Fig. 12. (A) Invagination and convergent extension lead to formation of a cylindrical intermediate. Arrows at left indicate convergent extension movements of cells within the notochord plate and its invagination, which occur simultaneously during late stage I and stage II. Dorsal is down and ventral is up. (B). Schematic view of an early-mid stage II notochord plate showing how individual cells extend their interior edges across the faces of adjacent notochord cell neighbors. (C). Textbook view of how an isolated cell crawls on a flat external substratum. (1) Localized actin-dependent protrusive forces (blue arrows) cause the leading edge to extend relative to adhesive contacts with the underlying substratum; (2) New adhesive contacts (green ovals) form at the leading edge with the underlying substratum, and subsequently stabilize through various mechanisms, including lateral clustering of adhesion proteins, and association with the underlying cortical cytoskeleton; (3) actin/myosin-dependent contractile forces within the cortical or interior cytoplasm (red arrows) set up a tug of war between different sites of attachment to the substratum. Directional movement occurs when this tug of war is biased to favor consolidation of leading edge attachments and release of adhesions at the rear (Chen, 1981; Jay et al., 1995; Palecek et al., 1996). (D) How the same machinery might operate within a monolayer epithelium. Each polygonal cell represents a cross section through an epithelial cell somewhere below the apical surface, each vertex represents an interior (basolateral) edge, analogous to the leading edge in (C), which attempts to extend (blue arrows) between adjacent neighbors. Homophilic associations between cadherin proteins replace the integrin-based adhesion used by most mesenchymal cells, but the underlying mechanics are entirely analogous. For simplicity, we consider contractile forces only within the cortex. (E) Mediolaterally biased protrusion (blue arrows) drives cells away from their preferred circular cross-sectional shapes. The cortical contractile forces that act to restore these shapes within each cell (red arrows) are joined by adhesive contacts to make contractile chains that span the width of the notochord plate and cause it to become longer and narrower.
