Fig. 7. FGFR-ERK signaling in limb buds. Dp-ERK staining in forelimb region at 9.0 dpc (A), 9.5 dpc (B,C) and 10.5 dpc (D). Initially, dp-ERK is detected in surface ectoderm (se) overlying lateral plate mesoderm (A). As limb bud outgrowth continues (B-D), a gradient of dp-ERK is seen in mesenchyme directly beneath the apical ectodermal ridge (AER). Transverse sections through hindlimbs at 9.5 dpc (E), 10.0 dpc (F) and 10.5 dpc (G). (H,I) Domains of FGF-dependent dp-ERK in developing limbs and proposed directions of FGF signaling shown with arrows. Fgfr1 expression is shown in yellow, Fgfr2 expression in brown and dp-ERK regions in red. Blue arrows indicate FGF signaling in the proximodistal directions, which is proposed by current models of limb development (Martin, 1998). Dp-ERK staining in the dorsal and ventral surface ectoderm suggest the possibility of mesenchyme to surface ectoderm FGF signaling or autocrine signaling within the surface ectoderm, as indicated by green arrows. Gradients of dp-ERK in mesenchyme beneath the surface ectoderm suggests FGF signaling from the surface ectoderm to mesenchyme (purple arrows). Scale bars: 50 µm.