Fertilization and early embryology: Intracytoplasmic single sperm injection of 1-day-old unfertilized human oocytes

Although the average fertilization rate in most in-vitro fertilization(IVF) centres is 60–70%, there are cases of complete orvirtually complete fertilization failures. The aim of our workwas to study the fertilization and the subsequent cleavage characteristicsof 1-day-old human oocytes treated by intracytoplasmic singlesperm injection (ICSI) after failing to fertilize during thestandard IVF procedure. A total of 115 metaphase II 1-day-oldunfertilized oocytes were collected from 23 patients. No additionaltreatment was applied to the oocytes or to the semen sample.A single spermatozoon from the patient's husband was injectedinto the cytoplasm of each of these oocytes 21–33 h afterovum retrieval. Injected oocytes were observed at 16–18h and again 42–44 h after the ICSI procedure. Of the injectedoocytes, 92% (n = 106) were intact after ICSI, 38% (n = 44)had two distinct pronuclei and there was no difference in thefertilization rate of oocytes when andrological and non-andrologicalpatients were compared. Similarly, there was no difference inthe fertilization rate after ICSI where patients with acceptableor good (> 15%) fertilization after standard IVF were comparedto patients who had poor (<15%) fertilization after IVF.There was no significant difference in the sperm concentrationor in the progressive forward motility (a + b motility) in thesegroups except where a + b motility of andrological and non-adrologicalpatients was compared. The majority (84%) of the normally fertilizedoocytes cleaved and most (77%) of these embryos showed <20%fragmentation 2 days after the ICSI procedure. From this studyit can be concluded that 1-day-old metaphase II oocytes whichhave failed to be fertilized after standard IVF procedure canbe fertilized and cleave when ICSI is performed on them theday after oocyte retrieval.