Detailed analysis of cytoplasmic strings in human blastocysts: new insights

Abstract

Summary The aim of this study was to determine if there was an association between the presence of cytoplasmic strings (CS) and their characteristics, with blastocyst quality, development and clinical outcome in human blastocysts. This two-centre cohort study was performed between July 2017 and September 2018 and involved a total of 1152 blastocysts from 225 patients undergoing in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI). All embryos were cultured in Embryoscope+ and were assessed for CS using time-lapse images. A single assessor examined all blastocysts and reviewed videos using the EmbyroViewer® Software. Blastocyst quality was assessed on day 5 of embryo development. The number of CS, location and duration of their activity was recorded on days 5/6. A positive association between the presence of CS in human blastocysts with blastocyst quality was identified. Blastocysts with a higher number of CS present, were of higher quality and were in the more advanced stages of development. Top quality blastocysts had CS activity present for longer, as well as having a higher number of vesicles present travelling along the CS. Blastocysts that had CS present, had a significantly higher live birth rate. This study has confirmed that a higher number of CS and vesicles in human blastocysts is associated with top quality blastocysts and is not a negative predictor of development. They had a higher number of CS present that appeared earlier in development and, although ceased activity sooner, had a longer duration of activity. Blastocysts with CS had a significant increase in live birth rate.

The importance of cytoplasmic strings during early human embryonic development

“…Cytoplasmic strings were detected in 81.2% of the examined embryos, which is almost two times higher than the CS+ embryo ratio (43.9%) published by Ebner et al (2020 ). However, the presence of CS is similar in scale to the results of Eastick et al (2021 ) just in the case of the proportion of CS after conventional IVF and ICSI fertilisation, which suggests that the method of fertilisation does not affect the formation of CS ( Eastick et al, 2022b ).…”

“…However, embryos in the CS+ group developed into a higher quality blastocyst. Although it seems that other working groups who examine cytoplasmic strings do not apply traditional morphology assessment for early embryo morphology evaluation, the use of Gardner’s blastocyst morphology assessment revealed similar results ( Eastick et al, 2022b ).…”

“…The observation of cytoplasmic strings in human embryos by other embryologists also suggests that bulb-like vesicle transport occurs along the strings (Ebner et al, 2020) in a bidirectional movement (antero-and retrograde) (Eastick et al, 2022b). Ebner and his colleagues also investigated the relationship between cytoplasmic extension and the blastocyst collapse, as well as the characterisation of filaments.…”

In Celebration of Women in Molecular and Cellular Reproduction

“…However, embryos in the CS+ group developed into a higher quality blastocyst. Although it seems that other working groups who examine cytoplasmic strings do not apply traditional morphology assessment for early embryo morphology evaluation, the use of Gardner's blastocyst morphology assessment revealed similar results (Eastick et al, 2022b). The implantation rate was also higher in the case of CS+ embryos, but this difference was not significant.…”