Hedgehog signaling patterns the tracheal branches

The elaborate branching pattern of the Drosophila tracheal system originate s from ten tracheal placodes on both sides of the embryo, each consisting o f about 80 cells. Simultaneous cell migration from each tracheal pit in six different directions gives rise to the stereotyped branching pattern. Each branch contains a fixed number of cells. Previous work has shown that in t he dorsoventral axis, localized activation of the Dpp, Wnt and EGF receptor (DER) pathways, subdivides the tracheal pit into distinct domains. We pres ent the role of the Hedgehog (Hh) signaling system in patterning the trache al branches. Hh is expressed in segmental stripes abutting the anterior bor der of the tracheal placodes, Induction of patched expression, which result s from activation by Hh, demonstrates that cells in the anterior half of th e tracheal pit are activated. In hh-mutant embryos migration of all trachea l branches is absent or stalled. These defects arise from a direct effect o f Hh on tracheal cells, rather than by indirect effects on patterning of th e ectoderm, Tracheal cell migration could be rescued by expressing Hh only in the tracheal cells, without rescuing the ectodermal defects. Signaling b y several pathways, including the Hh pathway, thus serves to subdivide the uniform population of tracheal cells into distinct cell types that will sub sequently be recruited into the different branches.