Electron microscopic and immunoelectron microscopic demonstration of pancreatic polypeptide cells in glucagonoma: colocalization of pancreatic peptide and glucagon in single secretory granules

Multiple polypeptide hormone expression in pancreatic islet cell carcinomas derived from phosphoenolpyruvatecarboxykinase-SV40 T antigen transgenic rats

Transgenic rats carrying a PEPCK-SV40 large T-antigen (TAg) transgene rapidly develop numerous pancreatic islet cell neoplasms, the cells of which express TAg. Although many of the larger neoplasms contain relatively undifferentiated cells, many tumors contain areas of well-differentiated cells with abundant endoplasmic reticulum (ER) and secretory granules for endocrine hormones like those observed in normal pancreatic islets. In the well-differentiated lesions, glucagon-producing alpha-cells, insulin-producing beta-cells, and somatostatin-producing delta-cells are readily identifiable morphologically under the electron microscope. Beta-cells were observed in all normal and hyperplastic islets, and nests of these cells were scattered throughout the larger neoplasms. These nests varied from small clusters of epithelium-like cells that stain intensely for insulin, to sheets of small, basophilic cells that stain more diffusely for the hormone. Alpha-cells were also present in all of the normal and hyperplastic islets, but in larger hyperplastic islets, the peripheral localization was absent. Larger neoplasms contained many nests of glucagon-expressing cells, as well as scattered glucagon-producing single cells. Delta-cells were rarely observed in the hyperplastic islets and in the neoplasms. Blood-glucose levels were unaltered in the transgenic animals relative to their nontransgenic litter mates. Thus although these islet cell neoplasms express several polypeptide hormones, there is no obvious clinical effect of such expression in vivo.

Pancreatic endocrine tumour: a 22-year clinico-pathological experience with morphological, immunohistochemical observation and a review of the literature

The clinico-pathological features of 53 Chinese patients (27 males; 26 females) with pancreatic endocrine tumours were studied. The age range was from 14 to 78 years old (mean: 48 years) with the modal peak in the sixth decade for both sexes. Pancreatic endocrine tumours accounted for 14% of the primary pancreatic tumours operated on in Queen Mary Hospital. The autopsy incidence was 0.11%. Seventy-two per cent (38 cases) of the tumours were clinically functioning, comprising 33 insulinomas, three gastrinomas and two glucagonomas. A rare case of malignant gastrinoma associated with Cushing's syndrome was also documented. The functional tumours were seen in the younger patients. The calculated annual incidence of clinically significant tumours was approximately 0.2 per 100,000 population. There was no correlation between the site, functional status and histological patterns of the tumours. Seventy-two per cent of the tumours showed a trabecular pattern. Calcification was present in 5.7% (three cases); two such cases being gastrinomas. Amyloid was found in 25% of tumours, chiefly (92%) in the insulinomas. The main difficulty encountered in diagnosis was distinguishing between solid and cystic tumours of the pancreas. The incidence of malignancy was 15% and the histological features were poor predicative indicators of malignant potential. The metastatic pancreatic endocrine tumours were often detected in the liver and lymph nodes. Immunohistochemical stains showed evidence of multi-hormone production in 18% of cases and all tumours showed a positive reaction to at least one of the six markers, namely, neuron-specific enolase (NSE), chromogrannin (CG), synaptophysin (SYN), insulin (INS), glucagon (GLU) or somatostatin (SOM). The three panendocrine markers (NSE, SYN, CG) were satisfactory for initial screening of the endocrine nature of the tumours if used in combination, as 92% of tumours were positive for at least one of these three markers.

Tumors of the endocrine/neuroendocrine system: an overview

For the sake of discussion, the markedly diversified tumors of the endocrine/neuroendocrine system are classified as those originating in classic epithelial endocrine organs (eg, adrenal cortical adenomas), from the diffuse endocrine cells (eg, jejunal carcinoid tumors), or from clusters of these cells (eg, islet cell tumors); and those arising from neurosecretory neurons (eg, neuroblastoma) or paraganglia (eg, carotid body tumor). Although traditional transmission electron microscopy is useful for identifying neurosecretory or endosecretory granules as such, with few exceptions (eg, insulin-containing granules with a complex paracrystalline core) it is not possible to ascribe a granule type (size, shape, or ultrastructure) to a distinct nosologic entity or secretory product because of their overlapping fine structures in different cell types. Immunoelectron microscopy methods utilizing colloidal gold-labeled secondary antibodies can be used to localize virtually any antigen (peptide or neuroamine) to a specific neurosecretory or endosecretory granule or other cell structure. General endocrine/neuroendocrine cell markers such as neuron-specific enolase, the chromogranins, and synaptophysin are useful in identifying neuroendocrine differentiation in a neoplasm using routine immunohistochemical procedures. The current relevance of the APUD concept of Pearse as well as the biologic importance of endocrine/neuroendocrine secretory products such as bombesin and insulinlike growth factors also are discussed.

Coexpression of insulin and somatostatin in single secretory granules of a pancreatic endocrine tumor

Pancreatic endocrine tumors, examined by immunohistochemistry, were found to be associated with multihormonal production, and recent studies, performed by employing double immunostaining methods, reported the coexpression of hormones in single cells or in single secretory granules. These findings have been attributed to the heterogeneity of the neoplastic cell population, characterized by the emergence of clones with different phenotypes, and were considered a sign of cell dedifferentiation or malignancy. In this study we describe a case of pancreatic endocrine tumor that showed focal colocalization of insulin and somatostatin in single secretory granules, by means of double labelling immunoelectron microscopy. We think that this observation can be linked to the hypothesis of those authors who speculated upon the appearance of polycrine cells in human fetal pancreas during embryogenesis.

Ultrastructural immunolabeling in the evaluation, diagnosis, and characterization of neuroendocrine neoplasms

Neuroendocrine neoplasia represents a heterogenous entity with variable morphologic light microscopic expressions. In many cases a definite diagnosis is easily made by light microscopic examination, but in some cases this does not suffice. In the latter instances, immunocytochemistry, ultrastructural examination, or both are required to diagnose a neuroendocrine neoplasm. However, basing a diagnosis of neuroendocrine neoplasia exclusively on the results obtained from immunocytochemical or ultrastructural evaluation of these tumors may not be entirely accurate in some instances. Ultrastructural immunolabeling plays a key role in accurately defining localization of immunoreactive substances in well-characterized neuroendocrine neoplasms, can assess colocalization of antigenic epitopes, helps define specificity and significance of immunocytochemistry results obtained at the light microscopic level, and is more sensitive than light microscopic immunocyto-chemistry. Some evolving diagnostic entities can be further characterized by utilization of ultrastructural labeling techniques. Controversies concerning the neuroendocrine nature of electron-dense structures identifiable at the ultrastructural level can be readily and accurately resolved. By providing a way to evaluate combined immunomorphologic parameters, ultrastructural immunogold labeling can settle important questions pertaining to neuroendocrine neoplasia. The present article illustrates a series of cases where the above-mentioned applications were tested.

Colocalization of pancreatic polypeptide and insulin in secretory granules of a pancreatic endocrine neoplasm

A 45-year-old woman presented with clinical symptoms of hypoglycemia of 4 months duration. Laboratory testing confirmed hyperinsulinemia; mild hypercalcemia and hypergastrine-mia were also documented. At the time of operation, 3 pancreatic endocrine neoplasms were found, and a diagnosis of multiple endocrine neoplasia type I was made. Immunohistochem-istry and immunoelectron microscopy showed all the tumors to be plurihormonal, each containing three or more of the following: insulin, glucagon, somatostatin, pancreatic polypep-tide, gastrin, and serotonin. Electron microscopy of 2 tumors revealed numerous atypical granules. In 1 tumor, pancreatic polypeptide and insulin were colocalized in secretory granules by dual-staining immunoelectron microscopy. To our knowledge, this combination of hormones has not been described previously in pancreatic endocrine neoplasms and suggests that such neoplasms, like mature pancreatic endocrine cells, may originate from pluripotential common precursor cells.

Islet cell tumor

A case of islet cell tumor occurring in a patient with the multiple endocrine neoplasia type I syndrome is reported. Immunostaining for insulin was strongly positive in the tumor cells. Numerous dense-core granules of endocrine caliber were identified ultrastructurally. Morphometric analysis of the secretory granules in 20 islet cell tumors gave a granule size of 182 +/- 52 nm (mean +/- standard deviation).

Upper abdominal neoplasms: an introduction and selected references

The organs in the upper half of the abdominal cavity can give rise to a highly heterogeneous collection of neoplasms that encompasses a broad spectrum of clinical features, light microscopic appearances, immunostaining properties, and fine structure. Because of the anatomic proximity of the viscera in this region, the differential diagnosis of an upper abdominal mass may include consideration of tumors that arise from the stomach, duodenum, liver, kidney, adrenal, and pancreas. There are many distinctive ultrastructural features among the various tumors that can be used effectively by the diagnostic electron microscopist, but it is also important to take into account the considerable overlap in fine structure that occurs among different tumors and among similar tumors from different organs. In the session on upper abdominal neoplasms, the electron microscopic characteristics of the common tumors of each viscus were reviewed before the presentation of individual case studies that incorporated original observations and illustrated some unusual entities. The selection of cases that follows is inevitably small and arbitrary, but the cases demonstrate the contribution that electron microscopy can make in the study of an upper abdominal mass and illustrate some of the similarities that can be encountered among tumors of different organs.