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Analysis of primuline, thioflavine S and thioflavine T acid and basic azo group-free thiazole dyes showed that they were built up from a number of components which were characterized by physicochemical methods. It was found that the isolated components, as well as related substances of known chemical composition, have characteristic staining properties. Factors involved in the staining mechanism of the dyes and of components—dye concentration, pH, aggregation of the dye molecules—were investigated and their roles are discussed. Selectivity of these fluorescent staining methods was also studied.

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In the course of a routine clinical check up of the 54 year old male a splenic well circumscribed tumor like mass of 12 cm in diameter was discovered. Splenectomy with removal of splenic hilar lymph nodes and liver wedge biopsy were performed. Four years later the patient is symptom free. In the removed spleen the tumor like lesion showed a pattern consistent with the diagnosis of inflammatory pseudotumor. However, besides lymphocytes, plasma cells, macrophages, eosinophils and myofibroblasts a high number of slightly polymorphic, frequently binucleated cells positive for CD21 and CD23 were seen. These cells which were scattered or formed smaller or larger groups and fascicles were considered to represent follicular dendritic reticulum cells (FDRCs) and the lesion a FDRC tumor. Flow cytometric DNA ploidy analysis showed a hyperdiploid cell population inside the tumor like lesion. Besides FDRC tumors of high and of intermediate malignancy the present case may represent a low grade type of moderate proliferation activity. The FDRCs of the lesion and a few smaller spindle cells were EBER positive indicative of the presence of EBV. No EBER positive cells were seen in the uninvolved spleen.