This study extends the reports of spatiotemporal regulation of a7 expression during mouse embryonic development to include the cochlear sensory structure, as well as confirms the extensive expression of this nAChR in the ascending central auditory system. The novel finding that in addition to expression of a7 GFP in developing sensory cells of the cochlear structure and neuronal cells of the spiral ganglion, there is also considerable expression by nonsensory cells. Cells of the spiral prominence and ligament, Deiters' cells, and some Hensen's cells. Despite overall agreement between our studies and those using in situ hybridization (e.g., Happe and Morley 1998; Morley and Happe 2000) , these nonsensory cells were not reported previously to express a7. However, these comparisons are incomplete because the earlier studies did not necessarily show the comparable structures or the developmental stages at times where we observed peak a7 GFP expression. Also, our method of detecting GFP as a marker of a7 expression offers improved sensitivity and resolution that has previously not been available for this nAChR.