Fig. 5. Following r3 removal, positional identity marker expression is unchanged within the neuroepithelium, but depends on r4 NCC density within r3* mesenchyme. (A-F) Dorsal views of two embryos (A-C and D-F) in which r4 cells were labelled with DiI at the time of r3 removal and the distribution of migrating r4 cells was examined 20 hours later. These embryos were subsequently labelled with Hoxa2 riboprobe. NCCs from r4 express Hoxa2 as they migrate along their normal pathway towards ba2 (C,F). (A-C) Under conditions of high density, aberrantly migrating r4 NCCs continue to express Hoxa2 within r3* mesenchyme (arrows) and even within ba1 (arrowheads). (D-F) However, when relatively small numbers of DiI-labelled r4 cells enter r3* mesenchyme (arrow in D) they no longer maintain Hoxa2 expression in ectopic locations (arrow in F). (G) Hoxb1 expression within r4 is unaffected 20 hours after r3 removal and although our previous dye-labelling experiments show that some r4 cells repopulate r3*, we found no evidence of Hoxb1-expressing cells within r3*. (H,I) EphA4 is normally expressed by r3 and r5. Some embryos were processed for EphA4 in situ hybridisation immediately after r3 removal (0 hours) to demonstrate the clean removal of r3 (H). Other embryos were processed for EphA4 in situ 20 hours after surgery and demonstrate that none of the cells repopulating r3* express EphA4 (I). ba1, branchial arch 1; ba2 branchial arch 2.