Depletion of Smad1 or Smad5 causes distinct embryonic phenotypes. (A) Embryos were either control (wild-type [wt]; row 1) or injected with 2 ng translation-blocking Smad1 MO (S1MO), which causes a phenotype first visible at the bud stage by elongation of the embryo (row 2). This phenotype is more severe starting around 18 hpf, with a ragged dorsal edge, neural degeneration, and a widened notochord (row 2; ). By 24 hpf, heart, brain, and endoderm defects are visible ( indicate neural degeneration and yolk stalk regression). These embryos go on to die at 3 to 4 dpf. Alternatively, embryos were injected with 1 ng translation-blocking Smad5 MO (S5MO), which causes a known dorsalization phenotype (row 3) similar to that of characterized smad5 mutants. This phenotype is first visible around the 5-somite stage with a tail defect () and a more caudal arrangement of somites. By the 18 hpf stage, the curvature of the tail is visible, and by 24 hpf, the lack of ventral tissues is evident (). The Smad5 MO phenocopies the piggytail mutant (row 4). Each panel is a representative photograph of phenotype and stage shown. More than 200 embryos were observed for each panel. (B-E) Double knockdown of both Smad1 and Smad5 at half-dose causes lethality at 15 hpf. Shown are wt (B,D) and double-morphant (S1MO + S5MO) (C,E) embryos at 15 hpf and 24 hpf, as indicated. The phenotype just prior to death has combined features of each individual knockdown. Less than 1% of double morphant embryos survive to 24 hpf, as in panel E. Approximately 150 double-injected embryos were observed, and a representative is shown.