(a) Representative scanning electron micrograph of proximal and distal regions of E6.5 BPs. There is no apparent stereocilia formation at this stage in either region. (b) Micrograph of E6.5 BP explant cultured 7 days in control medium. Proximal and distal regions are indicated. (c) Quantification of the number of stereocilia that formed in the proximal and distal regions of treated BPs. HCs form fewer stereocilia in both proximal and distal regions in RA-treated BPs. HCs form more stereocilia in proximal and distal regions of citral-treated BPs. *P<0.05; one-way ANOVA with Newman–Keuls post-test. (d–i) Representative micrographs of HCs that form in the proximal and distal regions of E6.5 BPs cultured for 7 days in control media (d,g), with RA (e,h), or with citral (f,i). Total number of BPs analysed are n = 8 for control; n = 6 for RA; n = 8 for citral. Graph shows mean±s.e.m. Scale bars in b, 100 µm; a, c–h, 1 µm.