Pathfinding analysis in a glia-less gcm mutant in Drosophila.

Many lines of evidence suggest that glial cells function as guide post cells for axonal pathfinding. However, due to the difficulty in completely eliminating glial cells during development, their functions in axonal pathfinding have not been critically evaluated. In Drosophila gcm mutant embryos, glial cells were genetically eliminated providing us with a unique opportunity to investigate glial functions in nervous system formation. We showed that even in the absence of glial cells the initial axonal extension of pioneer neurons was essentially normal. However, at later stages, axon bundle formation and pathfinding were disturbed in the absence of glial cells, and abnormal migration of glial cells led to misrouting of axons. This indicates that glial cells are required for correct pathfinding at later stages. We propose that glial cells function in a stage-specific manner; they are not required for the initial extension of pioneers but essential for the subsequent extension of pioneers and followers as well as axon bundle formation.