Fig. 4

Mosaic analysis reveals cell autonomous requirements for, and affects of, STAT3 activation upon CaP motoneuron pathfinding. White arrowheads in all panels mark donor cells that did not differentiate into neurons; most are EVL cells. Panels A–L show embryos at 27 h post-fertilization (p.f.). Embryos in panels M–P are at 48 h p.f. In all images, rostral is to the left, and dorsal is up. (A–L) Cell transplantation chimeras show a cell autonomous requirement for STAT3 in CaP motoneuron pathfinding and axon bifurcation upon constitutive activation. (A, C, E, G, I, K, and L) Donor motoneurons transplanted into host embryos revealing axon projections of the donor neurons. (B, D, F, H, and J) ZNP1 staining of primary motoneurons present in the preceding panel. (B) Wild-type donor CaP transplanted into a wild-type host showing that the transplant technique does not alter axon projections or donor viability. (C) Wild-type donor CaP and MiP showing normal axon paths in a STAT3 MO host. (E) Donor CaP motoneuron expressing wild-type STAT3 showing that STAT3 expression does not alter pathfinding trajectory. (G) Dominant negative STAT3 expressing CaP motoneuron showing pathfinding failure (open arrowhead) in a wild-type host. (I) Constitutively active STAT3 expressing CaP neuron shows bifurcated axon projecting both ventrally as well as along the dorsal pathway (arrow). (K) A group of donor motoneurons transplanted into a wild-type host. In no case did expression of any STAT3 variant affect MiP or RoP axon projections. The RoP and MiP pair enclosed by the dashed circle exemplifies donor neurons that were not included in any analysis since they occupy the same hemisegment and donor cell–cell interactions cannot be excluded. The lone MiP motoneuron that is labeled is the only neuron in that hemisegment originating from donor cells and was thus included in cell counts. (L) Representative donor RoP motoneuron. In condition was RoP pathfinding affected. (M–P) Expression of constitutively active STAT3 from a neurospecific promoter leads to ectopic collateral formation. (M) Normal axon projection of a CaP motoneuron expressing wild-type STAT3 from a neurospecific promoter. (O) Expression of constitutively active STAT3 produces a bifurcated axon projecting both ventrally as well as along the dorsal pathway (arrow). (N and P) ZNP1 staining of primary motoneurons present in the preceding panel. (Q–S) Schematic representation of CaP axon projections shown in the above panels.