Fig. 7

AChR clusters form but do not mature or persist in the absence of axons. (A) An HB9:mGFP^+/+ fish at 24 hpf. Out of six segments shown (asterisks), axons extend in only two. (B-E) HB9:mGFP^+/+ fish at 19 (B), 22 (C), 24 (D) and 48 (E) hfp, counterstained with BTX (red). Bands of AChR clusters formed in segments that were not innervated, but did not persist. AChRs did cluster following outgrowth of secondary axons (E). Asterisks in B indicate aneural clusters; asterisk in D indicates muscle pioneers without clusters. (F) Analysis of cluster number in segments that were not innervated (Un) and in innervated segments (In) of HB9:mGFP^+/+ fish. Segments that were not innervated contained the same number of clusters as innervated segments at 19 hpf, but contained nearly twice as many by 22 hpf. In the absence of innervation, most clusters disperse by 24 hpf (n=10-35 segments per bar, average=16; P=0.82 at 19 hpf, P<0.0001 at 22 hpf, P<0.0001 at 24 hpf). (G) Clusters present in innervated segments and in segments that were not innervated were similar in length at 19 and 22 hpf, but differed at 24 hpf (n=30-102 clusters per bar, average=60; P=0.31 at 19 hpf, P=0.92 at 22 hpf, and P<0.05 at 24 hpf). Scale bar in B: 10 µm for A-E.