Fig. 9. A model of restricted Wg movement along cell surface. Secreted extracellular Wg is bound and stabilized by glypicans Dally and Dlp on the cell surface. The differential concentration of Wg from producing cells to receiving cells drives Wg diffusion along cell surface to more distal cells through disassociation/re-association with HSPGs. In the absence of Dally and Dlp, the Wg protein can no longer move further. It either stays on cell membrane or diffuses into disc lumen, or is degraded. The thin black arrows at the top of this diagram indicate the displacement of Wg from one GAG chain to another. The double-headed arrow indicates the lateral movement of glypicans on the cell membrane.