Planarian flatworms have amazing regenerative abilities. Tissue fragments from almost anywhere in their anatomically complex bodies can regenerate into complete, perfectly proportioned animals, a feat that makes planarians ideal for the study of regenerative organogenesis. Now, on , Alejandro Sánchez Alvarado and colleagues provide the first detailed description of the excretory system of Schmidtea mediterranea, which consists of internal protonephridial tubules composed of specialised epithelial cells. Using α-tubulin antibodies to stain cilia in the planarian's excretory system and screens of gene expression patterns in whole mounts, the researchers show that protonephridial tubules form a complex branching structure that has a stereotyped succession of cell types along its length. Organ regeneration originates from a precursor structure that undergoes extensive branching morphogenesis, they report. Moreover, in an RNAi screen of signalling molecules, they identify EGF signalling as a crucial regulator of branching morphogenesis. Overall, these results establish the planarian protonephridia as a model system in which to study the regeneration and evolution of epithelial organs.
