Fig. 3. A sequential change in progenitor-cell gene expression in the rhombic lip distinguishes laterally from medially fated roof plate cells. (A-D) Doubly transgenic Wnt1::cre; Cre-responsive ßgal indicator embryos, with ßgal detected by X-gal staining. (A-C) Dorsal whole-mount views showing ßgal activity throughout the entire hindbrain roof plate epithelium (hRPe; arrows), both medially and laterally. Black broken lines demarcate the rhombic lip. Inset in A shows cre mRNA in the lower rhombic lip at E9.25 (arrowhead); Wnt1 and Wnt1::cre are expressed in dorsal hindbrain neuroepithelium, but not in hRPe, from E8 onwards. (D) Transverse section taken at the rhombomeric level of the white broken line in C, revealing ßgal+ hRPe cells medially (med) and laterally (lat). (E-H) Doubly transgenic Gdf7::cre; R26R embryos, with ßgal detected by X-gal staining. (E-G) Dorsal whole-mount views showing ßgal in laterally (arrows) but not medially located hRPe. Inset in E shows little to no cre mRNA in the rhombic lip at E9.25, with Gdf7 and cre expression starting between E9.25 and E9.5. (H) Transverse section taken at the rhombomeric level of the white dashed line in G, revealing ßgal+ hRPe cells laterally only (arrowheads). Black broken line demarcates the rhombic lip. (I) Schematic of hRPe fields at E11.5. Medially located hRPe (yellow) has a history of Wnt1 expression, whereas laterally located hRPe (blue) has a history of Wnt1 and Gdf7 expression. ov, otic vesicle; hb, hindbrain; med, medial; lat, lateral.