SUMMARYMicroridges, elongated 3D protrusions arranged in maze-like patterns on zebrafish skin cells, form by the accretion of simple precursor projections. Modeling and in vivo experiments showed that cortical contractions promote the coalescence of precursors into microridges by reducing membrane tension. ABSTRACTCellular protrusions create complex cell surface topographies, but biomechanical mechanisms regulating their formation and arrangement are largely unknown. To study how protrusions form, we focused on the morphogenesis of microridges, elongated actin-based structures projecting from the apical surfaces of zebrafish skin cells that are arranged in labyrinthine patterns. Microridges form by accreting simple finger-like precursors. Live imaging demonstrated that microridge morphogenesis is linked to apical constriction. A non-muscle myosin II (NMII) reporter revealed pulsatile contractions of the actomyosin cortex; inhibiting NMII demonstrated that contractions are required for apical constriction and microridge formation. A biomechanical model suggested that contraction reduces surface tension to permit the fusion of precursors into microridges. Indeed, reducing surface tension with hyperosmolar media promoted microridge formation. In anisotropically stretched cells, microridges formed by precursor fusion along the stretch axis, which computational modeling explained as a consequence of stretch-induced cortical flow. Collectively, our results demonstrate how contraction within the 2D plane of the cortex patterns 3D cell surfaces.