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Gallbladder carcinoid tumor in a formalin-fixed, paraffin-embedded section (left: HE, right: EM). A small polypoid pancreatic polypeptide-immunoreactive carcinoid tumor was totally embedded in paraffin. The tumor cells possess 200 nm-sized neuroendocrine granules with low electron density. The presence of a particulate structure such as neuroendocrine granules is well recognized ultrastructurally even after paraffin sectioning. Bar = 500 nm.
