ASSESSMENT OF FERTILIZATION FAILURE AND ABNORMAL FERTILIZATION AFTER INTRACYTOPLASMIC SPERM INJECTION (ICSI)

The assessment of fertilization is an important part of intracytoplasm ic sperm injection (ICSI) and oocytes are routinely examined about 17 h after injection using Nomarski differential interference contrast op tics. However, it is not possible to conclusively determine the aetiol ogy of fertilization anomalies in this manner, so cytological studies were undertaken to determine the causes of failed and abnormal fertili zation after ICSI. Oocytes which exhibited no evidence of fertilizatio n, one pronucleus (PN) or 3 PN were fixed in glutaraldehyde, stained w ith Hoechst 33342 and examined by fluorescence microscopy to identify PN, metaphase chromosomes, sperm heads and polar bodies. A total of 42 8 unfertilized oocytes were examined from 170 ICSI cycles. Overall, 82 % of these unfertilized oocytes were still at metaphase II (non-activa ted) while the remaining 18% were activated and had 1 PN and two polar bodies. The majority (71%) of the metaphase II oocytes contained a sw ollen sperm head, which indicates that the spermatozoon was correctly injected but the oocyte did not activate and complete its second meiot ic division. The swollen sperm head was located among the metaphase ch romosomes in 4.3% of these oocytes, while in some cases (6.6%), the sp erm chromosomes had undergone premature chromosome condensation (PCC). Other aetiologies of failed fertilization in these metaphase oocytes were ejection of the spermatozoon from the oocyte (19%) and complete f ailure of sperm head decondensation (10%). A similar pattern of anomal ies was found in 1 PN oocytes, although the ratios were different (swo llen sperm head, 51%; ejection of the spermatozoon, 19%; undecondensed sperm head, 30%). Seventy abnormally fertilized oocytes were also exa mined, of which 63 had 3 PN and a single polar body, indicating that t he unextruded second polar body developed into the third PN. In conclu sion, the present study demonstrates that the principal cause of ferti lization failure after ICSI is failure of oocyte activation and not ej ection of the spermatozoon from the oocyte. It is also apparent that f urther studies are needed to elucidate the mechanisms that control ooc yte activation and sperm head decondensation in injected oocytes.