Characteristics of the primary mesothelioma cell cultures. Left panel: Hematoxylin and eosin staining of primary cell cultures. Middle panels: Representative immunohistochemical stains of mesothelioma cell culture pellets that were formalin-fixed and paraffin-embedded. NCI-Meso16, NCI-Meso19 and NCI-Meso21 cells were strongly and diffusely positive for mesothelin, calretinin and WT1. NCI-Meso18 cells expressed primarily calretinin in a smaller percentage of cells, whereas NCI-Meso17 cells showed a small percentage of WT-1 positive cells. Right panel: Electron microscopic images of primary tumor cells which exhibit numerous long and complex surface villi (thick arrows) that are characteristic of mesothelioma cells. The thin arrow in the electron microscopic image of the NCI-Meso21 cells points to condensed aggregates of intermediate filaments indicative of tonofilamens, shown in detail at a higher magnification in the inset.