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Fig. 4. Somite epithelialisation and patterning requires both Meox genes. In situ hybridization analysis of Mesp2 (A,B), paraxis (C-F) and Dll1 (G-J) expression in control (A,C,E,G,I) and double Meox1-/-;Meox2-/- mutant (B,D,F,H,J,) embryos at E9.5. In A,B,E-N the caudal end of the embryo is towards the right. (A,B) Dorsal aspects of Mesp2 expression in the rostral presomitic mesoderm. Control (A) and mutant (B) embryos have similar expression profiles. Expression of paraxis in whole-mount preparations (C,D) and para-sagittal sections (E,F). In controls (C,E), paraxis is expressed throughout the epithelial somites, but double mutant embryos (D,F) express paraxis in a dorsally restricted domain. Furthermore, this expression is not maintained in older, more rostral, somites. Expression of Dll1 in caudal (G,H) and rostral (I,J) somites. In control embryos (G), Dll1 is expressed at high levels in presomitic mesoderm (black arrowhead), the caudal halves of newly formed somites (arrows) and the neural tube (white arrowhead). In mutants (H), Dll1 is expressed at high levels in the presomitic mesoderm (black arrowhead), but its expression in somites is virtually extinguished, whereas the neural tube expression remains (white arrowhead). (I) Dll1 is expressed in the myotome (arrows) of anterior differentiated somites of control embryos, and in the neural tube (white arrowhead). In mutants (J), the myotome expression is fused ventrally (arrows). (K-N) In situ hybridization analysis of Epha4. (K) In control embryos, Meox1+/-;Meox2+/-, dorsal views of Epha4 expression show a broad stripe (arrowhead) in the most rostral presomitic mesoderm (in the next somite to form) and a narrow stripe of expression (arrow) in the rostral half of the most recently formed somite. In embryos with one wild-type Meox allele, Meox1+/-;Meox2-/- (L) and Meox1-/-;Meox2+/- (M), the rostral somite stripe of expression (arrow) is less refined. In Meox1-/-;Meox2-/- mutants (N) the rostral half-stripe of Epha4 expression is absent and only the posterior stripe (arrowhead) is visible.