Fig. 1. Spatial and temporal patterns of phosphorylated ERK in postimplantation mouse embryos. (A-J) Embryos from indicated stages (5.0-10.5 dpc) were fixed and stained for dp-ERK immediately upon dissection to preserve endogeneous domains of signaling. (K,L) Embryos (9 dpc) were cultured in the absence (K) or presence (L) of 50 µM U0126 (MEK inhibitor) for 45 minutes prior to dp-ERK labeling to confirm staining was specific for the phosphorylated form of ERK. (A-F) Monochrome epifluorescence images of Cy3-dpERK labeled embryos. (G-L) DAB-HRP-dp-ERK stained embryos. Sustained ERK activation (color-coded with red lettering) is observed in the ectoplacental cone (epc), extra-embryonic ectoderm (exe), branchial arches (ba), frontonasal processes (fnp), tailbud (tb), limb buds (lb), forebrain (fb), midbrain-hindbrain boundary (mhb), foregut (fg) and liver primordia (l). Brief ERK activation (pink lettering) is seen in the distal tip of the epiblast (dt), allantoic bud (al), blood island mesoderm (bi), headfold mesoderm (hfm), heart primordia (h), sinus venosus (sv), dorsal aorta (da), intersomitic vessels (iv), eye primordia (ey), ear primordia (er), nasal pits (np), caudal region of somites (cds) and ganglia (g). Scale bars: 50 µm in A-F; 400 µm in G-L.

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