Fig. 1. Mechanisms of left-right (LR) patterning in mouse embryo. (A) Three steps in the generation of LR asymmetry: a symmetry-breaking event in the node, the patterning of the lateral plate mesoderm (LPM) and asymmetric organogenesis. The black arrow on the left represents a time course during development, from earlier embryonic (E7.5) stages to later ones (E10-11.5). (B) Posterior view of a mouse embryo at E8.0 showing Nodal expression in the node and left LPM. (C) A transverse section taken at the level indicated by the red lines in B. The location of pit cells, crown cells, paraxial mesoderm (PAM), LPM, endoderm and prospective floor plate are shown. Arrows indicate how signals are transferred during LR patterning. Asymmetric signal(s) generated by the leftward flow in the node (black wavy lines) might be transferred to the left LPM, either through the endoderm (pink line) or through the PAM (red line). According to the determinant-transporting model, an unknown molecule produced in the node/perinodal cells is secreted into the node cavity and transported towards the left side, where its signal may be transduced by the endoderm and finally by the LPM (pink line). Alternatively, cells in or near the node may sense the mechanical stress and send an unknown signal(s) to the left LPM thorough the PAM (red line). It is also possible that the perinodal (crown) cells respond to the chemical determinant and send a signal via the red route.