Protein synthesis during limb regeneration in the axolotl.

A study has been made of limb regeneration in the axolotl using two-dimensional gel electrophoresis of proteins labelled with [35S] methionine. In the early stages of regeneration seven proteins are identified which are specific to the mesenchyme and thirteen which are specific to the epidermis. These is very little change in the gel pattern until the onset of overt cytodifferentiation upon which the muscle and cartilage become substantially different both from each other and from the blastemal mesenchyme. The gel pattern of mesenchyme from the larval limb bud is almost identical to that of the blastemal mesenchyme, while the pattern of the limb-bud epidermis differs somewhat from the blastemal epidermis. A careful search has been made for differences which might be associated with 'positional information' by comparing forelimb with hindlimb, proximal with distal and anterior with posterior. The differences which have been found are all associated with differences in visible cellular composition or in rates of differentiation, rather than with position per se. It is concluded that positional codings cannot be detected by this technique. On the basis of biological experiments, six criteria are proposed by which to assess future searches for positional codings.