Axial Rotation Comprises Concurrent Twisting and Bending, Each Differentially Regulated by TGF-β Signaling

Axial rotation (AR), a morphogenetic movement that reshapes the body axis, is widely observed in chordates, including mice and rats. AR involves complex three-dimensional deformations; however, its geometric characteristics and regulatory mechanisms remain poorly understood. Here, using the chordate Ciona robusta ( Ciona intestinalis type A), we demonstrate that AR consists of two components—leftward bending and clockwise twisting along the anterior–posterior axis—differentially regulated by TGF-β signaling. A comparison between chorionated and dechorionated embryos revealed that dechorionation randomized the bending direction, while twisting remained consistently clockwise. Inhibition of TGF-β signaling with SB431542 randomized both deformations. Quantitative analysis of twisting angles indicated uniform clockwise twisting along the axis, peaking during the tailbud stage and proceeding in the tail region, independent of the tip, trunk, or myofibril patterning. Although overall twisting was reduced under TGF-β inhibition, the tail exhibited disorganized twisting. The sum of absolute twisting-angle differences in every 10 μm remained comparable to the wild type (WT). This suggests that twisting is intrinsically generated, while TGF-β signaling aligns local twisting into a coordinated global direction. Our findings dissected the mechanisms of AR in Ciona and highlight the multilayered regulation underlying the morphogenesis of the chordate body plan and providing a foundation for understanding its biomechanical and molecular bases.