Genetic analysis of Laminin A in Drosophila: extracellular matrix containing laminin A is required for ocellar axon pathfinding

Genetic analysis of the Laminin A (LamA) gene in Drosophila reveals that distinct classes of sensory axons have different requirements for extracellular matrix (ECM) containing laminin A versus epithelial cell surfaces. In the eye-antenna imaginal disc, the nerve from the three simple eyes (ocelli) to the brain is pioneered by a population of transient ocellar neurons whose axons extend on an ECM that covers and connects the disc epithelium and brain. Axons from neighboring mechanosensory (bristle) neurons extend under the ECM in direct contact with the surface of the disc cells, and pioneer a different axon pathway that enters the brain in a different location. In LamA mutants, the ocellar pioneer axons display striking pathfinding defects, while neighboring bristle axons appear normal; the ocellar pioneers usually extend in the proper direction, adhering to the epithelium and sometimes fasciculating with mechanosensory axons, but they invariably fail to reach the brain.