Fig. 2. Comparison of Fgf8 mRNA expression and Fgf8^lacZ ß-gal activity relative to AHF development. (A) Fgf8 mRNA, as seen by in situ hybridization, is expressed in the presumptive AHF, but not the primary (1°) heart field (arrow) as early as E7.75. (B,D,F,H,J) Fgf8^lacZ expression, as seen by ß-gal activity, is observed in AHF cells from E7.75 through E10.5 in whole mount. (C,C') Expression is later seen in the OT and the contiguous SM at E9.0 in whole-mount (C) and mid-sagittal section (C'). Fgf8 mRNA levels in the OT and SM become reduced at E9.5 (E,E'), and is no longer detectable in AHF cells at E10.5 (G,I). Fgf8^lacZ expression at E8.5 in whole-mount (K) and in transverse sections (K1-K3). ß-Gal activity is noted in the SM (black arrowheads) and in the outflow tract (OT). Mid-sagittal sections of Fgf8^lacZ embryos reveal ß-gal activity in the PE and SM at E9.0. At E9.5, ß-gal activity is noted in the SM and OT (F'). At E10.0, Fgf8^lacZ-positive cells are observed in the pharyngeal CM in whole mount (L) and in coronal section (L'), which are contiguous with the OT. OT, outflow tract; SM, splanchnic mesoderm; CM, core mesoderm; PE, pharyngeal endoderm; LV, left ventricle; SV, sinus venosus.