Actin-based protrusions vary in morphology, stability, and arrangement on cell surfaces. Microridges are laterally-elongated protrusions arranged in maze-like patterns on mucosal epithelial cells that rearrange dynamically by fission and fusion. To characterize how microridges mature and investigate the mechanisms driving fission and fusion, we imaged microridges in the maturing skin of zebrafish larvae. After their initial development, microridges continued to lengthen and microridge alignment became increasingly well ordered. Imaging F-actin and Non-Muscle Myosin II (NMII) revealed that microridge fission and fusion were associated with local NMII activity in the apical cortex. Inhibiting NMII blocked rearrangements, reduced microridge density, and altered microridge spacing. High-resolution imaging revealed that individual cortical NMII minifilaments are tethered to protrusions, often connecting adjacent microridges. NMII minifilaments connecting the ends of microridges fused them together, whereas minifilaments oriented perpendicular to microridges severed them or pulled them closer together. Our findings demonstrate that as cells mature, microridges continue to remodel and form an increasingly orderly arrangement through a process orchestrated by cortical NMII contraction.
