Full developmental potential of mammalian preimplantation embryos is maintained after imaging using a spinning-disk confocal microscope

Blastocyst Cell Number and Allocation Affect the Developmental Potential and Transcriptome of Bovine Somatic Cell Nuclear Transfer Embryos

Cloning cattle using somatic cell nuclear transfer (SCNT) is inefficient. Although the rate of development of SCNT embryos in vitro is similar to that of fertilized embryos, most fail to develop into healthy calves. In this study, we aimed to identify developmentally competent embryos according to blastocyst cell composition and perform transcriptome analysis of single embryos. Transgenic SCNT embryos expressing nuclear-localized HcRed gene at day 7 of development were imaged by confocal microscopy for cell counting and individually transferred to recipient heifers. Pregnancy rates were determined by ultrasonography. Embryos capable of establishing pregnancy by day 35 had an average of 117 ± 6 total cells, whereas embryos with an average of 128 ± 5 cells did not establish pregnancy (P < 0.05). A lesser average number of 41 ± 3 cells in the inner cell mass (ICM) also resulted in pregnancies (<0.05) than a greater number of 48 ± 2 cells in the ICM. Single embryos were then subjected to RNA sequencing for transcriptome analysis. Using weighted gene coexpression network analysis, we identified clusters of genes in which gene expression correlated with the number of total cells or ICM cells. Gene ontology analysis of these clusters revealed enriched biological processes in coenzyme metabolic process, intracellular signaling cascade, and glucose catabolic process, among others. We concluded that SCNT embryos with fewer total and ICM cell numbers resulted in greater pregnancy establishment rates and that these differences are reflected in the transcriptome of such embryos.

Pre-Implantation Bovine Embryo Evaluation-From Optics to Omics and Beyond

Approximately 80% of the ~1.5 million bovine embryos transferred in 2021 were in vitro produced. However, only ~27% of the transferred IVP embryos will result in live births. The ~73% pregnancy failures are partly due to transferring poor-quality embryos, a result of erroneous stereomicroscopy-based morphological evaluation, the current method of choice for pre-transfer embryo evaluation. Numerous microscopic (e.g., differential interference contrast, electron, fluorescent, time-lapse, and artificial-intelligence-based microscopy) and non-microscopic (e.g., genomics, transcriptomics, epigenomics, proteomics, metabolomics, and nuclear magnetic resonance) methodologies have been tested to find an embryo evaluation technique that is superior to morphologic evaluation. Many of these research tools can accurately determine embryo quality/viability; however, most are invasive, expensive, laborious, technically sophisticated, and/or time-consuming, making them futile in the context of in-field embryo evaluation. However accurate they may be, using complex methods, such as RNA sequencing, SNP chips, mass spectrometry, and multiphoton microscopy, at thousands of embryo production/collection facilities is impractical. Therefore, future research is warranted to innovate field-friendly, simple benchtop tests using findings already available, particularly from omics-based research methodologies. Time-lapse monitoring and artificial-intelligence-based automated image analysis also have the potential for accurate embryo evaluation; however, further research is warranted to innovate economically feasible options for in-field applications.

Micronucleus formation during early cleavage division is a potential hallmark of preimplantation embryonic loss in cattle

In assisted reproductive technology (ART)-derived embryos of non-rodent mammals, including humans and cattle, chromosome segregation errors are highly likely to occur during early cleavage division, resulting in aneuploidy, including mosaicism. However, the relationship between chromosomal segregation errors during early cleavage and subsequent embryonic development has not been detailed in these mammals. In the present study, we developed non-invasive live-cell imaging of chromosome segregation dynamics using a histone H2B-mCherry mRNA probe in bovine preimplantation embryos. Chromosome segregation errors in early cleavage affected blastocyst formation. Especially, embryos that underwent abnormal chromosome segregation (ACS) with multiple or large micronucleus formation rarely developed into blastocysts. Embryos with the severe ACS had prolonged cell cycle duration. After transfer of blastocysts with live-cell imaging of chromosome segregation to ten cows, six became pregnant and four of them gave full-term offspring. Interestingly, two of them were derived from blastocysts with ACS. Hence, chromosomal segregation errors with micronucleus formation during early cleavage can be a fatal hallmark of preimplantation embryogenesis in cattle. This technique has shown potential for understanding the relationship between chromosome segregation error and subsequent embryo development, and for selecting viable ART-derived embryos for medical and livestock production.

The effect of discrete wavelengths of visible light on the developing murine embryo

PURPOSE

A current focus of the IVF field is non-invasive imaging of the embryo to quantify developmental potential. Such approaches use varying wavelengths to gain maximum biological information. The impact of irradiating the developing embryo with discrete wavelengths of light is not fully understood. Here, we assess the impact of a range of wavelengths on the developing embryo.

METHODS

Murine preimplantation embryos were exposed daily to wavelengths within the blue, green, yellow, and red spectral bands and compared to an unexposed control group. Development to blastocyst, DNA damage, and cell number/allocation to blastocyst cell lineages were assessed. For the longer wavelengths (yellow and red), pregnancy/fetal outcomes and the abundance of intracellular lipid were investigated.

RESULTS

Significantly fewer embryos developed to the blastocyst stage when exposed to the yellow wavelength. Elevated DNA damage was observed within embryos exposed to blue, green, or red wavelengths. There was no effect on blastocyst cell number/lineage allocation for all wavelengths except red, where there was a significant decrease in total cell number. Pregnancy rate was significantly reduced when embryos were irradiated with the red wavelength. Weight at weaning was significantly higher when embryos were exposed to yellow or red wavelengths. Lipid abundance was significantly elevated following exposure to the yellow wavelength.

CONCLUSION

Our results demonstrate that the impact of light is wavelength-specific, with longer wavelengths also impacting the embryo. We also show that effects are energy-dependent. This data shows that damage is multifaceted and developmental rate alone may not fully reflect the impact of light exposure.

Watching the embryo: Evolution of the microscope for the study of embryogenesis

Embryos and microscopes share a long, remarkable history and biologists have always been intrigued to watch how embryos develop under the microscope. Here we discuss the advances in microscopy which have greatly influenced our current understanding of embryogenesis. We highlight the evolution of microscopes and the optical technologies that have been instrumental in studying various developmental processes. These imaging modalities provide mechanistic insights into the dynamic cellular and molecular events which drive lineage commitment and morphogenetic changes in the developing embryo. We begin the journey with a brief history of microscopy to study embryos. First, we review the principles and optics of light, fluorescence, confocal, and electron microscopy which have been key techniques for imaging cellular and molecular events during embryonic development. Next, we discuss recent key imaging modalities such as light-sheet microscopy, which are suitable for whole embryo imaging. Further, we highlight imaging techniques like multiphoton and super resolution microscopy for beyond light diffraction limit, high resolution imaging. Lastly, we review some of the scattering-based imaging methods and techniques used for imaging human embryos.

3D convolutional neural networks-based segmentation to acquire quantitative criteria of the nucleus during mouse embryogenesis

During embryogenesis, cells repeatedly divide and dynamically change their positions in three-dimensional (3D) space. A robust and accurate algorithm to acquire the 3D positions of the cells would help to reveal the mechanisms of embryogenesis. To acquire quantitative criteria of embryogenesis from time-series 3D microscopic images, image processing algorithms such as segmentation have been applied. Because the cells in embryos are considerably crowded, an algorithm to segment individual cells in detail and accurately is needed. To quantify the nuclear region of every cell from a time-series 3D fluorescence microscopic image of living cells, we developed QCANet, a convolutional neural network-based segmentation algorithm for 3D fluorescence bioimages. We demonstrated that QCANet outperformed 3D Mask R-CNN, which is currently considered as the best algorithm of instance segmentation. We showed that QCANet can be applied not only to developing mouse embryos but also to developing embryos of two other model species. Using QCANet, we were able to extract several quantitative criteria of embryogenesis from 11 early mouse embryos. We showed that the extracted criteria could be used to evaluate the differences between individual embryos. This study contributes to the development of fundamental approaches for assessing embryogenesis on the basis of extracted quantitative criteria.

Active H3K27me3 demethylation by KDM6B is required for normal development of bovine preimplantation embryos

The substantial epigenetic remodeling that occurs during early stages of mammalian embryonic development likely contributes to reprogramming the parental genomes from a differentiated to a totipotent state and activation of the embryonic genome. Trimethylation of lysine 27 of histone 3 (H3K27me3) is a repressive mark that undergoes global dynamic changes during preimplantation development of several species. To ascertain the role of H3K27me3 in bovine preimplantation development we perturbed the activity of KDM6B, which demethylates H3K27me3. Knockdown of maternal KDM6B mRNA inhibited the reduction in global levels of H3K27me3 from 2-cell to 8-cell embryo stages and compromised development to the blastocyst stage; embryos that developed to the blastocyst stage had fewer inner cell mass (ICM) and trophectoderm (TE) cells. In addition, the transcriptome of KDM6B knockdown embryos was altered at the 8-cell stage and characterized by downregulation of transcripts related to transcriptional regulation, chromatin remodeling, and protein catabolism. Inhibiting the catalytic activity of KDM6B with a specific small molecule inhibitor also prevented the global decrease in H3K27me3 and compromised development to the blastocyst stage. These results indicate that histone demethylation activity, mediated by KDM6B, is required for the global decrease in H3K27me3, correct activation of the embryonic genome, and development to the blastocyst stage in bovine embryos.

Viable offspring after imaging of Ca2+ oscillations and visualization of the cortical reaction in mouse eggs

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During mammalian fertilization, egg Ca 2+ oscillations are known to play pivotal roles in triggering downstream events such as resumption of the cell cycle and the establishment of blocks to polyspermy. However, viable offspring have not been obtained after monitoring Ca 2+ oscillations, and their spatiotemporal links to subsequent events are still to be examined. Therefore, the development of imaging methods to avoid phototoxic damage while labeling these events is required. Here, we examined the usefulness of genetically encoded Ca 2+ indicators for optical imaging (GECOs), in combination with spinning-disk confocal imaging. The Ca 2+ imaging of fertilized mouse eggs with GEM-, G-, or R-GECO recorded successful oscillations (8.19 ± 0.31, 7.56 ± 0.23, or 7.53 ± 0.27 spikes in the first 2 h, respectively), similar to those obtained with chemical indicators. Then, in vitro viability tests revealed that imaging with G- or R-GECO did not interfere with the rate of development to the blastocyst stage (61.8 or 70.0%, respectively, vs 75.0% in control). Furthermore, two-cell transfer to recipient female mice after imaging with G- or R-GECO resulted in a similar birthrate (53.3 or 52.0%, respectively) to that of controls (48.7%). Next, we assessed the quality of the cortical reaction (CR) in artificially activated or fertilized eggs using fluorescently labeled Lens culinaris agglutinin fluorescein isothiocyanate. Multicolor imaging demonstrated that the first few Ca 2+ spikes are sufficient for the completion of the CR and subsequent hardening of the zona pellucida in mouse eggs. These methods provide a framework for studying Ca 2+ dynamics in mammalian fertilization.

Long-term live-cell imaging of mammalian preimplantation development and derivation process of pluripotent stem cells from the embryos

Mammalian fertilization is a process in which two highly specialized haploid gametes unite and endow totipotency to the resulting diploid zygote. This is followed by cell proliferation and the onset of differentiation during the brief period leading up to implantation. In these processes, a number of cellular components and structures are regulated spatially and temporally, as seen in repeated cell division, cell cycle progression, and epigenetic reprogramming. In mammals, the numbers of oocytes and embryos that can be collected are very limited. Therefore, analyses of molecular mechanisms are hampered because of difficulties in conducting biochemical analyses on such limited material. Furthermore, immunostaining methods require cell fixation and are insufficient for understanding ontogeny, because the processes observed in fertilization and early embryonic development progress in time-dependent manners and each phenomenon is connected with others by cause-and-effect relationships. Consequently, it is important to develop an experimental system that enables molecular imaging without affecting embryonic development. To achieve the above advantages, especially retrospective and prospective analyses, we have established a live-cell imaging system that enables observations under minimally invasive conditions. Using this approach, we have succeeded in visualizing and predicting the developmental potential of embryos after various perturbations. We also succeeded in imaging embryonic stem (ES) cell derivation in natural conditions. In this review, we describe a brief history of embryonic imaging and detailed protocols. We also discuss promising aspects of imaging in the fields of developmental and stem cell biology.

Fluorescence cell imaging and manipulation using conventional halogen lamp microscopy

Technologies for vitally labeling cells with fluorescent dyes have advanced remarkably. However, to excite fluorescent dyes currently requires powerful illumination, which can cause phototoxic damage to the cells and increases the cost of microscopy. We have developed a filter system to excite fluorescent dyes using a conventional transmission microscope equipped with a halogen lamp. This method allows us to observe previously invisible cell organelles, such as the metaphase spindle of oocytes, without causing phototoxicity. Cells remain healthy even after intensive manipulation under fluorescence observation, such as during bovine, porcine and mouse somatic cell cloning using nuclear transfer. This method does not require expensive epifluorescence equipment and so could help to reduce the science gap between developed and developing countries.

Dynamic, long-term in vivo imaging of tumor-stroma interactions in mouse models of breast cancer using spinning-disk confocal microscopy

Tumors contain many components in addition to the cancer cells, including blood vessels, fibroblasts, and immune cells. Genetic studies and tumor biopsies have generated insights into the importance of these stromal components for cancer progression. However, it remains a challenge to reveal the dynamic interactions among the distinct tumor components within live animals. Studies involving multiphoton microscopy allow direct imaging of cellular movement in live mice, but multiphoton microscopy is expensive, complex, and usually relies on a single excitation wavelength for all fluorophores. This article describes a method for intravital imaging using a microlensed spinning-disk confocal microscope. Although tissue penetration with spinning-disk confocal microscopy is lower than with multiphoton microscopy, image acquisition with this method is very rapid, so artifacts from respiratory motion are avoided. Photobleaching and phototoxicity are low, and multicolor acquisition is cheaper and easier than with multiphoton microscopy. This article discusses various aspects of experimental setup, as well as methods for addressing technical barriers, such as generating and working with multiple tumor microenvironments within individual live mice, image collection, and long-term anesthesia.

Housekeeping gene transcript abundance in bovine fertilized and cloned embryos

The objective of this study was to compare housekeeping gene expression levels, relative to total mRNA, across different stages of bovine preimplantation development in embryos generated by IVF and somatic cell nuclear transfer (SCNT). We first analyzed the levels of total RNA recovered from different stages of preimplantation development. A similar RNA level was observed from oocytes to 16-cell stage embryos with a significant increase at morula and blastocyst stages. Then we used an absolute mRNA determination method that accounts for the RNA level in the embryo by quantifying copies of transcripts normalized to loaded cDNA amount. The number of housekeeping genes mRNA copies per nanogram of cDNA was compared among samples obtained from different stages of preimplantation IVF-derived embryos. None of the genes analyzed (GAPDH, PPIA, ACTB, RPL15, GUSB, and Histone H2A.2) maintained constant levels throughout preimplantation development, indicating that they are not suitable for normalizing gene expression across developmental stages. We then compared expression of housekeeping genes between IVF and SCNT embryos at different embryonic stages. We found different levels of transcript abundance between IVF and SCNT embryos for GAPDH, RPL15, GUSB, and ACTB. On the other hand, Histone H2A.2 and PPIA were similar between IVF and SCNT embryos at each stage analyzed, although they varied across stages as previously mentioned.

SWI/SNF-Brg1 regulates self-renewal and occupies core pluripotency-related genes in embryonic stem cells

The SWI/SNF-Brg1 chromatin remodeling protein plays critical roles in cell-cycle control and differentiation through regulation of gene expression. Loss of Brg1 in mice results in early embryonic lethality, and recent studies have implicated a role for Brg1 in somatic stem cell self-renewal and differentiation. However, little is known about Brg1 function in preimplantation embryos and embryonic stem (ES) cells. Here we report that Brg1 is required for ES cell self-renewal and pluripotency. RNA interference-mediated knockdown of Brg1 in blastocysts caused aberrant expression of Oct4 and Nanog. In ES cells, knockdown of Brg1 resulted in phenotypic changes indicative of differentiation, downregulation of self-renewal and pluripotency genes (e.g., Oct4, Sox2, Sall4, Rest), and upregulation of differentiation genes. Using genome-wide promoter analysis (chromatin immunoprecipitation) we found that Brg1 occupied the promoters of key pluripotency-related genes, including Oct4, Sox2, Nanog, Sall4, Rest, and Polycomb group (PcG) proteins. Moreover, Brg1 co-occupied a subset of Oct4, Sox2, Nanog, and PcG protein target genes. These results demonstrate an important role for Brg1 in regulating self-renewal and pluripotency in ES cells.

Activation of bovine somatic cell nuclear transfer embryos by PLCZ cRNA injection

The production of cloned animals by the transfer of a differentiated somatic cell into an enucleated oocyte circumvents fertilization. During fertilization, the sperm delivers a sperm-specific phospholipase C (PLCZ) that is responsible for triggering Ca(2)(+) oscillations and oocyte activation. During bovine somatic cell nuclear transfer (SCNT), oocyte activation is artificially achieved by combined chemical treatments that induce a monotonic rise in intracellular Ca(2)(+) and inhibit either phosphorylation or protein synthesis. In this study, we tested the hypothesis that activation of bovine nuclear transfer embryos by PLCZ improves nuclear reprogramming. Injection of PLCZ cRNA into bovine SCNT units induced Ca(2)(+) oscillations similar to those observed after fertilization and supported high rates of blastocyst development similar to that seen in embryos produced by IVF. Furthermore, gene expression analysis at the eight-cell and blastocyst stages revealed a similar expression pattern for a number of genes in both groups of embryos. Lastly, levels of trimethylated lysine 27 at histone H3 in blastocysts were higher in bovine nuclear transfer embryos activated using cycloheximide and 6-dimethylaminopurine (DMAP) than in those activated using PLCZ or derived from IVF. These results demonstrate that exogenous PLCZ can be used to activate bovine SCNT-derived embryos and support the hypothesis that a fertilization-like activation response can enhance some aspects of nuclear reprogramming.

Cellular reprogramming for the creation of patient-specific embryonic stem cells

The success of somatic cell nuclear transfer in mammals has opened the possibility to dedifferentiate cells from a patient into embryonic stem cells and in doing so, potentially generate all different cells and tissues of the human body. These cells could be later transplanted to the same patient without immune rejection. Whereas this principle has been demonstrated in laboratory animals, it is yet to be shown to work in primates. Herein we discuss the probability of somatic cell nuclear transfer becoming a real therapeutic alternative as well as the potential emerging dedifferentiation approaches that may eventually replace it.