Live imaging of mouse embryos

Analysis of Mid- to Late-Gestation Phenotypes in Mice

Mid- to late gestation is characterized by tissue differentiation, maturation, organogenesis, and growth, and many mutant genes have detrimental effects during this phase of development. The outcome may be lethal before birth or may be compatible with life but result in birth defects. Some of the common causes of death during late gestation are hematopoietic defects, cardiovascular problems, and placental insufficiency. Many morphological abnormalities, lethal or not, can be investigated with gross and histological analyses or by visualization of the developing skeleton. Molecular characterization of mutant phenotypes, guided by the expression pattern of the mutant gene, can reveal disruptions in gene expression patterns of known developmental genes. Cell proliferation and cell death assays will reveal disruptions in cellular dynamics. Various modalities of 3D imaging of intact embryos can provide volumetric information about mutant phenotypes.

Live imaging of echinoderm embryos to illuminate evo-devo

Echinoderm embryos have been model systems for cell and developmental biology for over 150 years, in good part because of their optical clarity. Discoveries that shaped our understanding of fertilization, cell division and cell differentiation were only possible because of the transparency of sea urchin eggs and embryos, which allowed direct observations of intracellular structures. More recently, live imaging of sea urchin embryos, coupled with fluorescence microscopy, has proven pivotal to uncovering mechanisms of epithelial to mesenchymal transition, cell migration and gastrulation. However, live imaging has mainly been performed on sea urchin embryos, while echinoderms include numerous experimentally tractable species that present interesting variation in key aspects of morphogenesis, including differences in embryo compaction and mechanisms of blastula formation. The study of such variation would allow us not only to understand how tissues are formed in echinoderms, but also to identify which changes in cell shape, cell-matrix and cell-cell contact formation are more likely to result in evolution of new embryonic shapes. Here we argue that adapting live imaging techniques to more echinoderm species will be fundamental to exploit such an evolutionary approach to the study of morphogenesis, as it will allow measuring differences in dynamic cellular behaviors - such as changes in cell shape and cell adhesion - between species. We briefly review existing methods for live imaging of echinoderm embryos and describe in detail how we adapted those methods to allow long-term live imaging of several species, namely the sea urchin Lytechinus pictus and the sea stars Patiria miniata and Patiriella regularis. We outline procedures to successfully label, mount and image early embryos for 10–16 h, from cleavage stages to early blastula. We show that data obtained with these methods allows 3D segmentation and tracking of individual cells over time, the first step to analyze how cell shape and cell contact differ among species. The methods presented here can be easily adopted by most cell and developmental biology laboratories and adapted to successfully image early embryos of additional species, therefore broadening our understanding of the evolution of morphogenesis.

Imaging the onset of oscillatory signaling dynamics during mouse embryo gastrulation

A fundamental requirement for embryonic development is the coordination of signaling activities in space and time. A notable example in vertebrate embryos is found during somitogenesis, where gene expression oscillations linked to the segmentation clock are synchronized across cells in the presomitic mesoderm (PSM) and result in tissue-level wave patterns. To examine their onset during mouse embryo development, we studied the dynamics of the segmentation clock gene Lfng during gastrulation. To this end, we established an imaging setup using selective plane illumination microscopy (SPIM) that enables culture and simultaneous imaging of up to four embryos (‘SPIM- for-4’). Using SPIM-for-4, combined with genetically encoded signaling reporters, we detected the onset of Lfng oscillations within newly formed mesoderm at presomite stages. Functionally, we found that initial synchrony and the first ∼6-8 oscillation cycles occurred even when Notch signaling was impaired, revealing similarities to previous findings made in zebrafish embryos. Finally, we show that a spatial period gradient is present at the onset of oscillatory activity, providing a potential mechanism accounting for our observation that wave patterns build up gradually over the first oscillation cycles.

Summary: A versatile light-sheet imaging setup enabling simultaneous live imaging of multiple mouse embryos for 48 h, an approach that offers insight into the onset of oscillatory signaling dynamics and the segmentation clock.

Live Imaging of the Dynamics of Mammalian Neural Crest Cell Migration

Neural crest cells (NCC) are a multipotent cell population that play an important role in vertebrate development. Often touted as the fourth-germ layer, NCC are induced at the border of the neural and non-neural ectoderm during the neurulation phase of embryogenesis. NCC undergo an epithelial to mesenchymal transition (EMT) that facilitates their delamination and migration throughout the embryo. After reaching their final destination, NCC then differentiate into numerous distinct cell types including neurons and glia, melanocytes, and craniofacial chondrocytes and osteoblasts. Research into the signals and mechanisms regulating each step of NCC development has been instrumental to our understanding of vertebrate development, evolution, and disease. However, studying the single and collective cellular dynamics of mammalian NCC migration has proven difficult due to the challenges accessing, and limitations visualizing, NCC within an embryo that develops in utero. The following chapter describes methods for studying the dynamics of cranial NCC migration in whole mouse embryos and in two-dimensional (2D) and 3D explant cultures of the neural plate, but these methods can be adapted for NCC at any axial level of the embryo.

Leukocyte Trafficking and Hemostasis in the Mouse Fetus in vivo: A Practical Guide

In vivo observations of blood cells and organ compartments within the fetal mammalian organism are difficult to obtain. This practical guide describes a mouse model for in vivo observation of the fetal yolk-sac and corporal microvasculature throughout murine gestation, including imaging of various organ compartments, microvascular injection procedures, different methods for staining of blood plasma, vessel wall and circulating cell subsets. Following anesthesia of pregnant mice, the maternal abdominal cavity is opened, the uterus horn exteriorized, and the fetus prepared for imaging while still connected to the placenta. Microinjection methods allow delivery of substances directly into the fetal circulation, while substances crossing the placenta can be easily administered via the maternal circulation. Small volume blood sample collection allows for further in vitro workup of obtained results. The model permits observation of leukocyte-endothelial interactions, hematopoietic niche localization, platelet function, endothelial permeability studies, and hemodynamic changes in the mouse fetus, using appropriate strains of fluorescent protein expressing reporter mice and various sophisticated intravital microscopy techniques. Our practical guide is of interest to basic physiologists, developmental biologists, cardiologists, and translational neonatologists and reaches out to scientists focusing on the origin and regulation of hematopoietic niches, thrombopoiesis and macrophage heterogeneity.

Tissue morphodynamics shaping the early mouse embryo

Generation of the elongated vertebrate body plan from the initially radially symmetrical embryo requires comprehensive changes to tissue form. These shape changes are generated by specific underlying cell behaviors, coordinated in time and space. Major principles and also specifics are emerging, from studies in many model systems, of the cell and physical biology of how region-specific cell behaviors produce regional tissue morphogenesis, and how these, in turn, are integrated at the level of the embryo. New technical approaches have made it possible more recently, to examine the morphogenesis of the mouse embryo in depth, and to elucidate the underlying cellular mechanisms. This review focuses on recent advances in understanding the cellular basis for the early fundamental events that establish the basic form of the embryo.

Optical coherence tomography guided microinjections in live mouse embryos: high-resolution targeted manipulation for mouse embryonic research

The ability to conduct highly localized delivery of contrast agents, viral vectors, therapeutic or pharmacological agents, and signaling molecules or dyes to live mammalian embryos is greatly desired to enable a variety of studies in the field of developmental biology, such as investigating the molecular regulation of cardiovascular morphogenesis. To meet such a demand, we introduce, for the first time, the concept of employing optical coherence tomography (OCT)-guide microinjections in live mouse embryos, which provides precisely targeted manipulation with spatial resolution at the micrometer scale. The feasibility demonstration is performed with experimental studies on cultured live mouse embryos at E8.5 and E9.5. Additionally, we investigate the OCT-guided microinjection of gold–silica nanoshells to the yolk sac vasculature of live cultured mouse embryos at the stage when the heart just starts to beat, as a potential approach for dynamic assessment of cardiovascular form and function before the onset of blood cell circulation. Also, the capability of OCT to quantitatively monitor and measure injection volume is presented. Our results indicate that OCT-guided microinjection could be a useful tool for mouse embryonic research.

Optical coherence tomography guided microinjections in live mouse embryos: high-resolution targeted manipulation for mouse embryonic research

The ability to conduct highly localized delivery of contrast agents, viral vectors, therapeutic or pharmacological agents, and signaling molecules or dyes to live mammalian embryos is greatly desired to enable a variety of studies in the field of developmental biology, such as investigating the molecular regulation of cardiovascular morphogenesis. To meet such a demand, we introduce, for the first time, the concept of employing optical coherence tomography (OCT)-guide microinjections in live mouse embryos, which provides precisely targeted manipulation with spatial resolution at the micrometer scale. The feasibility demonstration is performed with experimental studies on cultured live mouse embryos at E8.5 and E9.5. Additionally, we investigate the OCT-guided microinjection of gold–silica nanoshells to the yolk sac vasculature of live cultured mouse embryos at the stage when the heart just starts to beat, as a potential approach for dynamic assessment of cardiovascular form and function before the onset of blood cell circulation. Also, the capability of OCT to quantitatively monitor and measure injection volume is presented. Our results indicate that OCT-guided microinjection could be a useful tool for mouse embryonic research.

Contrast imaging in mouse embryos using high-frequency ultrasound

Ultrasound contrast-enhanced imaging can convey essential quantitative information regarding tissue vascularity and perfusion and, in targeted applications, facilitate the detection and measure of vascular biomarkers at the molecular level. Within the mouse embryo, this noninvasive technique may be used to uncover basic mechanisms underlying vascular development in the early mouse circulatory system and in genetic models of cardiovascular disease. The mouse embryo also presents as an excellent model for studying the adhesion of microbubbles to angiogenic targets (including vascular endothelial growth factor receptor 2 (VEGFR2) or αvβ3) and for assessing the quantitative nature of molecular ultrasound. We therefore developed a method to introduce ultrasound contrast agents into the vasculature of living, isolated embryos. This allows freedom in terms of injection control and positioning, reproducibility of the imaging plane without obstruction and motion, and simplified image analysis and quantification. Late gestational stage (embryonic day (E)16.6 and E17.5) murine embryos were isolated from the uterus, gently exteriorized from the yolk sac and microbubble contrast agents were injected into veins accessible on the chorionic surface of the placental disc. Nonlinear contrast ultrasound imaging was then employed to collect a number of basic perfusion parameters (peak enhancement, wash-in rate and time to peak) and quantify targeted microbubble binding in an endoglin mouse model. We show the successful circulation of microbubbles within living embryos and the utility of this approach in characterizing embryonic vasculature and microbubble behavior.

Mathematically guided approaches to distinguish models of periodic patterning

How periodic patterns are generated is an open question. A number of mechanisms have been proposed--most famously, Turing's reaction-diffusion model. However, many theoretical and experimental studies focus on the Turing mechanism while ignoring other possible mechanisms. Here, we use a general model of periodic patterning to show that different types of mechanism (molecular, cellular, mechanical) can generate qualitatively similar final patterns. Observation of final patterns is therefore not sufficient to favour one mechanism over others. However, we propose that a mathematical approach can help to guide the design of experiments that can distinguish between different mechanisms, and illustrate the potential value of this approach with specific biological examples.

Algorithms for improved 3-D reconstruction of live mammalian embryo vasculature from optical coherence tomography data

BACKGROUND

Robust reconstructions of the three-dimensional network of blood vessels in developing embryos imaged by optical coherence tomography (OCT) are needed for quantifying the longitudinal development of vascular networks in live mammalian embryos, in support of developmental cardiovascular research. Past computational methods [such as speckle variance (SV)] have demonstrated the feasibility of vascular reconstruction, but multiple challenges remain including: the presence of vessel structures at multiple spatial scales, thin blood vessels with weak flow, and artifacts resulting from bulk tissue motion (BTM).

METHODS

In order to overcome these challenges, this paper introduces a robust and scalable reconstruction algorithm based on a combination of anomaly detection algorithms and a parametric dictionary based sparse representation of blood vessels from structural OCT data.

RESULTS

Validation results using confocal data as the baseline demonstrate that the proposed method enables the detection of vessel segments that are either partially missed or weakly reconstructed using the SV method. Finally, quantitative measurements of vessel reconstruction quality indicate an overall higher quality of vessel reconstruction with the proposed method.

CONCLUSIONS

Results suggest that sparsity-integrated speckle anomaly detection (SSAD) is potentially a valuable tool for performing accurate quantification of the progression of vascular development in the mammalian embryonic yolk sac as imaged using OCT.

Live confocal microscopy of the developing mouse embryonic yolk sac vasculature

Understanding of mouse embryonic development is an invaluable resource for our interpretation of human embryology. Traditional imaging approaches such as immunofluorescence and in situ hybridization are excellent methods for characterizing gene expression and morphology but lack the ability to reveal the dynamic morphogenesis. Furthermore, mammalian embryonic development occurs in utero, which bars our ability to visualize development in dynamics. With the use of live confocal microscopy, vital fluorescent reporters, and embryo culture methods, we can observe cell migration, proliferation, differentiation, and cell-cell interaction in live developing wild-type and mutant embryos. In this chapter, we will discuss how confocal microscopy can be used to visualize the developing vasculature and hemodynamics of the mouse embryonic yolk sac. We will describe fluorescent protein reporter mouse models allowing to image yolk sac vessel development and blood flow, live embryo culture approaches, and confocal time-lapse imaging methods to study vascular morphology and hemodynamics in early embryos.

Quantitative imaging of cell dynamics in mouse embryos using light-sheet microscopy

Single/selective-plane illumination, or light-sheet, systems offer several advantages over other fluorescence microscopy methods for live, 3D microscopy. These systems are valuable for studying embryonic development in several animal systems, such as Drosophila, C. elegans and zebrafish. The geometry of the light path in this form of microscopy requires the sample to be accessible from multiple sides and fixed in place so that it can be rotated around a single axis. Popular methods for mounting include hanging the specimen from a pin or embedding it in 1-2% agarose. These methods can be particularly problematic for certain samples, such as post-implantation mouse embryos, that expand significantly in size and are very delicate and sensitive to mounting. To overcome the current limitations and to establish a robust strategy for long-term (24 h) time-lapse imaging of E6.5-8.5 mouse embryos with light-sheet microscopy, we developed and tested a method using hollow agarose cylinders designed to accommodate for embryonic growth, yet provide boundaries to minimize tissue drift and enable imaging in multiple orientations. Here, we report the first 24-h time-lapse sequences of post-implantation mouse embryo development with light-sheet microscopy. We demonstrate that light-sheet imaging can provide both quantitative data for tracking changes in morphogenesis and reveal new insights into mouse embryogenesis. Although we have used this approach for imaging mouse embryos, it can be extended to imaging other types of embryos as well as tissue explants.

Contrast-enhanced molecular ultrasound differentiates endoglin genotypes in mouse embryos

Targeted ultrasound contrast imaging has the potential to become a reliable molecular imaging tool. A better understanding of the quantitative aspects of molecular ultrasound technology could facilitate the translation of this technique to the clinic for the purposes of assessing vascular pathology and detecting individual response to treatment. The objective of this study was to evaluate whether targeted ultrasound contrast-enhanced imaging can provide a quantitative measure of endogenous biomarkers. Endoglin, an endothelial biomarker involved in the processes of development, vascular homeostasis, and altered in diseases, including hereditary hemorrhagic telangiectasia type 1 and tumor angiogenesis, was the selected target. We used a parallel plate perfusion chamber in which endoglin-targeted (MBE), rat isotype IgG2 control and untargeted microbubbles were perfused across endoglin wild-type (Eng+/+), heterozygous (Eng+/-) and null (Eng-/-) embryonic mouse endothelial cells and their adhesion quantified. Microbubble binding was also assessed in late-gestation, isolated living transgenic Eng+/- and Eng+/+ embryos. Nonlinear contrast-specific ultrasound imaging performed at 21 MHz was used to collect contrast mean power ratios for all bubble types. Statistically significant differences in microbubble binding were found across genotypes for both in vitro (p<0.05) and embryonic studies (p<0.001); MBE binding was approximately twofold higher in Eng+/+ cells and embryos compared with their Eng+/- counterparts. These results suggest that molecular ultrasound is capable of reliably differentiating between molecular genotypes and relating receptor densities to quantifiable molecular ultrasound levels.

Vascular development and hemodynamic force in the mouse yolk sac

Vascular remodeling of the mouse embryonic yolk sac is a highly dynamic process dependent on multiple genetic signaling pathways as well as biomechanical factors regulating proliferation, differentiation, migration, cell-cell, and cell-matrix interactions. During this early developmental window, the initial primitive vascular network of the yolk sac undergoes a dynamic remodeling process concurrent with the onset of blood flow, in which endothelial cells establish a branched, hierarchical structure of large vessels and smaller capillary beds. In this review, we will describe the molecular and biomechanical regulators which guide vascular remodeling in the mouse embryonic yolk sac, as well as live imaging methods for characterizing endothelial cell and hemodynamic function in cultured embryos.

Towards 3D in silico modeling of the sea urchin embryonic development

Embryogenesis is a dynamic process with an intrinsic variability whose understanding requires the integration of molecular, genetic, and cellular dynamics. Biological circuits function over time at the level of single cells and require a precise analysis of the topology, temporality, and probability of events. Integrative developmental biology is currently looking for the appropriate strategies to capture the intrinsic properties of biological systems. The "-omic" approaches require disruption of the function of the biological circuit; they provide static information, with low temporal resolution and usually with population averaging that masks fast or variable features at the cellular scale and in a single individual. This data should be correlated with cell behavior as cells are the integrators of biological activity. Cellular dynamics are captured by the in vivo microscopy observation of live organisms. This can be used to reconstruct the 3D + time cell lineage tree to serve as the basis for modeling the organism's multiscale dynamics. We discuss here the progress that has been made in this direction, starting with the reconstruction over time of three-dimensional digital embryos from in toto time-lapse imaging. Digital specimens provide the means for a quantitative description of the development of model organisms that can be stored, shared, and compared. They open the way to in silico experimentation and to a more theoretical approach to biological processes. We show, with some unpublished results, how the proposed methodology can be applied to sea urchin species that have been model organisms in the field of classical embryology and modern developmental biology for over a century.

Live imaging, identifying, and tracking single cells in complex populations in vivo and ex vivo

Advances in optical imaging technologies combined with the use of genetically encoded fluorescent proteins have enabled the visualization of stem cells over extensive periods of time in vivo and ex vivo. The generation of genetically encoded fluorescent protein reporters that are fused with subcellularly localized proteins, such as human histone H2B, has made it possible to direct fluorescent protein reporters to specific subcellular structures and identify single cells in complex populations. This facilitates the visualization of cellular behaviors such as division, movement, and apoptosis at a single-cell resolution and, in principle, allows the prospective and retrospective tracking towards determining the lineage of each cell.

How blood vessel networks are made and measured

Tissue and organ viability depends on the proper systemic distribution of cells, nutrients, and oxygen through blood vessel networks. These networks arise in part via angiogenic sprouting. Vessel sprouting involves the precise coordination of several endothelial cell processes including cell-cell communication, cell migration, and proliferation. In this review, we discuss zebrafish and mammalian models of blood vessel sprouting and the quantification methods used to assess vessel sprouting and network formation in these models. We also review the mechanisms involved in angiogenic sprouting, and we propose that the process consists of distinct stages. Sprout initiation involves endothelial cell interactions with neighboring cells and the environment to establish a specialized tip cell responsible for leading the emerging sprout. Furthermore, local sprout guidance cues that spatially regulate this outward migration are discussed. We also examine subsequent events, such as sprout fusion and lumenization, that lead to maturation of a nascent sprout into a patent blood vessel.