Fig. 2. T-Cre-mediated recombination results in Fgf8 inactivation specifically in mesodermal lineages. (A) Genetic cross generating T-Cre; Fgf8^Flox/2,3 mutants. (B) Cre-mediated recombination deletes exons 2 and 3, generating the Fgf8^2,3 null allele. The probe indicated was used in ISH of embryos in panels C-G and hybridizes only to mRNA generated from the unrecombined Fgf8^Flox allele. (C-G) Lateral views of whole-mount ISH at ages indicated. Mutant embryos are on the right of, or above, the control. Fgf8 expression is minimally affected in E6.5 mutants (C), but is greatly diminished in E7.5 mutants (D) and absent in E8.0 mutants (E). (F,G) Fgf8 is expressed in neural epithelium and surface ectoderm structures at E8.5 (F) and E9.5 (G), but is absent in somites (asterisks), tailbud and nephrogenic cord (G). aer, apical ectodermal ridge; anr, anterior neural ridge; cp, commissural plate; hp, heart progenitors; hf, headfold; mhb, mid/hindbrain junction; nm, nephrogenic mesenchyme (cord); ps, primitive streak; tb, tail bud. Horizontal bars indicate the extraembryonic/embryonic border.