Fig. 7. Lbx1 downregulates myoD. Embryos were injected with lbx1 mRNA in one cell at the two-cell stage and stained for myoD (A-E), pax3 (F-J) or lbx1 (K-M). Red staining is the ß-galactosidase lineage tracer. At stage 22, myoD is downregulated on the injected side (A), while pax3 expression is higher (F). These embryos are shown from a dorsal view, with the injected side on top. Transverse sections of stage 24 embryos show a complete loss of myoD expression on the injected side (right side), despite there being an enlarged somite (B), while pax3 expression is higher on the injected side (right side) (G). At stage 33, expression of myoD is still downregulated in regions of the ß-galactosidase lineage tracer (C-E). The uninjected side is shown in C, injected side in D, and a transverse section in E (injected side to the right). pax3 expression is also still upregulated at stage 33 in regions of the lineage tracer (H,I). The uninjected side is shown in H, injected side in I, and a transverse section in J (injected side to the right). At stage 24, endogenous lbx1 expression is lost on the injected side of the embryo (L), when compared with the uninjected side (K). At stage 33, ectopic lbx1 expression in the posterior somites (red staining) fails to upregulate endogenous lbx1 expression (M). (N) A stage 26, tadpole injected with lbx1-splice MO (right side) shows a mild upregulation of myoD in the ventrolateral domain of the somite (arrow). (O-T) Transverse sections of stage 31 (O-Q) and 41 (R-T) tadpoles injected with lbx1 into one cell at the two-cell stage were stained for injected lbx1 mRNA (O,R), myoD (P,S) and p27 (Q,T, arrow in Q indicates region of strong expression on the uninjected side). When injected lbx1 mRNA is still present (O), myoD and p27 are repressed on the injected side (P,Q). When the injected mRNA is no longer detectable (R), myoD and p27 are upregulated on the injected side (S,T).