Congenital insulinoma (nesidioblastoma): ultrastructural evidence for histogenesis from pancreatic ductal epithelium.

An insulinoma removed from a neonate with proved hyperinsulinemia was examined with the electron microscope. The tumor was composed of pancreatic ductal epithelium and beta cells in direct contact and in lobular units. The pattern of the tumor, the morphologic evidence of beta cell-type activity in ductal cells, and the similarity of the arrangement of tumor cells to that observed in islet regeneration after subtotal pancreatectomy in the rat indicate that this tumor originated from ductal epithelium.