The thymus in the tail regeneration of Xenopus laevis tadpoles

The tadpoles of the frog Xenopus laevis show significant regeneration capacity and are useful models to examine cellular and molecular mechanisms underlying the appendage regeneration. After amputation, most of the tail can be rebuilt during the larval life and this ability is gradually reduced as development proceeds towards metamorphosis. Previous studies demonstrated different morpho-functional responses to tail amputation of different aged X. laevis tadpoles. Unlike stage (st) 50, an high percentage of st 55/56 larvae showed limited regenerative efficiency and malformed and shorter new tails were observed. The immune cells were found to take part in the response to tissue injury, in determining the inflammation degree and success of repair process. In this work, the thymus from tail amputated Xenopus larvae (st 50 and st 55/56) was investigated by histochemical and immunohistochemical reactions. The examination of st 50 revealed changes of thymic architecture, compared to unoperated controls, characterized by an increased number of multicellular epithelial cysts, mucous and myoid cells, and cells, mainly located in the medulla, immunoreactive to anti-TNF-a antibody. A significant higher number in cortical apoptotic figures was only detected 1 day after tail cut. The thymic structural modifications were more marked, and observed throughout tail regeneration process, in most of the older tadpoles. The cellular responses included significant increase of apoptotic pictures and reduction of the medullary area where large epithelial cysts containing secretory material and cell debris were seen. At the end of regeneration process the organ size was found to be reduced of about 40%. Compared to unoperated controls, a higher number of TNF-a immunoreactive cells was also observed. These findings show that tail cut provokes a stimulation of the thymic function and induction of molecules critical for organ constitutive processes indicating a possible role of the lymphocyte component in control of Xenopus tail regenerative quality outcomes. The stage-dependent events occurring in regenerating tail microenvironment, i.e. degree of inflammatory response, may be related to thymic structural modifications.