Morphological and cellular aspects of tail and limb regeneration in lizards. A model system with implications for tissue regeneration in mammals.

The present review deals with the analysis of the cytological processes occurring during tissue regeneration in the tail and limb of lizards. These reptiles are considered as a model to understand the process of tissue regeneration in all amniotes. The review begins with some evaluative considerations on the origin of tail regeneration in comparison with the failure of limb regeneration, a unique case among amniotes. The formation of the tail in the embryo and the possible accumulation of stem cells in autotomy planes of the tail are discussed. The histological and ultrastructural processes occurring during blastema formation and tail regeneration and during limb cicatrization are presented. The comparison stresses the scarce to absent inflammatory reaction present in the tail in contrast to the massive inflammatory response in the limb leading to scarring. In fact the experimental inducement of a strong inflammation in the tail also leads to scarring. The importance of the nervous system in stimulating tail regeneration in lizards is emphasized. The presence of growth factors and extracellular matrix proteins during wound healing of the tail and limb is introduced. The review concludes by stressing the importance of the lizard model of tissue regeneration for medical studies and applications.