Figure 6. Expression of MafB and FoxP2 in V1-INs in early embryos. V1-INs are labeled in red in all panels (tdTomato), MafB- (A, B, E, F, I, J, M, and N) or FoxP2-immunoreactivities (C, D, G, H, K, L, O, and P) in green (FITC) and calbindin-IR in white (Cy5, B, D, F, H, J, L, N, and P). Solid lines delineate the embryonic spinal cord, while dashed lines mark the midline. Dotted lines indicate the edge of the progenitor area. A, B, The few calbindin-IR V1-INs present at E10.5 contain weak or no MafB expression. MafB expression is mostly present in motoneurons at this age (asterisk in A). C, D, No Foxp2 immunoreactivity is visible at E10.5. E, F, At E11.5 MafB expression is partially downregulated from motoneurons and it starts to be present in laterally located calbindin-IR V1-derived Renshaw cells (arrows). G, H, At E11.5 we can also first detect Foxp2 in V1-INs; these are mostly located exiting the progenitor area in the intermediate zone (arrow in G). None of these cells express calbindin (H). I, J, At E12.0 MafB expression is greatly downregulated from motoneurons and is present in most V1-derived Renshaw cells. A group of dorsal horn MafB-positive neurons exit the progenitor area at this age (asterisk in I). K, L, At E12.0 we detected FoxP2 in V1-INs leaving the progenitor area and also in V1-INs that have already migrated ventrally and display more complex morphologies. FoxP2 V1-INs are never calbindin positive (L). M, N, By E12.5 MafB is present in Renshaw cells, a few dorsally located V1-INs, and dorsal horn INs. O, P, At E12.5 Foxp2 is present in V1-INs located mostly dorsal and medial to calbindin-IR V1-INs. Scale bars: (in A, C, E, G) A--H, 50 μm; (in I, K, M, O), I–P, 100 μm.