Morphological and Cellular Aspects of Tail and Limb Regeneration in Lizards

The present review deals with the analysis of the cytological processess 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 evolutive considerations on the origin of tail regeneration in comparison to 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 autotomous planes of the tail are discussed. The histological and ultrastructural processess occuring during blastema formation and tail regeneration and during limb cicatrization are presented. The comparison stresses the scarse to absent inflammatory reaction present in the tail vs the massive inflammatory response in the limb leading to scarring. In fact the experimental induction 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 stressing the importance of the lizard model of tissue regeneration for medical studies and applications.

Zusammenfassung
The present review covers a very neglected field in regeneration studies, namely, tissue and organ regeneration in reptiles, especially represented by the lizard model of regeneration. The term regeneration is intended here as the ability of an adult organism to recover damaged or completely lost body parts or organs. The process of recovery is further termed restitutive regeneration when the lost part is reformed and capable of performing the complete or partial physiological activity performed by the original, lost body part. Lizards represent the only amniotes that at the same time show successful organ regeneration, in the tail, and organ failure, in the limb (Marcucci 1930a, b; Simpson 1961, 1970, 1983). This condition offers a unique opportunity to study at the same time mechanisms that in different regions of the same animal control the success or failure of regeneration. The lizard model is usually neglected in the literature despite the fact that the lizard is an amniote with a basic histological structure similar to that of mammals, and it is therefore a better model than the salamander (an a- mniote) model to investigate regeneration issues.

Inhalt
Regeneration in Reptiles and Its Position Among Vertebrates.- Tail Regeneration: Ultrastructural and Cytological Aspects.- Limb Regeneration: Ultrastructural and Cytological Aspects.- Conclusion and Perspectives: Implications for Human Regeneration.