Fig. 1. (A) A schematic representation of the egg at stage 9-10 of Drosophila oogenesis. The oocyte at the posterior part of the egg chamber is covered by a layer of follicle cells. The nucleus of the oocyte is located at its dorsoanterior cortex. D, V, A and P denote the dorsal, ventral, anterior and posterior sides of the egg chamber, respectively. (B) Cell communication between the oocyte and the follicle cells is initiated by the Gurken signal. The spatial distribution of the oocyte-derived Gurken is determined by a combination of localized release, diffusion and binding of Gurken to the EGF receptors uniformly distributed in the follicular epithelium. (C) Positive feedback loop in the follicle cells. Gurken stimulates the EGF receptors in the follicle cells, which activate the MAPK in the follicle cells; MAPK contributes to the cytoplasmic degradation of CF2, a transcription factor that negatively regulates the expression of rhomboid. Rhomboid, an intracellular protease, participates in processing and activation of Spitz, another EGFR ligand. The secreted Spitz further stimulates EGFR in the follicle cells. (D) Negative feedback loop in the follicle cells: high levels of EGF receptor and MAPK activation, via transcription factor Pointed, induce the expression of argos. Argos is a secreted inhibitor of EGFR. Extracellular concentrations of Argos, Gurken and Spitz jointly regulate the level of EGFR activation in the follicle cells.
