Fig. 3. Defective somitogenesis in the absence of Meox gene function. (A,B) Thin resin sagittal sections of the caudal region of E9.5 embryos. Unlike those of control Meox1^+/-;Meox2^+/- embryos (A), newly formed somites from double mutant Meox1^-/-;Meox2^-/- embryos (B) are irregularly shaped and sized (compare sizes of bars), not organized into epithelial spheres and the basal lamina that normally surrounds each somite is no longer evident between somites (black arrows), although it is present dorsal and ventral to somites (white arrows). (C,D) Transverse rostral sections of E10.5 embryos. The epithelial dermomyotome (arrow), characteristic of mature differentiated somites in controls (C), is absent in Meox1;Meox2 double homozygous mutants (D). n, neural tube, sc, sclerotome. (E,F) Longitudinal sections of E10.5 embryos. The segmented organization of adjacent sclerotomes (dashed lines) in controls (E) is absent in double Meox mutants (F). Furthermore, the anteroposterior polarity of each sclerotome, consisting of a rostral half (r) and denser caudal half (c) is not apparent. The epithelial dermomyotome in controls (E, arrows) is again not evident in mutants (F). (G,H) Para-sagittal sections of control and mutant embryos at E10.5. In control embryos, the dorsal root ganglia (DRG) are regularly sized and shaped (G, arrows); by contrast, they are uneven in size and spacing and often fused in mutants (H, arrows).