(A, B) Pax3Cre-driven recombination in the dorsal neural folds and neural tube, detected from E8.5 by direct YFP-reporter expression (A), and by immunofluorescence in transverse sections of the closing neural tube at E9.5 (B). After ss20, recombination is also detected in the dorsal SE (red arrows), but not at earlier stages (red crosses). Note also recombination in cells of the ventral NE (red arrowheads; see also Figure 3—figure supplement 1). At least three different embryos were analysed for each stage. (C, D) Pax3Cre-Rac1 mutants display open spina bifida at E11.5 (C, white arrowheads and inset, quantified in Table 1) and delayed PNP closure from ss24-27 onwards (D, **p<0.001 – see Figure 3—source data 1 for raw values and statistical details). (E, F) SEMs of the PNP fusion point of control embryos show predominantly ruffles and filopodia at ss15-22 and ruffles at ss23-30, whereas Pax3Cre-Rac1 mutants show ruffles and filopodia at ss15-22 and absent protrusions at ss23-30 (E, quantified in F, p=0.29604 for ss15-22 and **p=0.0002 for ss23-30). A – Absent or incipient protrusions, F – Filopodia only (or predominantly), RF – mixture of Ruffles and Filopodia (or filopodia emanating from ruffles), R – Ruffles only (or predominantly). Scale bars: 100 µm (A and B), 1 mm (C) and 10 µm (E).