Developmental Status of Human Oocytes that Fail to Cleave After Intracytoplasmic Sperm Injection

Background: Oocyte maturity is crucial for the success of intracytoplasmic sperm injection (ICSI). However, the oocyte appearance under a light microscope is the only indicator of maturity. Examining the chromatin status of the oocytes that fail to fertilize after ICSI may enrich the knowledge about maturity and possible causes of fertilization failure. Objectives: To investigate the developmental progress of human oocytes that fail to cleave after ICSI, and the stages at which the progress has been arrested. Methods: An observational study of human oocytes from women having ICSI cycles. Oocytes that failed to cleave after ICSI were collected and immunostained to visualize the meiotic spindles and chromatin using immunofluorescence microscopy. Results: 100 failed oocytes were successfully immunostained. Oocytes showed the following developmental stages: anaphase-I (4%), telophase-I (9%), prometaphase-II (14%), metaphase-II (24%), anaphase-II (33%), one pronucleus (6%), two pronuclei (9%), and syngamy (1%). The meiotic spindle was seen in most of the prometaphase-II and metaphase-II oocytes. Sperm chromatin showed different levels of decondensation in the oocytes. Meiotic spindles were absent in MII oocytes that showed condensed sperm chromatin. At least one polar body was identifiable in all of the oocytes studied. Many apparently MII oocytes haven’t reached the MII stage at ICSI time, despite having a visible first polar body. Conclusions: The appearance of a polar body in the oocyte doesn’t guarantee that the oocyte has reached the MII stage. Oocyte cleavage may be arrested at any developmental stage from sperm injection to syngamy.