Lipid-rich blastomeres in the two-cell stage of porcine parthenotes show bias toward contributing to the embryonic part

scm6A-seq reveals single-cell landscapes of the dynamic m6A during oocyte maturation and early embryonic development

N⁶-methyladenosine (m⁶A) has been demonstrated to regulate RNA metabolism and various biological processes, including gametogenesis and embryogenesis. However, the landscape and function of m⁶A at single cell resolution have not been extensively studied in mammalian oocytes or during pre-implantation. In this study, we developed a single-cell m⁶A sequencing (scm⁶A-seq) method to simultaneously profile the m⁶A methylome and transcriptome in single oocytes/blastomeres of cleavage-stage embryos. We found that m⁶A deficiency leads to aberrant RNA clearance and consequent low quality of Mettl3^Gdf9 conditional knockout (cKO) oocytes. We further revealed that m⁶A regulates the translation and stability of modified RNAs in metaphase II (MII) oocytes and during oocyte-to-embryo transition, respectively. Moreover, we observed m⁶A-dependent asymmetries in the epi-transcriptome between the blastomeres of two-cell embryo. scm⁶A-seq thus allows in-depth investigation into m⁶A characteristics and functions, and the findings provide invaluable single-cell resolution resources for delineating the underlying mechanism for gametogenesis and early embryonic development.

Fertilization and Cleavage Axes Differ In Primates Conceived By Conventional (IVF) Versus Intracytoplasmic Sperm Injection (ICSI)

With nearly ten million babies conceived globally, using assisted reproductive technologies, fundamental questions remain; e.g., How do the sperm and egg DNA unite? Does ICSI have consequences that IVF does not? Here, pronuclear and mitotic events in nonhuman primate zygotes leading to the establishment of polarity are investigated by multidimensional time-lapse video microscopy and immunocytochemistry. Multiplane videos after ICSI show atypical sperm head displacement beneath the oocyte cortex and eccentric para-tangential pronuclear alignment compared to IVF zygotes. Neither fertilization procedure generates incorporation cones. At first interphase, apposed pronuclei align obliquely to the animal-vegetal axis after ICSI, with asymmetric furrows assembling from the male pronucleus. Furrows form within 30° of the animal pole, but typically, not through the ICSI injection site. Membrane flow drives polar bodies and the ICSI site into the furrow. Mitotic spindle imaging suggests para-tangential pronuclear orientation, which initiates random spindle axes and minimal spindle:cortex interactions. Parthenogenetic pronuclei drift centripetally and assemble astral spindles lacking cortical interactions, leading to random furrows through the animal pole. Conversely, androgenotes display cortex-only pronuclear interactions mimicking ICSI. First cleavage axis determination in primates involves dynamic cortex-microtubule interactions among male pronuclei, centrosomal microtubules, and the animal pole, but not the ICSI site.

Fat-containing cells are eliminated during Dictyostelium development

Triacylglycerol is a universal storage molecule for metabolic energy in living organisms. However, Dictyostelium amoebae, that have accumulated storage fat from added fatty acids do not progress through the starvation period preceding the development of the durable spore. Mutants deficient in genes of fat metabolism, such as fcsA, encoding a fatty acid activating enzyme, or dgat1 and dgat2, specifying proteins that synthesize triacylglycerol, strongly increase their chances to contribute to the spore fraction of the developing fruiting body, but lose the ability to produce storage fat efficiently. Dictyostelium seipin, an orthologue of a human protein that in patients causes the complete loss of adipose tissue when mutated, does not quantitatively affect fat storage in the amoeba. Dictyostelium seiP knockout mutants have lipid droplets that are enlarged in size but reduced in number. These mutants are as vulnerable as the wild type when exposed to fatty acids during their vegetative growth phase, and do not efficiently enter the spore head in Dictyostelium development.

Summary: In contrast to many living organisms, storage fat is not beneficial for Dictyostelium cells when progressing through starvation and subsequent development of a dormant stage.

Random Allocation of Blastomere Descendants to the Trophectoderm and ICM of the Bovine Blastocyst

The first lineage specification during mammalian embryo development can be visually distinguished at the blastocyst stage. Two cell lineages are observed on the embryonic-abembryonic axis of the blastocyst: the inner cell mass and the trophectoderm. The timing and mechanisms driving this process are still not fully understood. In mouse embryos, cells seem prepatterned to become certain cell lineage because the first cleavage plane has been related with further embryonic-abembryonic axis at the blastocyst stage. Nevertheless, this possibility has been very debatable. Our objective was to determine whether this would be the case in another mammalian species, the bovine. To achieve this, cells of in vitro produced bovine embryos were traced from the 2-cell stage to the blastocyst stage. Blastocysts were then classified according to the allocation of the labeled cells in the embryonic and/or abembryonic part of the blastocyst. Surprisingly, we found that there is a significant percentage of the embryos (∼60%) with labeled and nonlabeled cells randomly distributed and intermingled. Using time-lapse microscopy, we have identified the emergence of this random pattern at the third to fourth cell cycle, when cells started to intermingle. Even though no differences were found on morphokinetics among different embryos, these random blastocysts and those with labeled cells separated by the embryonic-abembryonic axis (deviant pattern) are significantly bigger; moreover deviant embryos have a significantly higher number of cells. Interestingly, we observed that daughter cells allocation at the blastocyst stage is not affected by biopsies performed at an earlier stage.

The Principal Forces of Oocyte Polarity Are Evolutionary Conserved but May Not Affect the Contribution of the First Two Blastomeres to the Blastocyst Development in Mammals

Oocyte polarity and embryonic patterning are well-established features of development in lower species. Whether a similar form of pre-patterning exists in mammals is currently under hot debate in mice. This study investigated this issue for the first time in ovine as a large mammal model. Microsurgical trisection of unfertilized MII-oocytes revealed that cortical cytoplasm around spindle (S) contained significant amounts of total maternal mRNAs and proteins compared to matched cytoplast hemispheres that were located either near (NS) or far (FS) -to-spindle. RT-qPCR provided striking examples of maternal mRNA localized to subcellular substructures S (NPM2, GMNN, H19, PCAF, DNMT3A, DNMT1, and STELLA), NS (SOX2, NANOG, POU5F1, and TET1), and FS (GCN) of MII oocyte. Immunoblotting revealed that specific maternal proteins DNMT3A and NANOG were asymmetrically enriched in MII-spindle-half of the oocytes. Topological analysis of sperm entry point (SEP) revealed that sperm preferentially entered via the MII-spindle-half of the oocytes. Even though, the topological position of first cleavage plane with regard to SEP was quite stochastic. Spatial comparison of lipid content revealed symmetrical distribution of lipids between 2-cell blastomeres. Lineage tracing using Dil, a fluorescent dye, revealed that while the progeny of leading blastomere of 2-cell embryos contributed to more cells in the developed blastocysts compared to lagging counterpart, the contributions of leading and lagging blastomeres to the embryonic-abembryonic parts of the developed blastocysts were almost unbiased. And finally, separated sister blastomeres of 2-cell embryos had an overall similar probability to arrest at any stage before the blastocyst (2-cell, 4-cell, 8-cell, and morula) or to achieve the blastocyst stage. It was concluded that the localization of maternal mRNAs and proteins at the spindle are evolutionarily conserved between mammals unfertilized ovine oocyte could be considered polar with respect to the spatial regionalization of maternal transcripts and proteins. Even though, the principal forces of this definitive oocyte polarity may not persist during embryonic cleavages.

Preimplantation embryo metabolism and culture systems: experience from domestic animals and clinical implications

Despite advantages of in vitro embryo production in many species, widespread use of this technology is limited by generally lower developmental competence of in vitro derived embryos compared to in vivo counterparts. Regardless, in vivo or in vitro gametes and embryos face and must adjust to multiple microenvironments especially at preimplantation stages. Moreover, the embryo has to be able to further adapt to environmental cues in utero to result in the birth of live and healthy offspring. Enormous strides have been made in understanding and meeting stage-specific requirements of preimplantation embryos, but interpretation of the data is made difficult due to the complexity of the wide array of culture systems and the remarkable plasticity of developing embryos that seem able to develop under a variety of conditions. Nevertheless, a primary objective remains meeting, as closely as possible, the preimplantation embryo requirements as provided in vivo. In general, oocytes and embryos develop more satisfactorily when cultured in groups. However, optimization of individual culture of oocytes and embryos is an important goal and area of intensive current research for both animal and human clinical application. Successful culture of individual embryos is of primary importance in order to avoid ovarian superstimulation and the associated physiological and psychological disadvantages for patients. This review emphasizes stage specific shifts in embryo metabolism and requirements and research to optimize in vitro embryo culture conditions and supplementation, with a view to optimizing embryo culture in general, and culture of single embryos in particular.