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. Author manuscript; available in PMC: 2009 Feb 1.
Published in final edited form as: Circ Res. 2008 Aug 1;103(3):234–243. doi: 10.1161/CIRCRESAHA.108.175331

Figure 1.

Figure 1

Schematic overview of morphological events demonstrating the involvement of mechanical forces. (Top row) Early mammalian embryos determine their right from their left by creating fluid flow in one direction. Cilia located in a specialized node at the midline propel flow to the left. The embryo rapidly senses the fluid flow and upregulates the signaling molecule, nodal, on the left side only (shown in blue). Inherited defects in cilia motor proteins result in randomization of left-right patterning. In humans, this leads to Kartegener’s syndrome in 50% of patients. (Second row) During embryonic vasculogenesis, remodeling of immature vessels requires shear stress created by fluid flow. Before the beginning of fluid flow, embryonic mesodermal cells form “blood islands” outside the embryo. After the embryonic heart tube begins to function, fluid begins to flow and rapidly reshapes the blood islands into mature branched arteries and veins. This process specifically requires fluid shear stress. (Third row) Gastrulation, the first major morphological change in the embryo of the fruit fly D. melanogaster, is controlled by contraction of nonmuscle myosin. Before gastrulation, the embryo forms a single layer of cells arranged in a cylindrical egg (shown in cross-section, left panel), and myosin is localized to the inner surface. At the start of gastrulation, myosin relocalizes to the outer surface of a few cells, and its contraction pulls the outer portions of these cells together. This compresses the inner portions of the cells, pushing them inwards and creating a bulge (middle panel). Soon the cell layer has folded within itself (right panel). (Bottom row) Differentiation of mesenchymal stem cells is controlled by the level of nonmuscle myosin contraction inside the cell and the stiffness of its surroundings. Culturing cells on hard substrates will activate nonmuscle myosin contraction and promote differentiate towards osteoblasts. Culturing cells on soft substrates, or minimizing contact with hard surfaces, promotes differentiation towards adipocytes and chondrocytes.