Fig. 1. Gene expression of BMP4 in developing chick inner ear, stages 11–19, by whole-mount in situhybridization. The stages of embryos illustrated are as follows: (A) 12 somites, stage 11; (B,C) 21 somites, stage 13; (D,G) 26 somites, stage 16; (E) 28 somites, stage 16; (F, H, I) 30 somites, stage 17; and (J) 40 somites, stage 19. At stage 11 (A), BMP4 expression was detected in the medial and posterior margin of the otic placode (arrowheads). By stage 13 (B), BMP4 expression was present in the dorsal and posterior margin of the otic cup (black arrowheads). Some transcripts were detected in the ventral margin of the otic cup (white arrowheadsin B), but the majority of this hybridization signal was found to be contributed by ectoderm adjacent to the otic cup shown in a transverse section (C) of an embryo the same age asB (stage 13). Arrowheads inC point to hybridization signal within the otic epithelium, and the arrow points to signal in the adjacent ectoderm. At 26 somites (stage 16; D), the expression in the dorsal and posterior margin remained whereas the expression in the ventral portion of the otic epithelium had expanded (arrowheads in D, G). By 28 somites (stage 16; E), a positive streak appeared in the anterior otocyst (arrowhead) and the posterior hybridization signal became more restricted (black arrow). In addition, the dorsal portion of retina (single arrow) and the olfactory placode (double arrows) were also positive. A slightly older embryo (30 somites, stage 17; F) showed a similar pattern, with the arrowhead pointing to the anterior streak and the arrow pointing to the beginning of the posterior focus. In this embryo (F), the otocyst was closed as indicated by the notch shown in a transverse section (H). The hybridization signal from the anterior streak was present within the otic epithelium (arrow inH). Hybridization signal in the posterior focus was also within the otic epithelium, as shown in a more posterior section (arrow in I). By stage 19, two BMP4-positive foci were evident in the otocysts (arrowsin J). Scale bars: A,B, D–F, J, 200 μm; C, G–I, 100 μm.