Effect of regulation of oocyte cytoplasmic volume during fertilization on clinical outcomes of single-embryo transfer

Abstract

Using a time-lapse monitoring system, we observed that the oocyte cytoplasmic volume altered in the duration between sperm penetration and pronuclear (PN) fading. The present study aimed to determine the oocyte cytoplasmic volume at 5 morphokinetic events during fertilization and whether its regulation influences the clinical outcomes. Retrospective, single-center, cohort study. A total of 311 patients (311 cycles; mean age: 35.1 ± 3.7 years) that underwent minimal-stimulation in vitro fertilization (mini-IVF) followed by freshly cleaved single-embryo transfer (SET) from August 2017 to September 2018 were retrospectively analyzed. Retrieved oocytes were inseminated by intracytoplasmic sperm injection (ICSI) after meiotic spindles were confirmed. Oocytes were cultured in a time-lapse incubator (EmbryoScope+®, Vitrolife) after ICSI. The oocyte cytoplasmic volume corresponded to the oocyte major axis cross-sectional area (μm2). Measurements were recorded at 5 morphokinetic events: after ICSI (tICSI), time before 2nd polar body (PB) extrusion (tPB2b), time of 2nd PB extrusion (tPB2), time before PN fading (tPNfb), and time of PN fading (tPNf). The mean areas of oocytes at the morphokinetic events were compared (Study 1). In addition, the rates of change of oocyte cytoplasmic volume from tPB2b to tPNfb (area of tPNfb / tPB2b: group A), tPB2b to tPNf (area of tPNf / tPB2b: group B), and tPNfb to tPNf (area of tPNf / tPNfb: group C) were calculated. A multivariable logistic regression analysis was performed, which includes the significant confounding factors and yields adjusted odds ratios (aORs) and 95% confidence intervals (CIs), to evaluate the correlation between oocyte cytoplasmic volume change and clinical pregnancy (gestational sac observation) after SET (Study 2). P-values <0.05 were considered statistically significant. Study 1: The mean area of oocytes at tICSI, tPB2b, tPB2, tPNfb, and tPNf were 11,452, 10,826, 10,587, 10,237, and 10,308 μm2, respectively. The oocyte areas at tPNfb and tPNf were significantly smaller than those at tICSI, tPB2b, and tPB2 (P <0.05). Study 2: The multivariable logistic regression analysis showed that clinical pregnancy had significant associations with group A (area of tPNfb / tPB2b, aOR: 4.8, 95% CI: 1.07–23.08, p<0.05) and group B (area of tPNf / tPB2b, aOR: 7.3, 95% CI: 1.22–47.58, p<0.05), but not with group C (area of tPNf / tPNfb, aOR: 2.08, 95% CI: 0.30–14.1, p=0.4549). A significant decrease in oocyte cytoplasmic volume was observed from sperm penetration to PN fading. In addition, there were significant associations between clinical pregnancy and the degree of cytoplasmic volume change from 2nd PB extrusion to PN fading. These results suggest that the regulation of oocyte cytoplasmic volume during fertilization would influence oocyte competence, which may predict successful pregnancy after SET.