FIGURE 3.

Mechanical properties of the notochord are regulated by cell polarity. (A) The early notochord is a short, thick, stiff, rod-like structure made up of a multiline of cells. (i) Polarized F-actin drives cells to migrate and intercalate to form a single line. (ii) Ventrally polarized actomyosin drives notochord bending toward the ventral side. (iii) An equator-localized contractile ring drives notochord cell elongation. (B) The notochord forms as a long, thin, single cell-line cord. Its stiffness is low but flexibility is high, which facilitates rapid embryonic development in the narrow space inside the chorion. (iv) AB polarity is built during extracellular lumen formation. (v) Notochord cells migrate bidirectionally along the notochord sheath. (vi) Notochord cells form an endothelial-like single layer. (C) A single lumen-filled notochord tube increases tissue stiffness and flexibility, which enables the swimming of Ciona larva in seawater.