Phenotypic effects of blocking translation of aplnra mRNA. A-C: Lateral (A) and dorsal axial views (B, C) of a representative embryo at 75% epiboly injected with MO oligonucleotide inhibiting translation of aplnra mRNA. Anterior is up. This embryo is stained to show cells transcribing tbx16 in order to reveal paraxial presomitic mesoderm. The position of the arrested epibolic margin is indicated by arrowheads in A. Note the presence of tbx16-expressing cells in the axis (white arrows in B, enlarged in C), and apparently scattered in the hypoblast (arrowheads in B), and notches in the margin (asterices in B, C). D-E: Ectopic structures resembling somites (es) in the retarded tails of embryos with reduced aplnra mRNA translation but that complete epiboly. These ectopic structures are clearly seen in a lateral view (E, anterior to top, dorsal to right). The yolk extension (ye) and chorion (ch) are indicated. A transverse optical section of the tail tip shows ectopic structures resembling somites (es) dorsal to the spinal cord (D). F-G: YSL nuclei, labelled with Sytox Green dye (white dots) move vegetally with the blastoderm margin in an embryo injected with Cont MO at 1 mM (F) but move ahead of the margin in an embryo injected with MO3 at 1 mM (G). Yellow dashed lines indicate the position of the blastoderm margin. Embryos are oriented with animal pole to top. Each image is a composite of images captured under epifluorescence and bright field illumination. H-I: Acridine orange staining of a Cont MO-injected embryo (H) and an embryo injected with MO3 at 1 mM at the 1-cell stage (I). The latter embryo shows a slightly higher level of apoptosis, primarily in the head (asterisk). Note the retarded tail formation. Anterior is left and dorsal is up. J-L: Embryos were injected with Cont MO at 1 mM or with a series of MO2 concentrations (1 mM, 0.5 mM, 0.25 mM) blocking aplnra mRNA translation at the one cell stage or into the animal pole of the yolk cell at the 32-cell stage (1 mM yolk injection). The injected embryos were observed at 8 (a), 14 (b) and 24 hpf (c) and assessed for the extent to which they had undergone epiboly and, if they completed epiboly, for the length of their tail extension. Developmental stage descriptors under the histograms refer only to epiboly or tail extension relative to that of a wild type embryo at 28.5°C. Size bars are 100 μm except for D, E, 50 μm.