Expression ofHoxDGenes in Developing and Regenerating Axolotl Limbs

Hoxgenes play a critical role in the development of the vertebrate axis and limbs, and previous studies have implicated them in the specification of positional identity, the control of growth, and the timing of differentiation. Axolotl limbs offer an opportunity to distinguish these alternatives because the sequence of skeletal differentiation is reversed along the anterior–posterior axis relative to that of other tetrapods. We report that during early limb development, expression patterns ofHoxDgenes in axolotls resemble those in amniotes and anuran amphibians. At later stages, the anterior boundary ofHoxd-11expression is conserved with respect to morphological landmarks, but there is no anterior–distal expansion of the posterior domain ofHoxd-11expression similar to that observed in mice and chicks. Since axolotls do not form an expanded paddle-like handplate prior to digit differentiation, we suggest that anterior expansion of expression in higher vertebrates is linked to the formation of the handplate, but is clearly not necessary for digit differentiation. We also show that the 5′HoxDgenes are reexpressed during limb regeneration. The change in the expression pattern ofHoxd-11during the course of regeneration is consistent with the hypothesis that the distal tip of the regenerate is specified first, followed by intercalation of intermediate levels of the pattern. BothHoxd-8andHoxd-10are expressed in non-regenerating wounds, butHoxd-11is specific for regeneration. It is also expressed in the posterior half of nerve-induced supernumerary outgrowths.