Immunolocalization of tumor suppressors arhgap28 and retinoblastoma in the lizard Podarcis muralis suggests that they contribute to the regulated regeneration of the tail

Tail regeneration in lizards is an outstanding and unique postembryonic morphogenetic process. This developmental process is regulated by poorly known factors, but recent studies have suggested that it derives from a balanced activity between oncoproteins and tumor suppressors. Transcriptome and expression data have indicated that arhgap28 and retinoblastoma proteins are among the main tumor suppressors activated during tail regeneration. However, their cellular localization is not known. Therefore, in the present immunohistochemical study, two proteins have been detected in various tissues at the beginning of their differentiation. Both proteins are present especially in the new scales, axial cartilage, and muscle bundles of the regenerating tail, the main tissues forming the new tail. Sparse or occasionally labeled cells are observed in the blastema, but intense labeling is seen in the basal layers of the wound (regenerating) epidermis and in external differentiating epidermal layers. Numerous keratinocytes also show a nuclear localization for both proteins, suggesting that the latter may activate a gene program for tissue differentiation after the inhibition of cell multiplication. Based on microscopic, molecular, experimental, and in vitro studies, a hypothesis on the "inhibition of contact" among the apical cells of the blastema and those of proximal differentiating tissues is proposed to explain the permanence of an active blastema only at the apex of the regenerating tail without tail growth can degenerate into a tumorigenic outgrowth.