LEGS OR TAILS - RETINOIDS AND HOMEOSIS IN FROGS

A recent paper((1)) has once again highlighted the abilities of the vi tamin A-related retinoid molecules to induce bizarre effects during de velopment. Retinoid treatment of metamorphosing tadpoles, whose tails have been amputated, results in the development of supernumerary ectop ic limbs at the site of amputation. This effect was originally describ ed by Mohanty-Hejmadi et al.((2)) in the marbled balloon frog (Uperodo n systoma). Their observations have now been confirmed and extended - in considerable detail - in the common European species Rana temporari a((1)). In this paper, the con centration of retinoid, the duration of treatment, and developmental stage at which treatment is required for the generation of ectopic limbs have been examined, as has the morpho logy of the ectopic structures generated. The original observation, an d these recent data, are exciting from two viewpoints. Firstly, this e ffect represents the most striking homeotic transformation so far desc ribed in vertebrates, and secondly, it indicates a possible interactio n between retinoid and thyroid hormone receptor signalling pathways in vivo.