Immunogold detection of chromogranin A in the neuroendocrine tumor

State of the art in antigen retrieval for immunohistochemistry

The masking effects of antigens by chemical fixation, processing, embedding media interactions, represent a serious problem for immunohistochemical purposes. Fortunately, different approaches in antigen retrieval exist. These techniques are relatively recent and continuously expanding. This review focuses on the present state of the art in antigen retrieval methods for immunohistochemistry in light and electron microscopy. Moreover, a brief discussion on the chemical aspects of fixation, mechanism of retrieval, as well as its efficacy, is given.

Diagnostic value of microtubule-associated protein-2 (MAP-2) for neuroendocrine neoplasms

Microtubule associated proteins (MAPs) are major components of cytoskeleton proteins associated with microtubule assembly. Microtubule associated protein-2 (MAP-2), a component of the MAP family, has been shown to be specifically expressed in neuronally differentiated cells, and has been previously used as a sensitive and specific marker for neurons. Immunoreactivity of MAP-2 has been demonstrated in most neuroendocrine and neuroectodermal related neoplasms such as small cell carcinoma, large cell neuroendocrine carcinoma, carcinoid tumor of the lung, Merkel cell carcinoma of the skin, medulloblastoma, neurocytoma of the central nervous system, extrapulmonary small cell carcinoma and carcinoid tumor, and malignant melanomas. This report details the diagnostic value of MAP-2 on tumors with neuroendocrine differentiation and neoplasms derived from the neural crest.

An antigen retrieval method using an alkaline solution allows immunoelectron microscopic identification of secretory granules in conventional epoxy-embedded tissue sections

Immunoelectron microscopy using chromogranin A-specific antibodies has been proposed as an efficient technique for identification of secretory granules (SGs) in tumor cells with evidence of apparent neuroendocrine differentiation. Using an antigen retrieval (AR) method, we succeeded in immunolabeling SGs with antibodies in ultrathin sections of routinely processed epoxy-embedded blocks of tissue. Samples of an insulinoma were fixed in 2% glutaraldehyde, postfixed in 1% OsO(4), and embedded in epoxy resin. Ultrathin sections were immunostained with chromogranin A-specific antibodies and gold-conjugated second antibodies. There was no significant labeling in the absence of AR. Neither etching with sodium metaperiodate nor microwave irradiation of ultrathin sections in citrate buffer (pH 6.0) or in EDTA buffer (pH 8.0) was effective in improving the efficiency of immunolabeling. However, ultrathin epoxy-embedded sections that were microwaved in alkaline solution (pH 10) were adequately labeled (5.2 +/- 0.34 particles per SG). Moreover, considerably improved efficiency of immunostaining was achieved by microwaving sections in alkaline solution (pH 10) with subsequent immunostaining at 60C (12.2 +/- 0.51 particles per SG). This method can also be applied to epoxy-embedded sections obtained from formalin-fixed, paraffin-embedded blocks of tissue and was even valid for an old epoxy-embedded block of tissue prepared 15 years previously.

Microtubule-associated protein-2: a new sensitive and specific marker for pulmonary carcinoid tumor and small cell carcinoma

Microtubule-associated proteins (MAPs) are a major component of cytoskeleton family proteins associated with microtubule assembly. MAP-2 has been shown to be specifically expressed in neuronally differentiated cells. Pulmonary neuroendocrine carcinomas such as carcinoid tumors and small cell carcinomas are derived from neuroendocrine cells. We hypothesize that neuroendocrine cells may also express MAP-2, and therefore, MAP-2 may be used as a marker for pulmonary carcinomas of neuroendocrine differentiation. To investigate the utility of using MAP-2 expression to separate pulmonary neuroendocrine from non-neuroendocrine tumors, we examined the expression of MAP-2 immunohistochemically in 100 cases of pulmonary carcinomas. The immunoperoxidase method with antigen retrieval was used to characterize the expression of MAP-2, chromogranin, synaptophysin, and neuron-specific enolase in 25 small cell carcinomas, 25 carcinoid tumors, 25 adenocarcinomas, and 25 squamous cell carcinomas. All tumors were lung primaries. All 25 cases of carcinoid tumors (100%) as well as 23 of 25 cases (92%) of small cell carcinomas were positive for MAP-2. Four of 25 cases (16%) of adenocarcinomas were positive for MAP-2 and synaptophysin. Among the 25 squamous carcinomas, 4 cases (16%) were positive for MAP-2, 2 cases (8%) were positive for synaptophysin, 11 cases (44%) were positive for neuron-specific enolase, and none was positive for chromogranin. In conclusion, MAP-2 is a new sensitive and specific marker for the pulmonary tumors of neuroendocrine differentiation. We recommend that MAP-2 be added to immunohistochemical panels to separate non-neuroendocrine from neuroendocrine lung tumors.

Immunoelectron microscopy for chromogranin A in small cell neuroendocrine carcinoma of lung

To see if immunoelectron microscopy can improve localization of neurosecretory granules, postembedding immunolabelling for chromogranin A was performed on 15 examples of small cell anaplastic (neuroendocrine) carcinomas primary in lung, five cases of bronchopulmonary carcinoids, and two cases of pheochromcytoma; both the carcinoids and pheochromocytomas had neurosecretory granule-rich cytoplasm by routine electron microscopy and some neurosecretory granules were identified in each of the small cell carcinomas. Immunolabeling for chromogranin A resulted in many colloidal gold particles over cytoplasmic secretory granules in both pheochromcytomas and four of the carcinoids. One carcinoid that was focally positive by immunoperoxidase staining was negative by immunoelectron microscopy. None of the 15 cases of small cell carcinoma stained for chromogranin A using immunoperoxidase techniques, but three had a small number of secretory granules weakly labeled by the anti-chromogranin A/colloidal gold complex. Immunoelectron microscopy, at least using standard glutaraldehyde fixation and epoxy resin embedding, does not increase the sensitivity of neurosecretory granule identification in small cell neuroendocrine carcinomas of the lung. Results of other studies of this neoplasm suggest that despite transcription of chromogranin genes, synthesis of the specific protein may occur to a limited extent or not at all.

The theoretical ratio of immunogold labeling of deplasticized epoxy sections and acrylic sections

The purpose of this study was to predict the ratio of immunogold labeling of deplasticized epoxy sections and LR-White sections on the basis of theoretical considerations. Tissues used in experiments were pancreas, pituitary, kidney, thyroid, and fibrin. Antigens used as test proteins were glucagon, somatostatin, thyroglobulin, chromogranin A, ACTH (= Adrenocorticotropt hormone), amyloid A, and fibrinogen. These are proteins of different sizes. The quotient labelingdeplasticized/labelingLR-white was deduced theoretically and compared to measurements based on immunogold experiments to obtain a theoretical model with acceptable correlation to the measurements. This study describes a theory--expressed mathematically--for what happens at the molecular level in immunoelectron microscopy at the surface of deplasticized epoxy sections and acrylic sections. The theory explains why we normally get about the same amount of immunogold labeling using LR-White sections (acrylic resin) and deplasticized epoxy sections. Taking the nuances into account, the theory indicates increased usefulness of deplasticized epoxy sections when the diameter of the protein carrying the epitopes decreases.

Immunoelectron microscopic identification of human parvovirus B19

A mildly macerated, hydropic fetus was delivered spontaneously at 25 weeks gestation by a multigravidous black mother. At autopsy, gross and microscopic evidence suggested fetal anemia, and excessive extramedullary erythroblastosis was present generally. Erythroid precursor cells in the pulmonary capillary circulation contained eosinophilic nuclear inclusions consistent with human parvovirus B19 infection. Transmission electron microscopy on osmicated Epon lung sections showed lucent, granular chromatin corresponding to the inclusions. In rare foci only these sections contained paracrystalline arrays of 20-nm virions. In the same cells, similar virions were seen within the cytoplasm, both randomly distributed and in paracrystalline aggregates. Postembedding immunoelectron microscopy, done on formalin-fixed lung embedded in a hydrophilic resin, showed positive labelling over the nuclear inclusions, and also localized the viral capsid antigen to cytoplasmic virion aggregates. The nuclear aggregates suggest that the virus would reach the circulation after cell lysis whereas those in cytoplasm suggest that parvovirus also may be excreted from infected cells before they lyse.

Reliability of criteria for ultrastructural identification of neuroendocrine granules

For full diagnostic use to be made of the neurosecretory granule, the range of sizes, forms, and staining qualities for this cytoplasmic organelle, along with the extent of its expression in various neoplasms, must be established. Neurosecretory type granules occasionally occur in nonneuroendocrine tumors. A series of carcinoids of the lung provides a model for assessing the morphologic types of cytoplasmic granules identified by antibodies to chromogranin A and immunogold labeling. The results show that granule structure in tumors is pleomorphic. Despite having sizes within the expected range, many labeled and, indeed, unlabeled secretory granules are atypical, particularly in structural form. Cell-to-cell variation in the proportion of even typical neurosecretory granules labeling for chromogranin A is the rule. Studies correlating biochemical, immunohistochemical, electron microscopic, and perhaps in situ hybridization characteristics are required to define better the criteria for unequivocal identification of neurosecretory granules in tumors.