Abstract: Objective To study the ultrastructure changes of human unfertilized oocytes and to investigate why human oocytes failed to fertilize after in vitro fertilization (IVF) /intracytoplasmic sperm injection (ICSI). Methods Gathered unfertilized oocytes after IVF/ICSI and normal matured oocytes were handled with fixed, dehydrated, embedded, ultrathined and observed by transmit electron microscopy(TEM) ,and to compare the difference of ultrastructure among the three groups .Results The thickness of zona pellucida of group IVF(14.7±2.4μm) was thicker than group normal (10.1±2.1)μm (P<0.05). In IVF group, cortical granules were dispersed in the oocytes cortex, only a part of which were beneath the oolemma. In other two groups, there was one row of dense cortical granules. The rate of abnormal mitochondria of group IVF (41.2%) was higher than other two groups (P<0.05). The oocyte cortex contained matured and immatured Golgi complex in group IVF, the presence of Golgi complex still forming cortical granules. Conclusions Organelles appeared in abnormal position was related to oocytes cytoplasmic maturation, which led to fail in fertilization. Failed fertilization in conventional IVF was due to absence of acrosome reaction and abnormal dense of the zona pellucida.

Key words: fertilization failure, oocyte, in vitro fertilization