Visceral organ morphogenesis via calcium-patterned muscle constrictions

The sex of organ geometry

Organs have a distinctive yet often overlooked spatial arrangement in the body1-5. We propose that there is a logic to the shape of an organ and its proximity to its neighbours. Here, by using volumetric scans of many Drosophila melanogaster flies, we develop methods to quantify three-dimensional features of organ shape, position and interindividual variability. We find that both the shapes of organs and their relative arrangement are consistent yet differ between the sexes, and identify unexpected interorgan adjacencies and left-right organ asymmetries. Focusing on the intestine, which traverses the entire body, we investigate how sex differences in three-dimensional organ geometry arise. The configuration of the adult intestine is only partially determined by physical constraints imposed by adjacent organs; its sex-specific shape is actively maintained by mechanochemical crosstalk between gut muscles and vascular-like trachea. Indeed, sex-biased expression of a muscle-derived fibroblast growth factor-like ligand renders trachea sexually dimorphic. In turn, tracheal branches hold gut loops together into a male or female shape, with physiological consequences. Interorgan geometry represents a previously unrecognized level of biological complexity which might enable or confine communication across organs and could help explain sex or species differences in organ function.

Spiking at the edge: Excitability at interfaces in reaction-diffusion systems

Excitable media, ranging from bioelectric tissues and chemical oscillators to forest fires and competing populations, are nonlinear, spatially extended systems capable of spiking. Most investigations of excitable media consider situations where the amplifying and suppressing forces necessary for spiking coexist at every point in space. In this case, spikes arise due to local bistabilities, which require a fine-tuned ratio between local amplification and suppression strengths. But, in nature and engineered systems, these forces can be segregated in space, forming structures like interfaces and boundaries. Here, we show how boundaries can generate and protect spiking when the reacting components can spread out: Even arbitrarily weak diffusion can cause spiking at the edge between two non-excitable media. This edge spiking arises due to a global bistability, which can occur even if amplification and suppression strengths do not allow spiking when mixed. We analytically derive a spiking phase diagram that depends on two parameters: i) the ratio between the system size and the characteristic diffusive length-scale and ii) the ratio between the amplification and suppression strengths. Our analysis explains recent experimental observations of action potentials at the interface between two non-excitable bioelectric tissues. Beyond electrophysiology, we highlight how edge spiking emerges in predator-prey dynamics and in oscillating chemical reactions. Our findings provide a theoretical blueprint for a class of interfacial excitations in reaction-diffusion systems, with potential implications for spatially controlled chemical reactions, nonlinear waveguides and neuromorphic computation, as well as spiking instabilities, such as cardiac arrhythmias, that naturally occur in heterogeneous biological media.

Apical-basal polarity in the gut

Apical-Basal polarity is a fundamental property of all epithelial cells that underlies both their form and function. The gut is made up of a single layer of intestinal epithelial cells, with distinct apical, lateral and basal domains. Occluding junctions at the apical side of the lateral domains create a barrier between the gut lumen and the body, which is crucial for tissue homeostasis, protection against gastrointestinal pathogens and for the maintenance of the immune response. Apical-basal polarity in most epithelia is established by conserved polarity factors, but recent evidence suggests that the gut epithelium in at least some organisms polarises by novel mechanisms. In this review, we discuss the recent advances in understanding polarity factors by focussing on work in C. elegans, Drosophila, Zebrafish and Mouse.

TubULAR: tracking in toto deformations of dynamic tissues via constrained maps

A common motif in biology is the arrangement of cells into tubes, which further transform into complex shapes. Traditionally, analysis of dynamic tissues has relied on inspecting static snapshots, live imaging of cross-sections or tracking isolated cells in three dimensions. However, capturing the interplay between in-plane and out-of-plane behaviors requires following the full surface as it deforms and integrating cell-scale motions into collective, tissue-scale deformations. Here, we present an analysis framework that builds in toto maps of tissue deformations by following tissue parcels in a static material frame of reference. Our approach then relates in-plane and out-of-plane behaviors and decomposes complex deformation maps into elementary contributions. The tube-like surface Lagrangian analysis resource (TubULAR) provides an open-source implementation accessible either as a standalone toolkit or as an extension of the ImSAnE package used in the developmental biology community. We demonstrate our approach by analyzing shape change in the embryonic Drosophila midgut and beating zebrafish heart. The method naturally generalizes to in vitro and synthetic systems and provides ready access to the mechanical mechanisms relating genetic patterning to organ shape change.

Cell contractility in early animal evolution

Tissue deformation mediated by collective cell contractility is a signature characteristic of animals. In most animals, fast and reversible contractions of muscle cells mediate behavior, while slow and irreversible contractions of epithelial or mesenchymal cells play a key role in morphogenesis. Animal tissue contractility relies on the activity of the actin/myosin II complex (together referred to as 'actomyosin'), an ancient and versatile molecular machinery that performs a broad range of functions in development and physiology. This review synthesizes emerging insights from morphological and molecular studies into the evolutionary history of animal contractile tissue. The most ancient functions of actomyosin are cell crawling and cytokinesis, which are found in a wide variety of unicellular eukaryotes and in individual metazoan cells. Another contractile functional module, apical constriction, is universal in metazoans and shared with choanoflagellates, their closest known living relatives. The evolution of animal contractile tissue involved two key innovations: firstly, the ability to coordinate and integrate actomyosin assembly across multiple cells, notably to generate supracellular cables, which ensure tissue integrity but also allow coordinated morphogenesis and movements at the organism scale; and secondly, the evolution of dedicated contractile cell types for adult movement, belonging to two broad categories respectively defined by the expression of the fast (striated-type) and slow (smooth/non-muscle-type) myosin II paralogs. Both contractile cell types ancestrally resembled generic contractile epithelial or mesenchymal cells and might have played a versatile role in both behavior and morphogenesis. Modern animal contractile cells span a continuum between unspecialized contractile epithelia (which underlie behavior in modern placozoans), epithelia with supracellular actomyosin cables (found in modern sponges), epitheliomuscular tissues (with a concentration of actomyosin cables in basal processes, for example in sea anemones), and specialized muscle tissue that has lost most or all epithelial properties (as in ctenophores, jellyfish and bilaterians). Recent studies in a broad range of metazoans have begun to reveal the molecular basis of these transitions, powered by the elaboration of the contractile apparatus and the evolution of 'core regulatory complexes' of transcription factors specifying contractile cell identity.

From cells to form: A roadmap to study shape emergence in vivo

Organogenesis arises from the collective arrangement of cells into progressively 3D-shaped tissue. The acquisition of a correctly shaped organ is then the result of a complex interplay between molecular cues, responsible for differentiation and patterning, and the mechanical properties of the system, which generate the necessary forces that drive correct shape emergence. Nowadays, technological advances in the fields of microscopy, molecular biology, and computer science are making it possible to see and record such complex interactions in incredible, unforeseen detail within the global context of the developing embryo. A quantitative and interdisciplinary perspective of developmental biology becomes then necessary for a comprehensive understanding of morphogenesis. Here, we provide a roadmap to quantify the events that lead to morphogenesis from imaging to image analysis, quantification, and modeling, focusing on the discrete cellular and tissue shape changes, as well as their mechanical properties.

Spatio-temporal patterning of extensile active stresses in microtubule-based active fluids

Microtubule-based active fluids exhibit turbulent-like autonomous flows, which are driven by the molecular motor powered motion of filamentous constituents. Controlling active stresses in space and time is an essential prerequisite for controlling the intrinsically chaotic dynamics of extensile active fluids. We design single-headed kinesin molecular motors that exhibit optically enhanced clustering and thus enable precise and repeatable spatial and temporal control of extensile active stresses. Such motors enable rapid, reversible switching between flowing and quiescent states. In turn, spatio-temporal patterning of the active stress controls the evolution of the ubiquitous bend instability of extensile active fluids and determines its critical length dependence. Combining optically controlled clusters with conventional kinesin motors enables one-time switching from contractile to extensile active stresses. These results open a path towards real-time control of the autonomous flows generated by active fluids.

Patterning of morphogenetic anisotropy fields

Orientational order, encoded in anisotropic fields, plays an important role during the development of an organism. A striking example of this is the freshwater polyp Hydra, where topological defects in the muscle fiber orientation have been shown to localize to key features of the body plan. This body plan is organized by morphogen concentration gradients, raising the question how muscle fiber orientation, morphogen gradients and body shape interact. Here, we introduce a minimal model that couples nematic orientational order to the gradient of a morphogen field. We show that on a planar surface, alignment to a radial concentration gradient can induce unbinding of topological defects, as observed during budding and tentacle formation in Hydra, and stabilize aster/vortex-like defects, as observed at a Hydra's mouth. On curved surfaces mimicking the morphologies of Hydra in various stages of development-from spheroid to adult-our model reproduces the experimentally observed reorganization of orientational order. Our results suggest how gradient alignment and curvature effects may work together to control orientational order during development and lay the foundations for future modeling efforts that will include the tissue mechanics that drive shape deformations.

Morphogenetic Roles of Hydrostatic Pressure in Animal Development

During organismal development, organs and systems are built following a genetic blueprint that produces structures capable of performing specific physiological functions. Interestingly, we have learned that the physiological activities of developing tissues also contribute to their own morphogenesis. Specifically, physiological activities such as fluid secretion and cell contractility generate hydrostatic pressure that can act as a morphogenetic force. Here, we first review the role of hydrostatic pressure in tube formation during animal development and discuss mathematical models of lumen formation. We then illustrate specific roles of the notochord as a hydrostatic scaffold in anterior-posterior axis development in chordates. Finally, we cover some examples of how fluid flows influence morphogenetic processes in other developmental contexts. Understanding how fluid forces act during development will be key for uncovering the self-organizing principles that control morphogenesis.

Controlling the shape and topology of two-component colloidal membranes

Changes in the geometry and topology of self-assembled membranes underlie diverse processes across cellular biology and engineering. Similar to lipid bilayers, monolayer colloidal membranes have in-plane fluid-like dynamics and out-of-plane bending elasticity. Their open edges and micrometer-length scale provide a tractable system to study the equilibrium energetics and dynamic pathways of membrane assembly and reconfiguration. Here, we find that doping colloidal membranes with short miscible rods transforms disk-shaped membranes into saddle-shaped surfaces with complex edge structures. The saddle-shaped membranes are well approximated by Enneper's minimal surfaces. Theoretical modeling demonstrates that their formation is driven by increasing the positive Gaussian modulus, which in turn, is controlled by the fraction of short rods. Further coalescence of saddle-shaped surfaces leads to diverse topologically distinct structures, including shapes similar to catenoids, trinoids, four-noids, and higher-order structures. At long timescales, we observe the formation of a system-spanning, sponge-like phase. The unique features of colloidal membranes reveal the topological transformations that accompany coalescence pathways in real time. We enhance the functionality of these membranes by making their shape responsive to external stimuli. Our results demonstrate a pathway toward control of thin elastic sheets' shape and topology-a pathway driven by the emergent elasticity induced by compositional heterogeneity.

A squash and a squeeze

Advanced imaging techniques reveal details of the interactions between the two layers of the embryonic midgut that influence its ultimate shape.