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De-endothelialization is the main stimulus of intimal hyperplasia following vascular injury. In this study we investigated the time course of re-endothelialization in balloon injured and photodynamic therapy (PDT) treated aortas of New Zealand white rabbits. Twenty rabbits underwent balloon denudation of the abdominal aorta and were then sacrificed in groups of 4 animals 0, 1, 2, 4 and 8 weeks later. Twenty more rabbits underwent similar balloon denudation and were treated immediately afterwards with photodynamic therapy using the photosensitizer metatetrahydroxy phenyl-chlorin and endovascular illumination with 652 nm light. PDT treated rabbits were also sacrificed in groups of 4 animals at the same time intervals. A further 4 rabbits were sacrificed without any treatment to act as normal controls. The vasculature was perfusion fixed at 100 mmHg with 10% formal saline. The abdominal aortas were retrieved and five sections were cut from each aorta at 1 cm intervals, embedded in wax, sectioned and stained for endothelial cells using the Avidin Biotin complex/horseradish peroxidase technique for use with the monoclonal primary antibody CD31 from the clone JC70. Endothelial covering was measured using a light microscope and Magiscan image analysis system. Normal arteries showed a near full (92.1% plus or minus 3.0, mean plus or minus SEM) endothelial covering. Endothelium was removed completely after both balloon injury and PDT. In balloon injury alone there was progressive endothelial regrowth with (54.1 plus or minus 7.2) covering at 8 weeks. In contrast, endothelial regrowth was retarded in the aortas treated with balloon injury and PDT, with only (7.1 plus or minus 2.9) of covering at 8 weeks. The slow pace of re-endothelialisation is consistent with greater production of intimal hyperplasia in PDT treated vessels.
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