Plexin A3 controls where and when collateral CaP axon branches form.

(A, B) Time-lapse of a wild-type CaP axon traversing the ventral myotome between 22 hpf and 30 hpf. (C, D) Time-lapse of a wild-type sibling embryo, demonstrating that branching begins at ∼30 hpf, typically with axonal bifurcation (arrow) followed by axon collateral formation (arrowhead). (E, F) In plexin A3 mutants, collateral branches (arrowhead) form precociously between 22 and 30 hpf. (G, H) In plexin A3 mutants collateral formation plateaus between 30 hpf and 38 hpf. (I) Quantification of CaP primary axon branches between 22–30 hpf. (J) Quantification of CaP primary axon branches between 30–38 hpf. (H) Quantification of the spatial location of the first proximal collateral relative to wild-type at 48 hpf. Error bars indicate SEM. Scale bar, 15 µm.