Examination of microvilli (MV) by electron microscopy (EM) may help distinguish epithelial malignant mesotheliomas (EMM) from metastatic adenocarcinomas (AC). The goal of this study was to assess the diagnostic utility of microvillous length to width (l/w) measurements of EMM and AC, and to determine if ultrathin sections can be used to accurately assess lengths of villi not completely contained within a single section. Therefore, in addition to the usual ultrathin (600-800 A) sections used for standard transmission EM, thicker (up to 1 mu) sections were also prepared for study by scanning transmission EM. Seven cases of EMM and 3 of AC were analyzed. In each case, length and width were measured for all MV with identifiable bases (18-60 MV/case), using an electronic planimeter, and the mean l/w ratio calculated. The dimensionless l/w ratio is independent of magnification and does not require calibration, facilitating intercase comparison. In the 3 AC, the mean l/w was 5.39, versus 11.44 in the 7 EMM cases. Four EMM were analyzed using both standard ultrathin and thicker sections, disclosing a thin section l/w of 13.34 and a thick section of 12.26, supporting the confidence of measurements made from standard ultrathin sections. Examination of data also showed that equally good separation of EMM from AC could be obtained using the mean ratio of only the 10 longest MV (16.11 versus 8.93). By these techniques, EMM often may be distinguishable from AC, with a l/w ratio twice as large in EMM and a mean l/w greater than 11 supportive of a diagnosis of EMM.
