The mammalian kidney responds to partial nephrectomy with glomerular and tubular hypertrophy, but without renal regeneration. In contrast, renal regeneration in lower vertebrates is known to occur. Understanding the underlying mechanisms of renal regeneration is highly important; however, a serviceable animal model has not been developed. A neonephrogenic zone has been identified in the European lesser spotted dogfish, Scyliorhinus caniculus (Hentschel H. Am J Anat 190: 309-333, 1991), as well as in the spiny dogfish Squalus acanthias and the little skate, Leucoraja erinacea. The zone features the production of new nephrons complete with a countercurrent system. To analyze this nephrogenic region of elasmobranch fish further, a renal reduction model was established. The neonephrogenic zone in the adult kidney of the little skate resembles the embryonic metanephric kidney and contains stem cell-like mesenchymal cells, tips of the branching collecting duct system, and outgrowth of the arterial system. Four stages of nephron development were analyzed by serial sections and defined: stage I, aggregated mesenchymal cells; stage II, S-shaped body-like structure with high-prismatic epithelial cells; stage III, segmental nephron segregation; stage IV, functioning nephron. The stages were analyzed after partial nephrectomy. In addition, cell proliferation was assessed by incorporation of bromo-deoxyuridine (BrdU). New nephrons developed in animals undergoing partial nephrectomy. Growth was greatly stimulated in the nephrogenic zone, both in the remnant tissue and in the contralateral kidney within 10 wk. Mesenchymal cell aggregates increased significantly per renal cross-section compared with controls (stage I, 0.64 +/- 0.28 versus 0.27 +/- 0.25; P < 0.005; n = 10 animals per group). The same was the case for S-shaped body-like cysts (stage II, 0.24 +/- 0.19 versus 0.08 +/- 0.09; P < 0.02). Cellular proliferation in the neonephrogenic zone of the contralateral kidney was also greatly enhanced (14.42 +/- 3.26 versus 2.64 +/- 1.08 BrdU-positive cells per cross-section, P < 0.001). It is concluded that the skate possesses a nephrogenic zone containing stem cell-like mesenchymal cells during its entire life. Partial nephrectomy induces renal growth by accelerating nephrogenesis. This unique model may facilitate understanding renal regeneration.