ULTRASTRUCTURAL EVIDENCE FOR BLOOD MICROVESSELS DEVOID OF AN ENDOTHELIAL-CELL LINING IN TRANSPLANTED PANCREATIC-ISLETS

The aim of the present study was to investigate, at the ultrastructura l level, the process of revascularization of freshly isolated islets o r cultured islets after transplantation under the kidney capsule of sy ngeneic mice, Native islets in adult pancreases from mice, pigs, and h umans contained only capillaries with fenestrated endothelium. However , the endothelial cell lining was disrupted in both freshly isolated a nd cultured mouse islets, Shortly after transplantation (6 weeks) appr oximately 80% of graft microvessels contained no endothelial cell lini ng, Similar data on microvessel morphology were found when fetal porci ne islet-like cell clusters were implanted into athymic nude mice Re-e ndothelialization was a slow process, with 25% of the microvessels sti ll lacking endothelium 6 months after transplantation of cultured mous e islets or islet-like cell cluster. However, when freshly isolated mo use islets are used only 25% of microvessels within the islet graft la cked endothelium 6 weeks after implantation. We suggest that capillari es damaged during islet isolation may provide a preformed channel, ser ving as a scaffold for newly formed islet graft blood vessels. The pre sence of non-endothelialized microvessels, with an associated lack of barrier function, might make transplanted islets more prone to thrombo sis or an attack by the immune system This Provides a tentative explan ation for the increased vulnerability of islet grafts when compared wi th whole pancreas transplants,