Fig. 6. Nkx2.5^Cre lineage and Fgf8^lacZ-expressing cells are reduced in Nkx2.5^Cre/+; Fgf8^flox/-; R26R and Nkx2.5^Cre/+; Fgf8^flox/lacZ mutants, respectively. (A-F) Whole-mount Nkx2.5^Cre lineage analysis from E8.5 to E9.5 indicates a drastic reduction of lacZ-positive cells in the mutant class (B,D,F, black arrowheads) when compared with normal controls (A,C,E, black brackets). Histological sections at E9.5 (E',F') reveal a reduction of the Nkx2.5^Cre lineage in the pharyngeal endoderm (PE) in addition to the splanchnic mesoderm (SM), both of which are thinned with apparent reduced cell number. Ventral views (G,H) clearly demonstrate the reduction of the Nkx2.5^Cre lineage in the CM and SM. Fgf8^lacZ-positive cells are also markedly reduced, but present, in SM and OT in the mutant class at E9.5 (J,J') compared with controls (I,I'). Pharyngeal arches are referred to by their respective numbers (1, 2, 3/4); OT, outflow tract; LV, left ventricle; CM, core mesoderm; V, ventricle.