Fig. 6

Ectopic Fgf8a redirects pouch outgrowth. (A-P) Still images from time-lapse recordings monitoring the development of the third pouch in wild-type embryos with mosaic mesodermal mis-expression of a nkx2.5:Fgf8a-GFP transgene (two examples are shown: A-D and E-H) or a control nkx2.5:GFP transgene (I-L), as well as wnt11r mutants with nkx2.5:Fgf8a-GFP mis-expression (M-P) (see supplementary material Movie 4). her5:mCherryCAAX labels endodermal cell membranes in red. Recordings started at 25hpf (T0), and subsequent stills were taken every 2h. nkx2.5:Fgf8a-GFP-expressing clones transiently diverted developing third pouch cells (arrows) in four out of six embryos, whereas nkx2.5:GFP-expressing clones had no effect on adjacent third pouch cells (arrowheads) in all three of the embryos examined. nkx2.5:Fgf8a-GFP-expressing clones failed to attract wnt11r^-/- endodermal cells (arrowheads) in all embryos (n=3). (Q) A two-step model of Tbx1 function in pouch morphogenesis. Tbx1 promotes wnt11r and fgf8a expression in distinct domains of the mesoderm, with Wnt11r initiating pouch morphogenesis through epithelial destabilization and Fgf8a guiding subsequent pouch outgrowth. Scale bar: 20μm (A-P).