Quantitative analysis of the effect of retinoids on facial morphogenesis

Retinoids have been applied to stage 20 chick embryos by using beads that act as controlled release carriers. With beads soaked in high concentrations of all-trans-retinoic acid, the face, in addition to the wing, is affected. Severe bilateral clefting of the primary palate results; the upper beak is completely missing, whereas the lower beak is unaffected. By using a scoring system that quantitates the severity of the upper beak defect, dose-response curves have been obtained. With beads soaked in progressively higher concentrations of retinoic acid, the upper beaks are increasingly truncated. The synthetic retinoid TTNPB also causes beak defects and is 30 times more potent than all-trans-retinoic acid. By removing beads soaked in retinoids at different times after implantation, the effect of varying the length of exposure of the developing face to retinoids has been examined. The production of beak defects is a two-step process and only a short exposure to retinoid is required to produce clefting. There are striking similarities in the dose-time relationships of retinoid treatments that are required to bring about beak defects and pattern changes in the limb. The outgrowth and development of spatially defined patterns of connective tissue within the upper beak appear analogous to processes involved in limb morphogenesis. We propose that retinoids may act by the same mechanisms in both systems. An unsolved puzzle is why retinoids specifically affect the development of the upper beak.