HISTOGENESIS AND ULTRASTRUCTURE OF PANCREATIC-ISLET GRAFT MICROVASCULATURE - EVIDENCE FOR GRAFT REVASCULARIZATION BY ENDOTHELIAL-CELLS OF HOST ORIGIN

In previous studies we have demonstrated that syngeneic and xenogeneic pancreatic islet grafts are revascularized within a 10 to 14-day peri od after transplantation. With the combined use of intravital and elec tron microscopy, as well as immunohistochemistry using a set of specie s-specific or -crossreacting antibodies to endothelial cell antigens, we investigated 1) the origin of the endothelium of the newly formed c apillaries in free pancreatic islet isografts (hamster --> hamster) an d xenografts (rat --> hamster), and 2) the ultrastructural characteris tics of these microvessels. Intravital microscopy demonstrated that ne wly formed microvessels grow from the vascular bed of the host muscle tissue into the islet grafts. Immunohistochemical analysis of host tis sue and transplanted islets with antibodies against factor VIII (recog nizing both hamster and rat factor VIII), bovine PECAM-1 (CD31; endoCA M, crossreacting with hamster but not rat PECAM-1), and rat ICAM-1 (CD 54, noncrossreacting with hamster ICAM-1) showed that the transplanted rat islets were revascularized by endothelium of hamster Chest) origi n. At an ultrastructural level, the endothelial lining of the newly fo rmed microvessels showed diaphragmatic fenestration, a characteristic feature of endothelial cells of pancreatic islets in situ. On the basi s of these findings we suggest that pancreatic islet transplantation m ay tate a unique position in the field of organ transplantation, since the generally proposed mechanisms of endothelial cell-dependent antig en recognition as a trigger of graft rejection may not be transferred to islet grafts, containing microvessels lilted by endothelial cells o f host origin.