Improved live birth rates in blastocysts with “cytoplasmic strings”

Researchers in a 2022 study found microscopic structures called “cytoplasmic strings,” that can be seen in embryo photos, are associated with improved pregnancy outcomes.

Cytoplasmic strings are specialized projections that seem to connect cells of the inner cell mass (ICM) and trophectoderm. Their role is not clear, but they may be remnants of trophectoderm cells migrating to different regions of the blastocyst. They may also be involved in communication between cells.

Ma et al. (2022), in their retrospective study, compared 1,267 single blastocyst transfers that had time-lapse data between 2018 and 2019 in China. There were 1,002 blastocyst transfers with cytoplasmic strings and 265 transfers without.

Check out my complete guide to embryo grading and success rates to learn more about embryo development, grading and success rates.

---

Cytoplasmic strings are less common in low quality embryos

They compared the frequency of embryos with cytoplasmic strings based on the grade of the ICM and trophectoderm. Blastocysts with A or B graded ICMs and trophectoderm were significantly more likely to contain cytoplasmic strings compared to C grades.

They also compared the expansion of the embryos and those with an expansion of “2” were less likely to contain cytoplasmic strings compared to expansion 3, 4, 5 or 6.

Embryos with cytoplasmic strings have improved pregnancy outcomes

Women who transferred blastocysts containing cytoplasmic strings had a higher live birth rate compared to those who didn’t (48.5% vs 35.5%, p=0.003). There were also statistically significant increases in clinical pregnancies, but no differences in miscarriage rate.

They also compared pregnancy outcomes based on the number of cytoplasmic strings. Blastocysts with more than 4 cytoplasmic strings had the highest clinical pregnancy and live birth rates, which was statistically significant. There was a decrease in live birth rates in embryos with 3-4 cytoplasmic strings (statistically significant), although the authors didn’t elaborate on this.

There were no differences in birth weights in babies born from embryos with or without cytoplasmic strings.

Conclusions

This study found that cytoplasmic strings were more likely to be found in fair/good quality blastocysts, and they were associated with increases in clinical pregnancy and live birth rates. Embryos with more than 4 cytoplasmic strings had the best outcomes.

Reference

Ma BX, Yang L, Tian Y, Jin L, Huang B. Cytoplasmic strings between ICM and mTE are a positive predictor of clinical pregnancy and live birth outcomes: A time-lapse study. Front Med (Lausanne). 2022 Jul 28;9:934327. doi: 10.3389/fmed.2022.934327. PMID: 35966868; PMCID: PMC9366165.

 

---

About Embryoman

Embryoman (Sean Lauber) is a former embryologist and the founder of Remembryo, an IVF research and fertility education website. After working in an IVF lab in the US, he returned to Canada and now focuses on making fertility research more accessible. He holds a Master’s in Immunology and launched Remembryo in 2018 to help patients and professionals make sense of IVF research. Sean shares weekly study updates on Facebook, Instagram, and Reddit regularly. He also answers questions on Reddit or in his private Facebook group.

---