Blastulation timing is associated with differential mitochondrial content in euploid embryos

Preimplantation genetic screening (PGS) and assessment of mitochondrial content (MC) are current methods for selection of the best embryos for transfer. Studies suggest that time-lapse morphokinetics (TLM) may also be helpful for selecting embryos more likely to implant. In our study, we sought to examine the relationship between TLM parameters and MC to determine if they could be used adjunctively in embryo selection. We also examined the relationship between MC with ploidy and blastulation.

Cryopreserved human embryos at the zygote stage were thawed and cultured in a time-lapse system. Blastomere and trophectoderm biopsies were performed on days 3 and 6. Biopsied cells and all whole embryos from day 6 were analyzed for MC (ratio of mitochondrial to nuclear DNA) and ploidy using next-generation sequencing.

In embryos, MC per cell declined between day 3 and day 6. While early cleavage parameters did not predict MC, embryos with longer blastulation timing had higher MC on day 6. Day 6 MC was lower in euploid vs. aneuploid embryos and lower in blastocysts vs. arrested embryos.

A lower MC at the blastocyst stage was associated with euploid status and blastocyst formation, indicating better embryo quality compared to those with a higher MC. Higher MC in aneuploid and arrested embryos may be explained by slower cell division or degradation of genomic DNA over time. Blastulation timing may be helpful for selection of higher quality embryos. Combining blastulation timing and MC along with morphologic grading and euploid status may offer a new direction in embryo selection.