Fig. 2. (A) Positive- and negative-feedback loops in the EGFR/MAPK/Rhomboid/Spitz/Argos network. The level of MAPK activity regulates the expression of Rhomboid and Argos. The level of MAPK activity is determined by the extracellular concentration of EGFR ligands; EGFR is stimulated by Gurken and Spitz, and is repressed by Argos. Our model neglects additional positive- and negative-feedback loops identified in the interaction between the oocyte and the follicular epithelium. (B) The spatially distributed network of autocrine loops that mediates the oocyte/follicle cells interaction. The spatially distributed Gurken input leads to Argos and Spitz production; both of these signals are diffusing in a narrow gap between the oocyte and the follicular epithelium.