Pathfinding by sensory axons in Drosophila: substrates and choice points in early lch5 axon outgrowth.

We have examined the pattern of axon growth from the lateral chordotonal (lch5) neurons in the body wall of the Drosophila embryo and identified cellular substrates and choice points involved in early axon pathfinding by these sensory neurons. At the first choice point (TP1), the lch5 growth cones contact the most distal cells of the spiracular branch (SB) of the trachea. The SB provides a substrate along which the axons extend internally to the level of the intersegmental nerve (ISN). In the absence of the SB, the lch5 axons often stall near TP1 or follow aberrant routes towards the CNS. At the second choice point (TP2), the lch5 growth cones make their first contact with other axons and turn ventrally toward the CNS, fasciculating specifically with the motor axons of the ISN.