Summary: A guide to mechanics in development: unlock how forces, material properties and geometry shape the forms of life.

In this review, Weichselberger, Moore et al. discuss how physics-based approaches illuminate organoid development and homeostasis by integrating mechanical, chemical, and informational processes. They...

A distinction that rewards a promising and already fruitful scientific career.

Biological tissues acquire reproducible shapes during development through dynamic cell behaviors. These events involve the remodeling of cell contacts driven by active cytoskeletal contractile forces. However how cell-cell contacts remodel remains poorly understood because of lack of tools to directly apply forces at cell-cell contacts to produce their remodeling. Here we develop a dual-optical trap manipulation method to impose different force patterns on cell-cell contacts in the early epithelium of the Drosophila embryo. Through different push and pull manipulations at the edges of junctions, the technique allows us to produce junction extension and junction shrinkage. We use these observations to constrain and specify vertex-based models of tissue mechanics, incorporating negative and positive mechanosensitive feedback depending on the type of remodeling. We show that Myosin-II activity responds to junction strain rate and facilitates full junction shrinkage. Altogether our work provides insight into how stress produces efficient deformation of cell-cell contacts in vivo and identifies unanticipated mechanosensitive features of their remodeling. Significance statement The highly organized tissues and organs that form our body emerge from internal dynamic activities at the cellular level. Among such activities, cell shape changes and cell rearrangement, cell extrusion and cell division sculpt epithelial tissues into elongated sheets, tubes and spherical cavities. Remodeling of cell-cell contacts, powered by actomyosin contractility, is key to all these transformations. Although much is known about the molecular machinery and biochemical signals that regulate remodeling of cell contacts, there is a lack of approaches to directly probe the mechanics of cell contacts and therefore assess their ability to resist or deform in response to mechanical loads. We developed an experimental technique to manipulate and exert contractile and extensile forces to cell-cell junctions. Our results lead to a specific physical model of junctional mechanics, with implications in the modeling of collective cell behavior in epithelial tissues. ### Competing Interest Statement The authors have declared no competing interest.

A cellular and biophysical study on embryonic stem cell aggregates reveals that the endoderm can form by a three-step mechanism involving a Wnt/beta-catenin-mediated epiblast fragmentation, tissue flow and cell segregation.

Roadmap for the multiscale coupling of biochemical and mechanical signals during development, Pierre-François Lenne, Edwin Munro, Idse Heemskerk, Aryeh Warmflash, Laura Bocanegra-Moreno, Kasumi Kishi, Anna Kicheva, Yuchen Long, Antoine Fruleux, Arezki Boudaoud, Timothy E Saunders, Paolo Caldarelli, Arthur Michaut, Jerome Gros, Yonit Maroudas-Sacks, Kinneret Keren, Edouard Hannezo, Zev J Gartner, Benjamin Stormo, Amy Gladfelter, Alan Rodrigues, Amy Shyer, Nicolas Minc, Jean-Léon Maître, Stefano Di Talia, Bassma Khamaisi, David Sprinzak, Sham Tlili

Cell-cell adhesion is more than the sum of its molecular parts. In this perspective, Lenne, Rupprecht, and Viasnoff aim at bridging scales from adhesion molecules and actomyosin networks to junctional integrity and mechanical resistance of tissues and organs.

Epithelial tissues acquire their integrity and function through the apico-basal polarization of their constituent cells. Proteins of the PAR and Crumbs complexes are pivotal to epithelial polarization, but the mechanistic understanding of polarization is challenging to reach, largely because numerous potential interactions between these proteins and others have been found, without clear hierarchy in importance. We identify the regionalized and segregated organization of members of the PAR and Crumbs complexes at epithelial apical junctions by imaging endogenous proteins using STED microscopy on Caco-2 cells, human and murine intestinal samples. Proteins organize in submicrometric clusters, with PAR3 overlapping with the tight junction (TJ) while PALS1-PATJ and aPKC-PAR6β form segregated clusters that are apical of the TJ and present in an alternated pattern related to actin organization. CRB3A is also apical of the TJ and weakly overlaps with other polarity proteins. This organization at the nanoscale level significantly simplifies our view on how polarity proteins could cooperate to drive and maintain cell polarity. ### Competing Interest Statement The authors have declared no competing interest.

Shaping the animal body plan is a complex process that involves the spatial organization and patterning of different layers of cells. Recent advances in live imaging have started to unravel the cellular choreography underlying this process in mammals, however, the sequence of events transforming an unpatterned group of cells into structured territories is largely unknown. Here, using 3D aggregates of mouse embryonic stem cells, we study the formation of one of the three germ layers, the endoderm. We show that the endoderm can be generated from an epiblast-like state by a three-tier mechanism: a release of islands of E-cadherin expressing cells, their flow toward the tip of the 3D aggregate, and their segregation. In contrast to the prevailing view, this mechanism does not require epithelial to mesenchymal transitions and vice-versa but rather a fragmentation, which is mediated by Wnt/β- catenin, and a sorting process. Our data emphasize the role of signaling and cell flows in the establishment of the body plan. ### Competing Interest Statement The authors have declared no competing interest.

La recherche scientifique a besoin d’un plan d’urgence

Tissue remodeling during embryogenesis is driven by the apical contractility of the epithelial cell cortex. This behavior arises notably from Rho1/Rok induced transient accumulation of non-muscle myosin II (MyoII pulses) pulling on actin filaments (F-Actin) of the medio-apical cortex. While recent studies begin to highlight the mechanisms governing the emergence of Rho1/Rok/MyoII pulsatility in different organisms, little is known about how the F-Actin organization influences this process. Focusing on Drosophila ectodermal cells during germband extension and amnioserosa cells during dorsal closure, we show that the medio-apical actomyosin cortex consists of two entangled F-Actin subpopulations. One exhibits pulsatile dynamics of actin polymerization in a Rho1 dependent manner. The other forms a persistent and homogeneous network independent of Rho1. We identify the Frl/Fmnl formin as a critical nucleator of the persistent network since modulating its level, in mutants or by overexpression, decreases or increases the network density. Absence of this network yields sparse connectivity affecting the homogeneous force transmission to the cell boundaries. This reduces the propagation range of contractile forces and results in tissue scale morphogenetic defects. Our work sheds new lights on how the F-Actin cortex offers multiple levels of regulation to affect epithelial cells dynamics.

Calcium signaling plays a crucial role in the physiology of the organs but also in various aspects of the organogenesis of the embryo. High versatility of calcium signaling is encoded by the dynamic variation of intracellular calcium concentration. While the dynamics of calcium is important, little is known about it throughout the embryogenesis of the largest class of animals, insects. Here, we visualize calcium dynamics throughout embryogenesis of Drosophila using a fluorescent protein-based calcium indicator, GCaMP3, and report calcium transients in epithelium and neuronal tissues. Local calcium transients of varying duration were detected in the outer epithelium, trachea and neural cells. In addition, gap-junction-dependent calcium waves were identified at stage 16 in the outer epithelium and in the trachea at stage 17. Calcium transient waveform analysis revealed different characteristics as a function of the duration, location and frequency. Detailed characterization of calcium transients during embryogenesis of Drosophila will help us better understand the role of calcium signaling in embryogenesis and organogenesis of insects.

Morphogenesis relies on the active generation of forces, and the transmission of these forces to surrounding cells and tissues. Hence measuring forces directly in developing embryos is an essential task to study the mechanics of development. Among the experimental techniques that have emerged to measure forces in epithelial tissues, force inference is particularly appealing. Indeed it only requires a snapshot of the tissue, as it relies on the topology and geometry of cell contacts, assuming that forces are balanced at each vertex. However, establishing force inference as a reliable technique requires thorough validation in multiple conditions. Here we performed systematic comparisons of force inference with laser ablation experiments in three distinct Drosophila epithelia. We show that force inference accurately predicts single junction tensions, tension patterns in stereotyped groups of cells, and tissue-scale stress patterns, in wild type and mutant conditions. We emphasize its ability to capture the distribution of forces at different scales from a single image, which gives it a critical advantage over perturbative techniques such as laser ablation. Our results demonstrate that force inference is a reliable and efficient method to quantify the mechanics of epithelial tissues during morphogenesis.

Clément et al. investigate the mechanics underlying the irreversibility of morphogenetic deformations. They show that deformations caused by contractile pulses of myosin II or by optical tweezers are stabilized by dissipation on the minute timescale. Inhibiting actin turnover reduces dissipation and yields more reversible deformations.

Tissue morphogenesis relies on the production of active cellular forces. Understanding how such forces are mechanically converted into cell shape changes is essential to our understanding of morphogenesis. Here we use Myosin II pulsatile activity during Drosophila embryogenesis to study how transient forces generate irreversible cell shape changes. Analyzing the dynamics of junction shortening and elongation resulting from Myosin II pulses, we find that long pulses yield less reversible deformations, typically a signature of dissipative mechanics. This is consistent with a simple viscoelastic description, which we use to model individual shortening and elongation events. The model predicts that dissipation typically occurs on the minute timescale, a timescale commensurate with that of force generation by Myosin II pulses. We test this estimate by applying time-controlled forces on junctions with optical tweezers. Our results argue that active junctional deformation is stabilized by dissipation. Hence, tissue morphogenesis requires coordination between force generation and dissipation.

Summary: This Primer summarizes the current range of invasive and non-invasive techniques to measure forces in cells and tissues, and discusses their applications in developmental contexts.

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