Factors Influencing Isolation of Islets of Langerhans

More efficient methods of islet isolation must be developed for islet transplantation to become clinically routine. During collagenase dispersal of human pancreas, an amorphous, viscous, gellike material often develops and entraps large numbers of islets, thereby reducing the yield. When donor human pancreas is minced and treated with collagenase, the gel forms most abundantly if the digestion temperature is <35°C and if pH falls below 7.2 ± 0.2. Gel formation appears to be proportional to warm- or cold-ischemia time and may be related to tissue trauma during collection. Once gel has formed, trapped islets cannot be released by filtration, dilution, DNase, incubation temperature, or pH adjustment. These characteristics suggest that the material is gelatin derived from collagen released enzymatically from pancreatic stroma. We demonstrate that gelation is greatly reduced or eliminated when 1) the incubation medium includes glycerol—a common gelatin solvent—at 5% (vol/vol), 2) the minced tissue-to-total incubation volume ratio is ≥1:10, 3) free-islet exposure to pancreatic digestion products is minimized by frequent separation of islets, and 4) collagenase concentration is optimized by titration. Gelation is also minimized by maintaining 5) incubation temperature at 38 ± 1°C and 6) pH in the range 7.7–7.9. Variations in these physical and chemical conditions were analyzed by determining islet yields (stereoscopic microscope counts of serially diluted samples) and by insulin radioimmunoassay of acid alcohol extracts of isolated islets after separation through discontinuous Ficoll gradients. When isolation conditions are optimized as stated, we typically recover 3.3 ± 1.0 × 10⁴ islets/g pancreas, corresponding to >10⁶ islets per donor.