Histology, ultrastructure, and tissue culture of human insulinomas. 
Four cases of insulinoma were studied by electron microscopy.  Insulinoma cells were also cultured in vitro.  Ultrastructure was correlated with the content of immunoreactive insulin (IRI) and the percentage of proinsulin-like component (PLC) in the serum, tumors, and culture fluid.  The number and shape of secretory granules varied amont the samples, from the tumors having typical beta granules to poorly granulated tumors having a few atypical granules.  High levels of IRI in the serum and a high percentage of PLC in the serum and tumor, found in a patient with a virtually agranular tumor, combined with the fact of rare occurrence of emiocytosis, suggest a defective insulin storage mechanism and alternative routes of insulin release in insulinoma cells.