Morphogenetic movements driving neural tube closure in Xenopus require myosin IIB

Vertebrate neural tube formation involves two distinct morphogenetic events — convergent extension (CE) driven by mediolateral cell intercalation, and bending of the neural plate driven largely by cellular apical constriction. However, the cellular and molecular biomechanics of these processes are not understood. Here, using tissue-targeting techniques, we show that the myosin IIB motor protein complex is essential for both these processes, as well as for conferring resistance to deformation to the neural plate tissue. We show that myosin IIB is required for actin-cytoskeletal organization in both superficial and deep layers of the Xenopus neural plate. In the superficial layer, myosin IIB is needed for apical actin accumulation, which underlies constriction of the neuroepithelial cells, and that ultimately drive neural plate bending, whereas in the deep neural cells myosin IIB organizes a cortical actin cytoskeleton, which we describe for the first time, and that is necessary for both normal neural cell cortical tension and shape and for autonomous CE of the neural tissue. We also show that myosin IIB is required for resistance to deformation (“stiffness”) in the neural plate, indicating that the cytoskeleton-organizing roles of this protein translate in regulation of the biomechanical properties of the neural plate at the tissue-level.     

Measurement with an elastimeter of the stiffness of epithelial vesicles from pupal moth eye

Summary We have measured the stiffness of the monolayered epithelium which underlies the integument of an insect. Hollow vesicles of this epithelium, the hypodermis, formed during culture in a medium containing ecdysone, from squares of eye crescent integument excised from the moth Manduca sexta. Spherical vesicles were deformed by suction using an elastimeter; a plot of pressure vs. deformation has a slope which indicates the stiffness. This method allows a direct determination of the stiffness in a small patch of hypodermis at various places along the organism's surface. Vesicles produced from a sequence of sites along an adhesion gradient might show a corresponding sequence of stiffnesses. However, any difference in stiffness between vesicles from dorsal vs. ventral eye crescent was obscured by a large scatter in values for stiffness. Variation in the wall thickness of vesicles, and use of a scaling method involving elastic spheres, may underlie this scatter. The mean (±SD) corrected stiffness for all trials was 5.59±2.79 dynes/cm²/m deformation.