Coincident with this expression was strong staining of pioneering efferents that become separated into individually distinguished processes as they progress through the spiral ganglion (SG) to reach the external face of this sensory domain ( Fig. 2C ; see below). The staining of the epithelial cells of the lesser epithelial ridge intensifies thereafter (e.g., E15.5 in Fig. 2E ). At this stage, expression of a7 GFP by cells of the SG was in general only weakly observed in scattered cells (Fig. 2E ). By E16.5, a7 GFP expression continues to increase in cells of the lesser epithelial ridge of the prosensory domain where OHC and Deiters' cells can now be distinguished (Fig 2F and G and insert) . Cells throughout the SG were also revealed by expression of a7 GFP by this developmental stage. Pillar cells do not express a7 GFP and there were no identifiable efferent processes labeled by the expression of this receptor at this stage or thereafter (see the following sections). the a7 gene (Chrna7) was modified using homologous recombination to add a C-terminus epitope tag (hemagglutinin [HA] ) and inserted into the 3′ terminus of Chrna7 a reporter bicistronic internal ribosome entry sequence (IRES)-tau fusion to enhanced green fluorescent protein (eGFP) fusion protein cassette (a7 GFP ; see Methods and ). This construct was subsequently altered by replacing the tau:GFP cassette with the Cre-recombinase gene (a7 Cre ). (B, C) The visualization of the Chrna7 transcription using immunological detection of GFP compared with background. Shown are sagittal sections of the cochlear sensory structures of an E16.5 a7 GFP embryo in (B) and at greater magnification in (C). The panels on the left are stained for GFP expression (see Methods), whereas the image on the right shows an adjacent serial section that received the same staining treatment, only primary antibody was omitted. Photographs were collected at the same gain and exposure. The asterisk identifies cochlear ducts and the arrow points to the spiral prominence and the arrow head points to cell giving rise to the outer hair cells and Deiters' cells. Abbreviations are SG, spiral ganglion; and tg, trigeminal ganglion. In (B), the bar = 100 lm and in (C), the bar = 400 lm. (D) Examples of colabeling for a7 GFP (green) and anti-HA (HA) in cells associated with the spiral ganglion at E16.5. Examples of double-labeled cells are identified by with arrows. Some processes are also colabeled (arrow head). Bar = 50 lm.