Fst transcripts were present in the lesser epithelial ridge (LER) region of the cochlea in an increasing base-to-apex gradient during neonatal (A,B) and embryonic development (C-H). (A,B) At P0 and P8, Fst expression was barely detectable in the basal cochlear turns (A1,B1, asterisks), but was evident in the lateral side of the LER in the middle turns (A2,B2, brackets) and its expression domain included the entire LER region in the apical turns (A3,B3, brackets). Insets in A1–B3 show expression domains of Myo15 or Atoh1 in the adjacent sections to indicate the location of hair cells. Arrowheads indicate the lateral border of differentiating hair cells. (C,D) Fst expression patterns at E15.5 were comparable to those observed at P0 and P8. Fst expression domains largely overlapped that of Bmp4 in the LER area. (E1–H3) The increasing gradient of Fst expression patterns in the cochlea was already apparent in the cochlear primordium at E10.5 (F3,G3,H3, asterisk, brackets). Fst expression was not detectable in the presumptive vestibular organs including the lateral crista (E1–E3). Expression domains of Bmp4 (E1,F1,G1,H1) and Lfng (E2,F2,G2,H2) indicated the lateral and medial regions of the developing cochlea, respectively. The schematic diagram indicates the level of sections for the pictures shown in E1–H3. Scale bar in A (200 µm) applies to B-D; scale bar in A1 (50 µm) applies to A1–A3, B1–B3, C1–C3, and D1–D3; Scale bar in E1 (100 µm) applies to E1–H3.