Fig. 1.

Schematic diagrams of neuromuscular organization in developing zebrafish embryos. A, Side view of four myotomes at different stages of development; rostral to the left and dorsal to the top in this and all subsequent side views. The myotome on the left (1) shows the extent of axonal outgrowth by CaP (blue) and MiP (orange) at ∼20 h. The CaP axon has reached the distal end (broken line) of the common pathway (a), whereas the MiP growth cone has not exited the spinal cord (sc). The second myotome (2) shows CaP and MiP at ∼21 h. The CaP axon has extended ventrally beyond the common pathway onto its cell-specific pathway (b), whereas MiP has a ventral process on the common pathway and a dorsal collateral axon on the MiP pathway (c). Myotome 3 shows CaP and MiP at ∼26 h. CaP innervates ventral myotome and MiP innervates dorsal myotome; MiP has retracted its ventral process from the common pathway. By 16 h (myotome 4), three to six identifiable muscle pioneers (mps) are recognizable at the distal end of the common pathway. B, Transverse section of developing zebrafish embryo showing the positions of CaP axons relative to the muscle pioneers (mps) at ∼20 h.nc, Notochord. Scale bar, 25 μm.