Model for cell confinement regulation of lumen morphogenesis. In highly adhesive substrates, such as collagen I, low cell confinement induces cell spreading, promoting formation of peripheral nuclear and cortical actin fibers and maintaining the centrosome positioned at the cell center and nuclei close to the junctions. Cell confinement prevents cell spreading, contractility is reduced, and centrosomes reposition toward the cell junctions where the initial lumen forms. Peripheral actin contractility depends on LKB1-mediated regulation of myosin II activity through Rho–ROCK, whereas contractility-independent centrosome positioning is controlled by aPKC activity at the junctions. MT, microtubule; N, nuclei.