Cytoplasmic strings in human blastocysts: hypotheses of their role and implications for embryo selection

• Published in: Human reproduction (Oxford) 2024-11, Vol.39 (11), p.2453-2465
• Main Authors: Marconetto, Anabella, Innocenti, Federica, Saturno, Gaia, Taggi, Marilena, Chiappetta, Viviana, Trio, Samuele, De Falco, Felicia, Albricci, Laura, Coticchio, Giovanni, Ahlström, Aisling, Fiorentino, Giulia, Maggiulli, Roberta, Vaiarelli, Alberto, Zuccotti, Maurizio, Rienzi, Laura, Cimadomo, Danilo
• Format: Article
• Language: English
• Subjects: Adult; Aneuploidy; Blastocyst; Cytoplasm - metabolism; Embryo Culture Techniques; Embryo Implantation - physiology; Embryo Transfer - methods; Embryonic Development - physiology; Female; Fertilization in Vitro; Humans; Pregnancy; Retrospective Studies; Vitrification
• ISSN: 0268-1161; 1460-2350; 1460-2350
• DOI: 10.1093/humrep/deae226

What are the implications of the presence cytoplasmic strings (Cyt-S) and their quantity and dynamics for the pre-implantation development of human blastocysts? Cyt-S are common in human embryos and are associated with faster blastocyst development, larger expansion, and better morphological quality. 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. 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. 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. 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