Figure 2. Chemotactic Orientation in Competing Ligand Gradients Predicted by the Model: Top panel.

Illustration of competing ligand gradients. Lower panels: Equilibrium orientation signals for cells expressing the indicated receptor types (normal/desensitizable or nondesensitizable) in competing gradients of L₁ and L₂. In each case, ΔLR1* to L₁ gradient, ΔLR2* to L₂ gradient, and ΔLR*_total at equilibrium are compared as a function of position. The shaded area indicates ΔLR*_total levels below the threshold for orientation, set at ±10. Above each plot is a summary of predicted cell orientation and positioning at equilibrium: Arrows indicate the orienting zones (where ΔLR*_total is above the threshold). Dashed lines indicate predicted target zones, stable regions of random migration (where ΔLR*_total is below the threshold and the cell will be drawn back if it migrates into the surrounding orienting regions). Asterisks indicate an unstable region of poor orientation (ΔLR*_total is below the threshold but the cell will be directed away from these zones if it migrates into the orienting region). (A) Equilibrium orientation of a cell expressing normal receptors to L₁ and L₂ (normal R1 and R2). ΔLR*_total is below the threshold for effective orientation within the parenthesis. (B) Equilibrium orientation of a cell expressing normal R1 and nondesensitizable R2 receptors. (C) Equilibrium orientation of a cell expressing nondesensitizable receptors to both L₁ and L₂ (nondesensitizable R1 and R2). The arrow indicates the unstable central position at which cells expressing nondesensitizable receptors experience no net orientation signal.