Retinoic acid gradient visualized for the first time in an embryo

In a ground-breaking study, researchers report a new technique that allows them to visualize the distribution of retinoic acid in a live zebrafish embryo, in real-time. This technique enabled them to observe two concentration gradients going in opposing directions along the head-to-tail axis of the embryo, thus providing long-awaited evidence that retinoic acid is a morphogen.