Hand-made cloned goat (Capra hircus) embryos—a comparison of different donor cells and culture systems

Back to the future: optimised microwell culture of individual human preimplantation stage embryos

Although in vitro culture of human embryos is a crucial step in assisted reproduction, the lack of focused research hampers worldwide standardisation and consistent outcomes. Only 1.2% of research papers published in five leading journals in human reproduction in 2019 focused on in vitro culture conditions, creating the impression that the optimisation process has approached its limits. On the other hand, in vitro culture of mammalian embryos is based on old principles, while there is no consensus on basic issues as density, time, medium change, gas atmosphere and small technical details including the way of drop preparation. This opinion paper aims to highlight and analyse the slow advancement in this field and stimulate research for simple and affordable solutions to meet the current requirements. A possible way for advancement is discussed in detail. Selection of embryos with the highest developmental competence requires individual culture and modification of the widely used "drop under oil" approach. Current use of three-dimensional surfaces instead of large flat bottoms is restricted to time-lapse systems, but these wells are designed for optical clarity, not for the needs of embryos. The size and shape of the original microwells (Well of the Well; WOW) offer a practical and straightforward solution to combine the benefits of communal and individual incubation and improve the overall quality of cultured embryos.

Back to the future: optimised microwell culture of individual human preimplantation stage embryos

Although in vitro culture of human embryos is a crucial step in assisted reproduction, the lack of focused research hampers worldwide standardisation and consistent outcomes. Only 1.2% of research papers published in five leading journals in human reproduction in 2019 focused on in vitro culture conditions, creating the impression that the optimisation process has approached its limits. On the other hand, in vitro culture of mammalian embryos is based on old principles, while there is no consensus on basic issues as density, time, medium change, gas atmosphere and small technical details including the way of drop preparation. This opinion paper aims to highlight and analyse the slow advancement in this field and stimulate research for simple and affordable solutions to meet the current requirements. A possible way for advancement is discussed in detail. Selection of embryos with the highest developmental competence requires individual culture and modification of the widely used "drop under oil" approach. Current use of three-dimensional surfaces instead of large flat bottoms is restricted to time-lapse systems, but these wells are designed for optical clarity, not for the needs of embryos. The size and shape of the original microwells (Well of the Well; WOW) offer a practical and straightforward solution to combine the benefits of communal and individual incubation and improve the overall quality of cultured embryos.

Cytoplasmic volume of recipient oocytes affects the nucleus reprogramming and the developmental competence of HMC buffalo (Bubalus bubalis) embryos

The present study was undertaken to examine the effects of cytoplasmic volume on nucleus reprogramming and developmental competence of buffalo handmade cloning (HMC) embryos. We found that both HMC embryos derived from ~150% cytoplasm or ~225% cytoplasm resulted in a higher blastocyst rate and total cell number of blastocyst in comparison with those from ~75% cytoplasm (25.4 ± 2.0, 27.9 ± 1.6% vs. 17.9 ± 3.1%; 150 ± 10, 169 ± 12 vs. 85 ± 6, P<0.05). Meanwhile, the proportions of nuclear envelope breakdown (NEBD) and premature chromosome condensation (PCC) were also increased in the embryos derived from ~150 or ~225% enucleated cytoplasm compared to those from ~75% cytoplasm. Moreover, HMC embryos derived from ~225% cytoplasm showed a decrease of global DNA methylation from the 2-cell to the 4-cell stage in comparison with those of ~75% cytoplasm (P<0.05). Furthermore, the expression of embryonic genome activation (EGA) relative genes (eIF1A and U2AF) in HMC embryos derived from ~225% cytoplasm at the 8-cell stages was also found to be enhanced compared with that of the ~75% cytoplasm. Two of seven recipients were confirmed to be pregnant following transfer of blastocysts derived from ~225% cytoplasm, and one healthy cloned calf was delivered at the end of the gestation period, whereas no recipients were pregnant after the transfer of blastocysts derived from ~75% cytoplasm. These results indicate that the cytoplasmic volume of recipient oocytes affects donor nucleus reprogramming, and then further accounted for the developmental ability of the reconstructed embryos.

Factors affecting the development of somatic cell nuclear transfer embryos in Cattle

Nuclear transfer is a complex multistep procedure that includes oocyte maturation, cell cycle synchronization of donor cells, enucleation, cell fusion, oocyte activation and embryo culture. Therefore, many factors are believed to contribute to the success of embryo development following nuclear transfer. Numerous attempts to improve cloning efficiency have been conducted since the birth of the first sheep by somatic cell nuclear transfer. However, the efficiency of somatic cell cloning has remained low, and applications have been limited. In this review, we discuss some of the factors that affect the developmental ability of somatic cell nuclear transfer embryos in cattle.

Examination of DNA methyltransferase expression in cloned embryos reveals an essential role for Dnmt1 in bovine development

In studies of somatic cell nuclear transfer (SCNT), the ability of factors within the oocyte to epigenetically reprogram transferred nuclei is essential for embryonic development of the clone to proceed. However, irregular patterns of X-chromosome inactivation, abnormal expression of imprinted genes, and genomic DNA hypermethylation are frequently observed in reconstructed embryos, suggesting abnormalities in this process. To better understand the epigenetic events underlying SCNT reprogramming, we sought to determine if the abnormal DNA methylation levels observed in cloned embryos result from a failure of the oocyte to properly reprogram transcription versus differential biochemical regulation of the DNA methyltransferase family of enzymes (DNMTs) between embryonic and somatic nuclei. To address this question, we conducted real-time quantitation of Dnmt transcripts in bovine preimplantation embryos generated though in vitro fertilization (IVF), parthenogenic activation, and SCNT. By the 8-cell stage, transcripts encoding Dnmt1 become significantly down-regulated in cloned embryos, likely in response to the state of genomic hypermethylation, while the de novo methyltransferases maintain an expression pattern indistinguishable from their IVF and parthenote counterparts. Depletion of embryonic/maternal Dnmt1 transcripts within IVF embryos using short-interfering RNAs, while able to lower genomic DNA methylation levels, resulted in developmental arrest at the 8/16-cell stage. In contrast, SCNT embryos derived from a stable, Dnmt1-depleted donor cell line develop to blastocyst stage, but failed to carry to term. Our results indicate an essential role for Dnmt1 during bovine preimplantation development, and suggest proper transcriptional reprogramming of this gene family in SCNT embryos.