Convergence and extension movements elongate tissues during development. Drosophila germ-band extension (GBE) is one example, which requires active cell rearrangements driven by Myosin II planar polarisation. Here, we develop novel computational methods to analyse the spatiotemporal dynamics of Myosin II during GBE, at the scale of the tissue. We show that initial Myosin II bipolar cell polarization gives way to unipolar enrichment at parasegmental boundaries and two further boundaries within each parasegment, concomitant with a doubling of cell number as the tissue elongates. These boundaries are the primary sites of cell intercalation, behaving as mechanical barriers and providing a mechanism for how cells remain ordered during GBE. Enrichment at parasegment boundaries during GBE is independent of Wingless signaling, suggesting pair-rule gene control. Our results are consistent with recent work showing that a combinatorial code of Toll-like receptors downstream of pair-rule genes contributes to Myosin II polarization via local cell-cell interactions. We propose an updated cell-cell interaction model for Myosin II polarization that we tested in a vertex-based simulation.DOI: http://dx.doi.org/10.7554/eLife.12094.001

22The collective behaviour of cells in epithelial tissues is dependent on their 23 mechanical properties. However, the contribution of tissue mechanics to wound 24 90 cell-based models, such as vertex models 26,27 , have rarely been applied to replicate 91 in vivo wound healing dynamics. We model each cell in the tissue as a two-92 dimensional polygon carrying variable tension on their edges, with bulk elasticity and 93 peripheral contractility ( Supplementary Fig. 2a-b, methods). We parameterised the 94 model so that edges contacting the wound gradually increase in tension compared to 95 5 the surrounding tissue, to mimic the assembly of the contractile actomyosin purse 96 string, causing wound edge junctions to reduce in length. To capture experimentally 97 observed fluctuations in junctional and purse-string MyoII 28 , we introduced 98 fluctuations in line tension at cell-cell interfaces and in the purse-string 99 ( Supplementary Fig. 2d). Without introducing intercalation events into the model, 100 simulated wounds are unable to close (Figs. 2a, c, Supplementary Video 3). By 101 contrast, when intercalations are enabled in the model ( Supplementary Fig. 2c, see 102 methods), wounds are able to close (Figs. 2b, d, Supplementary Video 4), supporting 103 our hypothesis that intercalations at the wound edge are necessary to drive wound 104 closure. 105The vertex model predicts that in the absence of intercalation, cells around 106 the wound become more elongated towards the centre of the wound (Fig. 2e) than in 107 simulations with intercalations enabled (Fig. 2f). In both cases, the cells initially 108 elongate as the purse-string contracts the wound. As cells begin to intercalate away 109 from the wound edge their shapes relax, reducing the elongation over time. Towards 110 the end of wound closure, many intercalations occur (Fig. 2b), at which point the 111 elongation rapidly decreases, and the cells return to a fully relaxed state after healing 112 (Fig. 2f). With intercalations disabled, the cells remain highly elongated ( Fig. 2e). 113This led us to hypothesise that wound edge intercalations play a crucial role in 114 maintaining cell shape and tissue patterning. Indeed, wing disc cells appear regularly 115 packed immediately after wound closure (Fig. 3a) and the polygon distribution of 116 wound edge cells is restored upon healing (Fig. 3b). The seamless closure we 117 observe is distinct from a number of in vivo 22,29 and in vitro 5,30 systems that can form 118 visible scar-like rosette structures upon closure. To test our vertex model's prediction 119 that intercalation preserves cell shape, we quantified cell elongation in the first three 120 6 rows of cells away from the wound in wing discs ( Fig. 3c, Supplementary Video 5).

Emergent physical properties of tissues are not readily understood by reductionist studies of their constituent cells. Here, we show molecular signals controlling cellular, physical, and structural properties and collectively determine tissue mechanics of lymph nodes, an immunologically relevant adult tissue. Lymph nodes paradoxically maintain robust tissue architecture in homeostasis yet are continually poised for extensive expansion upon immune challenge. We find that in murine models of immune challenge, cytoskeletal mechanics of a cellular meshwork of fibroblasts determine tissue tension independently of extracellular matrix scaffolds. We determine that C-type lectin-like receptor 2 (CLEC-2)–podoplanin signaling regulates the cell surface mechanics of fibroblasts, providing a mechanically sensitive pathway to regulate lymph node remodeling. Perturbation of fibroblast mechanics through genetic deletion of podoplanin attenuates T cell activation. We find that increased tissue tension through the fibroblastic stromal meshwork is required to trigger the initiation of fibroblast proliferation and restore homeostatic cellular ratios and tissue structure through lymph node expansion.

Many epithelial developmental processes like cell migration and spreading, cell sorting, or T1 transitions can be described as planar deformations. As such, they can be studied using two-dimensional tools and vertex models that can properly predict collective dynamics. However, many other epithelial shape changes are characterized by out-of-plane mechanics and three-dimensional effects, such as bending, cell extrusion, delamination, or invagination. Furthermore, during planar cell dynamics or tissue repair in monolayers, spatial intercalation between the apical and basal sides has even been detected. Motivated by this lack of symmetry with respect to the midsurface, we here present a 3D hybrid model that allows us to model differential contractility at the apical, basal or lateral sides. We use the model to study the effects on wound closure of solely apical or lateral contractile contributions and show that an apical purse-string can be sufficient for full closure when it is accompanied by volume preservation.

The ability of cells to exchange neighbours, termed intercalation, is a key feature of epithelial tissues. Intercalation is predominantly associated with tissue deformations that drive morphogenesis. More recently, however, intercalation that is not associated with large-scale tissue deformations has been described both during animal development and in mature epithelial tissues. This latter form of intercalation appears to contribute to an emerging phenomenon that we refer to as tissue fluidity—the ability of cells to exchange neighbours without changing the overall dimensions of the tissue. Here, we discuss the contribution of junctional dynamics to intercalation governing both morphogenesis and tissue fluidity. In particular, we focus on the relative roles of junctional contractility and cell–cell adhesion as the driving forces behind intercalation. These two contributors to junctional mechanics can be used to simulate cellular intercalation in mechanical computational models, to test how junctional cell behaviours might regulate tissue fluidity and contribute to the maintenance of tissue integrity and the onset of disease. This article is part of the Theo Murphy meeting issue ‘Mechanics of development’.

Summary Here we present EpiGraph, an image analysis tool that quantifies epithelial organization. Our method combines computational geometry and graph theory to measure the degree of order of any packed tissue. EpiGraph goes beyond the traditional polygon distribution analysis, capturing other organizational traits that improve the characterization of epithelia. EpiGraph can objectively compare the rearrangements of epithelial cells during development and homeostasis to quantify how the global ensemble is affected. Importantly, it has been implemented in the open-access platform Fiji. This makes EpiGraph very user friendly, with no programming skills required. Availability and implementation EpiGraph is available at https://imagej.net/EpiGraph and the code is accessible (https://github.com/ComplexOrganizationOfLivingMatter/Epigraph) under GPLv3 license. Supplementary information Supplementary data are available at Bioinformatics online.

Wound healing is characterized by the re-epitheliation of a tissue through the activation of contractile forces concentrated mainly at the wound edge. While the formation of an actin purse string has been identified as one of the main mechanisms, far less is known about the effects of the viscoelastic properties of the surrounding cells, and the different contribution of the junctional and cytoplasmic contractilities. In this paper, we simulate the wound healing process, resorting to a hybrid vertex model that includes cell boundary and cytoplasmic contractilities explicitly, together with a differentiated viscoelastic rheology based on an adaptive rest-length. From experimental measurements of the recoil and closure phases of wounds in the Drosophila wing disc epithelium, we fit tissue viscoelastic properties. We then analyse in terms of closure rate and energy requirements the contributions of junctional and cytoplasmic contractilities. Our results suggest that reduction of junctional stiffness rather than cytoplasmic stiffness has a more pronounced effect on shortening closure times, and that intercalation rate has a minor effect on the stored energy, but contributes significantly to shortening the healing duration, mostly in the later stages.

16Epithelial tissues are inevitably damaged from time to time and must therefore 17 have robust repair mechanisms. The behaviour of tissues depends on their 18 mechanical properties and those of the surrounding environment 1 . However, it 19 remains poorly understood how tissue mechanics regulates wound healing, 20 particularly in in vivo animal tissues. Here we show that by tuning epithelial cell 21 junctional tension, we can alter the rate of wound healing. We observe cells moving past each other at the wound edge by intercalating, like molecules in a fluid, resulting 1 in seamless wound closure. Using a computational model, we counterintuitively 2