To investigate the role of pericytes in growth of retinal microvascular endothelial cells(RMECs) in a co-culture system in order to understand some mechanism of angiogenesis in hypoxia induced retinal neovascular disorders. · METHODS: RMECs were isolated by a modified protocol using CD31 coated Dynabeads, and identified by immunocytochemical staining with anti-Factor VIII and CD31 antibodies. Rat retinal pericytes were isolated and characterized by immunofluorescent staining with PDGFR-β; and desmin antibodies. Pericytes and RMECs were cultured in a contact co-culture system both under normoxia and hypoxia by Millicell chamber. RMECs proliferation was evaluated by MTT and cell cycle assay with flow cytometry. RT-PCR was used to detect the alteration of KDR/Flk-1 mRNA level in RMECs under normoxia or hypoxia in the co-culture system. · RESULTS: Highly pured rat RMECs and pericytes were harvested with the modified isolating method. The two cell types were identified by positive Factor VIII, CD31 and PDGFR-β, desmin cytochemical staining respectively. RMECs proliferated significantly under hypoxia from 3 to 9 day with a maximal rate on day 6 (24.9%, P < 0.01) by MTT. In the co-culture system, the proliferation of RMECs was inhibited by pericytes. After 6 days exposure to hypoxia, the fraction of S-phase RMECs number was greatly increased by 43.9%(P < 0.01). In the co-culture system, RMECs proliferation was inhibited by pericytes through decreasing the fraction of S-phase cell number both under normoxia (3.6%, P <0.05) and under hypoxia (15.1%,P <0.01). KDR/Flk-1 mRNA level in single cultured RMECs was shown to increase approximately 1.3-fold when exposed to hypoxia. Compared with single cultured RMECs, co-culture with pericytes could decrease KDR/Flk-1 mRNA by 45.1% (P <0.05) and 27.7% (P <0.05) under normoxia and hypoxia condition respectively. · CONCLUSION: The present study demonstrated that pericytes could inhibit proliferation of RMECs under both normoxia and hypoxia. The inhibition effects of pericytes maybe, at least in part, due to downregulation of KDR/Flk-1 of RMECs. These findings confirm that pericytes could be a potential inhibitor in the pathogenesis of retinal neovascularization(RNV).