Immunolocalization indicates that both original and regenerated lizard tail tissues contain populations of long retaining cells, putative stem/progenitor cells.

The regeneration of the tail in lizards is likely sustained by stem/progenitor cells located in the stump after amputation of the tail. This microscopic and ultrastructural study shows the localization of 5-bromo-deoxy-uridine (5BrdU)-long retaining labeled cells in different tissues of the tail stump. These putative stem/progenitor cells are sparsely detected in the epidermis of scales, adipose tissue, intermuscle connective septa, myosatellite cells, and perichondrion of the vertebrae. Most of 5BrdU-labeled cells are present in the bone marrow of vertebrae as hemocytoblasts and reticulate cells, whereas more numerous myelocytes and polychromatophilic erythroblasts show a variable level of nuclear labeling. 5BrdU and tritiated-thymidine labeled and unlabeled hemopoietic cells are seen in circulating vessels and in the blastema where their maturation is completed. This observation indicates that the entire differentiation span of both white and red blood cells, at least during tail regeneration, lasts longer than 4 weeks. Labeled polychromatophilic erythroblasts and heterophilic and basophilic myelocytes are present in the synusoidal vessels of the regenerating tail. This study indicates that extravasating blood cells involved in immunity make large part of the forming blastema cell population, but are replaced by mesenchymal cells of different origin. The presence of long retaining labeled cells in tissues of the tail stump is likely connected to the production of blastema mesenchymal cells. Although no direct cell-lineage study has been done, histological, immunocytochemical, and autoradiographic studies have indicated that it is from these tissues that proliferating cells appear mainly localized after tail amputation and blastema formation.