Fig. 3.

A) Scanning electron micrographs from apical, middle and basal regions of Emx2^+/+ and Emx2^KO/KO neonatal mice. The bottom left panel shows 4 rows of hair cells running from left to right of the image. The 3 upper rows are composed of outer hair cells and the bottom row is composed of inner hair cells. One of the outer hair cells is marked by 2 asterisks, between which lies the V-shaped hair bundle composed of stereocilia. The cell surface inside the V is covered with microvilli whereas outside the V it is smooth. The hair bundles on the inner hair cells have a less pronounced V shape. Note that in Emx2^KO/KO mice there were fewer hair cells in less defined rows. Most hair cells existed singly or in pairs and lacked elements of their normal planar polarity. In morphological terms they appeared to differentiate normally if slightly slower than normal. Signs of delayed hair cell differentiation were most obvious in the apex at this stage. Scale bar = 20 μm. B) Counts of the numbers of hair cells per 100 μm of epithelium revealed no difference between Emx2^+/+ and Emx2^KO/+ pups but a decrease of about 55% in the Emx2^KO/KO pups (t-test — p < 0.001 between Emx2^KO/KO and both Emx2^KO/+ and Emx2^+/+ mice for all parts of the cochlea).