Myofibroblasts play an important role in chronic renal interstitial fibrosis. However, the origin and developmental mechanisms remain to be elucidated. The myofibroblasts may express various cytoskeletons during the development. Immunoexpressions of vimentin, desmin and alpha-smooth muscle actin (alpha-SMA) were analyzed using experimentally (cisplatin and unilateral ureteral obstruction) induced rat and spontaneous canine fibrotic kidneys or kidney-related cell lines incubated with transforming growth factor-beta1 (TGF-beta1), platelet-derived growth factor-BB (PDGF-BB) or their combination at various concentrations. In rat fibrotic kidneys, both renal epithelia and interstitial cells showed positive reactions to alpha-SMA and vimentin, supporting epithelial-mesenchymal transition (EMT) theory; however, renal epithelia did not react to desmin, though interstitial cells were reactive. Renal epithelia in canine fibrotic kidneys did not show a positive reaction to alpha-SMA, whereas interstitial cells reacted strongly to alpha-SMA; conversely, renal epithelia reacted strongly to desmin, but interstitial cells did not; vimentin expression was infrequently seen in renal epithelia and interstitial cells of canine kidneys. Exposure of TGF-beta1 to porcine renal epithelial cells (LLC-PK1), rat renal interstitial cells (NRK-49F), and rat immature mesenchymal cells (MT-9) dose-dependently increased selectively alpha-SMA-positive cell numbers. Moreover, PDGF-BB exhibited an additive effect on TGF-beta1-induced alpha-SMA expression in these cell lines when simultaneously added. alpha-SMA was the most plastic cytoskeleton under fibrogenic stimuli. This study shows that there are interspecies differences in cytoskeletal immunoexpressions of renal epithelia or interstitial cells between rat and canine fibrotic kidneys, and that the derivation of renal myofibroblasts may be heterogeneous, such as renal epithelia, interstitial cells or immature mesenchymal cells.