Fig. 4. Mesp2 function is cell autonomously required for rostral properties. (A-D) Expression of lacZ and Uncx4.1 transcripts at the site of initial somite formation in control (A,B) and Mesp2-null (C,D) chimeric embryos. In the control, lacZ-expressing cells are randomly distributed and Uncx4.1 expression is normal. In the Mesp2-null chimera, lacZ-expressing Mesp2-null cells at the rostral part of the incomplete segments (arrows in C) ectopically express Uncx4.1 (arrows in D). Lines indicate somite boundaries. (E,F) In the dermomyotome, Mesp2-null cells are mostly localized at the caudal end, and the Uncx4.1 expression pattern is normal. (G,H) In the sclerotome, the distribution of Mesp2-null cells results in expansion of Uncx4.1 expression (arrows). (I) The control chimeric fetus shows normal vertebrae. (J) The Mesp2-null chimeric fetus exhibits partial fusion of the neural arches. (K) The control chimeric fetus shows normal ribs. (L) The Mesp2-null chimeric fetus shows proximal rib fusion. Scale bars: 100 µm. C, caudal compartment; R, rostral compartment.