Fig. 7. The anterior region of r1 in Gbx2^neo homozygotes assumes isthmus-like characteristics. (A,B,E-H) Sagittal sections of E10.5 embryos (indicated by white lines in C and D). (C,D,I,J) Dorsal views of E10.5 mid/hindbrain regions. (K,L) Flatmounts of E10.5 lateral mid/hindbrain. (A,B) Normal isthmic Fgf8 expression (A) extends caudally into r1 in Gbx2^neo homozygotes (B) (also compare with Fig. 6A,B). (C-F) Math1 expression is absent throughout medial r1 and anterior rhombic lip regions in Gbx2^neo homozygotes. (G,H) Immunohistochemical staining using an anti-phospho-Histone H3 antibody reveals that the mitotically less active region of the isthmus r1 in control embryos (G) extends caudally in Gbx2^neo homozygotes (H). Red brackets indicate regions of reduced proliferation. (I-L) E10.5 embryos stained for cyclin D2 expression demonstrate that a region of reduced cyclin D2 expression (line in I and right brackets in J-L) is expanded caudally in mutants. c, caudal; CEP, cerebellum primordium; Mb, Midbrain; r, rostral; RL, rhombic lip; r1, rhombomere 1.