Electron Microscopy of Insulin Precipitates

Morphologic characteristics of insulin precipitates were examined by both scanning and transmission electron microscopy. Insulin precipitate obtained after 1 wk of in vivo storage in an implanted dog reservoir was compared with insulin precipitate produced in vitro by isoelectric precipitation, acid-freeze-heat precipitation, and motion-induced precipitation. Insulin precipitate produced in vivo had several morphologic forms, with spherical-lamellar structures predominating. In vitro isoelectric precipitated insulin produced microcrystalline material, whereas acid-freeze-heat and motion-induced precipitated insulin were associated with elongated fibrils. The morphologic appearance of the in vivo precipitated insulin was not entirely reproduced by any of the three in vitro methods of insulin precipitation. We conclude that insulin precipitation in vivo is a process that may involve more than one of the known mechanisms by which insulin precipitates in vitro.