Fig. 7. Model for the movement of Nodal activity during LR patterning. Three events involving Nodal are illustrated. Blue represents the domains that receive Nodal signals and in which Foxh1 is active. Red indicates the domains in which Nodal is expressed. Arrows indicate the directions of signal transfer. In the wild-type embryo (top row), Nodal produced in the node travels to left LPM and initiates Nodal expression in a small region adjacent to the node (1). At this stage, Lefty1 is expressed in a small region of PFP adjacent to the node (green). Lefty1 expression in this domain may be induced by Nodal produced in the node. (2) Nodal produced in the small region of left LPM diffuses along the anteroposterior axis in left LPM and thereby induces the expansion of Nodal expression. (3) Nodal produced in left LPM travels to the entire PFP region along the AP axis (blue), where it induces Lefty1 expression. In Foxh1^c/- embryos (bottom row), Nodal produced in the node is unable to initiate Nodal expression in left LPM because of the absence of Foxh1 in LPM. As a result, Nodal expression in left LPM and Lefty1 expression in PFP are absent.

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