Fig. 5

Mypt1 maintains motor neuron position.

At the onset of axonogenesis at ~19–20 hp CaP motoneurons in wildtype (A) and mypt1 mutant (B) embryos carrying the mnx1:mKate mnx1:mCD8-mKate transgene are located at the points where motor axons exit from the spinal cord (n = 26/27 CaP in wildtype, and n = 12/12 CaP in mypt1 mutants; arrowheads point to the nascent axons). Time-lapse imaging of CaP motoneurons from the onset of axon initiation at 19 hpf until the axons reached the horizontal myoseptum in mnx1:mCD8-GFP transgenic wildtype (C^1–5) and mypt1 mutant embryos (D^1–5). During the length of the movie (400 min) all wildtype (n = 10/10) and all mutant (n = 25/25) motoneurons retained their position above the spinal cord exit point. Time-lapse imaging of CaP motoneurons labeled with mnx1:mKate in mnx1:mCD8-GFP transgenic siblings (E' to E‴) and mypt1 mutant embryos (F' to F‴) after axons have reached the horizontal myoseptum at 21 hpf until axons have reached the ventral extent of the myotome. While 30/31 wildtype CaP motoneuron cell bodies (red arrowhead) stayed precisely above the exit point (white arrow), 32% (n = 7/22) mypt1 mutant motoneuron cell bodies shifted progressively rostrally (p = 0.006, Fisher exact). A stalling mypt1 mutant axon is seen in an adjacent hemisegment (white star in F).