Ablation of the a7 Cre -expressing cell lineage confirms a7 GFP expression during cochlear development Although a7 GFP expression was not detected in the developing cochlear structures until E13.5 (Fig. 2B) , as reported previously the earliest a7 expression we have defined is at P9.0 in rhombomeres 3 and 5 . Because cochlear morphogenesis includes signaling from rhombomere 5 (Liang et al. 2010) , the possibility of a7 GFP contributing to the development of this structure was examined. This was done using embryos from a7 Cre mice crossed with mice harboring the conditional ROSA26-loxp (diphtheria-A toxin (DTA; termed a7 Cre: DTA ; . In these embryos, a7 Cre: DTA -expressing cells and their direct lineages were ablated, thus revealing expression that could have been be missed by a7 GFP measurements ). An example of the cochlear structure at E16.5 taken from a7 Cre:DTA crosses is shown in Fig. 6 . Because there is only occasional overlap with a7 GFP (see Fig. 5E ), we used peripherin expression to aid in examining the fate of non-a7-expressing cells (Fig. 6A and B) . The overall patterning of the cochlear structure and the formation of major boney structures of the cochlea inclusive of the otic capsule and modiolus were intact, albeit somewhat distorted. The cochlear ducts were collapsed (Fig. 6B) , probably due to the absence or severe thinning of the distal lateral wall. Also absent was the sensory cell domain Figure 5 . The a7 GFP expression during cochlear innervation. Innervation of the developing cochlear structure is revealed by a7 GFP labeling. (A) An E13.5 sagittal section shows a group of efferent processes (arrow) that distribute to solitary fibers that are strongly labeled for a7 GFP expression (arrow heads). Cells of the putative sensory region (sr) and the spiral prominence (SP) are identified. (B) At greater magnification, these fibers (arrow heads) have a beaded appearance and project towards the base of sr. (C) The possible origin of the pioneering efferent fibers is suggested by the intense expression of a7 GFP in the E13.5 cell groups (arrow) located caudal to the trigeminal sensory nucleus (V) consistent with the early olive in this horizontal section through the posterior brain stem. At increased magnification (Insert), the cell clustering (arrow) and their projections (arrowhead) are identified. Serial sections (not shown) reveal continuity between these cells and those entering the cochlear structures (arrow heads). (D) E15.5 a7 GFP expression and colabeling with other neuronal process markers (red). The processes that express peripherin (arrow) end mostly in the vicinity of the inner hair cells (IHC). Occasional solitary fibers (arrow) extend towards the base of the outer hair cells (OHC) at the dorsal boarder of the Deiters' cells (D). (E) The E16.5 cochlear innervation pattern looks much the same as E15.5, although the peripherinlabeled fibers (arrow) are more distinct. These processes lack detectable a7 GFP expression. (F) Olivocochlear efferents identified by calcitonin generelated proteins (CGRP; arrows). (G) The E18.5 embryo exhibits afferents detected by a7 GFP expression (arrows). These extend from SG cells that are not colabeled with peripherin (not shown). (H) At birth (P0), there are distinctly labeled a7 GFP afferents (arrowhead) and peripherin-labeled efferents that extend to the Deiters' cells (D) and then turn (arrows) to contact the base of the OHCs. Hensen's cells are noted (H). (I) The P12 innervation pattern is similar to the P0. In this merged image of a7 GFP expression (green) and peripherin (red), many spiral ganglion (SG) cells and processes are labeled, but the labels only rarely overlap in the same processes (see insert). The a7 GFP identify mostly processes reaching the IHCs (arrow). Peripherin-labeled processes mostly terminate at the base of the outer hair cells (OHC) or onto the a7 GFP -labeled afferent fiber near the base of the IHC. Hensen's cells expressing a7 GFP is identified (H). The inset shows the sensory cell region at increased magnification. The arrows identify the a7 GFP -expressing afferent ending at the base of nonlabeled IHC, whereas the double arrow heads point to the peripherin-labeled terminal. Other peripherin processes extend to the base of the OHCs (individual arrow heads). (J) Diagrams as in Fig. 2 depicting the basic innervation patterns observed in this study. Green is a7 GFP and red is peripherin. Afferent (af) and efferents (ef). Bars = 50 lm containing presumptive OHCs and Deiters' cells ( Fig. 6C and D), as expected from results of a7 GFP expression (Figs. 2, 5) .