Fig. 11

Fig. 11 Cells expressing shh are present in a 1-cell-wide row in the trunk midline of cyc;ntl double mutants. Transverse sections with dorsal to the top, through 11-hr embryos (4- to 6-somite stage, right panels) and 15-hr embryos (11- to 12-somite stage, left panels). Whole- mount RNA in situ hybridization (Krauss et al., 1993) and epon sections of 7 um were prepared as described under Materials and Methods. shh is expressed the floor plate region (white arrowheads) and in the notochord primordium (dark arrowheads) at these stages in wild- type embryos (A, B). Only the notochord is labeled in cyc mutants (C, D). In ntl mutants (E, F), it was often difficult to assign the position of labeled cells as being within or just beneath the developing spinal cord. Generally only a single labeled cell is observed at the same location in cyc;ntl double mutants (G, H). We quantified the difference between ntl and cyc;ntl mutants by counting shh-expressing cells in the midline of sections through the trunk at 11 hr. The number of shh-expressing cells per section was determined to be 3.6 ± 0.4 (mean ± SEM) for ntl mutants (n = 3 embryos, 86 sections total) and 1.2 ± 0.1 for cyc;ntl mutants (n = 3 embryos, 65 sections). The difference is significant (P < 0.05; from a t test assuming unequal variances). Scale bar, 25 um.