Fig. 8. FGFR-ERK signaling in extra-embryonic ectoderm. Confocal images (400x) of dp-ERK stained embryos superimposed on DIC images at (A) 5.5 dpc, (B) 6.0 dpc, (C) 7.0 dpc and (D-E) 7.5 dpc. The entire extra-embryonic region is shown in A,B whereas the EPC has been removed in C-E. White brackets indicate extent of dp-ERK staining in extra-embryonic ectoderm. (E) Sagittal section of D showing dp-ERK in both the internal and peripheral layers of the extra-embryonic ectoderm. (F-H) Schematic diagram depicting the Fgfr2-expressing extra-embryonic ectoderm in yellow, the domains of dp-ERK in the extra-embryonic ectoderm in red and the location of the proposed FGF signaling sources in blue. Boxes in F-H demarcate the region of embryo shown in confocal images (A,B,E). Initially, a gradient of dp-ERK can be seen throughout the entire extra-embryonic ectoderm at 5.5 dpc (A,F). This gradient becomes reduced and spans a distance of six to nine cell diameters by 6.0 (B,G). The upper boundary sharpens and the band of dp-ERK is reduced to four to six cell-diameters by 7.0 (C) and to two to four cell-diameters by 7.5 dpc (D,H). In situs show expression pattern of Fgfr2 throughout the extra-embryonic ectoderm at 6.5 (I) and 7.5 dpc (J). In situs of Eomes at 5.5 (K), 6.5 (L) and 7.5 dpc (M) correspond closely to regions of ERK activation. (I-M) Adapted, with permission, from Ciruna and Rossant (Ciruna and Rossant, 1999).