Biomechanics of cell rearrangements in Drosophila

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ScienceDirect Biomechanics of cell rearrangements in Drosophila Boris Guirao1,2 and Yohanns Bellaı¨che1,2 To acquire their adequate size and shape, living tissues grow and substantially deform as they develop. To do so, the cells making up the tissue can grow and deform as well, but they can also divide, intercalate and die. Among those cell behaviors, cell intercalation, also named cell rearrangement, is a major contributor to the morphogenesis of many cohesive tissues since it enables tissues to drastically deform as they develop while keeping their cohesiveness and avoiding extreme deformation of their cells. Here we review the mechanical principles and biological regulations at play during cell rearrangements in Drosophila tissues by first describing them in other cellular materials and by categorizing them. We then briefly discuss their quantifications and their interplay with other cell processes.

Addresses 1 Institut Curie, PSL Research University, CNRS UMR 3215, INSERM U934, F-75248 Paris Cedex 05, France 2 Sorbonne Universite´s, UPMC Univ Paris 06, CNRS, CNRS UMR 3215, INSERM U934, F-75005, France Corresponding authors: Guirao, Boris ([email protected]), Bellaı¨che, Yohanns ([email protected])

Current Opinion in Cell Biology 2017, 48:113–124 This review comes from a themed issue on Cell dynamics Edited by Eugenia Piddini and Helen McNeill

leading to the rearrangements of more than four cells [6,7]. Here, we first briefly describe the fundamental mechanical principles underlying cell rearrangement in passive cellular materials such as foams, and in active materials like cell aggregates to put developing tissues in perspective. We then review how the process of junction shortening and elongation is regulated by mechanical forces and signaling in Drosophila epithelial tissues. Then, we present some of the methods used to quantify rearrangements, and extend our discussion to the interplays between cell rearrangements, cell divisions, apoptoses, and cell shape changes. Better understanding these interplays is critical to capture the dynamics of epithelial tissues during development, homeostasis and repair.

Mechanics of cell rearrangement in passive and active cellular materials Here we focus on a common type of in-plane cell rearrangement that only involves four cells (Figure 1a,b). Rearrangements are not specific to living tissues: they also occur in other cellular material such as dry foams [8,9], cell aggregates [10–12], cell cultures [13], but not in plants [14]. Even though the mechanisms of rearrangements may differ according to the type of material, it is instructive to review those mechanisms in foams and cell aggregates before addressing the more complex case of developing tissues, as many similarities exist between them [15–17].

http://dx.doi.org/10.1016/j.ceb.2017.06.004 0955-0674/ã 2017 Elsevier Ltd. All rights reserved.

Introduction A living tissue grows and substantially changes its shape as it develops. To give the tissue its adequate size and shape, a basic requirement for its proper functioning, the cells can themselves grow and deform, but they can also divide, intercalate and die. Among those four elementary cell behaviors, cell intercalation, also named cell rearrangement or T1 transition, is a major contributor to the morphogenesis of many tissues [1–5]. A basic in-plane cell rearrangement is a dynamic process that involves four cells: two neighboring cells sharing a junction that shortens until it disappears to be replaced by a new junction between the other two cells that appears and lengthens (Figure 1a,b). More complex types of in-plane rearrangements can take place, for instance involving ‘rosettes’ www.sciencedirect.com

A dry foam is a passive cellular material made of air bubbles separated by a thin soapy water film: its motion and deformation can only be powered by external forces. Notably, in a foam all interfaces have same surface tension, leading to angles of 120
at each tri-cellular interface (vertex) at mechanical equilibrium (Plateau’s law) (Figure 1d). The mechanics of individual rearrangement is well understood in foams [9,18,19]. Energetically, a rearrangement corresponds to the transition from an energy minimum to another one, passing by a maximum at the fourfold vertex state, which is unstable and therefore transient (Figure 1c). To overcome this energy barrier, the cells in the foam need to get deformed enough (beyond the ‘yield strain’ GY, typically around 20%) by anisotropic external forces that must be large enough (above the ‘yield stress’ s y) (Figure 1d). Thus, below the yield strain/stress, the foam behaves elastically, recovering its initial shape when relaxed, but beyond this threshold, the foam exhibits plastic behavior characterized by many rearrangements leading to a permanent material deformation [9]. The dynamics of the new Current Opinion in Cell Biology 2017, 48:113–124

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Cell rearrangements at cell scale: ‘stress/strain relaxing’ versus ‘stress/strain building’ rearrangements. (a) In-plane rearrangement occurring in Drosophila dorsal thorax: an AJ (cyan) shortens, leading to a transient fourfold vertex followed by a new AJ that forms (magenta) and lengthens. AJ are labelled with E-CadGFP; one image every 30 min starting at 11:30 hour after pupa formation (hAPF). (b) Sketch of three configurations corresponding to extrema of energy in (c). (c) Sketch of energy landscape commonly associated with the configurations in (b) [106]: an energy barrier of DE separates configurations 1 and 3 that correspond to two energy minima, while configuration 2 (fourfold vertex) corresponds to an energy maximum. For inert cellular materials like foams, configurations 1 and 3 lay at the bottoms of each energy well, respectively. For active materials like cell aggregates, junction tensions and lengths fluctuate due to cortex activity. These fluctuations set an energy scale (Efluct) that determines how far cell configurations can deviate from their equilibrium configurations 1 and 3 by exploring the energy wells. If Efluct > DE, fluctuations can trigger spontaneous rearrangements leading from 1 to 3. In tissues, it is unclear whether such energy landscape is relevant as biological regulations seem to redefine it constantly. (d-d00 ) Sketches of rearrangements that relax tissue stress (s) and cell elongation after an

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Biomechanics of cell rearrangements Guirao and Bellaı¨che 115

junction elongation is entirely controlled by the viscoelastic properties of the interface and the characteristic relaxation time is of the order of seconds [18,19]. In contrast to foams, cell aggregates are made of living cells, and are therefore active cellular material in the sense that they are out of equilibrium since energy is constantly produced and consumed to achieve mechanical work [17,20,21,22]. Cells can adhere via the interfaces or cell–cell junctions involving in particular cadherin–catenin adhesion complexes. Their cortex, that contains acto-myosin filaments, has contractile properties, and forces are exerted at interfaces. The amount of myosin at a membrane interface controls its contractility and tends to reduce the interface area, whereas the amount of adhesion molecules such as cadherin determines its adhesion and tends to increase the interface area. The balance of those two antagonist contributions results in the interface surface tension, which influences the interface size [23–26]. In contrast to foams, cell generally display different levels of cadherin and myosin resulting in varying tension at interfaces, and angles at tricellular junctions can substantially deviate from 120
(Figure 1d0 ). An important consequence of this activity is that levels of acto-myosin and cadherin fluctuate, resulting in fluctuating interface tension and size [10]. Those fluctuations set an energy scale allowing cells to explore the energy landscape in the vicinity of their equilibrium configuration (Figure 1c) and enable them to overcome some energy barriers, thereby leading to some spontaneous rearrangements (Figure 1d0 ). Therefore, and unlike foams, aggregates can undergo oriented rearrangements in the presence of an oriented external load generating stress in the aggregate even if this stress is smaller than their yield stress. The cadherin–catenin mediated adhesion between cells being much stronger than the one in foams, the characteristic time of rearrangement is much larger, being of the order of hours, and seems to depends on interfacial tensions that tend to increase energy barriers (Figure 1c) [10,16,17]. This characteristic time is well illustrated by the time required for two aggregates to fuse, which takes about 6 h [10].

Foams and cell aggregates have been important to better understand the general principles of material plasticity and mechanics of rearrangements in cellular materials simpler than living tissues. In addition, they provide a reference to which living tissues behaviors can be compared. In particular, the dynamics of junction shortening and lengthening is completely determined by the external strain applied to the material and by the mechanical properties of the interfaces (tension and viscoelasticity) of the material. As for developing tissues, they are among the active cellular materials displaying the most complex behaviors. Indeed, if they share many common obvious features with cell aggregates, cells in developing tissues will morph the tissue into its proper size and shape so it can properly function. To do so, tissues and cells deploys an arsenal of biological regulations such as signal transduction and mechanotransduction pathways. In particular, in epithelial tissues, cells are polarized along their apico-basal axis, but can also be polarized in the plane of the epithelium by the planar cell polarity pathway (PCP) that recruit proteins along specific junctions to define a preferred direction [27,28]. Thus, by recruiting molecular motors such as myosins at specific junctions, developing tissues are able to selectively increase the contractility at those junction [3,4] and generate active oriented rearrangements that can enable local regions of the tissue to autonomously elongate the tissue (Figure 1e) and/or to generate cell elongation and stress (Figure 1e’). An interesting signature of this behavior can be found in the first phase of the pupal wing development when cell deformation exceeds tissue deformation [29,30]. Therefore, as opposed to foam and aggregates where only ‘stress/strain relaxing’ rearrangements can occur (Figure 1d,d’), tissues, are in addition able to autonomously drive ‘stress/strain building’ rearrangements thanks to biological regulations (Figure 1e,e’). Naturally, cells in tissues can also undergo important stretch exerted by neighboring parts of the tissue, and if the nature of their response can vary, both junction shortening and junction lengthening seem to involve biological regulation (Figure 1d”). In Drosophila

(Figure 1 Legend Continued) external tissue elongation in a foam, in an aggregate and in a tissue. (d) In a foam, a first material stretch leaving the foam below its yield stress s y, elongates cells leading to elastic deformation of the foam. An additional material stretch now taking s beyond the yield stress creates many rearrangements in the direction of the stretch, thereby leading to plastic behavior and partial relaxation of cell elongated shapes and of the material stress back to s = s y in about a second. (d0 ) In a cell aggregate, cells can stretch substantially (up to aspect-ratios of 1/6). Even at s < s y and thanks to fluctuations driven by cortex activity, the stretch has lowered some energy barriers, and rearrangements can already occur and completely relax the material stress over several hours. (d00 ) In a tissue, the responses to an external stretch are varied and remain unclear. Cells can deform substantially and relax spontaneously their elongated shape within minutes, like in Drosophila embryos [31], but unlike in aggregates, cells can also keep their elongated shape for hours without rearranging [32–34,107] or starting to rearrange only after a 4 hour delay (in orange), then relaxing stress in about 2 hour [30], with a junction lengthening phase involving biological regulation (in purple) [35,68,69,70]. During the process, junction lengths can fluctuate. (e-e0 ) Rearrangements generating stress or strain in two limit scenarios, both requiring biological regulations to actively shorten the junction (red) and actively lengthen the new one (magenta), with fluctuations in junction lengths (arrows). (e) With free boundary conditions and low friction, the cells can rearrange freely while keeping their shape, thereby generating tissue contraction-elongation. (e0 ) Conversely, with fixed boundary conditions or high friction, the cell can rearrange but they are constrained in space and have to elongate orthogonally to the rearrangement in order to fit in, thereby generating tissue stress. In general cells in tissues will combine all three scenarios (d00 ,e,e0 ), namely undergoing deformation due to external anisotropic stress while actively rearranging with spatial constraints due to their neighbors. www.sciencedirect.com

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embryos, cells can deform substantially and relax their elongated shape and the associated stress within about 10 min (for example see, [31]). However, there are several evidence of tissues with cells remaining stretched for hours without relaxing via rearrangements like in suspended stretched monolayers of MDCK cells (observed over 3 h) [32], or in the wing pouch with cells stretched at the periphery of overgrowing clones (observed over 5 hours) [33,34]: almost no rearrangement occur, suggesting that additional signal or regulation exist to allow for relaxation of their shape. In the Drosophila pupal wing, as part of its developmental program, the hinge contraction drives the elongation of cells in the blade. Here as well, a substantial delay (about 4 hours) has been proposed before even the onset of rearrangements, that are then characterized by a relaxation timescale of about 2 hours [30], and a new junction lengthening phase that is also under biological regulation, as first shown in [35]. Indeed, this last work shows that, without the proper biological regulations, junctions can fluctuate around their fourfold vertex state, a state that is normally unstable according to the classic view of energy landscape of cell configurations (Figure 1c). Together with the examples of rearrangements generating stress/strain in the tissue, this last example illustrates well that it is unclear whether such energy landscapes are of any relevance for tissues since biological regulations seem to be constantly redrawing it completely. In what follows we review what has been recently discovered about the regulations of junction shortening and lengthening in several Drosophila epithelial tissues.

Regulation of junction shortening in epithelial tissues

In Drosophila, the mechanisms of cell rearrangements have been extensively studied during Drosophila germband elongation, which promotes embryo anterior-posterior (AP) axis elongation during gastrulation. Numerous recent elegant studies by several groups have led to an integrated model where both global tissue scale tensile stress and local regulation of cell contractility contributes to axis elongation via cell rearrangements (Figure 2). Combining global embryo imaging, laser ablation and simulations, recent findings have confirmed the proposed roles of external tensile forces in axis elongation [31,36]. Tensile stress mainly generated by the invagination of the endoderm elongates cells in the AP direction and also shorten dorso-ventral (DV) cell adherens junctions (AJs). To shorten AJs, such external tensile stress act in parallel with more local forces due to non-muscle Myosin II (MyoII) polarization (Figure 2a). Accordingly, several studies show that planar cell polarization of MyoII along the DV junctions, lead to their shortening to induce cell rearrangements [37,38]. Importantly, polarized MyoII distribution and junction shortening was also shown to power convergence-extension in gastrulation and Current Opinion in Cell Biology 2017, 48:113–124

neurulation [7,39–41]. Recent advances have further deciphered the mechanisms controlling the acto-myosin dynamics and the polarization of MyoII during cell rearrangements. Pulsatile acto-myosin flows promote the junction shortening as they flow towards the junction, while MyoII enrichment at the junction is proposed to be important for stabilization of the shortened junction [42], in a manner similar to the mechanisms initially described for apical constriction [43,44]. While the Frizzled PCP is not involved in MyoII polarization [45], Toll receptors and G-protein coupled receptors were recently shown to contribute to MyoII polarization and germband elongation [46,47]. Toll receptor family is involved in patterning, innate immunity, cell completion and morphogenesis [48]. In the Drosophila embryo, Toll-2, Toll-6 and Toll-8 are expressed in overlapping transversal stripes along the AP axis. They act redundantly to polarize MyoII at the DV junction and to promote cell junction shortening and tissue elongation. Importantly, Toll receptors can hetero-dimerized and the mis-expression of Toll receptor is sufficient to promote MyoII polarization. Together these important findings led to the proposition of an heterophilic Toll-code ensuring planar polarization of MyoII by a yet uncovered signal transduction pathway [46]. An in-depth analysis of MyoII polarization dynamics at the scale of the embryos has further validated the heterophilic Toll-code model based on Toll receptors and illustrated how such code can be robust to variation in cell numbers within each embryo segment [49]. Acting in parallel or downstream of Toll receptors, G coupled protein receptors (GPCRs) were also shown to be important to promote MyoII polarization [47]. The Fog ligand initially identified for its role in mesoderm invagination [50] activates the Smog and an unknown GPCR in the lateral epithelium. In turn these GPCRs promote MyoII apical-medial recruitment via the heterotrimeric Ga12/13 protein, which acts upstream of the Rho1 GTPase exchange factor, RhoGEF2. While Toll receptors and GPCRs are likely the most upstream regulators of MyoII driven shortening, several additional actors including the cytoskeleton regulator Shroom [51], the Abelson kinase [52] as well as the polarity protein Bazooka (Drosophila Par-3) [53], Canoe [54] and the adhesion molecule E-Cad [42,55] were shown to be involved in junction shortening. Lastly, the MyoII dependent contractility along DV junction promotes additional recruitment of MyoII along the DV axis, thereby reinforcing its planar polarity and promoting the formation of rosette [56]. A central theme in MyoII-dependent force production is the existence of MyoII pulsatile and flow behavior that contribute to ratchet-like mechanisms promoting and stabilizing cell deformation [57,58]. While the pulsatile and flow behavior are in part regulated by Rho GEF and GAP in the C. elegans zygote and in the Drosophila mesoderm [59,60], a self-organized biomechanical feedback has been www.sciencedirect.com

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Biological regulations of junction shortening and elongation during cell rearrangement. Known regulators controlling junction shortening (a) and elongation (b) in germband and pupal wing where both global external force and local forces (here shown as two distinct mechanisms for clarity) act in parallel to promote cell rearrangements. See text for additional details on their roles and mechanisms of action.

involved during cell shape change in the Drosophila germband. This feedback entails on the one hand the advection of the active form of the GTPase Rho1, the Rho kinase and MyoII (positive feedbacks), to generate a convergent accumulation of actomyosin, and on the other hand the dissociation of the MyoII minifilament associated with the local increase of density of the actomyosin network and the Myosin phosphatase activity (negative feedbacks) that reduce the advection and promote actomyosin disassembly [61]. The accumulation of MyoII at AJ has been so far the main focus of investigation in junction shortening. A recent report now demonstrates the active contribution of the basal domain of epithelial cells during rearrangements and during the formation of rosettes, namely rearrangements involving more than four cells [62]. It was observed that the basal sides of two of the rearranging cells migrate towards each other. Importantly the migration takes place prior the formation of the apical rosette. This migration is regulated by the Scr-42 kinase which activates the small GTPase Rac and increases F-Actin level in the basal protrusion. Reduced Scr-42 activity decreases germband elongation, highlighting the www.sciencedirect.com

importance of such basal processes. Together with previous results on the role of basal protrusions in the regulation of the convergence extension of the mouse neural plate [63], these findings illustrate that in epithelial tissues the basal side of the cells have more prominent roles than initially thought in the regulation of cell rearrangements. MyoII is not the only Myosin shown to contribute to cell rearrangements by increasing junction tension. The opposing gradients of the proto-cadherin Dachsous and the Golgi Kinase Four-jointed induced the planar polarization of the Dachs myosin, which binds to Dachsous [64]. Dachsous was shown to regulate cell rearrangements in the pupal wing in response to hinge contraction [65]. Furthermore, Dachs polarity is sufficient to increase cell junction tension leading to cell rearrangements in the dorsal thorax [66]. The mechanisms by which Dachs increases tension within tissue might be distinct from the one proposed for MyoII, since in vitro recombinant Dachs does not bind F-Actin in an ATPdependent manner, but rather modulates F-Actin organization by promoting the binding of Zyxin to F-Actin [67]. Current Opinion in Cell Biology 2017, 48:113–124

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Positive and negative regulations of junction lengthening The formation of a new AJ upon cell rearrangement and the regulation of its lengthening is critical since it participates in tissue dynamics and defines the final architecture of the tissue (Figure 2b). Negative and positive regulators of junction elongation have been identified. During pupal wing hexagonal packing, the formation of new short junction is associated with the recruitment of MyoII at the fourfold vertex, including at the newly formed junction about to lengthen [35]. The elongation of the junction correlates with a decrease of MyoII concentration on the elongation junction. Importantly loss of the tumor suppressor PTEN prevents MyoII level decrease and induce cell rearrangements fluctuating around the fourfold vertex state and junction oscillating between shortening and elongating [35]. In the germband embryo, the invagination of the posterior midgut was shown to provide a global mechanical force essential for junction elongation [68]. In addition to this longrange signal, a more local and integrated dynamics of the cells sharing the newly formed junctions was shown to be involved in junction lengthening [68,69]. Indeed, the pulsatile contractions of the cells anterior and posterior to the newly formed junction promotes vertex resolution and junction elongation, while dorsal and ventral cells contraction are important to maintain junction elongation [69]. Moreover, during junction elongation MyoII was shown to accumulate in the neighbor cells and laser ablation near the cell vertex where MyoII accumulates abrogates junction elongation, corroborating a role of MyoII in the neighbors for junction elongation [68]. A similar mechanism was shown to account for the elongation of the cell junction in the aminoserosa during dorsal closure [70]. These studies show that while the mechanisms of junction shortening are mainly driven by increased contractility at the cell junction and external force, distinct mechanisms exist to control the length and the dynamics of the elongating junctions.

Quantification of cell rearrangements While we have learned a lot about the local regulation of junction shortening and elongation in developing tissues, analyzing the role of cell rearrangements from the cell level to the tissue level is critical to better understand the mechanics and rheology of tissues [10,71,72,73]. This requires the development of methods to link their occurrence and the overall tissue deformation. In the past few years, the live imaging of biological systems made substantial progress making it now possible to image entire developing tissues over several hours of development while keeping a cellular resolution with time resolution of minutes [74]. Much progress has also been made regarding the processing of those images in term of projection, signal improvement, digitalization and tracking of cell membranes, making it know possible to rebuild cell contours, even in 3D, and follow them over time [75–83]. With those Current Opinion in Cell Biology 2017, 48:113–124

new powerful methods, it is now possible to establish cell neighbor relationships in time and determine which junction has disappeared or appeared, as well as its evolution, thereby allowing a full characterization of rearrangement dynamics at cell and tissue levels. There are several ways of quantifying rearrangements, and the choice made must depend on the kind of questions asked [23,29,30,35,36,65,66,68,84,85,86]. One simple way is to count the number of occurrences within a given time range [23,35,65,68,86], which can be for instance relevant to study how much a newly formed junction fluctuates around its fourfold vertex state [35] or to determine the fraction of junctions involved in a rearrangement [68]. However, this does not permit to assess the contribution of rearrangements to tissue morphogenesis as: (i) it leaves aside the intrinsic polarity of the rearrangement process when not associated to the junction angle; and (ii) if not filtered with lengthening thresholds, flickering junctions will add up in number although they do not contribute to tissue deformation as opposed to those leading to a new junction actually lengthening. To quantify the contribution of cell rearrangements over time (but also division, death, size and shape changes, tissue deformation, PCP, junctional stress . . . ) during morphogenesis, a more appropriate approach consists of using tensors that can carry information relative to amplitude, anisotropy and direction, and that are not impacted by fluctuating rearrangements which do not participate in tissue deformation. Such methods using tensors to those ends were initially developed to characterize deformations and rearrangements in foam flows [8,87,88], and have recently started to be used in developmental biology to characterize tissue morphogenesis [29,30,35,66, 85,89]. The main objective of those methods is to provide a balance equation that express the tissue deformation tensor as a sum of tensors quantifying the respective contributions of cell behaviors, which allows the study of their time evolution (Figure 3a,b) [29,30,85]. In [85], the authors estimate tissue deformation from the gradient of cell centroid movements, and they independently estimate cell shape changes by fitting cell outlines. From these two measurements, they infer by subtraction the combined contribution of the remaining behaviors that involve topological changes in the tissue, namely rearrangements, division and death, the latter two being negligible in their study. Alternatively, when all cells are segmented and tracked over time (Figure 3c,d), the links connecting neighbor cell centroids (Figure 3e-e0 ) can be used to independently measure the respective contributions of each cell behaviors to tissue deformation (Figure 3f), including rearrangements (Figure 3g) or cell size and shape changes (Figure 3h), and to plot them over time (Figure 3i) [29,30]. The role of those novel methods will expand in the future as numerous recent www.sciencedirect.com

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findings have discovered interplays between cell rearrangements and other cell behaviors during development, as detailed below.

Interplay between cell rearrangements and others cell behaviors Recent reports illustrate the existence of interplays between cell rearrangements and division or apoptosis. Reduced of Myc function induces the formation of loser cells that are eliminated via apoptosis [90–92]. Apoptosis is preceded by the intermingling of looser cells with surrounding wild-type winner cells. The intermingling is mediated by cell rearrangements that arise from the difference in cell junction tension due to change in actin dynamics [93]. In contrast, increased Myc activity prevents cell mixing by promoting clone rounding by regulating cell junction tension [94]. Very interestingly the Toll receptor was shown to be involved in cell competition, suggesting a possible general role in the regulation of cell rearrangements coupled to tissue morphogenesis or tissue fitness [95]. Cell division is also associated with the remodeling of the cell contacts and several interplays between cell rearrangements and cell division have been described. In many cases, the two daughter cells remain in close contact, a process driven by MyoII activity in the neighboring in response to the pulling force exerted by the dividing cell [96,97,98,99,100]. In the context of highly dynamics tissues, the daughter cells get separated by the neighboring cells, thereby generating additional cell rearrangements during cell division [99,101,102]. Such cell rearrangements induced by cell divisions contribute to the overall tissue movements or its final architecture [99,101,102]. Finally, tissue-scale MyoII cable

located at anterior-posterior compartment boundary were shown to prevent cell mixing due to cell divisions or cell rearrangements to maintain compartment boundaries [84,103,104]. The use of the advanced quantitative methods described above has also enabled to highlight the interplay between tissue deformation, cell divisions, cell shape changes and cell rearrangements. Thus, suppressing divisions in the Drosophila dorsal thorax was shown to impact tissue morphogenesis, as well as cell shape changes and cell rearrangements [29]. Also, disrupting wing attachment proximally or distally was shown to directly impact tissue deformation and all cell behaviors unequally [30]. In particular in the dorsal thorax, cell divisions facilitate tissue elongation either by directly promoting tissue elongation, if the division are oriented in the direction of tissue elongation or by promoting additional cell rearrangement if division are oriented orthogonally to the direction of tissue elongation [29]. These experimental observations agree with theoretical models suggesting that the ability of tissues to remodel via rearrangements, division or extrusion provide them with fluid properties and decrease their effective viscosities [10,72,73,105]. Nevertheless, and unlike aggregates, biological regulations seem to control most of those topological events in developing tissues by driving them, by permitting them, or in contrast by inhibiting them.

Future directions Outstanding progresses has been made in recent years on the biomechanics of cell rearrangements in Drosophila and several studies have demonstrated the conservation

(Figure 3 Legend) Cell rearrangements at tissue scale: interplay with tissue deformation and cell shape changes. (a) and (b) To better grasp the interplay between tissue deformation, cell rearrangements and cell shape changes in different types of cellular materials, but also to get used to the new coarse-graining methods [29,30,73,85], we first consider simple materials for which the deformation (G) only occurs through cell shape changes (S) and rearrangements strain (R), namely where G = R + S. For each quantity, we draw the expected qualitative profiles in each of the following cases. (a) Cellular materials responding to an external stretch of about 70% along one axis occurring in 2 h and relaxing their stress via rearrangements, assuming incompressibility and that the material does not rip. In foams (a, left), the material stretch (blue) elongates cells that quickly relax their shape by making rearrangements along the stretch axis (magenta), and the average cell elongation (cyan dashed) quickly reaches its limit value (20%, cyan dashed line). In aggregates (a, middle), the cells first deform substantially (up to aspect-ratios of 1/6) due to a higher yield strain and slower relaxation time of several hours via rearrangements. Note that, unlike in foams, cells can manage to fully relax their shape thanks to cortex fluctuations over hours. In tissues (a, right), much less is known about the response to an external stretch: cells can deform drastically and keep their elongated shape for hours without making rearrangements (4 hour delay suggested in the wing), then, when they can, cell relax their shape in about 2 hour [30]. In other tissues like in the germband of Drosophila embryo, cells can also undergo cell rearrangements within minutes, much faster than in cell aggregates (not represented) [31]. (b) Tissues making active oriented rearrangements along the same axis to produce the same amount of deformation with different boundary conditions. (b, left) active oriented rearrangements occurring with free boundaries resulting in no cell shape changes and purely extending the tissue. (b, middle), same process but this time with fixed boundaries: the tissue cannot elongate and cell elongate of the same amount, but in the orthogonal direction, thereby building up stress. (b, right) intermediate situation where cells first elongate but then manage to relax their elongated shapes to elongate the tissue. (c-i) Study of the interplay in a real proliferative tissue during development. (c-c0 ) Part of developing scutellum in Drosophila dorsal thorax at 11:30 (c) and 27 hour 30 hour APF (c0 ). (d) Cell tracking highlighting new junctions formed by rearrangements (magenta) and by division (green). Shades of green represent the number of division rounds undergone by each cell; white cells are microchaetes and macrochaetes. (e-e0 ) Networks of links used to characterize tissue and cell dynamics in each cellular patch (thick black contours) between 11:30 (e) and 27:30 hour APF (e0 ). (f-h) Tissue deformation G (f), cell rearrangement strain R (g) and cell shape changes S (h) in six regions of the tissue measuring 40  40 mm initially. Scale: circles and bars represent 16% or isotropic algebraic dilation and contraction-elongation, respectively. (i) Rates of tissue deformation (g), rearrangement strain (r), cell shape changes (s), division (d) and delamination (a) projected onto G direction in each region and averaged over the six regions; those rates satisfy the balance g = r + s + d + a. A real tissue exhibit fluctuations and complex behaviors: from 12 to 17 hour APF, tissue contraction-elongation occurs mostly via cell rearrangements; from 17 to 22 hour APF, the tissue extends mostly via cell shape changes; from 22 to 26 hour APF it occurs via a combination of division and rearrangements. Current Opinion in Cell Biology 2017, 48:113–124

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of the identified regulations. Yet several important questions remain on both the biological and the mechanical aspects of this process. On the biological side, junction remodeling has been mainly studied at the level of AJ. The conversed roles of basal protrusions contacts in junction remodeling will be a major area of investigation in the future. Furthermore, the existence of possible regulations at the level of septate junctions in Drosophila or tight junctions in vertebrates remain unknown. Accordingly, mechanisms coordinating the dynamics of adherens and septate/tight junction are not well explored. While junction shortening and elongation have been the main focus of investigation, short-lived structures such as fourfold vertices or rosettes have impeded the characterization of the possible mechanisms controlling cell rearrangements at these transient stages. An important step in the analysis will first be to better understand in more detail how E-Cad dynamics and AJ formation is regulated at tricellular junctions and how junctions that maintain epithelial barrier function are remodeled during cell rearrangements. On the mechanical side, our understanding of rearrangements in foams and aggregates suggests several lines of future research for cell rearrangements in tissues. The observations that, while in aggregates cells manage to spontaneously relax their stress, cells in tissue can undergo substantial stretch without rearranging, or end up rearranging by seemingly carefully controlling their junction shortening and elongation like in the Drosophila pupal wing, raise several questions: what kind of biomechanical mechanisms prevent cell to relax and how relaxation can be initiated? Is mechanosensing involved in this response or are there unknown signals produced at specific time triggering cell rearrangements as observed in the pupal Drosophila wing? Answering these questions will be critical to better understand stress relaxation by cell rearrangements in tissues and how a tissue can change its shape without dramatically deform its cells. Lastly, although rearrangements seem to be used by the tissue in two opposed situations, namely relax tissue stress or in contrast generate stress or deformation in the tissue, the biological regulations controlling junction shortening/ elongation in the case of cell rearrangement generating stress or strain is less understood. In the future, to better understand tissue dynamics, it will be important to define criteria to identify regions where the tissue deforms ‘passively’ due to external stretch from neighboring parts, and regions where the tissue deforms ‘actively’ by generating its own strain, possibly using the new methods at our disposal. Since tissues are complex materials that can mix both behaviors, namely external tissue stretch and active oriented rearrangements, like for instance in the Drosophila germband [31], the dorsal thorax [29], or the pupal wing [29,30], it will be interesting to determine to which extent each process contributes to tissue deformation in different systems. www.sciencedirect.com

Acknowledgements We thank F. Graner for comments and suggestions on the manuscript and F. Bosveld for the images of cells in displayed Figures 1 and 3. The research in Bellaı¨che lab is funded by the ANR-MaxForce, ERC (TiMoprh, 340784), ARC (SL220130607097), ANR-DEEP (11-LBX-0044, ANR-10-IDEX0001-02) and PSL grants as well as by the CNRS, the INERM and the Curie Institute.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

Weaire D, Rivier N: Soap, cells and statistics – random patterns in two dimensions. Contemp Phys 1984, 25:59-99.

2.

Keller R, Shook D, Skoglund P: The forces that shape embryos: physical aspects of convergent extension by cell intercalation. Phys Biol 2008, 5:015007.

3.

Walck-Shannon E, Hardin J: Cell intercalation from top to bottom. Nat Rev Mol Cell Biol 2014, 15:34-48.

4.

Heisenberg C-P, Bellaı¨che Y: Forces in tissue morphogenesis and patterning. Cell 2013, 153:948-962.

5.

Tada M, Heisenberg CP: Convergent extension: using collective cell migration and cell intercalation to shape embryos. Development 2012, 139:3897-3904.

6.

Zallen JA, Blankenship JT: Multicellular dynamics during epithelial elongation. Semin Cell Dev Biol 2008, 19:263-270.

7.

Lienkamp SS et al.: Vertebrate kidney tubules elongate using a planar cell polarity-dependent, rosette-based mechanism of convergent extension. Nat Genet 2012, 44:1382-1387.

8.

Marmottant P, Raufaste C, Graner F: Discrete rearranging disordered patterns, part II: 2D plasticity, elasticity and flow of a foam. Eur Phys J E 2008, 25:371-384.

9.

Cantat I et al.: Foams: Structure and Dynamics. Oxford University Press; 2013.

10. Marmottant P et al.: The role of fluctuations and stress on the effective viscosity of cell aggregates. Proc Natl Acad Sci U S A 2009, 106:17271-17275. 11. Schotz EM et al.: Glassy dynamics in three-dimensional embryonic tissues. J R Soc Interface 2013, 10:20130726. 12. Forgacs G et al.: Viscoelastic properties of living embryonic tissues: a quantitative study. Biophys J 1998, 74:2227-2234. 13. Nnetu KD et al.: The impact of jamming on boundaries of collectively moving weak-interacting cells. N J Phys 2012, 14:115012. 14. Fernandez R et al.: Imaging plant growth in 4D: robust tissue reconstruction and lineaging at cell resolution. Nat Methods 2010, 7:547-553. 15. Hayashi T, Carthew RW: Surface mechanics mediate pattern formation in the developing retina. Nature 2004, 431:647-652. 16. Gonzalez-Rodriguez D et al.: Soft matter models of developing tissues and tumors. Science 2012, 338:910-917. 17. Khalilgharibi N et al.: The dynamic mechanical properties of cellularised aggregates. Curr Opin Cell Biol 2016, 42:113-120. 18. Durand M, Stone HA: Relaxation time of the topological T1 process in a two-dimensional foam. Phys Rev Lett 2006, 97:226101. 19. Grassia P, Oguey C, Satomi R: Relaxation of the topological T1 process in a two-dimensional foam. Eur Phys J E Soft Matter 2012, 35:64. 20. Prost J, Ju¨licher F, Joanny JF: Active gel physics. Nat Phys 2015, 11:111-117. Current Opinion in Cell Biology 2017, 48:113–124

122 Cell dynamics

21. Mizuno D et al.: Nonequilibrium mechanics of active cytoskeletal networks. Science 2007, 315:370-373. 22. Battle C et al.: Broken detailed balance at mesoscopic scales in  active biological systems. Science 2016, 352:604. The authors present a method using statistical thermodynamics and video microscopy to identify out-of-equilibrium fluctuations at the cellular scale.

38. Blankenship JT et al.: Multicellular rosette formation links planar cell polarity to tissue morphogenesis. Dev Cell 2006, 11:459-470. 39. Nishimura T, Honda H, Takeichi M: Planar cell polarity links axes of spatial dynamics in neural-tube closure. Cell 2012, 149:10841097.

23. Rauzi M et al.: Nature and anisotropy of cortical forces orienting Drosophila tissue morphogenesis. Nat Cell Biol 2008, 10:1401-1410.

40. Nishimura T, Takeichi M: Shroom3-mediated recruitment of Rho kinases to the apical cell junctions regulates epithelial and neuroepithelial planar remodeling. Development 2008, 135:1493-1502.

24. Maitre JL et al.: Adhesion functions in cell sorting by mechanically coupling the cortices of adhering cells. Science 2012, 338:253-256.

41. Shindo A, Wallingford JB: PCP and septins compartmentalize cortical actomyosin to direct collective cell movement. Science 2014, 343:649-652.

25. Ka¨fer J et al.: Cell adhesion and cortex contractility determine cell patterning in the Drosophila retina. Proc Natl Acad Sci U S A 2007, 104:18549-18554.

42. Rauzi M, Lenne PF, Lecuit T: Planar polarized actomyosin contractile flows control epithelial junction remodelling. Nature 2010.

26. Lecuit T, Lenne PF: Cell surface mechanics and the control of cell shape, tissue patterns and morphogenesis. Nat Rev Mol Cell Biol 2007, 8:633-644.

43. Martin AC, Kaschube M, Wieschaus EF: Pulsed contractions of an actin-myosin network drive apical constriction. Nature 2009, 457:495-499.

27. Vladar EK, Antic D, Axelrod JD: Planar cell polarity signaling: the developing cell’s compass. Cold Spring Harb Perspect Biol 2009, 1:a002964.

44. Roh-Johnson M et al.: Triggering a cell shape change by exploiting preexisting actomyosin contractions. Science 2012, 335:1232-1235.

28. Simons M, Mlodzik M: Planar cell polarity signaling: from fly development to human disease. Annu Rev Genet 2008, 42:517540.

45. Zallen JA, Wieschaus E: Patterned gene expression directs bipolar planar polarity in Drosophila. Dev Cell 2004, 6:343-355.

29. Guirao B et al.: Unified quantitative characterization of  epithelial tissue development. eLife 2015, 4:e08519. This work provides a method to quantify local tissue deformation as well as the contribution of each cell behavior by using the network of links connecting cell centroids and its evolution: the tissue strain is expressed as a sum of strains for each cell behavior. Using non-invasive force inference, the authors show that in Drosophila dorsal thorax, division orientation globally follow tissue junctional stress and that suppressing divisions impede tissue morphogenesis, cell deformation and cell rearrangements. 30. Etournay R et al.: Interplay of cell dynamics and epithelial  tension during morphogenesis of the Drosophila pupal wing. Elife 2015, 4. This work provides a method to quantify local tissue deformation and the contribution of each cell behavior by using the network of triangles connecting the cell centroids around each tricellular junction: the tissue strain is expressed as a sum of strains for each cell behavior and a term of correlation; the role of Drosophila wing epithelium attachment (by extracellular matrix protein Dumpy) on tissue deformation, stress and cell behaviors is analyzed experimentally and theoretically. 31. Lye CM et al.: Mechanical coupling between endoderm  invagination and axis extension in Drosophila. PLoS Biol 2015, 13:e1002292. This work shows that endoderm invagination produces an external mechanical stress promoting cell elongation and favoring cell rearrangements in the germband. 32. Wyatt TP et al.: Emergence of homeostatic epithelial packing and stress dissipation through divisions oriented along the long cell axis. Proc Natl Acad Sci U S A 2015, 112:5726-5731. 33. Mao Y et al.: Differential proliferation rates generate patterns of mechanical tension that orient tissue growth. EMBO J 2013. (advance online publication). 34. LeGoff L, Rouault H, Lecuit T: A global pattern of mechanical stress polarizes cell divisions and cell shape in the growing Drosophila wing disc. Development 2013, 140:4051-4059. 35. Bardet P-L et al.: PTEN controls junction lengthening and stability during cell rearrangement in epithelial tissue. Dev Cell 2013, 25:534-546. 36. Butler LC et al.: Cell shape changes indicate a role for extrinsic tensile forces in Drosophila germ-band extension. Nat Cell Biol 2009, 11:859-864. 37. Bertet C, Sulak L, Lecuit T: Myosin-dependent junction remodelling controls planar cell intercalation and axis elongation. Nature 2004, 429:667-671. Current Opinion in Cell Biology 2017, 48:113–124

46. Pare AC et al.: A positional toll receptor code directs  convergent extension in Drosophila. Nature 2014, 515:523-527. This work shows that the Toll2, 6 and 8 receptors acts redundantly to promote germband elongation. The results indicate the existence of Tollheterophilic codes that promote MyoII planar polarization downstream of pair-rule genes. 47. Kerridge S et al.: Modular activation of Rho1 by GPCR signalling  imparts polarized myosin II activation during morphogenesis. Nat Cell Biol 2016, 18:261-270. This article identified roles of GPCRs signaling in polarization of MyoII. 48. Lindsay SA, Wasserman SA: Conventional and nonconventional Drosophila Toll signaling. Dev Comp Immunol 2014, 42:16-24. 49. Tetley RJ et al.: Unipolar distributions of junctional Myosin II  identify cell stripe boundaries that drive cell intercalation throughout Drosophila axis extension. Elife 2016, 5. In this work, the authors performed extensive study of the dynamics of MyoII during germband elongation. Their results agree with the existence of Toll codes and illustrate how it could be robust to cell number changes in parasegments. 50. Costa M, Wilson ET, Wieschaus E: A putative cell signal encoded by the folded gastrulation gene coordinates cell shape changes during Drosophila gastrulation. Cell 1994, 76:10751089. 51. Simoes Sde M, Mainieri A, Zallen JA: Rho GTPase and shroom direct planar polarized actomyosin contractility during convergent extension. J Cell Biol 2014, 204:575-589. 52. Tamada M, Farrell DL, Zallen JA: Abl regulates planar polarized junctional dynamics through beta-catenin tyrosine phosphorylation. Dev Cell 2012, 22:309-319. 53. Simoes Sde M et al.: Rho-kinase directs Bazooka/Par-3 planar polarity during Drosophila axis elongation. Dev Cell 2010, 19:377-388. 54. Sawyer JK et al.: A contractile actomyosin network linked to adherens junctions by Canoe/afadin helps drive convergent extension. Mol Biol Cell 2011, 22:2491-2508. 55. Levayer R, Lecuit T: Oscillation and polarity of E-cadherin asymmetries control actomyosin flow patterns during morphogenesis. Dev Cell 2013, 26:162-175. 56. Fernandez-Gonzalez R et al.: Myosin II dynamics are regulated by tension in intercalating cells. Dev Cell 2009, 17:736-743. 57. Gorfinkiel N: From actomyosin oscillations to tissue-level deformations. Dev Dyn 2016, 245:268-275. www.sciencedirect.com

Biomechanics of cell rearrangements Guirao and Bellaı¨che 123

58. Levayer R, Lecuit T: Biomechanical regulation of contractility: spatial control and dynamics. Trends Cell Biol 2012, 22:61-81. 59. Mason FM et al.: RhoA GTPase inhibition organizes contraction  during epithelial morphogenesis. J Cell Biol 2016, 214:603-617. This article describes one of the different mechanisms contributing to the pulsatile MyoII dynamics. 60. Nishikawa M et al.: Controlling contractile instabilities in the  actomyosin cortex. Elife 2017, 6. This article describes one of the different mechanisms contributing to the pulsatile MyoII dynamics. 61. Munjal A et al.: A self-organized biomechanical network drives  shape changes during tissue morphogenesis. Nature 2015, 524:351-355. This article describes one of the different mechanisms contributing to the pulsatile MyoII dynamics. 62. Sun Z et al.: Basolateral protrusion and apical contraction  cooperatively drive Drosophila germ-band extension. Nat Cell Biol 2017. This important article demonstrates that the basal sides of cells meet before the apical sides and that loss of basal actin protrusions impair cell rearrangements, thereby providing strong evidence for a critical role of the basal surface of cells in cell rearrangements in Drosophila.

76. Faure E et al.: A workflow to process 3D + time microscopy images of developing organisms and reconstruct their cell lineage. Nat Commun 2016, 7:8674. 77. Heemskerk I, Streichan SJ: Tissue cartography: compressing bio-image data by dimensional reduction. Nat Methods 2015, 12:1139-1142. 78. Cilla R et al.: Segmentation and tracking of adherens junctions in 3D for the analysis of epithelial tissue morphogenesis. PLoS Comput Biol 2015, 11:e1004124. 79. Khan Z et al.: Quantitative 4D analyses of epithelial folding during Drosophila gastrulation. Development 2014, 141:28952900. 80. Amat F et al.: Fast, accurate reconstruction of cell lineages from large-scale fluorescence microscopy data. Nat Methods 2014, 11:951-958. 81. Heller D et al.: EpiTools: an open-source image analysis toolkit for quantifying epithelial growth dynamics. Dev Cell 2016, 36:103-116. 82. Barbier de Reuille P et al.: MorphoGraphX: a platform for quantifying morphogenesis in 4D. Elife 2015, 4:05864.

63. Williams M et al.: Distinct apical and basolateral mechanisms drive planar cell polarity-dependent convergent extension of the mouse neural plate. Dev Cell 2014, 29:34-46.

83. Aigouy B, Umetsu D, Eaton S: Segmentation and quantitative analysis of epithelial tissues. Methods Mol Biol 2016, 1478:227239.

64. Hale R, Strutt D: Conservation of planar polarity pathway function across the animal kingdom. Annu Rev Genet 2015, 49:529-551.

84. Umetsu D et al.: Local increases in mechanical tension shape compartment boundaries by biasing cell intercalations. Curr Biol 2014, 24:1798-1805.

65. Aigouy B et al.: Cell flow reorients the axis of planar polarity in the wing epithelium of Drosophila. Cell 2010, 142:773-786.

85. Blanchard GB et al.: Tissue tectonics: morphogenetic strain rates, cell shape change and intercalation. Nat Methods 2009, 6:458-464.

66. Bosveld F et al.: Mechanical control of morphogenesis by fat/ dachsous/four-jointed planar cell polarity pathway. Science 2012, 336:724-727. 67. Cao Y, White HD, Li XD: Drosophila myosin-XX functions as an actin-binding protein to facilitate the interaction between Zyx102 and actin. Biochemistry 2014, 53:350-360. 68. Collinet C et al.: Local and tissue-scale forces drive oriented  junction growth during tissue extension. Nat Cell Biol 2015, 17:1247-1258. This article describes one of the different mechanisms regulating junction elongation. 69. Yu JC, Fernandez-Gonzalez R: Local mechanical forces  promote polarized junctional assembly and axis elongation in Drosophila. Elife 2016, 5. This article describes one of the different mechanisms regulating junction elongation. 70. Hara Y, Shagirov M, Toyama Y: Cell boundary elongation by  non-autonomous contractility in cell oscillation. Curr Biol 2016, 26:2388-2396. This article describes one of the different mechanisms regulating junction elongation. 71. Ranft J et al.: Fluidization of tissues by cell division and apoptosis. Proc Natl Acad Sci U S A 2010, 107:20863-20868. 72. Tlili S et al.: Colloquium: mechanical formalisms for tissue  dynamics. Eur Phys J E Soft Matter 2015, 38:121. The authors review the relevant theoretical ingredients that can be assembled to build a constitutive equation describing epithelial tissue rheology using the dissipation function formalism.  M et al.: Active dynamics of tissue shear flow. N J Phys 73. Popovic  2017, 19:033006. The authors develop a theoretical continuous model to describe shear flow in epithelial tissues where they analyze the tissue behavior in response: (i) to external forces; (ii) to active generation of shear stress or oriented cell rearrangement by the tissue. 74. Keller PJ: Imaging morphogenesis: technological advances and biological insights. Science 2013, 340:1234168. 75. Stegmaier J et al.: Real-time three-dimensional cell segmentation in large-scale microscopy data of developing embryos. Dev Cell 2016, 36:225-240. www.sciencedirect.com

86. Sato K et al.: Left-right asymmetric cell intercalation drives  directional collective cell movement in epithelial morphogenesis. Nat Commun 2015, 6:10074. In this work, active rearrangements involving Myosin II are suggested to drive Drosophila genitalia rotation. 87. Graner F et al.: Discrete rearranging disordered patterns, part II: 2D plasticity, elasticity and flow of a foam. Eur Phys J E 2008, 25:349-369. 88. Aubouy M et al.: A texture tensor to quantify deformations. Granular Matter 2003, 5:67-70. 89. Rozbicki E et al.: Myosin-II-mediated cell shape changes and cell intercalation contribute to primitive streak formation. Nat Cell Biol 2015, 17:397-408. 90. Levayer R, Moreno E: Mechanisms of cell competition: themes and variations. J Cell Biol 2013, 200:689-698. 91. de la Cova C et al.: Drosophila myc regulates organ size by inducing cell competition. Cell 2004, 117:107-116. 92. Moreno E, Basler K: dMyc transforms cells into supercompetitors. Cell 2004, 117:117-129. 93. Levayer R, Hauert B, Moreno E: Cell mixing induced by myc is  required for competitive tissue invasion and destruction. Nature 2015, 524:476-480. This works provides evidence for a role of Myc in the regulation of cell junction tension and its impact on cell architecture in the tissue. 94. Bosveld F et al.: Modulation of junction tension by tumor  suppressors and proto-oncogenes regulates cell–cell contacts. Development 2016, 143:623-634. This work provides evidence for a role of Myc in the regulation of cell junction tension and its impact on cell architecture in the tissue. 95. Meyer SN et al.: An ancient defense system eliminates unfit cells from developing tissues during cell competition. Science 2014, 346:1258236. 96. Founounou N, Loyer N, Le Borgne R: Septins regulate the contractility of the actomyosin ring to enable adherens junction remodeling during cytokinesis of epithelial cells. Dev Cell 2013, 24:242-255. Current Opinion in Cell Biology 2017, 48:113–124

124 Cell dynamics

97. Pinheiro D et al.: Transmission of cytokinesis forces via  E-cadherin dilution and actomyosin flows. Nature 2017. This article shows the existence of mechanosensing during division contributing to the regulation of cell architecture in the tissue. 98. Morais-de-Sa E, Sunkel C: Adherens junctions determine the apical position of the midbody during follicular epithelial cell division. EMBO Rep 2013, 14:696-703. 99. Firmino J et al.: Cell division drives epithelial cell  rearrangements during gastrulation in chick. Dev Cell 2016, 36:249-261. This article illustrates how a coupling between cell division and cell rearrangements contribute to tissue morphogenesis. 100. Herszterg S et al.: Interplay between the dividing cell and its neighbors regulates adherens junction formation during cytokinesis in epithelial tissue. Dev Cell 2013, 24:256-270. 101. Higashi T et al.: Maintenance of the epithelial barrier and  remodeling of cell-cell junctions during cytokinesis. Curr Biol 2016, 26:1829-1842. This article shows the existence of mechanosensing during division contributing to the regulation of cell architecture in the tissue.

Current Opinion in Cell Biology 2017, 48:113–124

102. Lau K et al.: Anisotropic stress orients remodelling of  mammalian limb bud ectoderm. Nat Cell Biol 2015, 17:569-579. This article illustrates how a coupling between cell division and cell rearrangements contribute to tissue morphogenesis. 103. Landsberg KP et al.: Increased cell bond tension governs cell sorting at the drosophila anteroposterior compartment boundary. Curr Biol 2009, 19:1950-1955. 104. Monier B et al.: An actomyosin-based barrier inhibits cell mixing at compartmental boundaries in Drosophila embryos. Nat Cell Biol 2009, 12:60-65. 105. Ranft J et al.: Fluidization of tissues by cell division and apoptosis. Proc Natl Acad Sci U S A 2010, 107:20863-20868. 106. Bi D et al.: Energy barriers and cell migration in densely packed tissues. Soft Matter 2014, 10:1885-1890. 107. Harris AR et al.: Characterizing the mechanics of cultured cell monolayers. Proc Natl Acad Sci U S A 2012, 109:16449-16454.

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