Fig. 4. Distribution of EphA and ephrin A in stage 25 hindbrains. (A) EphA3-AP in situ binding on a stage 25 hindbrain revealed ephrin A activity. The blue bracket indicates the rostrocaudal extent of the lateral ephrin A gradient in the caudal hindbrain. (B) Ephrin A2-AP in situ binding on a stage 25 hindbrain revealed EphA activity. The blue bracket indicates EphA3-positive domain correlating with the position of presumptive cC-VC neurons. A ventral column of cells adjacent to the floor plate is also EphA positive (see also lower panels in C). (C) Top panel shows a transverse section of a hindbrain with its cC-VC neurons retrogradely labelled (retrogradely labelled cC-VC neurons indicated by white arrowhead, and cC-VC commissures indicated by white arrow). The same section was subjected to ephrin A2-AP in situ binding, shown in the middle panel in C; black arrow shows the EphA-positive commissure. The top and middle panels were superimposed and DiI signals were traced onto the AP signals by red dots (bottom panel). cC-VC neurons and their axons appear to be EphA positive. Scale bar: 400 µm in A,B; 100 µm in C.