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Fig. 2 Comparison of the ptc1 and shh expression patterns during wild-type embryogenesis. Transcripts were revealed by in situ hybridization with antisense RNA probes to ptc1 (A,C,E,G,I,K,M,O,Q,S,U) or shh (B,D,F,H,J,L,N,P,R,T,V). (A,B) Dorsal views of embryos at the end of gastrulation. The arrowhead indicates a row of hypoblast cells adjacent to the axial mesoderm which express ptc1 at high levels. The asterisk indicates the anterior boundary of the ptc1 low-level expression domain in the presomitic mesoderm. (C,D) Lateral views of 10-somite (C) and 14- somite (D) stage embryos. Arrowheads denote ptc1 expression in somites. Asterisks delimit the expression of ptc1 in the neurectoderm all along the anteroposterior axis of the embryo. (E,F) 18-somite stage embryos. (G,H) 26-somite stage. Asterisks indicate the dorsal extension of ptc1 and shh expression domains in the diencephalon. Brackets indicate upregulation of ptc1 in rhombomeres 2, 4 and 6, a modulation of expression that is unrelated to shh expression (see also Fig. 5c). (I,J) Embryos at 48 hours of development. Asterisks indicate the position of the floor plate of the neural tube. Arrowheads indicate expression in the gut. golden mutant embryos, which are defective in pigmentation, were used to facilitate visualisation of ptc1 and shh expression. (K,L) 8-somite stage embryos. Dorsal view of flat preparations showing the complementarity between the paraxial expression of ptc1 and the axial expression of shh in the notochord and the tailbud. (M,N) Transverse sections through the trunk of embryos at 24 hours of development. (N) Sections at two different levels along the axis, the section on the right-hand side being more posterior. (O,P) Horizontal sections through the trunk of embryos at 24 hours of development. (Q,R) Dorsal view of the trunk of 36-hour-old embryos showing the expression of ptc1 and shh in the posterior region of the developing fins. (S,T) Detail of embryos shown in I and J, respectively. Asterisks indicate the position of the floor plate of the neural tube. Arrowheads indicate expression in the foregut. Expression in the hindbrain, when observed dorsally, was found to be restricted to periventricular cells in the midline (data not shown). (U,V) Frontal views of embryos shown in I and J, respectively. Arrowheads indicate expression in the foregut surrounding the developing mouth. Abbreviations : e, eye; fp, floor plate; hb, hindbrain; n, notochord; te, tectum; t, telencephalon; tb, tail bud; y, yolk.