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Background  How axon guidance signals regulate growth cone behavior and guidance decisions in the complex  in vivo  environment of the central nervous system is not well understood. We have taken advantage of the unique features of the zebrafish embryo to visualize dynamic growth cone behaviors and analyze guidance mechanisms of axons emerging from a central brain nucleus  in vivo .    Results  We investigated axons of the nucleus of the medial longitudinal fascicle (nucMLF), which are the first axons to extend in the zebrafish midbrain. Using  in vivo  time-lapse imaging, we show that both positive axon-axon interactions and guidance by surrounding tissue control initial nucMLF axon guidance. We further show that two guidance molecules, transient axonal glycoprotein-1 (TAG-1) and laminin-α1, are essential for the initial directional extension of nucMLF axons and their subsequent convergence into a tight fascicle. Fixed tissue analysis shows that TAG-1 knockdown causes errors in nucMLF axon pathfinding similar to those seen in a laminin-α1 mutant. However,  in vivo  time-lapse imaging reveals that while some defects in dynamic growth cone behavior are similar, there are also defects unique to the loss of each gene. Loss of either TAG-1 or laminin-α1 causes nucMLF axons to extend into surrounding tissue in incorrect directions and reduces axonal growth rate, resulting in stunted nucMLF axons that fail to extend beyond the hindbrain. However, defects in axon-axon interactions were found only after TAG-1 knockdown, while defects in initial nucMLF axon polarity and excessive branching of nucMLF axons occurred only in laminin-α1 mutants.    Conclusion  These results demonstrate how two guidance cues, TAG-1 and laminin-α1, influence the behavior of growth cones during axon pathfinding  in vivo . Our data suggest that TAG-1 functions to allow growth cones to sense environmental cues and mediates positive axon-axon interactions. Laminin-α1 does not regulate axon-axon interactions, but does influence neuronal polarity and directional guidance.

Transient axonal glycoprotein-1 (TAG-1) and laminin-α1 regulate dynamic growth cone behaviors and initial axon direction in vivo