Fig. 1

(A) Schematic representation of the skeleton of a ray in the zebrafish fin. Dermal bone forms two segmented, concave and opposing hemirays. The diagram illustrates the position of amputations, relative to the bifurcation of the lepidotrichia; "low cut" and "high cut" are performed one to two segments proximal and four to six segments distal to the origin of bifurcation, respectively. (B) Sequence of shh expression patterns after a low cut, leading to ray bifurcations: (B1) 2 dpa, an unbifurcated ray expresses one shh domain in the distal blastema; (B2) by 4-5 dpa shh is expressed in two domains, preceding the formation of a bifurcation; (B3) two newly formed sister rays each exhibit one shh expression domain; (B4) the process repeats itself to form a second round of bifurcations (not to scale). (C) Ectopic expression of EGFP 24 h after injection of the _pcmv5EGFP reporter construct in the tissue separating the branches of lepidotrichia (L) of two adjacent rays. Injections were performed at 2 dpa (high cut shown by the dotted line). (D-I) Normal bifurcating (D, E) and N-shh (F, G) or bmp2b (H, L) injected fused rays stained with Alcian blue. Constructs were injected in the region indicated by white arrows (F, H). (D, F, H) Whole-mount control and fused rays; proximal part of the fin is to the left. (E, G, I) Cryosections (16 μm) in the region of normal bifurcation (E) or fusion (G, I). Arrows in G and I indicate the region of ectopic bone matrix deposition. No ectopic bone deposition occurs in the mesenchyme interior of the lepidotrichia (L) as marked with asterisks. (Bar = 50 μm.)