Fig. 4. Spatiotemporal expression patterns of BMP pathway members in quail. (A,B) Bmp2 (white), detected at low levels in epithelium at HH23, is expressed across the mandible at HH25. (C) Bmp4 transcripts are restricted to epithelium at HH23 (arrow). (D) By HH25, Bmp4 transcripts are expressed only in mesenchyme (arrow). (E,F) Bmp5 is not expressed at HH23 but is detected at HH25 in mesenchyme. (G,H) Bmp7 expression is observed in both epithelium and mesenchyme at HH23 and HH25. (I,J) Alk2 is uniformly expressed throughout the mandible at HH23 and HH25. (K,L) Bmpr1a, although not detected at HH23, is expressed across the mandible by HH25. (M,N) Bmpr1b transcripts are concentrated in lateral mesenchyme and epithelium at HH23 and become localized to medial mesenchyme by HH25.