Fig. 3. Mesp2-null cells tend to be excluded from the epithelial region of the somites. (A) The control chimeric embryo undergoes normal somite formation and shows random distribution of labeled cells. The right panel is a high-power view of a somite. (B) In the Mesp2-null chimeric embryo, incompletely segmented somites are formed. Mesp2-null cells tend to be localized at the rostral and central region of these incomplete segments. Red arrows: wild-type cell clusters; blue arrows: Mesp2-null cell clusters. (C,D) Other sections indicating multiple small epithelial cell clusters (arrows). Note that Mesp2-null cells only partially contribute to the epithelial clusters (blue arrows). (E,F) A small number of Mesp2-null cells are distributed in the dermomyotome and are mostly localized at the caudal end. Scale bars: 100 µm.