Figure 5—figure supplement 1.

(A) Schematic of model incorporating endocytic trafficking of Nodal receptors. Simulations presented here were performed on a one-dimensional tissue with length 300 μm. New Oep is added to the surface with at constant rate k₃, and Oep is internalized via constitutive endocytosis of both bound and unbound receptors. Unbound receptors can be recycled to the surface or degraded. Ligand-bound receptors are not recycled. In panels B and C, simulations are presented as kymographs; each image column shows the state of the system with the source at the bottom and animal pole at the top. Time proceeds from left to right. (B) Simulation of a wild-type gradient. With continual receptor replacement, the system achieves a steady state gradient with length scale set by the balance between ligand diffusion and capture. The free ligand, free receptor and receptor-ligand complex concentrations are plotted from left to right in red, cyan and magenta, respectively. (C) Simulation of gradient formation in a zygotic oep mutant. Simulation details are identical to (B), but with receptor replacement rate (k₃) set to zero. The system fails to establish a steady state due to gradual consumption and degradation of receptors. As observed in the model presented in the main text, the Nodal ligand profile expands (red) to drive a propagating wave of signaling activity (magenta).