Fig. 1. A caudorostral wave expands RALDH2 anteriorly. (A) In situ hybridization indicates that Raldh2 is expressed posterior to Hensen's node (*) between HH4-6. At HH6-7 spots of RALDH2 blur the otherwise sharp anteroposterior (AP) boundaries of expression in the lateral mesoderm (white arrowheads). At HH7⁺ two faint arches of RALDH2 appear in the anterior lateral mesoderm (black arrow). These arches intensify at HH8^–-HH8⁺ and progress anteriorly. At HH9-10 RALDH2 arches join at the midline below the anterior intestinal portal (AIP) (star). (B) HH6. Strong Raldh2 expression spreads from Hensen's node in a postero-lateral direction (black arrowhead), whereas faint expression appears at the anterior border of the lateral mesoderm (white arrowhead). (C) Grayscale image of B submitted to morphometry. Horizontal bars indicate the limits of high and low Raldh2 expression and the interval between them defines anterior expansion of RALDH2. (D) HH8⁺. Strong arches of expression form in the anterior lateral mesoderm (black arrows). (E) Grayscale image of D submitted to morphometry. Upper horizontal bar marks the anterior limit of Raldh2 expression in the lateral mesoderm. Lower horizontal bar marks the boundary between the last formed somite and the unsegmented paraxial mesoderm. The interval between horizontal bars defines anterior expansion of RALDH2. (F) Expansion of RALDH2 begins slowly between HH6-7 coinciding with appearance of scattered spots of expression at the anterior lateral mesoderm. Expansion reaches maximal rates of development between HH7-8 when faint arches appear at the anterior lateral mesoderm. Thereafter, it tapers off as RALDH2 arches join at midline below the AIP between HH9-10. Data are presented as means±s.e.m. AE, anterior expansion.

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