Structure of supernumerary limbs

Abstract

The generation of supernumerary limbs by rotation of amphibian limb buds or regeneration blastemas is a phenomenon well known to embryologists^1,2. The recent revival of interest in this aspect of limb morphogenesis has been largely due to the stimulus provided by the polar coordinate model for control of limb outgrowth and pattern³ and subsequent efforts to test its validity. The geometrical rules postulated in the polar coordinate model make precise predictions about the handedness of supernumerary limbs. Following antero-posterior (AP) or dorso-ventral (DV) inversions, they should be of the same handedness as the stump, whether single or in pairs, but after inversion of both axes (APDV) pairs should be of opposite handedness³. It is surprising, therefore, that the structure of supernumerary limbs has never been studied in sufficient detail to report whether they really are normal limbs, for only the skeletal elements are examined in cleared whole mounts (see Fig. 1). This only allows an assessment of the AP axis, and to determine the dorsal and ventral surfaces (and hence the handedness), the curvature of the digits is noted. I therefore decided to study the muscle patterns of all types of supernumeraries, and report here that, contrary to expectation, the majority of APDV supernumeraries are not normal limbs but are mirror-image duplications in the dorso-ventral axis, that is, either double-dorsal or double-ventral. This presents problems for contemporary models of pattern formation in the limb, and issues a warning to those who use supernumerary limbs as an experimental tool to investigate other areas of developmental biology.