Fig. 7. Lineage tracing of Shh-expressing cells in the mouse node. (A,B) Posterior-ventral views of 5-6 somite stage mouse embryos showing (A) Sfpe2 reporter activity and (B) Shh expression in the (i) node and (ii) ventral midline. Asterisk in A marks ectopic X-gal staining in the somites. Sections generated through the embryos in A and B at the level of the (i) node and (ii) second most newly formed somite show that X-gal staining and Shh expression are restricted to the ventral (mesodermal) layer of the node and notochordal plate, respectively. (C-E) Shh and ß-gal immunostaining colocalize to the ventral layer of the node in a 5-6 somite stage embryo carrying the Sfpe2 reporter. (F,G) Ventral views of 5-6 somite stage mouse embryos showing (F) Ptc-lacZ and (G) Foxa2 expression in the (i) node and (ii) ventral midline. Sections taken through the embryos in F and G are at the level of the (i) node and (ii) second most newly formed somite. Both Ptc-lacZ staining and Foxa2 expression localize to the dorsal and ventral layers of the node and to their derivatives, the floor plate and notochord, respectively. Red arrowheads point to the expression of Ptc-lacZ and Foxa2 in the presumptive floor plate. DL, dorsal layer; VL, ventral layer.