Practice question

Dorsoventral polarity originates during oogenesis when Gurken, a TGF-alpha-like ligand, secreted from oocyte nucleus located dorsal-anterior position activates EGFR in adjacent follicle cells. Dorsal follicle fate represses Pipe sulfotransferase expression ventrally, restricting Pipe to ventral follicle cells that modify vitelline membrane. After fertilization, serine protease cascade downstream of Pipe cleaves Spätzle, activating Toll receptor ventrally. Toll signaling via Tube, Pelle degrades Cactus inhibitor, allowing Dorsal NF-kB transcription factor nuclear entry ventrally. Thus Gurken dorsalizes follicle cells and Dorsal patterns embryo ventrally, jointly establishing DV axis.