Figure 4: Sim1 is specifically expressed in the avian forelimb and is associated with flight feather development.

(a) Expression pattern of Sim1 in fore- and hindlimbs in chicken (HH32), gecko (23 dpo) and mouse (E13.5) embryos. Black arrowheads indicate specific expression in the posterior margin of the forelimb in chicken. (b–e) Expression of Sim1 (b,c), Shh (d) and Bmp7 (e) on the longitudinal sections of the chicken forelimb at HH35. (c) is a higher magnification image of boxed area in b. All photographs are oriented with distal to the right and posterior to the bottom. (f–i) Expression of Sim1 (f,g), Shh (h) and Bmp7 (i) on the transverse sections in the zeugopod region of the chicken forelimb at HH36. (g) is a higher magnification image of boxed area in f. White arrowheads indicate the flight feather buds. All photographs are oriented with dorsal to the right and posterior to the bottom. (j) Expression pattern of Sim1 and Bmp7 at HH30 and HH31. Neither the Sim1 expression nor the spot-like expression of Bmp7 was observed at HH30, whereas both of them were detected at HH31 (white bracket and arrowheads). Insets indicate the Sim1 expression from the posterior view. (k) Photos of the whole body (top) and the feathered foot (bottom) of an adult Cochin bantam. D1-4 incicate digits 1–4, respectively. Digit 4 is not seen from this angle because of heavily covering feathers. (l) Expression of Sim1 in the forelimb (top) and hindlimb (bottom) in the Cochin bantam embryo at HH34. Bracket indicates Sim1 expression in the hindlimb. Scale bars, 1 mm (a,b,j,l); 500 μm (c–i).