Oocyte membrane maturation and oocyte-sperm relationship: transmission electron microscopy study.

The success of in vitro maturation (IVM) depends greatly on the acquisition of immature oocytes. Immature oocytes in prophase I (PI) and metaphase I (MI), aspirated after controlled ovarian hyperstimulation, were incapable of fertilization, leading to a lower fertilization rate. Therefore, they must be evaluated on a fine structure level for their in vitro maturation (IVM) processes and their relationship with sperm. Oocyte membrane maturation and oocyte-sperm relationship were studied using transmission electron microscopy. A total of 55 human oocytes obtained from 20 patients at various times and 83 oocytes obtained from the dissected ovarians of female Wistar rats were used for transmission electron microscopy (TEM) evaluation. Despite being in either prophase I and metaphase I or in metaphase II, the oocytes were not fertilized after 48 h of incubation. At the various stages of maturation between PI and MII, the number and the size of microvilluses on the oocyte membrane increased as MII approached and decreased after full maturation. Oocyte activation was related to oocyte membrane maturation and has an effect on the oocyte sperm penetration.