The effects of Pou4f3 ddl mutation on the development of the cochlea is shown. A, B) Atoh1-LacZ remains expres-sed in undifferentiated hair cells near the apex at least until six weeks of age; B) counterstaining nerve fibers with OsO4 for myelin shows that many myelinated nerve fibers extend as radial fibers to the apex; C, D, F, G) immunocytochemistry for tubulin reveals that even after 6 to 14 weeks many pillar cells survive in a patchy distribution in particular near the apex; D) nerve fibers can be stained to the habenula perforate but rarely past this; E, H) staining for Myo VII, a reliable marker for hair cells, shows some immunopositivity in cells around pillar cells (stained with anti-tubulin, red). In the absence of a signal from Atoh1-LacZ, these cells likely reflect some upregulation in supporting cells around the pillar cells. G, I) SEM show loss of all structural features of the organ of Corti, except that the area of the organ of Corti is somewhat elevated, possibly due to the remaining pillar cells. ISS: inner spiral sulcus; OC: organ of Corti; OSS: outer spiral sulcus; SL: spiral limbus. Bar: 100 μm (A, B, C, F) and 10 μm (D, E, G, H, I, J).