The homeotic transformation of tails into limbs in Rana temporaria by retinoids.

The most remarkable of all the effects of retinoids on embryonic systems is the homeotic transformation of tails into legs which was recently reported using an Indian species of frog. Since then several attempts have been made to repeat these results on other species, notably Xenopus, with no success. Here I report the successful repetition of this homeotic transformation using Rana temporaria tadpoles treated with retinyl palmitate. The phenomenon is concentration-dependent, time-dependent, and stage-dependent. There is some difference in effect according to the tail amputation level. The limbs induced are always hindlimbs and there can be between 1 and 9 of them. There is a tendency to induce limbs in pairs so that even numbered groups are produced in considerable excess over odd numbered groups. As assessed by cartilage staining the majority of the limbs are normal in the proximodistal and anteroposterior axes. The other types of outgrowths induced are double-posterior limbs, posterior half-limbs (usually defective in the proximodistal axis), and spikes. As assessed by the anatomy of the muscle patterns some of these apparently normal limbs are duplicated in the dorsoventral axis. The stage and species dependencies of this phenomenon suggest that it may involve thyroid hormone receptors whose levels rise leading up to metamorphosis and whose interactions with retinoic acid receptors have recently been described.