Imaging of human islet vascularization using a dorsal window model

Background: The islets of Langerhans are micro-organs rich in blood vessels. The process of islet isolation and culture disrupts the vasculature of the islets. The reestablishment of an appropriate microvascular supply is an essential prerequisite for long-term survival and function of islet grafts. In this study, we examined the effects on the process of neovascularization of coating the islets with fibrin. Methods: Isolated human islets were stained using the dioctadecylindocarbocyanine (DII) dye. An aliquot of the human islets were embedded in 3-dimensional fibrin. Human islets (100 islets-equivalents) were transplanted into a mouse dorsal window model to evaluate angiogenesis over 17 days. Transplanted islets were divided into 2 groups: either free islets or islets coated with fibrin gel. Animals were imaged using intravital microscopy immediately and at 3, 4, 8, 11, and 17 days after surgery. The DII dye caused the islets to be fluorescent and visible using a rhodamine filter. Fluorescein isothiocyanate dextran was used to visualize vasculature structures surrounding the islets. Results: Human islets coated with fibrin demonstrated an early appearance of a network of immature blood vessels that produced a significantly higher density/unit area for neovascularization by day 8 after transplantation. Conclusion: Our preliminary data showed that fibrin played a role in early neovascularization and support to sustain development of new blood vessels. Fibrin formed a matrix that helped to maintain the 3-dimensional structure of, and therefore reducing the environmental stress on islets.