Takashi Kuno

In regular IVF, a portion of oocytes exhibit abnormal numbers of pronuclei (PN) that is considered as abnormal fertilization, and they are routinely discarded. However, it is known that abnormal ploidy still does not completely abandon embryo development and implantation. To explore the potential of cytoplasm from those abnormally fertilized oocytes, we developed a novel technique for the transfer of large cytoplasm between pronuclear-stage mouse embryos, and assessed its impact. A large volume of cytoplast could be efficiently transferred in the PN stage using a novel two-step method of pronuclear-stage cytoplasmic transfer (PNCT). PNCT revealed the difference in the cytoplasmic function among abnormally fertilized embryos where the cytoplasm of 3PN was developmentally more competent than 1PN, and the supplementing of fresh 3PN cytoplasm restored the impaired developmental potential of postovulatory “aged” oocytes. PNCT-derived embryos harbored significantly higher mitochondrial DNA copies, ATP content, oxygen consumption rate, and total cells. The difference in cytoplasmic function between 3PN and 1PN mouse oocytes probably attributed to the proper activation via sperm and may impact subsequent epigenetic events. These results imply that PNCT may serve as a potential alternative treatment to whole egg donation for patients with age-related recurrent IVF failure.

We have developed an automated device for the measurement of oxygen consumption rate (OCR) called Chip-sensing Embryo Respiratory Measurement system (CERMs). To verify the safety and the significance of the OCR measurement by CERMs, we conducted comprehensive tests using a mouse model prior to clinical trials in a human in vitro fertilization (IVF) program. Embryo transfer revealed that the OCR measured by CERMs did not compromise the full-term development of mice or their future fertility, and was positively correlated with adenosine triphosphate (ATP) production and the mitochondrial membrane potential (ΔΨm), thereby indirectly reflecting mitochondrial oxidative phosphorylation (OXPHOS) activity. We demonstrated that the OCR is independent of embryo morphology (the size) and number of mitochondria (mitochondrial DNA copy number). The OCR correlated with the total cell numbers, whereas the inner cell mass (ICM) cell numbers and the fetal developmental rate were not. Thus, the OCR may serve as an indicator of the numbers of trophectoderm (TE) cells, rather than number or quality of ICM cells. However, implantation ability was neither correlated with the OCR, nor the embryo size in this model. This can probably be attributed to the limitation that chimeric embryos contain non-physiological high TE cells counts that are beneficial for implantation. CERMs can be safely employed in clinical IVF owing to it being a safe, highly effective, non-invasive, accurate, and quantitative tool for OCR measurement. Utilization of CERMs for clinical testing of human embryos would provide further insights into the nature of oxidative metabolism and embryonic viability.