A functional midline with expression of shh is required for the ACFP. A, B, GFP expression domains including that in the floor plate indicating shh expression (arrowhead) and the ACFP marker (arrow) are present in wild type (A) and lost in flh/- embryos (B). C, D, The fin fold mesenchyme as indicated by msx-c expression is unaffected in flh-/- embryos. The ragged appearance of the fin fold is due to shrinkage during the whole mount in situ hybridization procedure. E, F, presence of residual floor plate cells (arrowheads) are coupled with rescue of the ACFP (arrows). Red arrowheads in F indicate the position of floor plate cells as detected in the fluorescent image. All images of caudal fin regions anterior to the left. Bright field views are shown on the left panels and fluorescence images using GFP filter on the right (A, B, E, F).