Tail regenerative capacity and thymus responses in Xenopus laevis tadpoles

Xenopus laevis provides a valuable model to examine cellular and molecular mechanisms involved in the progressive decline of regenerative potential that occurs towards metamorphosis. To better understand the involvement of cells of the immune system on the quality of tissue response to injury, we studied the morpho-functional changes in regenerating area and thymus of different staged (st 50 and st 55/56) X. laevis tadpoles after partial tail cut. Unlike younger larvae able to rebuild perfect tails, an high percentage of older tadpoles showed reduced regenerative capacity and the histochemical data demonstrated different morphological patterns in the repair events. In st 55/56, wound healing slowed down, inflammatory and angiogenetic responses and immunoreactivity to anti-inducible nitric oxide synthase antibody were stronger and persistent during the regeneration process. The new tails resulted malformed. The thymus of st 50 larvae, compared to that of unoperated controls, showed a significant higher number of cells with apoptotic figures only at 1 day after amputation, an increased number of medullary epithelial cysts, mucous and myoid cells and cells immunoreactive to anti-TNF-a antibody. A recovery to control patterns was also found. The thymic structural modifications were severe and observed throughout the process in the older tadpoles and included significant increase of apoptotic cells, reduction of medulla and a higher number of TNF- a immunoreactive cells. At the end of regeneration the thymus size was found to be reduced even up to about 40%. The findings indicate a stimulation of the thymic function after tail amputation and an induction of molecules critical for organ function suggesting a possible role of the lymphocyte component in control of Xenopus regenerative outcomes. The age-dependent events occurring in regenerates, i.e. degree of inflammatory response, may be related to thymic structural modifications.