Fig. 3

The spt^-;flh^- floor plate is nearly continuous. (A-D) Dorsal views of shh expression during early segmentation (6-8 somites, 12- 13 h, anterior to the top). shh is expressed in the developing notochord and floor plate of wild-type (A; Krauss et al., 1993) and spt^- (C) embryos. In flh^- embryos (B), shh expression is seen only in presumptive floor plate cells; however, in spt^-;flh^- embryos (D), shh is expressed in both in floor plate and in anterior midline mesodermal cells (presumptive notochord). The extent of shh expression in spt^-;flh^- midline mesoderm is variable; most spt^-;flh^- embryos look as shown (13/21, 62%); however, some spt^-;flh^- embryos have less extensive shh expression (3/21, 14%), whereas other spt^-;flh^- embryos with more extensive staining were recovered from the spt^- class (5/21, 24%). Slightly fewer spt^-;flh^- embryos were observed than expected (21/422 versus 26/422), raising the possibility that shh expression in few spt^-;flh^- embryos is so extensive that they are indistinguishable from spt^- embryos. (E-H) Lateral views of shh expression at the completion of the segmentation period (25 h, anterior to the left). In wild-type (E; Krauss et al., 1993) and spt^- (G) embryos, shh is expressed in the floor plate and posterior notochord. In spt^- embryos, shh-expressing cells are also found ventral to the notochord. shh expression in spt^-;flh^- embryos (H) reveals a nearly continuous floor plate, which is in great contrast to the patchy shh expression observed in the floor plate ‘islands’ of flh^- embryos (F; Halpern et al., 1995, 1997; Talbot et al., 1995; Odenthal et al., 1996; Beattie et al., 1997). The spt^-;flh^- embryo shown in H is a typical example (13/24, 54%); other spt^-;flh^- embryos have either more extensive (6/24, 25%) or less extensive (5/24, 21%) shh expression in the tail. Scale bars, 100 um.