Is salamander hindlimb regeneration similar to that of the forelimb? Anatomical and morphogenetic analysis of hindlimb muscle regeneration in GFP-transgenic axolotls as a basis for regenerative and developmental studies.

The axolotl Ambystoma mexicanum is one of the most used model organisms in developmental and regenerative studies because it is commonly said that it can reconstitute a normal and fully functional forelimb/hindlimb after amputation. However, there is not a publication that has described in detail the regeneration of the axolotl hindlimb muscles. Here we describe and illustrate, for the first time, the regeneration of the thigh, leg and foot muscles in transgenic axolotls that express green fluorescent protein in muscle fibers and compare our results with data obtained by us and by other authors about axolotl forelimb regeneration and about fore- and hindlimb ontogeny in axolotls, frogs and other tetrapods. Our observations and comparisons point out that: (1) there are no muscle anomalies in any regenerated axolotl hindlimbs, in clear contrast to our previous study of axolotl forelimb regeneration, where we found muscle anomalies in 43% of the regenerated forelimbs; (2) during axolotl hindlimb regeneration there is a proximo-distal and a tibio-fibular morphogenetic gradient in the order of muscle regeneration and differentiation, but not a ventro-dorsal gradient, whereas our previous studies showed that in axolotl forelimb muscle regeneration there are proximo-distal, radio-ulnar and ventro-dorsal morphogenetic gradients. We discuss the broader implications of these observations for regenerative, evolutionary, developmental and morphogenetic studies.