STUDY QUESTION What are the implications of the presence cytoplasmic strings (Cyt-S) and their quantity and dynamics for the pre-implantation development of human blastocysts? SUMMARY ANSWER Cyt-S are common in human embryos and are associated with faster blastocyst development, larger expansion, and better morphological quality. WHAT IS KNOWN ALREADY Cyt-S are dynamic cellular projections connecting inner cell mass and trophectoderm (TE) cells, that can be observed during blastocyst expansion. Their prevalence in human embryos has been estimated to be between 44% and 93%. Data relevant to their clinical implications and role in development are lacking, limited, or controversial. STUDY DESIGN, SIZE, DURATION Retrospective study conducted at a single IVF center between May 2013 and November 2014 and involving 124 pre-implantation genetic testing for aneuploidy cycles in a time-lapse incubator with ≥1 blastocyst biopsied and vitrified (N = 370 embryos assessed). These cycles resulted in 87 vitrified-warmed single-euploid blastocyst transfers. PARTICIPANTS/MATERIALS, SETTING, METHODS ICSI, continuous blastocyst culture (Days 5–7), TE biopsy of fully expanded blastocysts without Day 3 zona pellucida drilling, qPCR to assess uniform full-chromosome aneuploidies, and vitrification were all performed. Only vitrified-warmed euploid single-embryo-transfers were conducted. Blastocyst morphological quality was defined according to Gardner’s criteria. The AI-based software CHLOE™ (Fairtility) automatically registered timings from time of starting blastulation (tSB) to biopsy (t-biopsy, i.e. blastocyst full-expansion) as hours-post-insemination (hpi), embryo area (including zona pellucida in µm2), and spontaneous blastocyst collapses. One senior embryologist manually annotated Cyt-S presence, quantity, timings, and type (thick cell-to-cell connections and/or threads). All significant associations were confirmed through regression analyses. All couples’, cycles’, and embryos’ main features were also tested for associations with Cyt-S presence, quantity, and dynamics. MAIN RESULTS AND THE ROLE OF CHANCE About 94.3% of the patients (N = 117/124) had ≥1 embryo with Cyt-S. Out of a total of 370 blastocysts, 55 degenerated between blastulation and full-expansion (N = 55/370, 14.9%). The degeneration rate among embryos with ≥1 Cyt-S was 10.8% (N = 33/304), significantly lower than that of embryos without Cyt-S (33.3%, N = 22/66, P < 0.01). Of the remaining 315 viable blastocysts analyzed, 86% (N = 271/315; P < 0.01) had ≥1 Cyt-S, on average 3.5 ± 2.1 per embryo ranging 1–13. The first Cyt-S per viable embryo appeared at 115.3 ± 12.5 hpi (85.7–157.7), corresponding to 10.5 ± 5.8 h (0.5–31) after tSB. Overall, we analyzed 937 Cyt-S showing a mean duration of 3.8 ± 2.7 h (0.3–20.9). Cyt-S were mostly threads (N = 508/937, 54.2%) or thick cell-to-cell connections becoming threads (N = 382/937, 40.8%) than thick bridges (N = 47/937, 5.0%). The presence and quantity of Cyt-S were significantly associated with developmentally faster (on average 6–12 h faster) and more expanded (on average 2700 µm2-larger blastocyst’s area at t-biopsy) embryos. Also, the presence and duration of Cyt-S were associated with better morphology. Lastly, while euploidy rates were comparable between blastocysts with and without Cyt-S, all euploid blastocysts transferred from the latter group failed to implant (N = 10). LIMITATIONS, REASONS FOR CAUTION Cyt-S presence and dynamics were assessed manually on seven focal planes from video frames recorded every 15 min. The patients included were mostly of advanced maternal age. Only associations could be reported, but no causations/consequences. Lastly, larger datasets are required to better assess Cyt-S associations with clinical outcomes. WIDER IMPLICATIONS OF THE FINDINGS Cyt-S are common during human blastocyst expansion, suggesting their physiological implication in this process. Their presence, quantity and dynamics mirror embryo viability, and morphological quality, yet their role is still unknown. Future basic science studies are encouraged to finally describe Cyt-S molecular nature and biophysical properties, and Artificial Intelligence tools should aid these studies by incorporating Cyt-S assessment. STUDY FUNDING/COMPETING INTEREST(S) None. TRIAL REGISTRATION NUMBER N/A.

中文翻译：

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人囊胚中的细胞质串：它们的作用和对胚胎选择的影响的假设


研究问题：细胞质串 （Cyt-S） 的存在及其数量和动力学对人类囊胚植入前发育有什么影响？总结答案 Cyt-S 在人类胚胎中很常见，与更快的囊胚发育、更大的扩张和更好的形态质量有关。已知的 Cyt-S 是连接内部细胞团和滋养外胚层 （TE） 细胞的动态细胞投射，可以在囊胚扩增过程中观察到。它们在人类胚胎中的患病率估计在 44% 到 93% 之间。与其临床意义和发育作用相关的数据缺乏、有限或有争议。研究设计、规模、持续时间 2013 年 5 月至 2014 年 11 月期间在单个 IVF 中心进行的回顾性研究，涉及 124 次植入前基因检测，用于在延时培养箱中进行非整倍体周期，≥1 个囊胚活检和玻璃化（评估 N = 370 个胚胎）。这些循环导致 87 例玻璃化加热的单倍体囊胚转移。参与者/材料、设置、方法ICSI、连续囊胚培养（第 5-7 天）、完全扩张的囊胚的 TE 活检，无需第 3 天透明带钻孔、qPCR 以评估均匀的全染色体非整倍体和玻璃化。仅进行了玻璃化加热的整倍体单胚胎移植。囊胚形态学质量根据 Gardner 标准定义。基于 AI 的软件 CHLOE™ （Fairtility） 自动记录从开始爆破 （tSB） 到活检（t 活检，即囊胚完全扩增）的时间，作为授精后小时数 （hpi）、胚胎面积（包括 μm2 中的透明带）和自发性囊胚塌陷。 一位高级胚胎学家手动注释了 Cyt-S 的存在、数量、时间和类型（粗细胞间连接和/或线程）。通过回归分析确认所有显著关联。还测试了所有夫妇、周期和胚胎的主要特征与 Cyt-S 存在、数量和动力学的关联。主要结果和机会的作用 大约 94.3% 的患者 （N = 117/124） 有 ≥1 个带有 Cyt-S 的胚胎。在总共 370 个囊胚中，有 55 个在爆破和完全扩张之间退化 （N = 55/370,14.9%）。≥1 Cyt-S 胚胎的退化率为 10.8% （N = 33/304），显著低于无 Cyt-S 的胚胎 （33.3%，N = 22/66，P < 0.01）。在分析的其余 315 个活囊胚中，86% （N = 271/315;P < 0.01） 有 ≥1 个 Cyt-S，平均每个胚胎 3.5 ± 2.1，范围为 1-13。每个活胚胎的第一个 Cyt-S 出现在 115.3 ± 12.5 hpi （85.7-157.7） 处，对应于 tSB 后 10.5 ± 5.8 小时 （0.5-31）。总体而言，我们分析了 937 个 Cyt-S，显示平均持续时间为 3.8 ± 2.7 小时 （0.3-20.9）。Cyt-S 主要是线 （N = 508/937， 54.2%） 或粗细胞间连接变成线 （N = 382/937， 40.8%） 而不是粗桥 （N = 47/937， 5.0%）。Cyt-S 的存在和数量与发育更快 （平均快 6-12 小时） 和更扩大 （T 活检时囊胚面积平均大 2700 μm2） 胚胎显著相关。此外，Cyt-S 的存在和持续时间与较好的形态相关。最后，虽然有和没有 Cyt-S 的囊胚之间的整倍体率相当，但从后一组转移的所有整倍体囊胚都未能植入 （N = 10）。 限制，谨慎的原因 Cyt-S 的存在和动态是从每 15 分钟录制的视频帧中在 7 个焦平面上手动评估的。纳入的患者大多为高龄产妇。只能报告关联，但不能报告因果关系/后果。最后，需要更大的数据集来更好地评估 Cyt-S 与临床结果的关联。研究结果的更广泛意义 Cyt-S 在人类囊胚扩增过程中很常见，表明它们在这一过程中的生理意义。它们的存在、数量和动态反映了胚胎的活力和形态质量，但它们的作用仍然未知。鼓励未来的基础科学研究最终描述 Cyt-S 分子性质和生物物理特性，人工智能工具应通过结合 Cyt-S 评估来帮助这些研究。研究资金/竞争利益 无。试验注册号 N/A。