Interommatidial cells build a tensile collagen network during Drosophila retinal morphogenesis

Drosophila compound eye morphogenesis transforms a simple epithelium into an approximate hollow hemi-sphere comprised of -700 ommatidia, packed as tapering hexagonal prisms between a rigid external array of cuticular lenses and a parallel, rigid internal floor, the fenestrated membrane (FM). Critical to vision, photo -sensory rhabdomeres are sprung between these two surfaces, grading their length and shape accurately across the eye and aligning them to the optical axis. Using fluorescently tagged collagen and laminin, we show that that the FM assembles sequentially, emerging in the larval eye disc in the wake of the morphoge-netic furrow as the original collagen-containing basement membrane (BM) separates from the epithelial floor and is replaced by a new, laminin-rich BM, which advances around axon bundles of newly differentiated pho-toreceptors as they exit the retina, forming fenestrae in this new, laminin-rich BM. In mid-pupal development, the interommatidial cells (IOCs) autonomously deposit collagen at fenestrae, forming rigid, tension-resisting grommets. In turn, stress fibers assemble in the IOC basal endfeet, where they contact grommets at anchor-ages mediated by integrin linked kinase (ILK). The hexagonal network of IOC endfeet tiling the retinal floor couples nearest-neighbor grommets into a supracellular tri-axial tension network. Late in pupal develop-ment, IOC stress fiber contraction folds pliable BM into a hexagonal grid of collagen-stiffened ridges, concomitantly decreasing the area of convex FM and applying essential morphogenetic longitudinal tension to rapidly growing rhabdomeres. Together, our results reveal an orderly program of sequential assembly and activation of a supramolecular tensile network that governs Drosophila retinal morphogenesis.

Automated summary

Drosophila compound eye morphogenesis involves sequential assembly and activation of a supramolecular tensile network. This network is formed by the emergence of a new laminin-rich basement membrane, deposition of collagen at fenestrae by interommatidial cells, assembly of stress fibers in the endfeet of interommatidial cells, and contraction of these stress fibers to fold the pliable basement membrane into a hexagonal grid. This process applies essential morphogenetic tension to the rapidly growing rhabdomeres and governs retinal morphogenesis in Drosophila.