Oculomotor nerve requires an early interaction with muscle precursors for nerve guidance and branch patterning

Muscle function is dependent on innervation by the correct motor nerves. Motor nerves are composed of motor axons that extend through peripheral tissues as a compact bundle, but then diverge to create nerve branches to specific muscle targets. A transition point typically occurs as motor nerves grow near their targets, where the fasciculated nerve halts further growth, then later initiates branching to muscles. The motor nerve transition point is potentially an intermediate target acting as a guidepost to present specific cellular and molecular signals for navigation. Here we describe the navigation of the oculomotor nerve with respect to eye muscle precursor cells in mouse embryos. We found that the oculomotor nerve initially grew to the eye three days prior to the appearance of any eye muscles. The oculomotor axons spread to form a plexus within a mass of eye muscle precursors, then the nerve growth paused for more than two days. This plexus persisted during primary extraocular myogenesis, with a subsequent phase in which the nerve branched out to specific muscles. To test the functional significance of the nerve-precursor contact in the plexus, we genetically ablated muscle precursors early in nerve development, prior to nerve contact. Ablation of muscle precursors resulted in oculomotor nerve fibers failing to stop to form the plexus, but instead growing past the eye. In contrast, ablating the precursor pool at later stages, after the nerve has contacted the precursor cells, results in ectopic branching restricted near the eye. These results demonstrate that muscle precursors act as an intermediate target for nerve guidance, and are required for the oculomotor nerve to transition between nerve growth and distinct stages of terminal axon branching.