Fig. 3

hand2 and osr1 regulate the development of npas4l-expressing lateral vessel progenitors. In situ hybridization shows npas4l expression in wild-type control (A,C,E,G), hand2s6 mutant (B,F) and osr1el593 mutant (D,H) embryos; dorsal views, anterior to the left, at 6 som (A?D) and 12 som (E?F). (A?D) At 6 som, npas4l is expressed in a relatively medial territory (arrowhead) on each side of the wild-type (A,C), hand2s6 (B), and osr1el593 (D) posterior mesoderm. In osr1el593 mutants, npas4l is also expressed in relatively lateral cells (D, arrow), and its expression is increased in a distinct proximal region (D, asterisks). Lateral npas4l expression was observed in 84% of osr1el593 mutants (n = 31) and 2% of wild-type control siblings (n = 110); furthermore, the lateral expression observed in wild-type embryos was restricted to only a few cells at the anterior extent of the posterior mesoderm (Fig. S2C). Similarly premature expression of etv2 in lateral cells was seen in osr1 mutants previously (Perens et al., 2021). (E?H) At 12 som, npas4l is expressed in a relatively lateral territory (arrows) on each side of wild-type (E,G) and osr1el593 mutant (H) embryos; in osr1el593 embryos, npas4l expression remains increased proximally (H, asterisks). In hand2s6 mutants (F), npas4l is not expressed in this lateral territory (100%, n = 20). At this stage, a small amount of npas4l expression is still observed in the posterior medial vessel progenitors in hand2s6 mutants (H, arrowhead). n = 77 (A), 30 (B), 110 (C), 31 (D), 66 (E), 20 (F), 53 (G), 18 (H). Scale bar: 100 ?m.