Fig. 2

tbx6 is expressed by ventrolateral mesendodermal precursors in the early zebrafish gastrula. (A) A simplified fate map of the early zebrafish gastrula (Kimmel et al., 1990). At this stage the blastoderm is shaped as an inverted cup covering the animal pole of the yolk cell (gold). Mesoderm and endoderm arise from involuting cells initially present at the blastoderm margin (purple). The remaining cells give rise to ectoderm. Mesodermal tissues are formed from prospective tissue precursors that are organized in the following dorsal (D) to ventral (V) order: notochord (noto.), somites (som.), pronephros (pron.), blood (blo.). (B and B′) Oblique lateral views (animal pole up) of early gastrulae showing the distribution of tbx6 transcripts revealed by whole-mount in situ hybridization. tbx6 is expressed by a continuous arc of cells at the margin of the blastoderm, corresponding to the mesendodermal precursors. The dorsal view in B′ illustrates the absence of the tbx6 expression in dorsal marginal cells. (C) A section through the margin of an embryo in which tbx6 transcripts were detected by in situ hybridization (purple stain) and the No Tail protein was detected by immunohistochemistry (brown nuclear stain) reveals coexpression of the two genes in marginal cells. (D) In situ hybridization to detect transcripts of both tbx6 and the dorsalspecific gene goosecoid indicates that tbx6 is not expressed in the dorsal mesoderm. In this animal pole view of a midgastrula (7 h) embryo, the recently involuted goosecoid-expressing cells (*) appear as a knot of stained cells at the center of the region devoid of tbx6 gene expression. (E) An animal pole view of an early gastrula (5.5 h) embryo showing the distribution of both tbx6 and goosecoid transcripts. At the onset of gastrulation the domain of tbx6 expression (light blue arc) is continuous, and perhaps slightly overlapping, with the domain of goosecoid expression (dark blue staining, *). (F, G) tbx6 expression is down-regulated in embryos dorsalized by exposure to lithium ions. (F) A lateral view of tbx6 transcripts in a control early gastrula. (G) A lateral view of tbx6 transcripts in an early gastrula of an embryo that had been exposed at the 32- to 64-cell stage to 0.3 M LiCl for 10 min.