Quantifying the forces that drive zebrafish axis elongation and somitogenesis
The study of morphogenesis in animals is concerned with the question of how the structures composing the body acquire their form during embryonic development. As cells are able to sense and respond to biochemical as well as physical cues, a complete understanding of any morphogenetic process requires an understanding of genetic as well as mechanical factors at play. While many powerful tools have been developed for studying and manipulating genetics, the set of tools able to probe the mechanics of living systems is rather limited. In this thesis, I present my contributions to the technological advancement of droplets as in vivo and in situ mechanical sensors, as well as a series of measurements in zebrafish embryos characterizing stress patterning during two hallmark morphogenetic events: body axis elongation and somitogenesis. I discuss how these stress measurements, along with other quantifications, yield new insights into physical mechanisms underlying both body axis elongation and somite formation.
First, I discuss the development of a computational tool for automated surface analysis of volumetrically imaged particles which have been surface-coated with a fluorescent label. Prior to this work, droplets as force sensors had been developed as a proof-of-principle technology, but no framework for automated and accurate segmentation of a droplet surface and quantification of its surface curvature had been developed. Since droplet stress sensors work by relating anisotropies in surface normal stress to the observed variations in mean curvature on the droplet surface and a known value of surface interfacial tension, an automated method of transforming image data into stress measurements is required for any large scale implementation of the technique. Here, I demonstrate such an algorithm and characterize the performance in terms of accuracy on simulated and experimental data sets.
Next, I discuss a collaborative study of the mechanical conditions underlying body axis elongation in zebrafish. Here, I present droplet-based measurements of cell-scale and tissue-scale stress collected in the mesodermal progenitor zone (MPZ) and along the presomitic mesoderm (PSM) in zebrafish embryos at 10 somites stage. I show that local patterns of cell-scale stress are correlated on short timescales (~1 minute), while the amplitudes of these stresses are spatially uniform from the MPZ to the PSM. In contrast, tissue scale stresses increase from the posterior to the anterior end of the tissues. I also discuss these stress measurements in the context of experiments showing the existence of a spatially graded yield stress in the tissue. I explain how the regional tuning of cell-cell junctional tension fluctuations, cell volume fraction, and large-scale stresses gives rise to a rigidity gradient, which helps to explain how body axis elongation occurs.
Finally, I explore a puzzle introduced by the findings of the previous chapter: how are somites mechanically sculpted from the recently rigidified PSM? I motivate a mechanical model for somite boundary formation with experiments and simulation. Using live imaging, I quantify the morphological dynamics of somite boundary formation, and relate the time evolution of boundary straightening to the time evolution of enrichment of F-actin and myosin II at the forming border. To understand how stresses change over the formation period, I collect time course measurements of cell and tissue scale stresses using droplets. Here, we find average cell-scale stresses show no significant change, while tissue scale stress anisotropies re-orient and increase in amplitude as somite boundaries form. I also analyze the spatiotemporal distributions of T1 transition rates within a forming somite, and I discover a spatial gradient in the rates of cell-cell rearrangements. These experimental findings, in combination with simulations, suggest that increased actomyosin activity drives increases in tension at the nascent boundary, resulting in localized fluidization, allowing the somite to rapidly section off from the PSM.
Taken together, the years of research presented here demonstrate the value of droplet force sensor technology as a powerful tool for gaining insights into the physical mechanisms which guide morphogenesis. In case of axis elongation, we learned how fluctuating stresses at cellular scales tune material properties to allow uniaxial growth at the posterior end. In the case of somitogenesis, we learned how acute increases in stress anisotropies locally fluidize boundaries and enable physical segmentation of somites. In my final chapter, I reflect on my cumulative findings and discuss possible future directions of research.