Med12 functions with Cas/Sox32 and Foxa2 to regulate endodermal organ development. (A–C) Embryos from med12s432 heterozygote incrosses injected with cas mRNA were harvested at 90% epiboly and analyzed for sox17 expression. med12s432 homozygous mutant embryos overexpressing cas mRNA showed decreased induction of sox17 expression (C) compared to med12 heterozygous (B) and wild-type (A) embryos. (D–H′) Embryos from wild-type crosses and med12s432 heterozygote incrosses injected with foxa2 MO were harvested at 48 hpf and analyzed for foxa3 expression. Wild-type foxa3 expression reveals structures such as the liver (black arrow), pancreas (black arrowhead), and swim bladder (white arrowhead) (D). med12s432 mutants show an ambiguous ductal morphology (black bracket) and defective development of endodermal organs (E). foxa2 MO-injected embryos showed slight perturbation of organ development (F), while med12s432 heterozygous embryos injected with foxa2 MO showed a more severe malformation of organs (G), and med12s432 homozygous mutant embryos injected with foxa2 MO showed an almost complete absence of accessory organs, exhibiting a solid endodermal rod (H, H′). (I–K) Confocal projections of larvae from med12s432 heterozygote outcrosses in the Tg (fabp1a:dsRed, elastase:GFP; insulin:dsRed) heterozygote line injected with foxa2 MO (J and K) and analyzed at 96 hpf. While foxa2 MO-injected larvae showed slight perturbation of liver and exocrine pancreas (J), med12s432 heterozygous larvae with foxa2 MO injection showed a more severe malformation of these organs (K). Images A–C are dorsal views, anterior to the top; D–H′ are dorsal views, anterior to the left; I–K are ventral views, anterior to the top.