The left-right asymmetry pathway. (a) An 8.25-day mouse embryo showing asymmetric Nodal expression (in blue) at the node as well as in the left, but not the right, lateral plate, and asymmetric Cerl2 expression in purple. (b) A higher magnification of the node. The central pit of the node contains motile cilia (not shown) that drive a leftwards nodal flow (indicated by arrows). Surrounding the node are the node crown cells; these express Nodal (represented by blue dots) in an asymmetric fashion, with expression stronger on the left than on the right. An asymmetric calcium signal (represented by red stars) is also stronger on the left than the right. Downstream of nodal flow and asymmetric gene expression at the node, a 'leftness' signal is communicated several cell diameters to the left lateral plate mesoderm (LPM). Here it activates the Nodal signaling cascade, ultimately resulting in left-side-specific Pitx2 expression and asymmetric morphogenesis. The primary axes are shown: anterior-posterior (A-P); dorsal-ventral (D-V); left-right (L-R).