Embryo s-phase length analysis on first and second cell cycle and its relationship with reproductive outcome

OBJECTIVE: to analyse the length of the s-phase (synthesis of DNA), in the first cell cycle (ECC1) and in each blastomere of the second cell cycle (ECC2) of known implantation data (KID) embryos, and to relate them with reproductive outcome. DESIGN: Observational retrospective study of 1679 transferred embryos from 940 ICSI patients from our egg donation program between January 2011 and January 2014 in IVI Vigo and IVI Valencia, cultured in Embryoscope. Sperm samples under 1 million/ml were used as an exclusion criteria. MATERIALS AND METHODS: Only non-multinucleated embryos which either failed to implant or fully implanted were included in the study. The s-phase was calculated as the period while nuclei were visible, being ECC1 S-phase1⁄4tPNfading-tPNappearance, and ECC2 S-phase1⁄4 t2MONO1(f)t2MONO1(a). ANOVA test and c2-test were performed when applicable to assess the influence of the length of S-phase in the implantation rate. RESULTS: S-phase in both cell cycles were calculated in 904 KID embryos out of 1679, and therefore were analyzed. 32,74% (n1⁄4296) fully implanted (KID+), and 67,25% (n1⁄4608) failed to implant (KID-). The average length of s-phase in the ECC1 in the KID+ embryos was longer than in KID-, 15,50h v.s. 14,38h, and slightly shorter in ECC2, 8,35 in KID+ v.s. 8,60h in KID-. 46,1% (n1⁄4112) of the embryos implanted when the difference between both s-phases was greater than 7.96h, being 29,1% for those where the difference is between 5,96h-7,95h; 23,1% for those between 3,61h-5,95h; and 30% for those below 3,6h. CONCLUSIONS: Results show that the s-phase ECC1, nearly double the length of s-phase ECC2, and the greater the difference between both s-phase, the greater the embryo implantation, suggesting that the replication in the first cell cycle may implicate particularities related to the genome combination, and that those embryos which do not implant may not complete the replication of its genome during the first cycle producing a larger reduction in the duration of the second.