Fertilization and early embryology: Scanning electron microscopy of the zona pellucida of human oocytes during intracytoplasmic sperm injection (ICSI)

During intracytoplasmic sperm injection (ICSI) approximately 10% of all injected oocytes degenerate. The reason for this process is unknown. It has been speculated that the mechanical procedure of the insertion of the ICSI needle induces injuries to the zona pellucida which lead to the death of the cell. By scanning electron microscope (SEM), it could be shown that the surface structure of mature oocytes is extremely elastic so that the injection needle penetrates the zona pellucida without destroying the mesh-like or more compact surface. No tissue pieces or zona fragments were detectable. After a culture time of 15 min the penetration site on the zona was no longer easily visible. We believe that oocyte degeneration is not caused by the penetration of a glass needle into the ooplasm but by an injury to the meiotic spindle or by an excessive dose of fluid [polyvinylpyrrolidone (PVP) or medium] during sperm injection.