Acinar-endocrine cell tumor of the pancreas. Report of a pancreatic tumor containing both zymogen and neuroendocrine granules.

A case of pancreatic tumor with features of both an acinar cell and an endocrine cell tumor is presented. The histologic appearance of the tumor by light microscopy was consistent with the appearance of either an islet cell tumor or an acinar cell tumor. Electron microscopy revealed two distinct populations of membrane bound granules within the same tumor cells; namely, a small granule measuring 100-200 nanometers in diameter with a narrow halo separating it from the granule membrane, and a large granule measuring 400-500 nanometers in diameter and having a closely applied membrane. The smaller granules have the characteristic appearance of gastroenteropancreatic (GEP) endocrine-cell granules. The large granules are greater in size than neuroendocrine granules, their morphologic structure is most consistent with that of zymogen granules, and a positive lipase stain supported the presence of zymogen granules within the tumor. The occurrence of both neuroendocrine and zymogen granules within the same cell has been previously described in the "intermediate cell" of the pancreas, but not to our knowledge in a tumor of the pancreas. The finding of both endocrine and exocrine granules within a single cell indicates a histogenetic relationship between the pancreatic endocrine and exocrine cells, and may represent the first reported case of a neoplastic proliferation of intermediate cells.