View Program - 63rd Annual Drosophila Research Conference

Development of functional tissue morphologies requires precise spatiotemporal regulation of cellular processes that generate mechanical forces and modulate tissue material properties. Extensive work has revealed the existence of common cell biological strategies that are employed to shape epithelial sheets; however, the influence that tissue scale material/viscoelastic properties impart on morphogenetic processes is not understood. There is growing evidence that within a single cell, different organization states of the cytoskeleton result in different cellular viscoelastic properties, but how these cellular behaviors—in aggregate—influence tissue-scale material properties, is an open question. The Drosophila retina, which has long been used as a model system to understand the cell biological basis of tissue patterning, presents an intriguing opportunity to explore the cell biological and biophysical processes that coordinate to shape the fly eye at the tissue scale.

The fly retina is a neuroepithelium comprised of ~750 multicellular units called ommatidia, each of which comprises a core cluster of photoreceptor neurons surrounded by a supportive lattice of non-neuronal cells. During development, ommatidial units are positioned in a stereotyped curvature that is critical for the optical resolution of the adult compound eye. During pupal development, the apical surfaces of the photoreceptor cells involute and then elongate along the optical axis of the tissue in a process called apical expansion. We have recently discovered that the retina establishes its curvature prior to apical expansion onset, rather than concomitantly as it was previously assumed. This suggests that retinal curvature establishment is a discrete morphogenetic process driven by a distinct mechanism. Interestingly, coincident with curvature formation, the retinal epithelium reorganizes apical cell contacts and cell adhesion machinery, resulting in precise hexagonal packing of ommatidia. Live-imaging experiments of retinal epithelia during this period suggest that the tissue undergoes a transition from a viscous-fluid like state to an elastic-solid like state. This transition depends on both packing geometry of ommatidia in the epithelium and actomyosin contractility. This newly described morphogenetic process in the pupal retina provides a model to probe the relationship between changes in tissue material properties and final tissue morphology. Quantitative live-imaging coupled with perturbations to cytoskeletal networks in the developing retina can provide mechanistic insight into the cellular processes that tune tissue material properties during tissue morphogenesis.

Tissue scale viscoelastic properties influence 3-D organ morphology in the developing fly retina