Fig. 7. Rostro-caudal patterning of the sclerotome is disrupted in Mesp1/Mesp2 double-null chimeric embryos. (A) The control chimeric embryos exhibit normal stripe patterns of Uncx4.1 expression throughout the somite region. (B) The Mesp1/Mesp2 double-null chimeric embryos exhibit continuous Uncx4.1 expression in the ventral sclerotomal region. Note that caudal localization of Uncx4.1 expression is normal in the dermomyotome and dorsal sclerotome. The insets show a higher magnification of lumbar somites. (C,D) Adjacent sections showing that lacZ-expressing Mesp1/Mesp2 double-null cells express Uncx4.1. (E-H) The Mesp1/Mesp2 double-null chimeric fetus exhibits caudalization of the vertebrae and of the proximal ribs. (E) The control chimeric fetus shows normal metameric arrangement of the neural arches. (F) The Mesp1/Mesp2 double-null chimeric fetus shows severe fusion of the pedicles and the laminae of neural arches. (G) The control chimeric fetus has normal arrangement of ribs. (H) The double-null chimeric fetus shows severe fusion of the proximal elements of the ribs. Scale bars: 100 µm.