Met is required for secondary motoneuron formation. (a-b') gata2:GFP (green) transgenic embryos at 48 hpf labeled with antibody to Alcam (red). gata2:GFP is expressed in ventrally projecting SMNs, some of which also express Alcam (a); (a') shows only the GFP. met MO-injected embryos (b,b') show a severe decrease in SMNs, but Alcam labeling of the floor plate appears normal. A small number of SMNs form and make ventral projections (arrow). (b) Both markers shown; (b') only GFP shown. (c,c'). Some of the GFP-positive cells in met MO-injected embryos show interneuron-like axons. (c) An interneuron with an ascending axon; (c') an interneuron with a descending axon. The two GFP-positive cells to the right in (c') are SMNs. (d-g) gata2:GFP transgenic embryos (green) at 48 hpf labeled with Alcam antibody (red). Exposure to SB203580 (f) severely decreased the formation of SMNs relative to controls (d), but the overall architecture of the spinal cord appeared normal based on expression of GFP driven by the pax2a promoter (e,g). Scale bar, 20 μm in all panels except (c,c'), which is 10 μm.