Chromosomal stability of second polar bodies in mouse embryos

The second polar body contributes to the fate asymmetry in the mouse embryo

The polar bodies (PBs) are extruded microcells during oocyte meiosis and generally regarded as inessentials for embryonic development. Therefore, PBs have been widely used as important materials for pre-implantation genetic diagnosis in human. Here we report that the second PB (PB2) in the mouse zygote may play roles in cell-fate specification and post-implantation development. A subset of mRNAs encoding pluripotency-related factors are enriched in PB2. Nascent proteins may be synthesized in PB2 after fertilization and transport from PB2 to the zygote before the two-cell stage. The PB2-attached blastomere (pbB) at the two-cell stage, compared to the other blastomere (npbB), likely contributes more descendants to the inner cell mass (ICM) lineage in the blastocyst. Removal of PB2 from the zygote or transient blockage of material exchange between PB2 and the zygote by nocodazole treatment appears to cause a loss of the ICM fate bias of pbB. PB2 removal or nocodazole treatment also results in abnormal post-implantation development. Injection of PB2 lysate into pbB of PB2-removed two-cell-stage embryos may reset the cell-fate preference and rescue post-implantation development. Our data collectively suggest that PB2 would demarcate the earliest cell-fate asymmetry of the mouse zygote and be required for post-implantation development.

Developmental potential of 2n/3n mixoploid mouse embryos produced by fusion of individual second polar bodies and blastomeres of 2-cell embryos

Using 2n/3n mixoploid mouse embryos produced by fusion of individual second polar bodies (PB2s) with individual blastomeres of 2-cell embryos, the dynamics of PB2 nuclei in the host blastomeres during mitosis were examined and the fate of the 3n cell line in the mixoploid embryos was followed. Most of the PB2 nuclei were synchronised with the cell cycle of the host blastomeres and all chromosomes were incorporated into a single mitotic spindle. The majority of the mixoploid embryos developed to blastocysts with 3n cells. In conceptuses at Day 11.5 and Day 18.5 of gestation, 3n cells were recognised in both of the embryonic/fetal and placental tissues. When green fluorescent protein (GFP)-transgenic mice were used as a donor of PB2, GFP-positive 3n cells were found in more than 40% of morulae and blastocysts, indicating that the PB2 genome can be reactivated during the pre-implantation stage. GFP-positive 3n cells were non-randomly allocated in trophectoderm in blastocysts. These findings may explain the production mechanism of 2n/3n mixoploid human embryos, that is, a PB2 is incorporated into one daughter blastomere during the early cleavage period.

Polar body genome transfer for preventing the transmission of inherited mitochondrial diseases

Inherited mtDNA diseases transmit maternally and cause severe phenotypes. Currently, there is no effective therapy or genetic screens for these diseases; however, nuclear genome transfer between patients' and healthy eggs to replace mutant mtDNAs holds promises. Considering that a polar body contains few mitochondria and shares the same genomic material as an oocyte, we perform polar body transfer to prevent the transmission of mtDNA variants. We compare the effects of different types of germline genome transfer, including spindle-chromosome transfer, pronuclear transfer, and first and second polar body transfer, in mice. Reconstructed embryos support normal fertilization and produce live offspring. Importantly, genetic analysis confirms that the F1 generation from polar body transfer possesses minimal donor mtDNA carryover compared to the F1 generation from other procedures. Moreover, the mtDNA genotype remains stable in F2 progeny after polar body transfer. Our preclinical model demonstrates polar body transfer has great potential to prevent inherited mtDNA diseases.