Fig. 2

a Time lapse observations of the Erk activity domain in DREKA embryos following exposure to a low (50 μM) concentration of SU5402 (an inhibitor of the Fgf pathway). The time span between images is 4 min. (the period of the clock in this experiment is about 45 min.) The yellow lines point to the boundary of Erk activity which periodically jumps by about 50 μm except in the first period after administration of SU5402 where the jump is larger (labeled as SU). Scale bar: 50 μm. b–e Normal somitogenetic development in DREKA embryos (control). b Erk activity in DREKA embryos at 14 somite stage: dark nuclei point to high Erk activity. c The false color images represent the Erk activity averaged over a 30 μm perpendicular stack of images. Notice the low activity area in the pre-pattern region (shown by the bracket) at the anterior PSM. d In situ hybridization (ISH) against xirp2a (a marker of somite boundaries) in 30 hpf embryos. e Immuno-Histochemical (IHC) staining against Mesp2a, a determinant of the last somite boundary (arrow). f–i Somitogenetic development in DREKA embryos in presence of a high (200 μM) concentration of SU5402 from the 10 somite stage (arrow in (h)). Somitogenesis is impaired past 13 somite stage (indicated by a * in h) with unclear somite boundaries. f, g The Erk activity is repressed (low) throughout the PSM. h ISH against xirp2a in 30 hpf embryos. i The determinant of the last somite boundary (Mesp2a), shown by an arrow is absent past 13 somite stage in i but present in e. Scale bar: 50 μm.