Manipulating and imaging the early Xenopus laevis embryo

Imaging Methods in Xenopus Cells, Embryos, and Tadpoles

Xenopus is an excellent vertebrate model system ideally suited for a wide range of imaging methods designed to investigate cell and developmental biology processes. The individual cells of Xenopus are much larger than those in many other vertebrate model systems, such that both cell behavior and subcellular processes can more easily be observed and resolved. Gene function in Xenopus can be manipulated and visualized using a variety of approaches, and the embryonic fate map is stereotypical, such that microinjections can target specific tissues and cell types during development. Tissues, organotypic explants, and individual cells can also be mounted in stable chambers and cultured easily in simple salt solutions without cumbersome environmental controls. Furthermore, Xenopus embryonic tissues can be microsurgically isolated and shaped to expose cell behaviors and protein dynamics in any regions of the embryo to high-resolution live-cell imaging. The combination of these attributes makes Xenopus a powerful system for understanding cell and developmental processes as well as disease mechanisms, through quantitative analysis of protein dynamics, cell movements, tissue morphogenesis, and regeneration. Here, we introduce various methods, of both fixed and living tissues, for visualizing Xenopus cells, embryos, and tadpoles. Specifically, we highlight protocol updates for whole-mount in situ hybridization and immunofluorescence, as well as robust live imaging approaches including methods for optimizing the time-lapse imaging of whole embryos and explants.

Unite to divide - how models and biological experimentation have come together to reveal mechanisms of cytokinesis

Cytokinesis is the fundamental and ancient cellular process by which one cell physically divides into two. Cytokinesis in animal and fungal cells is achieved by contraction of an actomyosin cytoskeletal ring assembled in the cell cortex, typically at the cell equator. Cytokinesis is essential for the development of fertilized eggs into multicellular organisms and for homeostatic replenishment of cells. Correct execution of cytokinesis is also necessary for genome stability and the evasion of diseases including cancer. Cytokinesis has fascinated scientists for well over a century, but its speed and dynamics make experiments challenging to perform and interpret. The presence of redundant mechanisms is also a challenge to understand cytokinesis, leaving many fundamental questions unresolved. For example, how does a disordered cytoskeletal network transform into a coherent ring? What are the long-distance effects of localized contractility? Here, we provide a general introduction to 'modeling for biologists', and review how agent-based modeling and continuum mechanics modeling have helped to address these questions.

Prolonged in vivo imaging of Xenopus laevis

BACKGROUND

While live imaging of embryonic development over long periods of time is a well established method for embryos of the frog Xenopus laevis, once development has progressed to the swimming stages, continuous live imaging becomes more challenging because the tadpoles must be immobilized. Current imaging techniques for these advanced stages generally require bringing the tadpoles in and out of anesthesia for short imaging sessions at selected time points, severely limiting the resolution of the data.

RESULTS

Here we demonstrate that creating a constant flow of diluted tricaine methanesulfonate (MS-222) over a tadpole greatly improves their survival under anesthesia. Based on this result, we describe a new method for imaging stage 48 to 65 X. laevis, by circulating the anesthetic using a peristaltic pump. This supports the animal during continuous live imaging sessions for at least 48 hr. The addition of a stable optical window allows for high quality imaging through the anesthetic solution.

CONCLUSIONS

This automated imaging system provides for the first time a method for continuous observations of developmental and regenerative processes in advanced stages of Xenopus over 2 days. Developmental Dynamics 243:1011-1019, 2014. © 2014 Wiley Periodicals, Inc.

Blastocoel-spanning filopodia in cleavage-stage Xenopus laevis: Potential roles in morphogen distribution and detection

In the frog Xenopus laevis, dorsal-ventral axis specification involves cytoskeleton-dependent transport of localized transcripts and proteins during the first cell cycle, and activation of the canonical Wnt pathway to locally stabilize translated beta-catenin which, by as early as the 32-cell stage, commits nuclei in prospective dorsal lineages to the subsequent expression of dorsal target genes. Maternal ligands important for activating this dorsal-specific signaling pathway are thought to interact with secreted glypicans and coreceptors in the blastocoel. While diffusion between cells is generally thought of as sufficient to accomplish the distribution of secreted maternal ligands to their appropriate targets, signaling may also involve other potential mechanisms, including direct transfer of morphogens via membrane-bounded entities, such as argosomes, exosomes, or even filopodia. In Xenopus, the blastocoel-facing, basolateral surfaces where signaling interactions ostensibly take place have not been previously examined in detail. Here, we report that the cleavage-stage blastocoel is traversed by hundreds of extremely long cellular protrusions that maintain long-term contacts between nonadjacent blastomeres during expansion of the interstitial space in early embryogenesis. The involvement of these protrusions in early embryonic patterning is suggested by the discoveries that (a) they fragment into microvesicles, whose resorption facilitates considerable exchange of cytoplasm and membrane between blastomeres; and (b) they are active in caveolar endocytosis, a prerequisite for ligand-receptor signaling.