Persistent Fgf signalling is required for maintenance of myod expression and normal fast muscle differentiation. (A) Schematic showing timecourse of SU5402 treatment regimes (C,D,F,G,I,J,L,M) and control vehicle treatment (B,E,H,K). (B) Around 9 s, myod expression is clearly visible in each nascent somite (arrowheads). (C) Treatment with SU5402 for 30 minutes substantially reduces lateral myod expression (arrowheads). (D) Pre-treatment for 1 hour prevents all lateral expression (arrowheads). Inset in D shows adaxial myod expression in tailbud. (E) At 10-12 s, myod expression is well established in lateral cells of all somites. (F) Exposure to SU5402 for 30 minutes reduces lateral myod expression without noticeable effect on adaxial expression. (E-G) Dorsal flatmounts of posterior embryo (right) with nascent somites shown at high magnification (left). (G) Pre-treatment for 1 hour essentially ablates lateral myod expression. (H,I) SU5402 treatment for 2.5 hours from 10 s (I) abolishes new myod stripes (I, arrowheads; compare with control in H), but has less effect in anterior somites (arrows). Dorsal views of whole mount (left) and flat mount (right). There is similar expression in rostralmost somites (white arrows). (J) SU5402 treatment for 2.5 hours, followed by a 2-hour wash-out permits recovery of myod-expressing stripes in more than the four newly-formed somites (arrowheads). (K) At 23 s, mylz2 expression reveals abundant differentiated fast muscle. Lateral view, dorsal towards the left. (L,M) Moderate (L; 27/36 treated embryos) and severe (M; 5/36) examples of effect of late SU5402 application at 18 s to otherwise wild-type embryos. Anterior is towards top in all panels.
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