Effect of timing of oocyte denudation and micro-injection on survival, fertilization and embryo quality after intracytoplasmic sperm injection

In human in-vitro fertilization (IVF), the oocytes are surrounded bycumulus and corona cells at the time of insemination so that their maturitycannot easily be evaluated. The best IVF results are obtained if theoocytes are inseminated 2-6 h after retrieval. In the intracytoplasmicsperm injection (ICSI) procedure, the oocytes are denuded by enzymatic andmechanical treatment in order to be able to perform the injection. As aconsequence, the nuclear maturity of the oocytes can be evaluated and onlythose that have extruded the first polar body are injected. However,metaphase-II oocytes that have not yet reached cytoplasmic maturity cannotbe recognized. The purpose of this study was to investigate the effect ofdifferent timing of cumulus- corona cell removal and injection on theoutcome of ICSI. For this we allowed the oocytes to complete in-vitrocytoplasmic maturation in two different culture conditions: (i) surroundedby their cumulus and corona cells or (ii) totally denuded. We performedthree different studies on sibling oocytes obtained after a standardizedbuserelin/human menopausal gonadotrophin (HMG) protocol. We investigatedthe effect of early (1-2 h after retrieval) and late (5-6 h afterretrieval) oocyte denudation and injection on the survival andfertilization of the injected oocytes and on embryo cleavage afterfertilization. We found no statistically significant differences betweenearly and late injection, indicating that after a standardizedbuserelin/HMG protocol the metaphase-II oocytes do not need time forfurther cytoplasmic maturation. Furthermore, a different timing ofcumulus-corona cell removal has no effect on the outcome of ICSI,suggesting that the surrounding cells are not necessary for survival,fertilization and cleavage after ICSI.