Abstract
Abstract Study question This study aimed to investigate the influence of cytoplasmic strings (CSs) on the development and quality of the embryo. Summary answer Blastocysts with CSs exhibited accelerated development from t7 to the tB stage and a higher rate of good-quality on day 5. What is known already Time-lapse (TL) culture during in vitro fertilization is an established method for evaluating the morphological and morphokinetic features of embryos. CSs, identified through TL culture, are among the characteristics of developing embryos. Although the impact of CSs on embryo quality is recognized, their specific functions in human embryos remain unclear. Study design, size, duration This retrospective study was conducted from January 2022 to December 2022. A total of 1050 injected oocytes were cultured in the EmbryoScope® (Vitrolife),with images captured every 20 minutes for 5-6 days. CSs are observed the straight lines connecting the inner cell mass to the mural trophectoderm. On days 5-6, the presence of CSs was assessed in 610 blastocysts. Participants/materials, setting, methods The evaluation of differences in development rate and quality related to CSs was conducted using morphological or morphokinetic parameters: the time to second polar body extrusion (tPB2), time to pronuclei appearance and fading (tPNa, tPNf), time to reach 2–8 cells (t2, t3, t4, t5, t6, t7, t8, t9), time to the start of blastulation and blastocyst formation (tSB, tB). Statistical analysis employed Student’s t-test for developmental time and the Chi-square test for quality comparisons. Main results and the role of chance Blastocysts were categorized into Group 1 if they had CSs and Group 2 if they lacked CSs. No differences in developmental rates between the groups were observed until the t6 stage. However, from t7, the pre-cleavage stage, to the tB stage, the Group 1 exhibited a significantly faster developmental compared to the Group 2 (t7: 56.97h vs. 58.82h, p = 0.013; t8: 61.68h vs. 63.39h, p = 0.049; t9: 72.05h vs. 74.63h, p = 0.001; tSC: 82.32h vs. 85.56h, p < 0.001; tM: 92.60h vs. 95.48h, p < 0.001; tSB: 105.11h vs. 107.57h, p = 0.004 and tB: 114.73h vs.117.97h, p = 0.001). In examining of good-quality embryos (GQE) on day 5(≥ grade BB), there was a notable association in Group 1, demonstrating 71.6% compared to 28.4% in the Group 2 (p < 0.001).These findings indicate a positive correlation between the presence of CSs and embryo developmental progress and quality. Limitations, reasons for caution The determination of CSs relies on visual evaluation, containing the potential for subjective bias. Future research requires an automated time course monitoring system for more objective methods for identifying CSs to improve accuracy. Wider implications of the findings This research has advanced our comprehension of the impact of CSs on embryonic development. Moreover, the evaluation of CSs features suggests the possibility of their introduction as novel morphological parameter in future embryonic grading systems. Trial registration number Not Applicable
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