Signal transduction by a protease cascade

The dorsoventral axis of the Drosophila embryo is determined by a spatial c ue generated by ovarian somatic cells. This cue is communicated to the embr yo through an extracellular serine protease cascade active only on the vent ral side of the embryo. Studies of the proteases and somatically expressed proteins involved in thi s signalling process suggest a working model for how the protease cascade i s locally activated hours after the ovarian somatic cells have degenerated.