Structure-Activity Relationships of Retinoids in the Morphogenesis of Cutaneous Appendages in the Chick Embryo

We have evaluated the efficacy of 13 analogues of retinoic acid (retinoids) in producing in vivo feathers on the normally scaly foot integument (ptilopody) of the chick embryo. The retinoids were dissolved in dimethylsulfoxide and injected at doses from 1.25-250 micrograms into the amniotic cavity of embryos of 10 days of incubation. The results showed that the production of ptilopody requires specific retinoids, namely, those incorporating a free trans carboxylic end group. Indeed, the esterification of this acid end group (retinoid ethyl ester) leads to a delay in the effect of the product. Its substitution by an alcohol (retinol) or an aldehyde group (retinal), or its isomerization, leads to inactive products. The esterification by acetic acid of the alcohol terminal group of retinol leads to a moderately active compound (retinyl acetate). By contrast, modifications of the ring do not interfere with the efficacy of the retinoid, and modifications of the side chain by addition of one or two supplementary rings lead to an increase of the activity.