Oates et al., 2005 - Generation of segment polarity in the paraxial mesoderm of the zebrafish through a T-box-dependent inductive event. Developmental Biology   283(1):204-214 Full text @ Dev. Biol.

Fig. 1 Gene expression defects in the paraxial mesoderm of fused somites/tbx24 mutant embryos. Expression of (A) mespb, (B) papc, (C) fgf8, and (D) myoD mRNA in paraxial mesoderm and tailbud of wild-type (left panel) and fss/tbx24 mutant (right panel) embryos at 5–6 (A, B) and 10 (C, D) somite stages. Embryos are deyolked and flat mounted with anterior up. Arrowheads indicate somitic boundaries. (E–F) Comparison of notch5 expression in wild type and fss/tbx24 backgrounds at the 5 somite stage. (E) Expression of notch5 mRNA in caudal half of PM segments. From left to right, panels show cell nuclei, notch5 expression, and a merge of the two. (F) Paraxial notch5 expression is absent from fss/tbx24 mutant embryos, although retained in the notochord and intermediate mesoderm.

Fig. 2 Rescue of morphological boundary formation by wild-type cells in fused somites/tbx24 mutant hosts. Formation of morphological boundaries in PM of fss/tbx24 host embryos after transplantation of wild-type cells (green), shown in live embryos: dorsal view in panels A–B, anterior up; lateral views in panels C–F, anterior left. (A) Normal segmentation in 6-somite stage wild-type embryo after transplantation of wild-type cells into PM. (B) Morphological boundary formation (arrowheads) in sibling fss/tbx24 host associated with wild-type donor cell clusters. (C) Appearance of somite-like wild-type donor cell clusters at A/P level of somite 6 in paraxial mesoderm of fss/tbx24 host at 12-somite stage, showing strong boundary morphology (arrowheads). Top panel is DIC image, middle panel is fluorescent image of green transplanted wild-type cells and bottom panel is a merge. (D–F) Confocal sections through PM of wild-type (E) and fss/tbx24 (D, D′, F) embryos at 24 hpf. (D, D′) Arrangement of wild-type cells at high-density forming compact cell clusters in fss/tbx24 host embryos. (D) Section through lateral surface of cluster. Arrangement of wild-type cells at low density is indicated with arrows. (D2) More medial section through center of cluster, showing distinctive rostral morphological boundary (arrowheads), and interdigitation of wild-type with fss/tbx24 cells on caudal side of cluster (asterisks). (E) Ends of wild-type muscle fibers at low density align to segmental boundaries in the trunk of wild-type hosts (arrows). (F) Ends of wild-type muscle fibers at low density do not align in the trunk PM of fss/tbx24 host embryos (arrows). cns = central nervous system, pm = paraxial mesoderm, im = intermediate mesoderm, n = notochord.

Fig. 3 Expression of the rostral segment polarity marker genes mespb, papc, and fgf8 by wild-type cells in fused somites/tbx24 hosts. (A–B) Expression of mespb mRNA (red) in confocal sections of the right-hand side of the PM at the A/P level of the segment polarity zone in 8-somite stage embryos containing transplanted wild-type cells (green). (A) Normal mespb expression in wild-type embryos, arrows indicate transplanted cells expressing mespb. (B) Cell-autonomous mespb expression in wild-type donor cells in fss/tbx24 host PSM (arrows). (C–D′) Expression of papc mRNA (red) in confocal sections of the paraxial mesoderm at an A/P level spanning the segment polarity zone in 8-somite stage fss/tbx24 mutant host embryos containing transplanted wild-type cells (green). (C) papc expression associated with small wild-type donor cell clusters (arrows), in contrast to absence of papc on contralateral side. (C′) Higher magnification of C showing papc expression only in wild-type donor cells (arrows). (D) Striped expression of papc in large, high-density clone of wild-type cells (arrows) and location of papc expression in host cell (arrowhead). (D′) Higher magnification of region indicated by arrowhead in D showing papc expression in fss/tbx24 host cell (arrow). (E–F′) Expression of fgf8 mRNA (red) in confocal sections of the paraxial mesoderm at an A/P level spanning the segment polarity zone in an 18-somite (lateral view E, E′) and 6-somite stage (dorsal view F, F′) fss/tbx24 mutant host embryos containing transplanted wild-type cells (green). Location of E, E′ shown in diagrammatic form. (E) fgf8 expression in wild-type cells (arrows), dashed line indicates the dorsal extent of the embryo, the dotted lines delimit the paraxial mesoderm, and the asterisk marks the intermediate mesoderm. (E′) Higher magnification of fgf8 expressing region in panel C, arrows mark fgf8-positive wild-type donor cells. (F) Expression of fgf8 associated with wild-type donor cells in paraxial mesoderm, notochord delineated with dashed line and position of the lateral edge of embryo with a dotted line. (F′) Higher magnification of F, showing fgf8 expression in wild-type donor (arrows) and fss/tbx24 mutant host cells (arrowheads). c = central nervous system, m = paraxial mesoderm, y = yolk, n = notochord, pm = paraxial mesoderm.

Fig. 4 Induction of notch5 expression in fss/tbx24 host cells by wild-type neighbors. Expression of notch5 mRNA (red) in confocal sections of the PM of 5–6 somite stage fss/tbx24 host embryos containing transplanted cells (green) from wild-type donors. Embryos are flat mounted, anterior up. (A) notch5 is not expressed in the PM of fss/tbx24 host embryo with low density of wild-type donor cells. (B) High-magnification view of PM of fss/tbx24 host embryo containing high density of wild-type donor cells. Autonomous notch5 expression is seen in wild-type cells, and notch5 induction in numerous fss/tbx24 host cells (arrows) up to three cell diameters from the wild-type cell clones. (C) Periodic stripes of notch5 expression in fss/tbx24 host embryo containing 5 somite-like clusters of wild-type donor cells in the right-hand PM. Contralateral side does not express notch5. Arrows indicate the boundaries between clusters, arrowheads mark fss/tbx24 host PM cells expressing notch5. (C′) High magnification of the region of notch5 expression. Arrows and arrowheads as in panel C, asterisks mark the rostral half of cell clusters. Note high-level notch5 expression in the caudal part of each somite-like cluster. n = notochord, h = hindbrain, pm = paraxial mesoderm, im = intermediate mesoderm.

Acknowledgments:
ZFIN wishes to thank the journal Developmental Biology for permission to reproduce figures from this article. Please note that this material may be protected by copyright.

Reprinted from Developmental Biology, 283(1), Oates, A.C., Rohde, L.A., and Ho, R.K., Generation of segment polarity in the paraxial mesoderm of the zebrafish through a T-box-dependent inductive event, 204-214, Copyright (2005) with permission from Elsevier. Full text @ Dev. Biol.