Fig. 1.

Expression of BMP3 during normal chick limb development. A–L: Whole-mount in situ hybridization of BMP3 expression in developing chick legs (A–G) and histological sections of the whole-mount limbs (H–L). In cross-sections, dorsal is to the left and in transverse sections it is at the top. Hamburger and Hamilton (HH) stages are indicated. A: At stage 20, BMP3 is expressed in the developing somites but not in the hindlimb bud. B: At stage 23, BMP3 is expressed in the dorsal and ventral mesenchyme. C: At stage 24, BMP3 expression broadens and expands distally. D: Anterior view of stage 24 leg bud. BMP3 transcripts are excluded from the central core mesenchyme. E: At stage 25, BMP3 is expressed in both the proximal and distal mesenchyme. F: At stage 27, BMP3 expression outlines where the future zuegopod will form. G: At stage 31, BMP3 is expressed in the perichondrium and forming bone collar of the developing bones in the leg and foot and in the digit tips. H: Schematic diagram of a BMP3 hybridized limb bud showing the approximate levels of the cross- and transverse sections shown in I–L. I: Cross-section through stage 24 leg bud showing subepidermal expression of BMP3 in the dorsal and ventral limb mesenchyme and not in the central core mesenchyme. J: Transverse section of stage 24 leg bud. K: Transverse section of stage 25 leg bud. BMP3 continues to be expressed in the subepidermal mesenchyme and begins to prefigure the cartilage elements. L: Transverse section of stage 27 leg bud. BMP3 is expressed in the developing perichondrium that outlines the forming leg bones. BMP, bone morphogenetic protein.