AIM: To study the regulatory mechanisms of sinusoidal regeneration after partial hepatectomy.METHODS: We investigated the expression of angiopoietin (Ang)-1, Ang-2, Tie-2, and vascular endothelial growth factor (VEGF) in regenerating liver tissue by quantitative reverse-transcription polymerase chain reaction (RT-PCR) using a LightCycler (Roche Diagnostics) and also immunohistochemical staining after 70% hepatectomy in rats. In the next step, we isolated liver cells (hepatocytes,sinusoidal endothelial cell (SEC), Kupffer cell, and hepatic stellate cells (HSC)) from regenerating liver tissue by in situ collagenase perfusion and counterflow elutriation,to determine potential cellular sources of these angiogenic factors after hepatectomy. Proliferation and apoptosis of SECs were also evaluated by proliferating cell nuclear antigen (PCNA) staining and the terminal deoxynucleotidyl transferase d-uridine triphosphate nick end labeling (TUNEL) assay, respectively.RESULTS: VEGF mRNA expression increased with a peak at 72 h after hepatectomy, decreasing thereafter.The expression of Ang-1 mRNA was present at detectable levels before hepatectomy and increased slowly with a peak at 96 h. Meanwhile, Ang-2 mRNA was hardly detected before hepatectomy, but was remarkably induced at 120 and 144 h. In isolated cells, VEGF mRNA expression was found mainly in the hepatocyte fraction.Meanwhile, mRNA for Ang-1 and Ang-2 was foundin the SEC and HSC fractions, but was more prominent in the latter. The PCNA labeling index of SECs increased slowly,reaching a peak at 72 h, whereas apoptotic SECs were detected between 120 h and 144 h.CONCLUSION: Ang-Tie system, together with VEGF,plays a critical role in regulating balance between SEC proliferation and apoptosis during sinusoidal regeneration after hepatectomy. However, the VEGF system plays a more important role in the early phase of sinusoidal regeneration than angiopoietin/Tie system.