Fig. 6. Relationship of SHH gene expression to the developing chick OMC and pRN. (A-C) PHOX2A and SHH gene expression at E 2.5 (A) and E5 (B,C), shown in whole-mount preparations (A,B) and in transverse section (C). The SHH source is confined to the midline at E2 (A), but from E3 expands within the ventricular layer (asterisk in C) to overlie the OMC (B,C). (D) E5 midbrain wholemount demonstrating that ventricular SHH expression also overlies the BRN3A+ pRN. Midbrain BRN3A labeling is first detected at E4 (data not shown). FP, floor plate of hindbrain; IS, isthmus; OMC, oculomotor complex; rFP, rostral, or midbrain, floor plate; RN, red nucleus; tec, tectum.