The effect of denudation and injection timing in the reproductive outcomes of ICSI cycles: new insights into the risk of in vitro oocyte ageing

STUDY QUESTION: Does the time elapsed between oocyte pick-up (OPU) and denudation or injection affect the probability of achieving a live birth (LB) in ICSI cycles? SUMMARY ANSWER: Prolonged oocyte culture before denudation (>4 h) was associated with an increase in clinical pregnancy (CP), LB and cumulative LB (CLB) rates when compared with earlier denudation timings. WHAT IS KNOWN ALREADY: Oocyte maturation is a complex and dynamic process involving structural and biochemical modifications in the cell necessary to support fertilization and early embryo development. While meiotic competence is easily identifiable by the presence of an extruded first polar body, cytoplasmic maturation cannot be assessed microscopically. Culturing oocytes with their surrounding cumulus cells (CCs) prior to ICSI can enhance the completion of in vitro cytoplasmic maturation; conversely, prolonged culture may induce cell degeneration. The optimal culture intervals prior to oocyte denudation and/or injection have not yet been established and may prove relevant for the improvement of ICSI reproductive outcomes. STUDY DESIGN, SIZE, DURATION: This is a single-centre retrospective cohort analysis of 1378 ICSI cycles performed between January 2005 and October 2018. Data were categorized according to: (i) the time interval between OPU and denudation (<3 h, 3-4 h and >= 4 h), (ii) the time interval between denudation and ICSI (<1.5 h, 1.5-2 h, >= 2 h) and (iii) the time interval between OPU and ICSI (<5 h, 5-6 h and >= 6 h). The effect of these timings on fertilization, CP, LB and CLB rates were compared. The culture intervals between different procedures were dependent exclusively on laboratory workload. PARTICIPANTS/MATERIALS, SETTING, METHODS: ICSI cycles performed in women younger than 40 years old using autologous gametes with at least one metaphase II injected oocyte were included. The effect of oocyte culture duration prior to denudation and injection of the oocytes was compared using multivariable regression accounting for potential confounding variables. MAIN RESULTS AND THE ROLE OF CHANCE: Fertilization and oocyte damage rate after ICSI was found to be independent of the time interval to denudation (<3 h, 3-4 h and >= 4 h) and/or injection (<5 h, 5-6 h and >= 6 h). Extending oocyte culture before denudation significantly improved CP (29.5%, 42.7% and 50.6%, respectively), LB (25.1%, 34.4% and 40.7%, respectively) and CLB rates (26.0%, 36.1% and 42.2%, respectively), particularly if the time interval was at least 4 h. Additionally, LB (31.7%, 35.8% and 27.4%, respectively) and CLB rates (34.2%, 36.6% and 27.7%, respectively) were also dependent on the time from OPU to injection. LIMITATIONS, REASONS FOR CAUTION: This study is limited by its retrospective nature and potential unmeasured confounding cannot be excluded. Furthermore, the effect of even shorter or longer periods of culture before denudation and/or injection were not evaluated and should not be extrapolated from these results. WIDER IMPLICATIONS OF THE FINDINGS: Our findings propose new evidence of a previously unrecognized protective effect of the CCs-oocyte interactions in human ART, raising the question of a possible downstream effect in embryogenesis which significantly affects LB rates. Additionally, this is the first study to suggest a negative effect of further extending culture before ICSI on LB and CLB rates, thus potentially allowing for the narrowing of an optimal ICSI time interval. Simple strategies such as the establishment of more effective time frames to perform these procedures and adjusting laboratory practice may prove beneficial, ultimately improving ICSI reproductive outcomes. STUDY FUNDING/COMPETING INTEREST(S): None.