The purification of human pancreatic islets before transplantation relies on the density-dependent separation of islets from exocrine fragments after collagenase digestion of the donor pancreas. The results vary among pancreases despite increasing automation of the digestion and purification processes, reflecting variations in the overlapping densities of islets and contaminating exocrine tissue.
Hypothermic storage of both the pancreas and the pancreatic digest alters cell volumes and tissue densities, thereby affecting islet purification. By biochemical analysis of the isopycnic distribution of islets and exocrine tissue fragments from 23 human pancreases on linear continuous density gradients, the effect of various solutions for cold storage of pancreatic digest was studied. The use of the University of Wisconsin cold storage solution, which resulted in a significant decrease in digest volume (P=0.006) and increase in the densities of both exocrine tissue (P=0.001) and islets (P=0.005), produced a significant improvement in islet purity compared with tissue culture medium (P=0.035), predominantly due to the inclusion of a colloid, which increased the difference in density between exocrine tissue and islets. The addition of large molecular weight cellular impermeants without alteration in the concentration of permeable anions produced no effect.
The results of this study support the concept that the use of solutions that minimize cell swelling throughout the process of islet purification would result in significant improvements in density-dependent islet separation, and that such solutions should contain a colloid.