Analysis of cellular behavior and cytoskeletal dynamics reveal a constriction mechanism driving optic cup morphogenesis

Abstract

Contractile actomyosin networks have been shown to power tissue morphogenesis. Although the basic cellular machinery generating mechanical tension appears largely conserved, tensions propagate in unique ways within each tissue. Here we use the vertebrate eye as a paradigm to investigate how tensions are generated and transmitted during the folding of a neuroepithelial layer. We record membrane pulsatile behavior and actomyosin dynamics during zebrafish optic cup morphogenesis by live imaging. We show that retinal neuroblasts undergo fast oscillations and that myosin condensation correlates with episodic contractions that progressively reduce basal feet area. Interference with lamc1 function impairs basal contractility and optic cup folding. Mapping of tensile forces by laser cutting uncover a developmental window in which local ablations trigger the displacement of the entire tissue. Our work shows that optic cup morphogenesis is driven by a constriction mechanism and indicates that supra-cellular transmission of mechanical tension depends on ECM attachment.Tissues and organs form into their final shapes because the cells in a developing embryo generate forces that alter their shape and position. Networks of fibres made from actin and myosin proteins generate these forces, and because the fibres can assemble in many different ways inside cells, they allow the cells to move and change shape in many different ways.Forces in some tissues can cause flat sheets of cells to bend. These sheets of cells are attached on one side (their “basal” surface) to a collection of membranes and molecules that are known as the extracellular matrix. When the cells in the sheet progressively shrink at their basal surface, causing the sheet to bend towards the extracellular matrix, this is known as basal constriction.Nicolás-Pérez et al. have used high-resolution imaging to record how basal constriction helps the optic cup – the main chamber of the eye – to form in zebrafish embryos. This imaging confirmed that a sheet of precursor cells progressively bends towards its basal surface to form the curved shape of the eyeball. Further analysis revealed that this basal constriction happens when myosin fibres accumulate in clusters along the basal surface of some of the precursor cells. The resulting contraction of the basal surface of the cells relies both on the tension generated by myosin inside the cell and on the cells being attached properly to the extracellular matrix.Using a laser beam, Nicolás-Pérez et al. also destroyed small parts of the basal surface of the retina. This procedure allows the mechanical tension distribution throughout the developing eye to be mapped. Laser ablations revealed a narrow time window during development when destroying small parts of the basal surface can cause the entire sheet of cells to relax, preventing it from curving to form the shape of the eye.Sheets of precursor cells are important building blocks of the nervous system, yet researchers only have limited knowledge of the processes that enable them to fold or bend into a final shape. As such, the findings of Nicolás-Pérez et al. will contribute to a wider understanding of how cells and tissues behave while the brain is forming.