Isolating and optimising transmural endothelialization as an independent mode of spontaneous vascular graft healing

Includes bibliographical references.Background: Poorly designed animal models with short, non-isolated grafts have led researchers to exclusively investigate transanastomotic endothelialization(TAE), which is known to be limited to the perianastomotic region in humans. Alternative spontaneous forms of endothelialization (fallout and transmural) need to be independently investigated and optimised to redirect future conduit research. Methods: All implants were performed in infra-renal rat aorta. TAE was determined at 2, 4, and 6 weeks (n=6/time-point) (ePTFE;ID1.7mm;IND15-25µm). High-porosity polyurethane (internal diameter 1.7mm- 150mm pore size) grafts were interposed between ePTFE grafts for 2, 4, 6, and 8 weeks (n=6/time point). Grafts were looped to increase their length and were implanted for 6, 8, 12, and 24 weeks (n=8/time-point). Perigraft isolation included (PU skin, 50µm, or 350µm wall thickness wrap, (IND30µm;ePTFE) to prevent transmural ingrowth; 12-weeks. The mid-graft was further surface-treated with heparin or heparin-VEGF/PDGF and implanted for 3 weeks (n=10/group). Explanted samples were analysed by light, immunofluorescence and scanning electron microscopy for vascularization and endothelialization. Results: TAE occurred at 0.6±0.4mm/wk., which slowed from 0.8±0.4 to 0.5±0.3mm/wk., [p=0.039] from 2 to 6 weeks. Straight composite grafts separation zones were too short to isolate transmural ingrowth beyond four weeks. However, a broad endothelial-free zone was preserved in all looped composite grafts (23.6±10.1, 23.7±8.2, 13.4±11.0, 10.5±5.6mm for 6, 8, 12, and 24 weeks respectively), [p=0.0031]. Mid-graft pre-confluence was reached by 6 weeks (55±45%) and confluence between week 12 and 24 (95.0±10.0% and 84.0±30.13%). The subintimal thickness did not increase (91.8±93.9 mm vs. 71.4±59.4 mm at 6 and 24 weeks, respectively; NS). All sealed grafts occluded. Mid-graft endothelialization was not influenced by 50µmFilmwrap (93±8.3%), but the 350µm-Wrap significantly reduced coverage (16±30%, pre Conclusions: Transmural endothelialization can be clearly distinguished from TAE in a high throughput rat model. A looped interposition graft model provides sufficient isolation length to separate the two events for up to half a year without an increase in intimal hyperplasia. Growth factor surface modification further enhances this form of healing, while perigraft isolation confirms that fallout endothelialization is insufficient to heal the mid-graft with confluence. These findings may be useful for the development of clinical peripheral vascular (including endoluminal) grafts, as TM endothelialization remains a viable healing mode in humans