Fig. 4. Bmp4 expression levels control beak depth and height. (A,B) Large ground finches have thick, broad and long beaks. (C) The embryonic beak of a ground finch exhibits high Bmp4 expression levels, which promote chondrogenesis and therefore increased beak height, length and depth (red arrow). (D) Misexpression of Bmp4 in the frontonasal process mesenchyme of chick embryos produces a noticeably broader and thicker upper beak, paralleling the beak morphology of the ground finch. (E) Alcian staining of chick embryos injected with RCAS-Bmp4 reveals enlarged skeletal elements in the upper beak. (F,G) Cactus finches have thinner, shorter and narrower beaks. (H) The embryonic beak of a cactus finch exhibits very little Bmp4 expression, and chondrogenesis of the beak is not as pronounced, which leads to an overall smaller beak. (I) Misexpression of noggin, a Bmp4 antagonist, in frontonasal process mesenchyme of chick embryos produces a noticeably thinner and narrower upper beak, paralleling the beak morphology of the cactus finch. (J) Alcian staining reveals stunted upper beak skeletal elements in chicken embryos injected with RCAS-noggin. (B-D,G-I) Reproduced, with permission, from Abzhanov et al. (Abzhanov et al. 2004). (E,J) Reproduced, with permission, from Wu et al. (Wu et al. 2004). (A,F) Courtesy of P. Grant, A. Abzhanov and C. Tabin.