Fig. 4. The dynamic pattern of EGFR activity detected with dp-ERK. (A-C) Spatiotemporal patterns of dp-ERK in three successive stages of invagination. Cells were labeled for dp-ERK (magenta), trh-lacZ (green), GFP-Moesin (green) expression; DNA is shown in gray scale; x-y and y-z sections of the same placode (labeled with a and b, respectively) are shown for each time point. Additional views of B are shown in Bc to Be. dp-ERK expression was initially detected in the nucleus and the cytoplasm of a few cells in the dorsal side of the tracheal placode (A, arrowhead) and subsequently expanded to fill the entire dorsal half of the placode (B). dp-ERK showed a nuclear localization in the peripheral region (arrowhead) and was concentrated in the apical cytoplasm in the central region. Note that Bc is a deeper optical section of Ba and Bd (position of the section is indicated by a thin white line in Bb). Apical constriction had not yet occurred and only a mild degree of cell boundary smoothing was observed at this stage (Bd). The dp-ERK signal was downregulated after invagination (C). (D,E) dp-ERK expression in Egfr^+/- (D) and Egfr^-/- (E) embryos. D',E' show the dp-ERK signal. (F) rho-lacZ expression was very similar to the pattern of dp-ERK. The tracheal placode (dashed outline) was marked with KNI (Kni). (G) The overlap of KNI and trh-lacZ expression. Scale bars: 10 µm.