Innervation of cochlear sensory cells follows a series of distinct steps that were in part revealed by a7 GFP visualization (Fig. 5 ). As noted, the first detection of a7 expression was in the prominently labeled efferent processes that appear to form bundles upon entering the SG and then disperse into small solitary fibers (E14.5; Figs. 5A and 2C,D). These solitary processes exhibit a beaded structure as they proceed to the base of the developing sensory cells (Fig. 5B) . The origin of these efferent fibers was examined in serial sections of the E14.5 hind brain. These fibers appear to originate from a cell grouping in the basal brain stem caudal to trigeminal nucleus V that could be distinguished by their transient a7 GFP expression (Fig. 5C ). These cells occur in clusters (Fig. 5C insert) and their prominently labeled processes can be followed using serial section sets to the cochlear structure where they give rise to the fiber bundles and the point of dissemination associated with the SG (Fig. 5C and insert) . The anatomical location of these cells suggest that these cells are within the forming olive complex, which is consistent with the reports of pioneering fibers that originate from the developing olive complex and extend to the developing cochlea (Zuo et al. 1999 ). These fibers were not detected after E15.5. During the E15.5-16.5 period, there was essentially no labeling of neuronal processes by a7 GFP (Fig. 5D-F) .