Fig. 3. Axial tendon progenitor specification and differentiation do not occur in the absence of myotome formation. Whole-mount in situ hybridization for Scx (A,B) and Fgf4 (C,D) at E10.5 on Myf5^+/-; Myod1^+/- (A,C) or Myf5^-/-; Myod1^-/- (B,D) embryos. Blue arrows indicate forelimb buds; green arrows, branchial arches. Section in situ hybridization for myogenin (E,F), Scx (G,H,K,L) and tendin (I,J) on transverse sections of E13.5 Myf5^+/-; Myod1^+/- (E,G,I,K) or Myf5^-/-; Myod1^-/- (F,H,J,L) embryos. (E-J) Transverse sections through vertebrae and epaxial muscles; (K,L) sections through forelimbs. Section in situ hybridization for myogenin (M,Q), Pea3 (N,R) or Fgf8 (O,S) at E10.5 on Myf5^+/-; Myod1^+/- (M-O) or Myf5^-/-; Myod1^-/- (Q-S) embryos. (P,T) Detection of phosphorylated ERK/MAPK on frontal sections at E10.5 on Myf5^+/-; Myod1^+/- (P) or Myf5^-/-; Myod1^-/- (T) embryos. Black and yellow arrows in N,P,R,T indicate anterior and posterior sclerotome; blue arrows in O indicate anterior and posterior dermomyotome. Scale bars: 50 µm.