Fig. 2. The sensory primordium in the cochlea is marked by p27^Kip1 and the zone of non-proliferating cells (ZNPC). Timed-mated pregnant animals were injected with BrdU at E12.5 (A,B), E13.5 (C,D) and E14.5 (E,F), and sacrificed 8 hours later. Adjacent sections from each stage were stained with antibody to either BrdU (A,C,E; green) or p27^Kip1 (B,D,F; red). (A,B) At E12.5, BrdU stained nuclei are seen throughout the cochlear epithelium, including at the site of future organ of Corti formation (A). p27^Kip1 is not expressed at this time (B). The site of future organ of Corti formation (arrows) was determined by staining for spiral ganglion cell axons containing ßIII-tubulin or neurofilament (data not shown). (C,D) At E13.5, BrdU-stained nuclei are absent from a region of the cochlear epithelium (brackets), here termed the zone of non-proliferating cells (ZNPC) (C). In an adjacent section, the ZNPC can be seen to correspond precisely with the expression domain of p27^Kip1 (D). (E,F) At E14.5, the ZNPC is still present (E) and the p27^Kip1 staining persists (F). Scale bar: 50 µm.