Tbx16 and Msgn1 act cell-autonomously in migration out of the tailbud. (A) Diagram of transplant scheme. Labelled undifferentiated cells are removed from donor embryos and placed in the ventral margin (fated to become tail somites) of unlabeled gastrulating embryos. B, C and E, F. Fluorescently labelled donor cells (red) overlaid on bright field images of host embryos at 24 h post-fertilization. (B) Wild-type donor cells in wild-type host. (C) MO donor cells in wild-type host. (E) Wild-type donor cells in MO host. (F) MO donor cells in MO host. Brackets and arrow indicate locations of donor cells. (D) Percentage of wild-type host embryos containing donor cells in somite, fin or epithelium, and undifferentiated groups. *: p<0.01 by Χ² test. (G) Percentage of MO host A–P body length containing donor cells. (Distance from posterior of embryo to anterior-most donor cell divided by total A-P body length.) *: p<0.01 by Anova. Bars show standard deviation. (H) 24 h post-fertilization MO host with somite-like organization of wild-type donor cells. Left panel is a single frame bright field image; middle is a Z projection through 4 µm of fluorescent dextran-labeled donor cells; right is a Z projection through 4 µm of embryos stained with a muscle myosin antibody. Arrows show somite-like structures formed from donor cells. Scale bar is 50 µm.

A novel tailbud explant method allows for high spatio-temporal imaging of migrating cells in vivo. (A) The posterior portion of an embryo in mid-somitogenesis is dissected away from the anterior tissue and yolk, mounted, and imaged. (B) Time lapse image series of an explant from a wild-type host expressing a fluorescent membrane marker driven by the tbx16 promoter (white) with wild-type donor cells (red) taken at mid-somitogenesis. Anterior is to the left with the notochord running down the middle of the tissue. These images correspond to frames from Movie S1. Scale bar=50 µm. (C) Time lapse image series of two cells in an explant from a wild-type embryo mosaically expressing the fluorescent actin marker LifeAct driven by the tbx16 promoter. Scale bar=10 µm.