Earlier second polar body transfer and further mitochondrial carryover removal for potential mitochondrial replacement therapy.

The second polar body (PB2) transfer in assisted reproductive technology is regarded as the most promising mitochondrial replacement scheme for preventing the mitochondrial disease inheritance owing to its less mitochondrial carryover and stronger operability. However, the mitochondrial carryover was still detectable in the reconstructed oocyte in conventional second polar body transfer scheme. Moreover, the delayed operating time would increase the second polar body DNA damage. In this study, we established a spindle-protrusion-retained second polar body separation technique, which allowed us to perform earlier second polar body transfer to avoid DNA damage accumulation. We could also locate the fusion site after the transfer through the spindle protrusion. Then, we further eliminated the mitochondrial carryover in the reconstructed oocytes through a physically based residue removal method. The results showed that our scheme could produce a nearly normal proportion of normal-karyotype blastocysts with further reduced mitochondrial carryover, both in mice and humans. Additionally, we also obtained mouse embryonic stem cells and healthy live-born mice with almost undetectable mitochondrial carryover. These findings indicate that our improvement in the second polar body transfer is conducive to the development and further mitochondria carryover elimination of reconstructed embryos, which provides a valuable choice for future clinical applications of mitochondrial replacement.