Microscopical observations on the regenerating tail in the tuatara Sphenodon punctatus indicate a tendency to scarring,but also influence from somatic growth

The process of tail regeneration in the tuatara (Sphenodon punctatus) is not entirely known. Similarity to and differences from lizard tail regenerations are indicated in the present histological and ultrastructural study. Regeneration is influenced by the animal's age and ambient temperature, but in comparison to that of lizards it is very slow and tends to produce outgrowths that do not reach the length of the original tail. Although microscopically similar to lizard blastemas, the mesenchyme rapidly gives rise to a dense connective tissue that contains few muscle bundles, nerves, and fat cells. The unsegmented cartilaginous tube forming the axial skeleton is not calcified after 5 months of regeneration, but calcification in the inner region of the cartilage, present after 10 months, increases thereafter. Amyelinic and myelinic peripheral nerves are seen within the regenerating tails of 2–3 mm in length and the spinal cord forms an ependymal tube inside a cartilaginous casing. Tissues of the original tail, like muscles, vertebrae and the adipose mass, are largely replaced by dense connective tissue that occupies most of the volume of the new tail at 5 and 10 months of regeneration. It is unknown whether the differentiation of the dense connective tissue is caused by the relatively low temperature that this species lives under or stems from a genetic predisposition toward scarring as with most other amniotes. Increases of muscle and adipose tissues seen in older regenerated tails derive from somatic growth of the new tail in the years following tail loss and not from a rapid regeneration process like that in lizards.