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Fig. 2 Normal expression patterns of mesp-a and mesp-b transcripts in zebrafish embryos. Embryos are oriented with anterior (animal pole) to the top (except for B, in which the embryo is viewed from the animal pole). (A,B) mesp-a expression at the shield stage. mesp-a is expressed in the blastoderm margin with a small gap (arrowhead, A). (B) Animal-pole view of an embryo double-stained with goosecoid (gsc). gsc expression is seen within the gap in mesp-a expression (arrowhead). (C-F) Dorsal views of embryos hybridized with a mesp-a probe at 80% epiboly (C), 100% epiboly (D) and 10- to 12-somite stages (E,F). As gastrulation proceeds the marginal expression expands along the animal-vegetal axis in the dorsal region (anterior edge indicated by arrowhead in C). However, the expression is rapidly down-regulated in most of the area except in the anterior border, leaving a pair of stripes (D). During the segmentation period, one or two stripes of expression are visible (arrowheads in E,F). (G-I) mesp-b expression at the shield stage (G) and 10- to 12-somite stages (H,I). No mesp-b expression is detected during gastrulation. During the segmentation period, two or three stripes of expression are visible (arrowheads in H,I). (J,K) Two-colour staining with DIG-labeled mesp-a and fluorescein-labeled mesp-b probes at the 10- somite stage. The embryo was first processed for mesp-b staining, then photographed (red in J), followed by mesp-a staining (blue in K). The expression domains of mesp-a and mesp-b overlap (arrowheads, K). Bars, 100 μm.