Composition of nephron tubules in health and disease. (A) (Left) Healthy tubule with an intact epithelium. (Right) After injury, tubules show the loss of epithelial cells into the luminal space, which denudes the basement membrane and is accompanied by local inflammation. Mesenchymal stem cells (MSCs) provide restorative paracrine effects to damaged tubules. (B) Models of intratubular regeneration in AKI include (left) the dedifferentiation of endogenous differentiated tubule epithelium and (right) the activation of renal stem/progenitor cells. (C) (Left) In CKD, tubular damage is not restored and this is accompanied by a prolonged inflammatory response and the activation of myofibroblasts. (Right) Over time, lesions from myofibroblasts replace once-healthy renal tissue and this fibrosis is associated with permanent nephron erosion.