Fig. 2. Primary cilia in the embryonic node and left-right axis specification. The illustration represents the ventral surface of the mouse embryonic node, viewed from the center of the node toward the left. Motile cilia (green) in the center of the node rotate in a clockwise direction. Because the cilia are positioned at an angle at the posterior end of the node cells, when clockwise-rotating cilia stroke toward the right they are close to the cell surface and flow is impeded; when they stroke to the left at the top of the arc, they are away from the cell surface and flow is unimpeded. This produces an asymmetric flow of fluid (red arrows) towards the left periphery of the node. At the left periphery of the node, fluid flow is sensed by mechanosensory cilia (blue), and/or secreted signaling molecules are concentrated by the flow and received by chemosensory cilia. These events, alone or in combination, cause intracellular Ca²⁺ levels to increase in cells on the left side of the node, which triggers a signal transduction pathway that controls the asymmetric expression of genes that establish the left-right axis. A, anterior; L, left; P, posterior; R, right.