Malignant Mesothelioma Electron Microscopy

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1 33 Malignant Mesothelioma Electron Microscopy Raoul Fresco In spite of recent advances in immunocytochemistry, electron microscopy continues to be the gold standard for the differential diagnosis of mesothelioma from other tumors affecting serosal surfaces (1). Three major morphologic subtypes of mesothelioma are well established: epithelial, sarcomatoid, and mixed or biphasic. Of these three types, electron microscopy has been the most contributory in the diagnosis of the epithelial variant and is less helpful in the sarcomatoid mesotheliomas (2,3). Epithelial Mesothelioma This subtype, which is the most common, is also the one most difficult to differentiate at the light microscopic level from peripheral lung or metastatic adenocarcinoma. At the ultrastructural level, the most characteristic feature of the neoplastic epithelial cells in the epithelial and mixed types of mesotheliomas is the distinctive morphology of its microvilli (4,5). Microvilli Unlike adenocarcinomas (6,7), which usually have relatively scarce short blunt microvilli (Fig. 33.1), mesotheliomas are characterized by the presence of abundant long, slender, undulating, and often bifurcating microvilli. These lack filamentous cores, core rootlets, and surface glycocalyx (fuzz); they measure approximately 0.1mm in diameter and up to 3mm in length (Fig. 33.2). Some investigators (8 10) combined the quantitative and qualitative features of tumor cell microvilli and developed a length to diameter ratio (MLDR) as a useful measurement to help distinguish mesotheliomas from adenocarcinoma. Thus, MLDR values over 15 are highly suggestive of mesothelioma, while most adenocarcinomas have MLDR values of under 10. However, some overlapping exists, and other ultrastructural criteria are used to confirm the diagnosis. Profuse microvilli frequently cover the entire tumor surface, best seen in cells aggregated in a papillary 508

2 R. Fresco 509 Figure Lung adenocarcinoma. Relatively scarce, short, blunt microvilli covered by fuzzy glycocalyx. Zymogen-like electron dense secretory granules are present, but no intermediate filaments are seen. Figure Epithelial mesothelioma. Abundant, long, slender at times, bifurcating microvilli. Cytoplasm shows bundles of intermediate filaments.

3 510 Chapter 33 Malignant Mesothelioma Electron Microscopy Figure Epithelial mesothelioma, papillary pattern. Profuse microvilli covering most of the cellular surfaces. One or more prominent nucleoli are present in the nuclei. pattern (Fig. 33.3). When the tumor cells form a tubular pattern, the microvilli line the luminal surfaces (Fig. 33.4). At times, these microvilli are seen to contact collagen fibrils without the interposition of basement membranes, a phenomenon first described by Dewar et al (7) and termed microvillus-collagen association by Ghadially et al (11,12) (Figs and 33.6). This interaction was once believed to be a diagnostic feature of mesotheliomas, and though it is most often seen in these tumors, it has also been reported to occur in adenocarcinomas (11). Intermediate Filaments Tonofilaments are present in the cytoplasm of a majority of mesotheliomas (Fig. 33.2). These are often aggregated in fibrillary bundles arising in relation to desmosomes or in the perinuclear zone, whereas they are rarely seen in adenocarcinomas of the lung (9,10,13). Cellular Junctions While desmosomes occur in both mesotheliomas and adenocarcinomas, long (more than 1 mm), so-called giant desmosomes, when present, favor a diagnosis of mesothelioma (Fig. 33.7) (14,15).

4 Figure Epithelial mesothelioma, tubular pattern. Numerous microvilli line the luminal surfaces. The abluminal surfaces show a basal lamina, beyond which are bundles of collagen fibers. Figure Epithelial mesothelioma, tubular pattern. Collagen fibers are seen in direct contact with the microvilli lining the lumen.

5 512 Chapter 33 Malignant Mesothelioma Electron Microscopy Figure Epithelial mesothelioma. Microvillus-collagen association. Figure Epithelial mesothelioma. Giant desmosome.

6 R. Fresco 513 Figure Sarcomatoid mesothelioma. Spindle-shaped tumor cells with elongated nuclei and abundant cisternae of rough surfaced endoplasmic reticulum. Sarcomatoid Mesothelioma While the epithelial type of mesothelioma can be confused with adenocarcinoma when examined with the light microscope, the sarcomatoid variant is likely to be confused with fibrosarcoma, even at the ultrastructural level. The mesenchymal-looking cells forming these tumors resemble fibroblasts; they have elongated nuclei with clumped chromatin and their cytoplasm contains numerous cisternae of rough endoplasmic reticulum (Fig. 33.8). No basal laminae, desmosomes, or microvilli can be recognized, but rudimentary intercellular junctions are often seen (16). Some of the cells may show myofibroblastic differentiation, with peripheral bundles of filaments bearing focal densities (17). Mixed (Biphasic-Transitional) Mesothelioma As their names imply, these tumors have features that are found in both epithelial and sarcomatoid subtypes. The two components may be closely intermixed or may occur in different parts of the same tumor,

7 514 Chapter 33 Malignant Mesothelioma Electron Microscopy features that may present diagnostic difficulties, even at the ultrastructural level, in view of the small size of samples usually studied by electron microscopy. This morphologic diversity demonstrates the multidirectional differentiation capability of mesothelial cells (18,19). Technical Considerations In the previous paragraph we alluded to what is a well-known problem confronting diagnostic ultrastructural pathologists, namely the inherent sampling error due to the small area available for examination under the electron beam. Often, the need for electron microscopy is realized only after all the biopsy material has been embedded in paraffin for light microscopy with no portion kept for ultrastructural studies. In such a case, a selected area of the paraffin block can be scooped out, deparaffinized in xylene, and transferred to absolute and then descending grades of alcohol and finally water, at which point it can be reprocessed for electron microscopy (20). The resulting ultrastructural preservation is in great part dependent on the quality of the primary formalin fixation. Our use of this methodology over Figure Mesothelioma. Formalin fixed, deparaffinized biopsy. Microvillus morphology and prominent desmosomes show excellent preservation, allowing correct diagnosis.

8 R. Fresco 515 the past few years has exceeded our expectations, and while we do not recommend it routinely as a substitute for conventional electron microscopic techniques, it certainly should be considered when optimally processed tissue is not available and diagnostic problems arise at the light microscopic level (Fig. 33.9). Viral Pathogenesis In view of the increasing evidence that at least some cases of mesotheliomas are associated with simian virus 40 (SV40) infection (21), it is interesting to note that some authors have detected tubuloreticular structures in two cases of pleural mesotheliomas (22). These intracytoplasmic structures, first believed to be viral in nature (23), can be traced to be continuous with the endoplasmic reticulum, suggesting a host cell response (24). They have been shown to be induced by a-interferon, and besides autoimmune diseases, where they were first reported, they are present in many viral infections (25,26). References 1. Hammar SP, Bockus DE, Remington FL, Rohrbach KA. Mucin-positive epithelial mesotheliomas: a histochemical, immunohistochemical, and ultrastructural comparison with mucin-producing pulmonary adenocarcinomas. Ultrastruct Pathol 1996;20: Battifora H. The pleura. In: Sternberg SS, ed. Diagnostic Surgical Pathology. New York: Raven Press, 1989: Battifora H, McCaughey WTE. Tumors of the Serosal Membranes: Atlas of Tumor Pathology, 3rd series, fascicle 15. Washington, DC: Armed Forces Institute of Pathology, Ghadially FN. Ultrastructural Pathology of the Cell and Matrix. London: Butterworths, 1982: Henderson DW. Transmission electron microscopy of malignant mesothelioma. MSA Bull 1993;23: Comin CE, de Klerk NH, Henderson DW. Malignant mesothelioma: current conundrums over risk estimates and whither electron microscopy for diagnosis? Ultrastruct Pathol 1997;21: Dewar A, Valente M, Ring NP, et al. Pleural mesothelioma of epithelial type and pulmonary carcinoma: an ultrastructural and cytochemical comparison. J Pathol 1987;152: Burns TR, Greenberg SD, Mace ML, et al. Ultrastructural diagnosis of epithelial malignant mesothelioma. Cancer 1985;56: Warhol MJ, Corson JM. An ultrastructural comparison of mesotheliomas with adenocarcinomas of the lung and breast. Hum Pathol 1985;16: Warhol MJ, Hickey WF, Corson JM. Malignant mesothelioma: ultrastructural distinction from adenocarcinoma. Am J Surg Pathol 1982;6: Ghadially FN, McCaughey WT, Perkins DG, Rippstein P. Diagnostic value of microvillus-matrix associations in tumors. J Submicrosc Cytol Pathol 1992;24: Ghadially FN, McCaughey WT, Perkins DG, et al. Morphogenesis and frequency of microvillus-matrix associations in mesotheliomas and adenocarcinomas. MSA Bull 1993;23:

9 516 Chapter 33 Malignant Mesothelioma Electron Microscopy 13. Warhol MJ, Corson JM. An ultrastructural comparison of mesotheliomas with adenocarcinomas of the lung and breast. Hum Pathol 1985;16: Burns T, Johnson E, Cartwright J Jr, et al. Desmosomes of epithelial malignant mesothelioma. Ultrastruct Pathol 1988;12: Ghadially FN, Rippstein PU, Cavell S, Venance SL. Giant desmosomes in tumors. Ultrastruct Pathol 1995;19: Klima M, Bossart MI. Sarcomatous type of malignant mesothelioma. Pathology 1983;4: D Andiran G, Gabbiani G. A metastasizing sarcoma of the pleura composed of myofibroblasts. Prog Surg Pathol 1980;2: Ghadially FN. Diagnostic Electron Microscopy of Tumours, 2nd ed. London: Butterworths, 1985: Oury TD, Hammar SP, Roggli VL. Ultrastructural features of diffuse malignant mesotheliomas. Human Pathol 1998;29: Widehn S, Kindblom LG. A rapid and simple method for electron microscopy of paraffin-embedded tissue. Ultrastruct Pathol 1988;12: Carbone M, Pass HI, Rizzo P, et al. Simian virus 40-like DNA sequences in human pleural mesothelioma. Oncogene 1996;9: Bernaudin JF, Soler P, Basset F. Intracytoplasmic tubuloreticular structures in 2 cases of sarcoidosis and 2 cases of malignant pleural mesothelioma. Pathol Eur 1973;8: Grausz H, Earley LE, Stephens BG, Lee JC, Hopper J Jr. Diagnostic import of virus-like particles in the glomerular endothelium of patients with systemic lupus erythematosus. N Engl J Med 1970;283: Uzman BG, Saito H, Kasac M. Tubular arrays in the endoplasmic reticulum in human tumor cells. Lab Invest 1971;24: Mills AE, Emms M. Frequent occurrence of microtubuloreticular complexes encountered during routine ultrastructural examination at a children s hospital. Ultrastruct Pathol 1988;12: Sidhu GS. The ultrastructural markers of AIDS. MSA Bull 1993;23: