Fig. 1. Mesp1 and Mesp2 are co-expressed in the anterior PSM but have differing roles in somitogenesis. (A) Overlapping expression of Mesp1 and Mesp2 is revealed by in-situ hybridization using the left and right halves of the same embryo. The lines show most recently formed somite boundaries. (B-C) A Mesp1-null embryo (B) shows the same normal somite formation as a wild-type embryo (C). Arrowheads indicate somite boundaries. (D) In Mesp2-null embryos, no somite formation is observed but Mesp1 is expressed at comparable levels to wild type, although its expression is anteriorly extended and blurred. (E-H) Mesp1-null embryos show normal rostro-caudal patterning of somites. (E,F) Expression of a caudal half marker, Uncx4.1. (G,H) Expression of a rostral half marker, EphA4. The lines indicate presumptive or formed somite boundaries and the dotted line indicates approximate position of somite half boundary.