研究開発・学会発表

Study question
Is it possible to reliably predict blastocyst development on day 2 after fertilization in order to reduce the occurrence of monozygotic twinning and to reduce the duration of in-vitro embryo culture? 

Summary answer
It is possible to predict the likelihood of blastocyst development based on the following criteria. The size of cellular fragmentation is less than 26µm in diameter, 2PN zygotes undergo normal twofold cell division, cell vacuoles are not larger than 10µm in diameter and reverse cleavage is not observed.

What is known already
It has been reported that blastocyst culture causes a higher rate of monozygotic twinning and a delay in embryo development compared to in-vivo embryo growth. The presence of cellular vacuoles (larger than 10µm) and reverse cleavage are known to diminish blastocyst development.

Study design, size, duration
A retrospective study of 351 2PN zygotes, that were observed by a time-lapse system until day 5 of development conducted at the Hanabusa Women’s Clinic from January 2014 to December 2014.

Participants/materials, setting, methods
The size of fragmentation and the presence of vacuoles at the time of the first cleavage were recorded. The rates of blastocyst development and high-grade blastocyst formation (>3AA) were analysed in embryos, with or without normal twofold cell division, and the detection of vacuoles and reverse cleavage.

Main results and the role of chance
The cutoff size of fragments that distinguished normal from abnormal cleaved cells was defined as <26µm. Based on this criterion, the rate of blastocyst formation from 2PN zygotes with normal twofold cell division without large vacuoles was 97.2% (104/107) and was significantly higher than those with reversed cleavage (50.8% (33/65), p<0.001), erratic cell division into 3-4 cells (56.7% (56/99), p<0.001), or 5 and more cells (13.5%(10/74), p<0.001). The rate of high-grade blastocyst formation (>3AA) from 2PN zygotes with normal twofold cell division without large vacuoles was 42.1% (45/107) and was also significantly higher than those with reversed cleavage (4.6% (3/65), p<0.001), erratic cell division into 3-4 cells (6.1% (6/99), p<0.001), or 5 and more cells (0% (0/74), p<0.001).

Limitations, reasons for caution
It is not known whether chromosomes are present in the larger fragments and abnormally cleaved blastomeres. Further studies are required to determine pregnancy rates and take-home baby rates based on the criteria used in the present work.

Wider implications of the findings
As this method can predict blastocyst development with high accuracy from day 2 embryos, blastocyst culture may no longer be necessary for choosing viable embryos. This new method simplifies the procedure for selecting the best embryos, without labor-intensive protocols, such as measuring the time of cleavage and the duration of each cell stage.