SUMMARYFluctuations in the shape of amnioserosa (AS) cells during Drosophila dorsal closure (DC) provide an ideal system with which to understand contractile epithelia, both in terms of the cellular mechanisms and how tissue behaviour emerges from the activity of individual cells. Using quantitative image analysis we show that apical shape fluctuations are driven by the medial cytoskeleton, with periodic foci of contractile myosin and actin travelling across cell apices. Shape changes were mostly anisotropic and neighbouring cells were often, but transiently, organised into strings with parallel deformations. During the early stages of DC, shape fluctuations with long cycle lengths produced no net tissue contraction. Cycle lengths shortened with the onset of net tissue contraction, followed by a damping of fluctuation amplitude. Eventually, fluctuations became undetectable as AS cells contracted rapidly. These transitions were accompanied by an increase in apical myosin, both at cell-cell junctions and medially, the latter ultimately forming a coherent, but still dynamic, sheet across cells. Mutants with increased myosin activity or actin polymerisation exhibited precocious cell contraction through changes in the subcellular localisation of myosin. thickveins mutant embryos, which exhibited defects in the actin cable at the leading edge, showed similar timings of fluctuation damping to the wild type, suggesting that damping is an autonomous property of the AS. Our results suggest that cell shape fluctuations are a property of cells with low and increasing levels of apical myosin, and that medial and junctional myosin populations combine to contract AS cell apices and drive DC.

Hedgehog (Hh) moves from the producing cells to regulate the growth and development of distant cells in a variety of tissues. Here, we have investigated the mechanism of Hh release from the producing cells to form a morphogenetic gradient in the Drosophila wing imaginal disk epithelium. We describe that Hh reaches both apical and basolateral plasma membranes, but the apical Hh is subsequently internalized in the producing cells and routed to the basolateral surface, where Hh is released to form a longrange gradient. Functional analysis of the 12-transmembrane protein Dispatched, the glypican Dally-like (Dlp) protein, and the Iglike and FNNIII domains of protein Interference Hh (Ihog) revealed that Dispatched could be involved in the regulation of vesicular trafficking necessary for basolateral release of Hh, Dlp, and Ihog. We also show that Dlp is needed in Hh-producing cells to allow for Hh release and that Ihog, which has been previously described as an Hh coreceptor, anchors Hh to the basolateral part of the disk epithelium.

The dynamic reshaping of tissues during morphogenesis results from a combination of individual cell behaviours and collective cell rearrangements. However, a comprehensive framework to unambiguously measure and link cell behaviour to tissue morphogenesis is lacking. Here we introduce such a kinematic framework, bridging cell and tissue behaviours at an intermediate, mesoscopic, level of cell clusters or domains. By measuring domain deformation in terms of the relative motion of cell positions and the evolution of their shapes, we characterize the basic invariant quantities that measure fundamental classes of cell behaviour, namely tensorial rates of cell shape change and cell intercalation. In doing so we introduce an explicit definition of cell intercalation as a continuous process. We demonstrate how spatiotemporal mapping of strain rates in three models of tissue morphogenesis leads to new insight into morphogenetic mechanisms. Our quantitative approach has broad relevance for the precise characterisation and comparison of morphogenetic phenotypes.

The Hedgehog (Hh) morphogenetic gradient controls multiple developmental patterning events in Drosophila and vertebrates. Patched (Ptc), the Hh receptor, restrains both Hh spreading and Hh signaling. We report how endocytosis regulates the concentration and activity of Hh in the wing imaginal disc. Our studies show that Ptc limits the Hh gradient by internalizing Hh through endosomes in a dynamin-dependent manner, and that both Hh and Ptc are targeted to lysosomal degradation. We also found that the ptc14 mutant does not block Hh spreading, as it has a failure in endocytosis. However, this mutant protein is able to control the expression of Hh target genes as the wild-type protein, indicating that the internalization mediated by Ptc is not required for signal transduction. In addition, we noted that both in this mutant and in those not producing Ptc protein, Hh still occurred in the endocytic vesicles of Hh-receiving cells,suggesting the existence of a second, Ptc-independent, mechanism of Hh internalization.

Halfway through embryonic development, the epidermis of Drosophila exhibits a gap at the dorsal side covered by an extraembryonic epithelium, the amnioserosa (AS). Dorsal closure (DC) is the process whereby interactions between the two epithelia establish epidermal continuity. Although genetic and biomechanical analysis have identified the AS as a force-generating tissue, we do not know how individual cell behaviours are transformed into tissue movements. To approach this question we have applied a novel image-analysis method to measure strain rates in local domains of cells and performed a kinematic analysis of DC. Our study reveals spatial and temporal differences in the rate of apical constriction of AS cells. We find a slow phase of DC, during which apical contraction of cells at the posterior end predominates, and a subsequent fast phase, during which all the cells engage in the contraction, which correlates with the zippering process. There is a radial gradient of AS apical contraction, with marginal cells contracting earlier than more centrally located cells. We have applied this analysis to the study of mutant situations and associated a particular genotype with quantitative and reproducible changes in the rate of cell contraction and hence in the overall rate of the process. Our mutant analysis reveals the contribution of mechanical elements to the rate and pattern of DC.

The Hedgehog (Hh) family of morphogenetic proteins has important instructional roles in metazoan development and human diseases. Lipid modified Hh is able to migrate to and program cells far away from its site of production despite being associated with membranes. To investigate the Hh spreading mechanism, we characterized Shifted (Shf) as a component in the Drosophila Hh pathway. We show that Shf is the ortholog of the human Wnt inhibitory factor (WIF), a secreted antagonist of the Wingless pathway. In contrast, Shf is required for Hh stability and for lipid-modified Hh diffusion. Shf colocalizes with Hh in the extracellular matrix and interacts with the heparan sulfate proteoglycans (HSPG), leading us to suggest that Shf could provide HSPG specificity for Hh. We also show that human WIF inhibits Wg signaling in Drosophila without affecting the Hh pathway, indicating that different WIF family members might have divergent functions in each pathway.

The tissues of a developing embryo are simultaneously patterned, moved and differentiated according to an exchange of information between their constituent cells. We argue that these complex self-organizing phenomena can only be fully understood with quantitative mathematical frameworks that allow specific hypotheses to be formulated and tested. The quantitative and dynamic imaging of growing embryos at the molecular, cellular and tissue level is the key experimental advance required to achieve this interaction between theory and experiment. Here we describe how mathematical modelling has become an invaluable method to integrate quantitative biological information across temporal and spatial scales, serving to connect the activity of regulatory molecules with the morphological development of organisms.

This study investigates the role of the homeobox gene Distal-less (Dll) in the development of the legs, antennae, and wings of Drosophila. Lack of Dll function causes a change in the identity of ventral appendage cells (legs and antennae) that often results in the loss of the appendage. Ectopic Dll expression in the proximal region of ventral appendages induces nonautonomous duplication of legs and antennae by the activation of wingless and decapentaplegic. Ectopic Dll expression in dorsal appendages produces transformation into corresponding ventral appendages; wings and halteres develop ectopic legs and the head-eye region develops ectopic antennae. In the wing, the exogenous Dll product induces this transformation by activating the endogenous Dll gene and repressing the wing determinant gene vestigial. It is proposed that Dll induces the development of ventral appendages and also participates in a genetic address that specifies the identity of ventral appendages and discriminates the dorsal versus the ventral appendages in the adult. However, unlike other homeotic genes, Dll expression and function is not defined by a cell lineage border. Dll also performs a secondary and late function required for the normal patterning of the wing.