Human embryos that get too much or too little retinoic acid, a derivative of vitamin A, can develop into babies with birth defects. New research at UC Irvine shows for the first time how embryonic cells may regulate levels of retinoic acid, giving scientists insight into how it acts as a signal between cells to control development of the brain, limbs and many other tissues in embryos. Thomas Schilling, Richard White, Qing Nie and Arthur Lander found that if the level of retinoic acid in zebrafish becomes too high, an enzyme called cyp26a1 degrades the excess and brings it back to normal. When levels drop too low, proteins called fibroblast growth factors, or FGFs, stop the retinoic acid from degrading as rapidly. The study appears Tuesday, Nov. 20, in the journal Public Library of Science Biology.