Module 8: Figure nodal flow hypothesis
The nodal fluid flow hypothesis for establishing left–right asymmetry.
The triangular depression that is the node contains cells, each of which has a single cilium. The node has two populations of cilia: there are motile cilia in the central region (yellow) that are surrounded by non-motile cilia (green). The central motile cilia are distinguished from the non-motile cilia by having the motor protein left–right dynein (lrd) that enables them to set up a rotary counter-clockwise motion that produces a directed flow of nodal fluid. This nodal fluid flow causes the non-motile cilia to bend over, and this mechanical deformation activates the mechanosensitive Ca2+ channel polycystin-2. The idea is that the left-to-right flow deforms the cilia to activate polycystin-2, thereby setting up a Ca2+ gradient specifically in the cells on the left side of the node (red cells). This is the symmetry-breaking event responsible for activating the genes such as lefty-1, lefty-2, nodal and Pitx2 that then go on to establish the left–right axis.