Fig. 2. Evolutionary conservation of the caudorostral wave of RALDH2. RALDH2 in situ hybridization of mouse embryos from early headfold (EHF) to somite stages. Note the sharp anteroposterior (AP) edges of RALDH2 expression in the EHF embryo. Anterior expansion of RALDH2 in the lateral mesoderm is underway at the 1-somite stage, but a more robust expression of RALDH2 is observed in embryos at the 2-somite stage. At the 5-somite stage RALDH2 expression advanced considerably in anterior lateral mesoderm. Black arrows indicate arches of Raldh2 expression in the lateral mesoderm.