural aspects of the bovine zona pellucida (ZP) of in vitro-matured (IVM) oocytes and in vitro-produced (IVP) em-bryos were studied in two experiments to find a tentative expla-nation for the zona’s barrier function against viral infection. In Experiment 1, the ultrastructure of the outer ZP surface was studied. The diameter (nm) and the number of the outer pores within an area of 5000 mm2 of 10 IVM oocytes, 10 zy-gotes, 10 8-cell-stage embryos, and 10 morulae were evaluated by scanning electron microscopy. In oocytes and morulae, the ZP surface showed a rough and spongy appearance with nu-merous pores. In zygotes, the ZP surface was found to have a smooth, melted appearance with only a few pores. In 8-cell-stage embryos, both surface patterns were found. The mean number (per 5000 mm2) and the mean diameter of the outer pores were different between the four stages of development (