Ectopic mdka expression blocks somitogenesis and promotes floorplate formation. (A) Noninjected control zebrafish embryo at 24 hpf. Location of eye (e) and mid-hindbrain boundary (arrow) is indicated. (B) A 24 hpf zebrafish embryo that had been injected with 100 pg of RNA encoding mdka at the two-cell stage. Head structures (eye, e; mid-hindbrain boundary, arrow) form normally, but the trunk is malformed. (C,D) Higher magnification of trunk regions of control (C) and injected (D) embryo. No chevron-shaped somite boundaries (as in control C, arrows) are visible in mdka-injected embryo (D). (E) Dorsal view of myoD expression in the adaxial mesoderm (small arrows) and posterior parts of somites (arrowhead) in a noninjected control embryo (anterior to the top) at 15 hpf. (F) In an embryo injected with 100 pg of mdka RNA, myoD expression is lost in the paraxial (arrowhead) and reduced in the adaxial (arrows) mesoderm. (G) Dorsal view of anterior neural plate in a control embryo at 13 hpf. fkd6 expression in premigratory neural crest cells. (H) A 13 hpf embryo injected with 100 pg of mdkb RNA with enhanced fkd6 expression in premigratory neural crest cells. (I) Dorsal view of posterior region of same noninjected embryo as in G. myoD expression in paraxial (arrow) and adaxial (arrow) mesoderm. (J) Posterior region of same mdkb-injected embryo as in H. Although the number of fkd6-expressing neural crest cells is increased (asterisk), myoD expression (arrow, arrowhead) and spacing of formed somites appears only slightly affected when compared with control (I) and mdka (F) injected embryos. (K) Transient expression of fkd6 in the floorplate (arrow) of a control embryo at 10 hpf (bud stage, dorsal view, anterior to the top). (L) Increase in number of fkd6-expressing floorplate cells in an embryo injected with 150 pg of mdka RNA. (M) Expression of f-spondin in medial floorplate cells of control embryo at 12 hpf (dorsal view, anterior to the top). (N) Increase in number of f-spondin-positive floorplate cells in embryo injected with 100 pg of mdka RNA.
