Fig. 4.

Retinoic acid sentinels in live embryos and responding to exogenous RA in half-embryo cultures. (A) Merged bright field and fluorescence images of live E8.25 and E9.5 embryos containing the retinoic response sentinel DR5-TA-CFP at the −60 and −228 sites of the endogenous Rosa26 promoter. Composite images were generated with Photoshop. Fluorescent domains are noted. (B) Half-embryo fragments from E8.5 embryos of the designated genotypes were cultured overnight with 1 μM RA or DMSO carrier (control). Images are shown before (left) and after (right) culture, revealing an anterior-ward extension of the fluorescent signal upon RA treatment for each construct (white arrows). In supplementary material Fig. S3, data are shown indicating that only RA extended the CFP expression domain anteriorly, and not the RA inhibitor AGN193109, BMP4, noggin, a TGFβ inhibitor or a MAPK inhibitor, demonstrating specificity.