Microencapsulated pancreatic islets: a pathologic study

Dog pancreatic islets isolated by an enzymatic digestion method were encapsulated in an alginate-poly L-lysine-alginate membrane. These microencapsulated pancreatic islets were cultured in vitro to study their ability of insulin secretion. Portions of these in vitro-cultured microencapsulated pancreatic islets were taken out for a viability dye exclusion study as well as for pathologic studies to correlate them with insulin secretion ability. We found that there was a strong correlation between them. Good insulin-secreting microcapsules showed well-preserved cell membranes and beta-cell granules. An in vitro culture for one to two days in RPMI-1640 made the islets more stable, the cellular surface became smoother and the beta-granules were in better shape. The microencapsulated pancreatic islets were also injected into the peritoneum of streptozotocin-induced diabetic CDF1 mice. Blood glucose levels dropped and stayed low for up to 60 days. But, when non-encapsulated dog pancreatic islets were used, the blood glucose levels remained low for only about 14 days. A small portion of the injected microcapsules were washed out at specific times for pathologic study. Up to 28 days after injection, only a few of the injected microcapsules showed pericapsular cellular infiltrate. However, after 56 days, most of the microcapsules showed dense pericapsular cellular infiltrate. Immunohistochemical analysis of these infiltrates showed that the majority of cells were fibroblasts and macrophages. Most of the cells located in the inner portion of the infiltrate were fibroblasts, while the macrophages were located mainly on the outer portion. Both scanning and transmission electron microscopy showed that the surface of the microcapsule outer wall was much smoother than the inner wall. The size of the microcapsules was approximately 0.6-0.8 mm and the thickness of the wall measured around 10 nm. The smaller the microcapsule is, the less chance there is of rupture with release of the xenographic islets. Once the wall of the transplanted microcapsules was ruptured, the inner surface showed more increased inflammatory cell and fibroblast infiltration than the outer surface.