The Drosophila tracheal system is a branched tubular structure that supplies air to target tissues. The elaborate tracheal morphology is shaped by two linked inductive processes, one involving the choice of cell fates, and the other a guided cell migration. We will describe the molecular basis for these processes, and the allocation of cell fate decisions to four temporal hierarchies. First, tracheal placodes are specified within the embryonic ectoderm. Subsequently, branch fates are allocated within the tracheal placodes, prior to migration. Localized presentation of the FGF ligand, Branchless, to tracheal cells that express the FGF receptor, Breathless, guides migration. Once cell migration is initiated, distinct cell fates are determined within each migrating branch. Finally, inhibitory feedback mechanisms ensure the correct assignment of these fates. Tracheal cell fate choices are determined by signaling cascades triggered by signals emanating from the tracheal cells, as well as by ligands produced by adjacent tissues.
Copyright 2000 John Wiley & Sons, Inc.