Cytoplasmic strings in human in-vitro embryos are a promising indicator for enhanced live-birth outcomes: a systematic review and meta-analysis.

Abstract

Cytoplasmic strings (CS) are transient filamentous connections which traverse the blastocoel, connecting the inner cell mass to the mural trophectoderm in blastocysts. Although they have been observed in animal studies for years, their role in embryogenesis remains unclear. At first, CS were considered abnormal in expanded blastocysts, but recent time-lapse studies on human in-vitro embryos suggest that CS may indicate higher developmental potential. Although CS are now included in morphological and morphokinetic guidelines, their impact on embryo viability remains inconclusive due to limited evidence. The aim of this review was to investigate whether CS in human in-vitro blastocysts influences IVF and pregnancy outcomes, primarily as indicated by livebirth rates. This systematic review and meta-analysis followed PRISMA guidelines. PubMed, Embase, Medline, Cochrane Library, and Web of Science were searched using key terms 'cytoplasmic strings' and related MeSH terms from 1946 till 30 September 2025 for all languages. Studies were selected per the PICO framework, screened independently, and back-reference searched. Inclusion criteria were studies on preimplantation embryos with CS. Meta-analysis required studies with control blastocysts without CS, and reporting on either the primary outcome, livebirth rate (LBR), or secondary outcomes, clinical pregnancy rate (CPR), embryo quality, morphokinetics, CS characteristics, and patient treatment characteristics. Non-human studies were excluded. Data were extracted for meta-analysis and represented in odds ratios (OR), standardized mean differences (SMD), and mean differences utilizing random-effects models on Revman v5.4. Bias was assessed via ROBINS-E and funnel plots, with subgroup analyses performed based on variables identified. From 1029 publications screened, 24 articles, including 15 observational studies with 14 datasets, were included. Meta-analysis of 13 studies (12 datasets) included 7583 blastocysts analysed for CS presence: 4417 exhibited CS (CS+), while 3166 did not (CS-). CS + blastocysts were associated with a higher LBR (OR = 1.88, 95% CI: 1.35, 2.61, I² = 55%, n = 2766 transferred blastocysts, 6 studies) and an increase in CPR (OR = 2.20, 95% CI: 1.29-3.75, I² = 59%, n = 1963, 5 studies). CS + blastocysts also demonstrated higher odds of good morphological quality (OR = 2.40, 95% CI: 1.17-4.92, I2 = 82%, n = 1967, 5 studies). Furthermore, this relationship was maintained with increased quantities of CS (blastocysts with ≥5 CS vs 1-4 CS: OR = 3.01, 95% CI: 2.22, 4.08, I2 = 0%, n = 1300, 3 studies), earlier CS onset, measured by hours post insemination (hpi), (SMD = -0.44, 95% CI: -0.58, -0.30, I2 = 9%, n = 972 blastocysts, 3 studies) and earlier disappearance of CS (SMD = -0.45, 95% CI: -0.59, -0.30, I2 = 13%, n = 962 blastocysts, 3 studies). Additionally, CS + blastocysts reached each successive morphokinetic stage progressively faster compared to CS - blastocysts, particularly post-blastulation (time of full blastocyst: SMD = -0.75, 95% CI: -0.99, -0.52, I2 = 53% n = 1525, 3 studies). GRADE assessment indicated very low certainty secondary to high heterogeneity and bias. Our meta-analysis demonstrated that CS in human in-vitro blastocysts is associated with improved IVF outcomes. Further research on the biological role of CS in embryogenesis and their influence on embryo viability is necessary. Following these answers, the integration of CS assessment into blastocyst evaluation protocols could improve embryo selection strategies and ultimately improve pregnancy success rates in IVF. PROSPERO (CRD420250652014).