Immunochemical identification of endocrine cell types in the streptozotocin nicotinamide-induced rat islet adenoma

The streptozotocin nicotinamide-induced rat islet adenoma is a benign encapsulated tumor that exhibits considerable histologic and endocrine variability. Individual tumors differ in their general pattern of cellular arrangement and often contain necrotic regions, ductlike structures and large amounts of interspersed connective tissue. The majority of tumors are polyhormonal. All contain insulin by radioimmunoassay and immunohistochemistry, although the amounts vary greatly. Glucagon and/or somatostatin immunoreactive cells are present in 62% of tumors by immunohistochemistry and in 86% by radioimmunoassay. Again, the amounts vary greatly. Pancreatic polypeptide immunoreactivity was localized in a small number of scattered cells in 31% of tumors studied. Tumors demonstrated considerable hormonal intra- and intertumor heterogeneity both by immunohistochemistry and by radioimmunoassay. From these data it is indicated that the tumor can neither be considered an "insulinoma" nor "pure" in endocrine composition. This histologic and endocrine variability must be taken into account when using the tumor as a source of islet mRNA or as a source of tissue in insulin biosynthetic work.