How morphological diversity arises from variations in biomechanical processes remains an open question. Although forces shape tissues, how force-generating systems differ across species to create diverse forms is unclear. Here, we combine comparative morphogenesis and active matter theory across six cnidarian species spanning 500 million years of divergence to identify the mechanical basis of larval shape diversity. We define species-specific configurations of mechanical modules-termed mechanotypes-that quantitatively predict larval shapes across taxa. We find that shape elongation is a simple trait at the mesoscale level, as its variation depends on one mechanical module, whereas shape polarity is a complex trait dependent on several modules. Perturbations mimicking interspecies regulatory differences reshape these modules, reprogramming larval morphology into forms resembling sister species. By establishing a mesoscale mechanical framework for cross-species comparison, this work reveals how variations in a limited set of tissue-scale parameters generate morphological diversity.
The range over which a morphogen gradient provides reliable positional information is limited by intrinsic noise. We identify a regulatory circuit that counteracts this constraint for Dpp, a BMP that organizes the anterior/posterior axis of Drosophila wings. The transcriptional repressor Brinker (Brk), a Dpp target, enhances positional precision by repressing Dad, an inhibitory Smad, thereby extending Dpp's effective range. Thus, Brk mediates a feedback circuit that selectively amplifies low-level Dpp signals as would a logarithmic amplifier. This circuit also achieves temporal integration, mitigating the inevitable noise penalty associated with amplification. Although a core component of BMP signaling in flies, Brk is found exclusively in insects. Phylogenetic and expression analyses in the apterygote insect Thermobia domestica suggest that Brk originated in insects and was incorporated into the BMP network in pterygotes, possibly to permit long-range signaling in wing primordia. Brk exemplifies how gene regulatory network (GRN) evolution can enhance developmental precision, thus opening the door to increased morphological complexity.
How cell fate decisions and tissue remodeling are coordinated to establish precise and robust patterns is a fundamental question in developmental biology. Here, we investigate this interplay during the refinement of Drosophila wing veins. We show by live imaging that vein refinement is driven initially by local tissue deformation, followed by cell fate adjustments orchestrated by a signaling network involving Notch, EGFR, and Dpp. Dynamic tracking of signaling reporter activity uncovers a wave of Notch signaling that converts wide crude proveins into thin stereotypical veins. Perturbing large-scale convergence and extension does not affect vein refinement, and optogenetically induced veins refine irrespective of their orientation, demonstrating that the signaling network suffices for refinement, independently of large-scale tissue flows. A minimal biophysical description recapitulates the signaling network's ability to coordinate vein refinement in various experimental situations. Our results illustrate how cell fate decisions are updated for robust patterning in a remodeling tissue.
Blood vessels expand and contract actively as they continuously experience dynamic external stresses from blood flow. The mechanical response of the vessel wall is that of a composite material: its mechanical properties depend on its cellular components, which change dynamically as the cells respond to external stress. Mapping the relationship between these underlying cellular processes and emergent tissue mechanics is an ongoing challenge, particularly in endothelial cells. Here we assess the mechanics and cellular dynamics of an endothelial tube using a microstretcher that mimics the native environment of blood vessels. The characterization of the instantaneous monolayer elasticity reveals a strain-stiffening, actin-dependent and substrate-responsive behaviour. After a physiological pressure increase, the tissue displays a fluid-like expansion, with the reorientation of cell shape and actin fibres. We introduce a mechanical model that considers the actin fibres as a network in the nematic phase and couples their dynamics with active and elastic fibre tension. The model accurately describes the response to the pressure of endothelial tubes.
Blood vessels expand and contract actively as they continuously experience dynamic external stresses from blood flow. The mechanical response of the vessel wall is that of a composite material: its mechanical properties depend on its cellular components, which change dynamically as the cells respond to external stress. Mapping the relationship between these underlying cellular processes and emergent tissue mechanics is an ongoing challenge, particularly in endothelial cells. Here we assess the mechanics and cellular dynamics of an endothelial tube using a microstretcher that mimics the native environment of blood vessels. The characterization of the instantaneous monolayer elasticity reveals a strain-stiffening, actin-dependent and substrate-responsive behaviour. After a physiological pressure increase, the tissue displays a fluid-like expansion, with the reorientation of cell shape and actin fibres. We introduce a mechanical model that considers the actin fibres as a network in the nematic phase and couples their dynamics with active and elastic fibre tension. The model accurately describes the response to the pressure of endothelial tubes.
We present a generic framework for describing interacting, spinning, active polar particles, aimed at modeling dense cell aggregates, where cells are treated as polar, rotating objects that interact mechanically with one another and their surrounding environment. Using principles from nonequilibrium thermodynamics, we derive constitutive equations for interaction forces, torques, and polarity dynamics. We subsequently use this framework to analyze the spontaneous motion of cell doublets, uncovering a rich phase diagram of collective behaviors, including steady rotation driven by flow-polarity coupling or interactions between polarity and cell position.
Mechanical interactions between cells play a fundamental role in the self-organization of organisms. How these interactions drive coordinated cell movement in three dimensions remains unclear. Here we report that cell doublets embedded in a three-dimensional extracellular matrix undergo spontaneous rotations. We investigate the rotation mechanism and find that it is driven by a polarized distribution of myosin within cell cortices. The mismatched orientation of this polarized distribution breaks the doublet mirror symmetry. In addition, cells adhere at their interface through adherens junctions and with the extracellular matrix through focal contacts near myosin clusters. We use a physical theory describing the doublet as two interacting active surfaces to show that rotation is driven by myosin-generated gradients of active tension whose profiles are dictated by interacting cell polarity axes. We also show that three-dimensional shape symmetries are related to broken symmetries of the myosin distribution in cortices. To test for the rotation mechanism, we suppress myosin clusters using laser ablation and generate new myosin clusters by optogenetics. Our work clarifies how polarity-oriented active mechanical forces drive collective cell motion in three dimensions.
The steroid hormone 20-hydroxy-ecdysone (20E) promotes proliferation in Drosophila wing precursors at low titer but triggers proliferation arrest at high doses. Remarkably, wing precursors proliferate normally in the complete absence of the 20E receptor, suggesting that low-level 20E promotes proliferation by overriding the default anti-proliferative activity of the receptor. By contrast, 20E needs its receptor to arrest proliferation. Dose-response RNA sequencing (RNA-seq) analysis of ex vivo cultured wing precursors identifies genes that are quantitatively activated by 20E across the physiological range, likely comprising positive modulators of proliferation and other genes that are only activated at high doses. We suggest that some of these "high-threshold" genes dominantly suppress the activity of the pro-proliferation genes. We then show mathematically and with synthetic reporters that combinations of basic regulatory elements can recapitulate the behavior of both types of target genes. Thus, a relatively simple genetic circuit can account for the bimodal activity of this hormone.
Segmenting cells within cellular aggregates in 3D is a growing challenge in cell biology, due to improvements in capacity and accuracy of microscopy techniques. Here we describe a pipeline to segment images of cell aggregates in 3D. The pipeline combines neural network segmentations with active meshes. We apply our segmentation method to cultured mouse mammary duct organoids imaged over 24 hours with oblique plane microscopy, a high-throughput light-sheet fluorescence microscopy technique. We show that our method can also be applied to images of mouse embryonic stem cells imaged with a spinning disc microscope. We segment individual cells based on nuclei and cell membrane fluorescent markers, and track cells over time. We describe metrics to quantify the quality of the automated segmentation. Our segmentation pipeline involves a Fiji plugin which implement active meshes deformation and allows a user to create training data, automatically obtain segmentation meshes from original image data or neural network prediction, and manually curate segmentation data to identify and correct mistakes. Our active meshes-based approach facilitates segmentation postprocessing, correction, and integration with neural network prediction.
Shape transformations of epithelial tissues in three dimensions, which are crucial for embryonic development or in vitro organoid growth, can result from active forces generated within the cytoskeleton of the epithelial cells. How the interplay of local differential tensions with tissue geometry and with external forces results in tissue-scale morphogenesis remains an open question. Here, we describe epithelial sheets as active viscoelastic surfaces and study their deformation under patterned internal tensions and bending moments. In addition to isotropic effects, we take into account nematic alignment in the plane of the tissue, which gives rise to shape-dependent, anisotropic active tensions and bending moments. We present phase diagrams of the mechanical equilibrium shapes of pre-patterned closed shells and explore their dynamical deformations. Our results show that a combination of nematic alignment and gradients in internal tensions and bending moments is sufficient to reproduce basic building blocks of epithelial morphogenesis, including fold formation, budding, neck formation, flattening, and tubulation.
Milinkovitch’s lab reveals that elephant skin cracks as a stiff epidermis bends over microscopic bumps, forming a water‑retaining network for cooling. Their study of the trunk’s simple deformation patterns is now informing flexible soft‑robotic grippers.
