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Pathology of Human Malignant Mesothelioma By Yasunosuke Suzuki THE EXISTENCE of malignant mesothe lioma was recorded before the end of the In 1953, Weiss' suggested that asbestos expo sure was responsible for the induction of malig nineteenth century. The pleural lesion reported nant mesothelioma in an asbestos worker. In by Wagner' in 1870 is now undoubtedly consid I960, Wagner et al.1 reported on the strong ered to be a malignant mesothelioma.1 In those association of asbestos exposure and malignant days, terminology for mesothelioma was a mesothelioma. They identilied multiple cases of matter of controversy, and the term "endothelio malignant pleural mesothelioma among resi ma" was frequently used for this tumor.2,2 dents in the Northwest Cape Province of South DuBray and Rosson2 proposed the term primary Africa, all but two of whom had had exposure to mesothelioma of the pleura in 1920, having asbestos dust. Since then, others have confirmed concluded that the tumors arose from the and extended these observations both epidemio surface of the parietal pleura.2 In 1931, Klem logically and in animal experiments.9'12 perer and Rabin4 described two cases from In the past, malignant mesothelioma was a Mount Sinai Hospital, and added three from the rare tumor occurring in 0.IT--0.01% of autop European literature. From these cases they sies.12 In the past 20 yr. however, the incidence of developed many of what are now accepted as the the tumor has been rising.2,9 12 Recent epidemio modern concepts of the pathology of diffuse logic studies9 l2IJ-,s indicate that there is a long malignant pleural mesothelioma. They con latent period (20-40 yr) from asbestos exposure cluded that (1) the localized giant tumor to the development of the tumor. These studies connected to the visceral pleura was usually have also shown that malignant mesothelioma benign originating from the sub-mesothelial can occur in -association with a wide range of connective tissue; (2) the gross anatomy of asbestos exposures, including direct occupation malignant mesothelioma usually was diffuse al, indirect occupational, and family contact originating from the mesothelial cells; and (3) exposures. Since submicroscopic asbestos fibrils malignant diffuse mesothelioma was highly vari can be seen by electron microscopy in the lungs able histologically due to the probable multipo of patients with mesothelioma who did not ex tentiality of the mesothelial cell, which could hibit clinical evidence of asbestosis, it is possible differentiate intQ_.various cell lines. After this that low level exposures to asbestos can induce report, the term malignant mesothelioma malignant mesothelioma. became popular. The biologic behavior of the Generally, malignant mesothelioma is consid tumor, such as its ability to differentiate into ered to be a malignant diffuse tumor arising distinct histologic types, remained of academic from the mesothelial cell. Alternate explana interest.1 9 tions, however, come from some experimental studies. In these studies, both the mesothelial From (he Environmental Sciences Laboratory, Depart ment of Community Medicine. and Department of Patholo gy, Mount Sinai School of Medicine of the Citv University cell and the submesothelial mesenchymal cell"'1' have been implicated as the precursors of various forms of malignant mesothelioma. of \ew V ork. .V('w York, S ) Supported by Research (/rants CA24.il l and CA29432 from the National Cancer Institute and Center Grant ES 00928 front (He National Institute ofEnvironmental Health Sciences, L.S Department of Health and Human Services Address reprint requests to Yasunosuke Suzuki. Environ Regardless of the cell of origin, the surface linings of three serosal cavities, pleura, peritone um, and pericardium, are known to be the major sites of malignant mesothelioma. The incidence of pericardial mesothelioma is considerably less mental Sciences Laboratory, Department of Community Medicine. Mount Sinai School of Medicine, Citv L'nisersttv of .Vh } ork. One Gustave Levy Place. Yew York. S Y 10029 c I VS I by Grime & Stratton. Inc* 1)09J '?54/S l;<)$()J (h)05$()2 OOjO frequent than pleural or peritoneal mesothelio ma. In material submitted to our laboratory, the ratio of plcural.pcritoneahpericardial mesothe liomas has been 57.IT, 39.5T, 1%, respectively. The site of origin of the remaining 2.4% could 268 Seminars m Oncology. Vol 8. No- 3 (September). 1980 PATHOLOGY OF MALIGNANT MESOTHELIOMA 269 not be determined, since both pleura and perito neum were extensively involved by the neoplastic process. In addition to these three major sites, rare mesotheliomas of the serosa of the genital tract and organs such as the tunica vaginalis, ovary, uterus, and fallopian tubes have been reported.18 27 Both benign and malignant forms have been seen. These mesotheliomas have not as yet been associated with asbestos exposure, with the possible exception of one patient with malig nant mesothelioma of the tunica vaginalis." Klemperer and Rabin4 classified mesothelio mas as either benign, which was usually local ized and histologically showed only a fibrous form, or malignant, which was usually diffuse, and three forms could be distinguished histologi cally: epithelial, biphasic (mixed), or mesenchy mal or fibrous (sarcomatous). On the other hand, Stout28 also divided mesothelioma into benign and malignant, but both were classified into dilTusc and localized. He further divided the localized and diffuse into epithelial, mesenchy mal, and biphasic forms on the basis of light microscopy. These diverse classifications were based on necropsy findings. DIAGNOSTIC CRITERIA A comprehensive approach using gross ana Fig, 1. Malignant pleural mesothelioma. Showing gross anatomical appearance of malignant pleural mesothelioma. Left pleura was totally infiltrated by neoplastic tissue. The lung parenchyma was compressed by diffuse, thick neoplastic tissue. {From Or. I.J. Selikoff's slide collection.) tomical observations, histology, histochemistry, electron microscopy, and possibly biochemistry of the fluid and cytology have been attempted for diagnosis of malignant mesothelioma. what localized. The tumor tissue of mesothe lioma is usually white and hard, but occasionally (lie tumor cannot be distinguished from the Gross "Appearance surrounding librotic tissue. Metastascs may be seen with malignant meso As shown in. Fig. I (pleural), and Fig. 2 thelioma.u"'"' Regional lymph nodes, lungs, I (peritoneal), malignant mesothelioma has a brain, adrenal glands, and liver arc potential tendency to develop along the serosa, with inva sites of metastascs from pleural mesothelioma. sion into surrounding tissues such as the subme- Abdominal and pelvic lymph nodes, liver, lung, sothelial connective tissue, and the chest wall, pleura, and pericardium are organs in which abdomen, diaphragm, and the subserosal paren metastascs from peritoneal mesothelioma may chyma of various organs such as lung, liver, and be detected. Both pleural and peritoneal meso intestines. Both pleural and peritoneal mesothe thelioma can infiltrate through the diaphragm liomas are frequently accompanied by hyaline into the other body cavity. Histologic evaluation I plaques in the perineoplastie libroiic tissue.' ,n of the metastascs sometimes demonstrates Pleural effusion and ascites are frequently considerable variability from the original tumor. observed and the fluid is usually bloody in The presence of metastascs can sometimes nature. Peritoneal mesothelioma generally show confuse the diagnosis grossly. However, diffuse a pattern of extensive intraabdominal spread process in the chest or abdomen should immedi even when first diagnosed. Occasionally malig ately bring to mind the possibility of mesothelio nant pleural mesothelioma may appear some- ma. 270 YASUNOSUKE SUZUKI Differential Diagnosis The absence of obvious primary tumor in any of the various organs such as lung, gastrointesti nal tract, pancreas, or ovaries, coupled with a diffuse mesotheliai tumor should alert one to the presence of a mesothelioma. Since other tumors can metastasize to the pleura or peritoneum, in the past many pathologists were unwilling to diagnose mesotheliomia except as a diagnosis of exclusion. There are now, however, specific histologic, histochemical and ultrastructural features that will allow for a more specific and antemortem diagnosis and these will be discussed in the respective sections. Histology Histologic evaluation is the most popular and important method to determine the diagnosis of malignant mesothelioma. In many series, the majority of tumors were diagnosed histologically from open or needle biopsies. Generally, the gross appearance is helpful and a greater quan tity of tissue is preferable. Thus, open biopsy is probably the preferred method for obtaining diagnostic material. This histology of pleural, peritoneal, and pericardial mesotheliomas do not differ much. In each, the tumor can be classified into three forms: epithelial, biphasic (mixed), and fibrous (mesenchymal) varieties. This classi fication is based on the characteristics of the individual neoplastic cells. Our data from Mount Sinai Hospital in New York based on 210 consecutive malignant mesotheliomas, showed Fig. 2. Malignant peritoneal mesothelioma. Abdominal sur face of diaphragm diffusely infil trated by fine neoplastic nodules. (From Or. I.J. Setikoff's slide collection.) that 67% were epithelial, 26% were biphasic (mixed), and 7% were of the fibrous (mesenchy mal) form. These mesotheliomas were classified as 177 definite (104 pleural, 66 peritoneal, 2 pericardial, and 5 both pleural and peritoneal) and 33 (16 pleural and 17 peritoneal) probable cases. Epithelial Form In this subtype of mesothelioma, the neoplas tic cells may show various epithelial arrange ments, such as papillary (Fig. 3a), tubular (Fig. 3b), tubulopapillary, cordlike (Fig. 3c) and sheetlike (Fig. 3d) patterns. Frequently, two or three patterns are mixed in a single case. The epithelial form, when present in sheetiike pattern, has occasionally been misjudged as poorly differentiated squamous cell carcinoma. However, neither intercellular bridging nor keratinization is found. The papillary and tubu lopapillary forms must be differentiated from metastatic adenocarcinoma of lung, ovary, and gastrointestinal tract. In general, the cytoplasm shows either eosino philic or basophilic tones, which are related to ultrastructural features as discussed below. In the epithelial form, cells frequently produce intracellular (signet ccll-likc) and intercellular vacuoles. Generally, these vacuoles represent "empty spaces" on Hematoxylin-eosin stains. Using specific histochemical stains, hyaluronic acid may be demonstrated in some of the spaces. The nucleus of the cells is round or ovoid, and PATHOLOGY OF MALIGNANT MESOTHELIOMA 27 * Fig. 3. Light micrographs showing histologic variability of the epithelial form, (a) Papillary pattern {H&E. 350L (bl Tubular pattern (H&E. x330). (c) Cord-like pattern (H&E. x ) (d) Sheet-like pattern {H&E. <530). {(a. bl adapted from American Journal of Pathology74 with permission.) the chromatin is generally fine. The nucleolus is usually well developed but fewer abnormal mitoses appear in mesothelioma than in other malignant tumors. The stroma of the tumor varies. Frequently hyaline plaque (acellular hyaline degeneration of proliferated collagen) is seen in the neoplastic tissue, and sometimes, pseudomyxomalous de generation of the stroma is observed. Biphasic (MixedI Form The biphasic or mixed form is characterized by the presence of the two different cell types, epithelial and mesenchymal: frequently there are cells intermediate between the two cell types. as shown in Fig. 4b. When a tumor -.hows histologic characteristics of this subtype, one may confidently diagnose it as malignant meso thelioma. Figures 4a. b, and c illustrate the histology of the biphasic form. Fpithcir.il cells show mg .1 tubular pattern are seen in Fig. -la ano atypical (fusiformI epithelial cells are seen m Fig 4b. Spindle shape fibrous cells are seen Fig. 4c These transitional cells will be charac terized at the level of ultrastructure (sec bciow i Small numbers of these intermediate ceils are occasionally identified within tumors classified as either the epithelial or fibrous variety The more one examines tissue sections of either lorm. the more likely one is to find these cells 11 ig "j 272 VASUNOSUKE SUZUKI Fig. 4. Light micrographs of biphasic form of malignant peritoneal mesothelioma, (a) Epithelial form tubular pattern, (b) Atypical epithelial cells, (c) Fibrosarcomatous cells (H&E. x 330.) (Adapted from American Journaf of Pathology1* with permission.) The identification of these intermediate "transi tional'' cells helps to establish the diagnosis of malignant mesothelioma. Fibrous Form The histology of the fibrous form is seen in l-'ig. (), The cells in this form arc spindle shaped and show a parallel arrangement and have an ovoid or elongated nucleus with well-developed nucleoli. The histologic aspects are similar to those of the fibrous portions of the biphasic (mixed) form, seen in Fig. 4c. Anaplastic features and plcomorphism arc usually not strik ing. Histochemistry Malignant mesothelioma cells can produce hyaluronic acid. Accordingly, hisiochemieal detection of this substance has been proposed for the diagnosis of the tumor. Meyer and Chaffee'" first reported that the pleural effusion of a patient with mesothelioma contained a large amount of hyaluronic acid, a variety of acid mucopolysaccharide. Histochcmically, colloidal iron or Alcian blue stains with or without hyaluronidase digestion have been used to study the presence of the material.' '4 K Figure 7a illus trates a section of mesothelioma stained with colloidal iron. The arrows in the illustration show the positively stained material in vacuoles. The positive material could be digested by hyalunidase (Fig. 7b) identifying it as hyaluronic acid. Hyaluronic acid can be found in both the cells and stroma of malignant mesothelioma. Hyal uronic acid content, particularly in the stromal tissue, can be detected in many other conditions. PATHOLOGY OF MALIGNANT MESOTHELIOMA 273 Fig. 5. Spindle shaped neoplastic cells (arrows) are shown. Although the case was classified as an example of the epithelial form, it is an example of the presence of the intermediate cells in the epithelial form (H&E, x350). (Adapted from American Journal of Pathology74 with permission.) such as bronchogenic carcinoma, mesenchymal tumors, inflammatory lesions, and young, active connective tissue. But our impression is that stromal hyaluronic acid seems to be greater in amount in mesothelioma than in the other condi tions described above. The exclusion of intracellular mucin (often produced in cells of adenocarcinoma of the lung or gastrointestinal tract) is also necessary for the diagnosis of mesothelioma, since mucin is not Fig. 6. Histology of fibrous form of malignant pleural mesothelioma. Cells are spindle shaped and in parallel arrangement. Also are reminescent of fibroblasts (H&E, y.350). Fig. 7. A systematic histochemical study of a case of malignant pleural mesothelioma, (a) Mowry's colloidal iron. Intracellular vacuoles (arrows) are filled with colloidal iron positive materials. Mowry's colloidal iron stain, *574. (b) Mowry's colloidal iron stain after hyaluronidase digestion. The material is digested by the enzyme before the staining. Accordingly, the intracellular vacuoles do not show the positive material by subsequent staining with colloidal iron. Digested material is therefore probably hyaluronic acid. Mowry's colloidal iron stain with hyaluronidase digestion, x574. (Adapted from American Journal of Pathology14 with permission.) usually produced in cells of mesothelioma. The presence of mucin will exclude mesothelioma. Periodic-acid-Schiff stain (PAS) with or without diastase digestion or mucicarmine stains arc frequently used for the histochemical identifica tion of mucin. Cells of mesothelioma may contain glycogen, but this, unlike mucin, is removed by diastase digestion. For histochemical evaluation of possible mesothelioma several stains are therefore useful: periodic-acid-Schiff stain (PAS) with or without diastase digestion (or mucicarmine stain), colloidal iron stains (or Alcian blue stains) with or without hyaluroni dase digestion. Biochemical Detection of Hyaluronic Acid Biochemical detection of hyaluronic acid in effusions has been proposed as a diagnostic crile- 274 ria of malignant mesothelioma.33,35'39 Effusions caused by other diseases, such as metastatic cancers and inflammatory diseases, are also known to be rich in hyaluronic acid.38-" There fore the identification of hyaluronic acid in the pleura is not diagnostic of mesothelioma. When present there is, however, generally greater amounts of hyaluronic acid found in malignant mesothelioma.39 A quantitative relationship be tween other diseases and mesothelioma has been demonstrated with dried pleural tissues of meso thelioma containing over 0.10 mg of hyaluronic acid per gram tissue while pleural tissues from carcinoma or fibrous pleura contained 0.02 to 0.03 mg/g of tissue.37 Thus a pleural or perito neal effusion with a large quantity of hyaluronic acid has a reasonable likelihood of being derived from mesothelioma. Cytology Since effusions are commonly seen in malig nant mesothelioma and since malignant cells may exfoliate into the effusions, cytological diagnosis of the tumor has been attempted using light and electron microscopy of the cell blocks.42"'7 Although many cytopathologists favor the use of cytopathology for diagnosis of malignant mesothelioma,42,46,47 some still remain critical because of the high false positive and negative rate.43-45 The well-differentiated float ing epithelial cell-of mesothelioma is usually difficult to separate from reactive mesothelial cells. Naylor44 has, therefore, recommended using combined cytologic and chemical analyses of effusions to strengthen the diagnosis of malig nant mesothelioma. However, the use of electron microscopy combined with specific cytochemical staining may make cytologic preparation suffi cient for diagnosis. Ultrastructural characteriza tion of floating mesothelioma cells has been proposed.48,49 AH three cell types have been iden tified by electron microscopy in effusions49 and will be discussed below. Immunopathologic Diagnosis Recently, immunopathologic diagnosis of malignant mesothelioma has been attempted.5051 Indirect immunofluorescence staining for carcinoembryonic antigen (CEA)-like material was negative in malignant pleural mesothelioma, but detected in bronchioloalveolar cell carcinoma and adenocarcinoma, suggesting that the test YASUNOSUKE SUZUKI may be useful in excluding these tumors from the differential diagnosis of malignant mesothe lioma.50 Antimesothelial cell serum was tested as a diagnostic reagent for malignant mesothelio ma,51 and mesothelioma cells were stained with the serum, suggesting that this technique may prove useful for antemortem diagnosis. These immunopathologic approaches have yet to be attempted in comparison with routine methods and judged for accuracy. It is likely, however, that these and similar techniques may become useful, particularly as the technology improves. Electron Microscopy Echevarria and Areans52 first Reported on the ultrastructure of malignant pleural mesothelio ma. Subsequently, electron microscopy of malig nant mesothelioma has been gaining acceptance for diagnostic purposes.18,24'27,48,49,50,52'57 Trans mission electron microscopy of ultrathin sections is the most commonly used method, but scanning electron microscopy has also been utilized.58,59 There are certain ultrastructural features of malignant mesothelioma that are unique and, will support the diagnosis. These will be discussed according to the various cell types. Epithelial Form The cells of the tubulopapillary form are quite similar in ultrastructure to hyperplastic mesothelium with polarity and presence of well-devel oped microvilli and a continuous basement membrane. Cells of the epithelial form (Fig. 8a) show microvilli formation and a basement membrane that are less well differentiated or discontinuous respectively when compared to hyperplastic mesothelium. The epithelial cell shown in Fig. 8c is a well-differentiated cell and contains intracellular vacuoles (large arrow). Again in this cell microvilli arc present. Glycogen granules can be observed in the epithe lial neoplastic cells. Microvilli may be covered by a fuzzy material (Fig. 8d); only occasionally the epithelial cells lack microvilli. Tonofilaments are frequently seen in the form of bundle structures (Figs. 8b and e), and they may be connected to the junctional structure (Fig. 8e). Cilia and mucinous secretory granules are not found in the mesothelial cells and their identification elimi nates mesothelioma. Unlike secretory granules containing mucin, the contents of the vacuoles in mesothelioma lack electron opacity. Mytochon- PATHOLOGY OF MALIGNANT MESOTHELIOMA 275 Fig. 8. (at (Jltrastructure of loss differentiated epithelial cells. Dark (Dt-D3) and clear (C) celts are easily seen. An arrow indicates a desmosome. The basement membrane is discontinuous and disappears beneath one of the neoplastic cells (Dj|. 0 x2900. (b) Higher magnification of the rectangle in (a) illustrating tonofilaments, x 11.600. (c) Well differentiated epithelial d form with intercellular gaps in the malignant pleural mesothelioma. Basement membrane is indicated at 8 by arrow. x3500. (d) Shows microvilli covered by ill-defined material, x65.000. (e) Shows a desmosome connected by tonofilaments. x 50,000. .c (f 1 Demonstrates intracytoplasmic vacuoles containing microvilli. *4-160. (Figures 8a-f adapted from American Journal of i- Pathology74 with permission.) 2 76 YASUNOSUKE SUZUKI dria are relatively large in size and the matrix is frequently pale. Lysosomal granules and osmiophilic lamellar structures can be rarely observed; these organelles are not characteristic of meso thelioma. The nucleus of the cells, regardless of cell type, show certain common features; the nucleus is filled with fine chromatin and a nucleolus is almost always well developed (Figs. 8a, 9a, and 12a). Microvilli are sometimes seen in the intracytoplasmic vacuole (Fig. 8f). Thus, for the differentiated epithelial form typical ultrastructural features include basement mem brane, microvilli, tonofilanients, rather large mitochondria, cell junctions, cytoplasmic glycog en, and occasionally microvilli within the intra cytoplasmic vacuoles. Less differentiated epithe lial cells lack one or more of these epithelial features. Biphasic or Mixed Form The epithelial variety of the biphasic form is the same in ultrastructure as the epithelial form. Electron microscopy of an atypical epithelial cell of the biphasic form is shown in Figs. 9a-c. In the atypical epithelial cell, epithelial characteris tics are not clearly seen with this low magnifica tion (Fig. 9a) but enlargement shows a discon tinuous basement membrane (Fig. 9b) and remnants of microvilli (Fig. 9c). This cell was thus classified as an intermediate cell. The ultra structure of a fibrous or mesenchymal cell is seen in Fig. 10. Like a fibroblast, the fibrous cell is K iVj Fig. 9. (a) Ultrastructure of an atypical epithelial or intermediate cell seen in biphasic form of malignant peri toneal mesothelioma, for which light micrographs were shown in Fig. 4. (b. c) Higher magnification of rectangles 1 and 2 in (a): a basement membrane (b) and the remnants of microvilli (c). (a) <1995; (b| x13. 968; (c| x3500. (Figures 9a-c adapted from American Journal of PathologyTA with permission.) Fig. 10. Ultrastructure of biphasic form of malignant pleural mesothelioma. A mesenchymal cell type. With an arrow indicated direct contact of the cell with another neoplastic cell. x3288. (Adapted from American Journal of Pathologyu with permission.) [ i K. (1 / si n rr ft ih occc PATHOLOGY OF MALIGNANT MESOTHELIOMA 277 Fig. 11. Ultrastructure of biphasic mixed form. Collagen fibrils can be seen in the intercellular space of the mesenchymal neoplastic cells, x 22.800. (Adapted from American Journal of Pathology1* with permission.) rich in the rough-surfaced endoplasmic reticu lum. There is, however, direct contact of two adjacent cell membranes (arrow). The intersti tial spaces may be filled with collagen fibrils (Fig. 11). Another mesenchymal cell is shown in Fig. 12a; this cell highly resembles a fibroblast, since there is no cell to cell contact. Figure 12b illustrates a cell intermediate in appearance between an epithelial form cell (Fig. 9a) and a fibrous cell (Fig. 12a). Direct contact of two adjacent cells, occasional appearance of tight junction (arrows) and lack of other epithelial appearances, siich as microvilli, a basement membrane, and lonofilaments, arc indications of the intermediate cell type. Electron microscopi cally, three cell types, the epithelial, interme diate, and mesenchymal, are identified in the biphasic or mixed form of mesothelioma. Fibrous' Form The ultrastructural features of a fibrous form cell (seen on light microscopy in Fig. 6) are shown in Fig. 13. The ultrastructure of this fibrous form cell is quite similar to that of the mesenchymal variety illustrated in the biphasic form (Figs. 10 and 12a). Kay cl al.M have stated that epithelial cells can be observed among the cells of the fibrous form. It is possible that occasional atypical epithelial (intermediate) Fig. 12. Electron micrographs of another biphasic form of malignant pleural mesothelioma. Showing both fibrous (a) and intermediate (b) cell types. Arrows in (b) show tight functions, (a. x3127;b, x3024.) . cells may be found in the fibrous form with the electron microscopic if a sufficiently large number of neoplastic cells were screened. Thus, the fibrous form is characterized by cells lacking the characteristics of the epithelial form cells Fig. 13. Electron micrograph showing fibrous form of malignant pleural mesothelioma, for which the light micro graph is shown in Fig. 6. Ultrastructural aspects of neoplas tic cells are similar to those of fibrous or intermediate cells in the biphasic form. x540O. 278 such as microvilli, basement membrane, and tonofilaments. They have the appearance of fibroblasts but frequently exhibit direct cell to cell contact. Only occasionally may intermediate cells be identified. YASUNOSUKE SUZUKI ELECTRON MICROSCOPY OF CELL BLOCKS FROM CYTOLOGY PREPARATIONS Although routine Papanicolaou stained prepa rations from effusions have been somewhat disappointing in their diagnostic accuracy for malignant mesothelioma, it is possible that cell blocks examined by electron microscopy may lead to a better diagnostic rate for this tumor. Ultrastructurc of the cells obtained from a meso thelioma effusion is shown in Figs. 16a, b, and c. Microvilli, tonofilaments, nucleoli, and junc tional structure typical of epithelial mesothe lioma are observed. The ultrastructural details in this case led to the diagnosis of malignant meso thelioma. Attention, however, still must be focused on the differentiation of mesothelioma cells from reactive mesothelial cells. Electron microscopy of the cell block is effective in iden tifying whether the cells are of mesothelial origin. However, differentiation of reactive mesothelial cells from well-differentiated epithe lial mesothelioma cells is not yet possible. For this reason, prior to electron microscopic obser vation, the malignancy should be confirmed with the cell block sections using light microscopy. Further study is needed to develop specific crite ria which will allow for the identification of mesothelioma from cell blocks obtained from effusions. I `ii .} * f f f ELECTRON MICROSCOPIC DIFFERENTIAL DIAGNOSIS OF MALIGNANT MESOTHELIOMA FROM METASTATIC TUMORS Occasionally there are pleural or peritoneal tumors which arc very difficult to differentiate mesothelioma from metastatic disease. Figure 14a illustrates the light microscopy of such a Fig. 14. (a) Light micrograph taken from a pleural tumor in which the histologic diagnosis was unclear pleural tumor. By light microscopy the diagnosis between metastatic adenocarcinoma and malignant meso in this case was controversial (malignant meso thelioma: epithelial form vs. metastatic adeno thelioma (H&, y 254}. Electron micrographs of neoplastic cells are quite revealing. However, (b) shows a tubular luminar filled with electron dense substance (mucopro- carcinoma). Figures 14b and c show the ultra- tein?) and a cell cytoplasm rich in lysosomal granules. structural details of this tumor. Unlike malig <7384. (c) Some intracytoplasmic vacuoles are also filled with electron dense substance. <14,840. These ultrastruc 1 Hi h. nant mesothelioma cells, the tubular lumen is tural details strongly suggest adenocarcinoma. (Adapted di' filled with electron opaque material, and the from American Journal of Pathology74 with permission.) of Sfi cytoplasm is rich in lysosomal granules (Fig. th PATHOLOGY OF MALIGNANT MESOTHELIOMA 14b). Intracytoplasmic vacuoles are also seen with electron dense material (Fig. 14c). The electron opaque material may represent mucin or a mucoprotein, which are frequently produced in adenocarcinoma. Histochemically, the mate rial was stained with PAS and was not digested by diastase. Both histochemical stains and the electron microscopic analyses lead to the conclu sion that this pleural tumor was not a malignant mesothelioma. Figure 15a illustrates the light microscopy of a pleural tumor that was originally diagnosed as malignant mesothelioma; 279 fibrous form. However, with electron microscopy (Fig. 15b) erythrophagocytosis, phagosomes, and poorly developed cell organoids are seen. These findings are not consistant with malignant mesothelioma but suggest malignant fibrous histiocytoma. These cases illustrate the value of performing these special techniques to distin guish between primary mesothelia! tumors and metastatic deposits. SCANNING ELECTRON MICROSCOPY In addition to conventional electron micro scopy, scanning electron microscopy has been applied to the diagnosis of malignant mesothelio ma.58'3'' There is a report that the microvilli of the mesothelioma cells are widely distributed and show variation in appearance, irregular overlapping and the presence of fused and branched forms.58 It is quite possible, however, that under a scanning electron microscope the appearance of the microvilli may vary in struc ture and development between the various cell types, since a transmission electron microscopy has revealed that the mesenchymal cell lacks the microvilli, and the intermediate cell is rarely equipped with the microvilli.14 For this reason, further careful studies are still needed to obtain detailed information on the scanning electron microscope appearances in the various cell types. Hopefully such studies will increase the diagnos tic utility of this analytical method. Fig. 15. (a) Light microscopy of a pleural tumor first diagnosed to be malignant mesothelioma (fibrous form}. H&E. *395. Electron micrograph of the pleural tumor, however, shows distinct features in (b). Ultrastructure is different from that of fibrous mesothelioma. There is a lack of cell contact and the presence of both erythrophagocytosis and phagosomes. <3819. This leads to the suggestion that this tumor is a malignant fibrous histiocytoma. PATHOGENESIS OF MALIGNANT MESOTHELIOMA Etiological Factors Since Wagner et al.8 reported the relation of asbestos exposure to malignant pleural mesothe lioma. their conclusion has been widely confirmed.'1 " 4064 Recently. Baris el al.1' have reported that fibrous erionite. a type of zeolite mineral, was responsible for an endemic oceurrcnce of pleural mesothelioma in Turkey. Animal studies have also indicated that various agents other than asbestos can induce malignant mesothelioma. Fibrous glass, brucite. and fibrous erionite are known to induce experimen tal mesotheliomas.61-66-75 Stanton et al."' have suggested that the carcinogenicity of asbestos is related primarily to the structural characteris tics represented by long (over 8 u). thin (less than I g) fibrous shapes, rather than its physico chemical properties. In addition to fibrous inor- 280 game agents, some organic compounds, includ ing diethylstibestrol benzo(o)pyrene, and ra dioactive substances such as ',9PuO, have been reported as carcinogenic agents in producing mesothelioma and other tumors.67-70 From such observations, one might suspect that agents other than asbestos can be implicated alone or as cocarcinogens in the evolution of mesothelioma. Further studies are obviously needed to define the various components in the etiologic develop ment of this tumor. Histogenesis of Malignant Mesothelioma Klemperer and Rabin provided an important basic concept concerning the histogenesis of mesothelioma. They explained in part that histocytologic variability of the tumor was due to the unique multipotential of the mesothelial cell, which could differentiate into various cell lines.6 Later, other investigators reported that cultured mesothelioma cells could transform into dif ferent cell types.11 77 One might question the applicability of tissue culture study for human mesothelioma. Tissue culture studies at Mt. Sinai Hospital of New York have revealed that cultured cells change strikingly from a cell containing hyaluronic acid (well differentiated epithelial form) to the nonhyaluronic acid containing cell of the poorly differentiated epithelial form, with appearance of many lipid granules. Such alterations may have resulted from artificial conditions in the medium. Nevertheless, the unique potential can be supported by light and electron microscopic evidence that the intermediate cells between the epithelial and mesenchymal cells arc frequently identified in mesothelioma. Unlike cultured cells, mesothelioma cells transplanted into nude mice have kept their original cellular character istics over I yr. with repetitive transplantation7' suggesting that there may be great differences between cells maintained in vitro and in vivo. The transplanted mesothelioma may be a good model for further study of pathobiologic mecha nisms. They may also be a model for testing antineoplastic therapies. Unfortunately we still lack data concerning the evolution from the normal mesothelium to malignant neoplastic mesothelium. and the role of the hyaline plaque in the development of malignant mesothelioma. We also lack informa tion about similarities and differences in histo- YASUNOSUKE SUZUKI Fig. 16. (a) Electron microscopy of a suspected neoplastic cell obtained from a peritoneal effusion in a patient with mesothelioma, <2000. (b) Higher magnifica tions of portions of (a) show well-developed microvilli and tonofilaments. Desmosomes are seen in (c). (b, x28.000; c. <54,600.) These findings, however, may still be confused with reactive mesothelial cells--see text. (Adapted from American Journal of Pathology1' with permission.) genesis of the tumors induced by different agents. Answers to these and similar questions will contribute to development of a better under standing of the biology of mesothelioma which in turn might lead to prevention, earlier diagnosis, or better applications of treatment. ACKNOWLEDGMENT The author would like lo express; his appreciation to Dr. Irving J SelikolT for his review and suggestions on this manuscript. PATHOLOGY OF MALIGNANT MESOTHELIOMA 281 REFERENCES 1. Wagner E: Das tuberkelahuliche Lymphadenoma. Arch Hcilk 11:497-525, 1870 2. DuBray ES, Rosson FB: Primary mesothelioma of the pleura; A clinical and pathologic contribution to the pleural malignancy, with report on a case. Arch 1m Med 26:715 737, 1920 3. Webster I: Methods by which mesothelioma can be diagnosed, in Glen HW (ed): Proceedings of Asbestos Symposium. Johannesburg, South Africa. Oct. 3-7, 1977. National Institute for Metallurgy, 1978, pp 3-8 4. Klemperer P, Rabin CB: Primary neoplasms of the pleura. A report of five cases. Arch Pathol 11:385-412, 1931 5. Churg J, SelikolT IJ: Geographic pathology of pleural mesothelioma in: Symposium on Geographic Pathology of Pulmonary Disease. 1967, pp 284-297 6. Maximov A; Ucber das Mcsothel (Deckzellen der serdsen Haute) und die Zcllen der scrosen Exsudatc. Arch Exp Zcllforsch 4:1-42, 1927 7. Weiss A; Plcurakrebs bei Lungcnasbcstose, in vivo morphologisch gesichert. Mcdizinische 1:93-94, 1953 8. Wagner JC, Sleggs CA, Marchand P: Diffuse pleural mesothelioma and asbestos exposure in the North Western Cape Province. Br J Ind Med 17:260-271, I960 9. Sclikoff IJ, Churg J, Hammond EC: Relation between asbestos exposure and neoplasia. N Engl J Med 272:560 565, 1965 10. SclikofT IJ, Hammond EC, Scidman H: Mortality experiences of insulation workers in the United States and Canada. Ann NY Acad Sci 3301:91-116. 1979 11. Newhouse ML, Thompson H: Mesothelioma of pleura' and peritoneum following exposure to asbestos in the London area. Br J Ind Med 22:261-269, 1965 12. Elmcs PC, McCaughey WTW, Wade OL: Diffuse mesothelioma of the pleura and asbestos. Br Med J 1:350 353,1965 13. Hochbcrg L; Endothelioma (mesothelioma) of the pleura. Am Rev Tuberc 63:150-175, 1951 14. Stumphius J: Asbestos bodies and mesothelioma. Ann Occup Ffyg 11:283-293, 1968 15. Anderson HA: Household contact asbestos neoplastic risk. Ann NY Acad Sci 271:311 -323. 1976 16. Shin ML. Firminger HI: Acute and chronic effects of intraperitoncal injection of two types of asbestos in rats with a study of the histogenesis and ultraslructurc of resulting mesotheliomas. Am J Pathol 70:291-314. 1973 17. Davis JMG: Histogenesis and line structure of peri toneal tumor produced in animals by injections of asbestos. J Natl Cancer Inst 52:1823-1837, 1974 18 Wood C. Bouchcllc WH: Benign mesothelioma simu lating a uterine leiomyoma. Am J Obstct Gvnccol 132:225 226, 1978 ' 19 l-ligiel Kancko M. Malignant mesothelioma of the tunica vaginalis propria testis in a patient with asbestos exposure. Cancer 37 1478 1484.1976 20 l-.imoto T. Inouc L Malignant fibrous mesothelioma of the tunica vaginalis. Cancer 39:2059 2066. 1977 21 Parmley TH, Woodrutf JD: The ovarian mesothelio ma. Am J Obstct Gynecol 120:234-241, 1974 22. Fishelovitch J, Mcira/ D. Keinan et al Malignant mesothelioma ol the testicular tunica vaginalis Br .1 Urol 47 208.1975 23. Evans N: Mesotheliomas of uterine and tubal serosa and tunica vaginalis testis. Am J Pathol 19:461 -471, 1943 24. Ferencay A, Fcnoglio J, Richart RM: Observations on benign mesothelioma of the genital tract (adenomatoid tumor): A comparative ultrastructural study. Cancer 30:244-260, 1972 25. Marcus JB, Lynn JA: Ultrastructural comparison of an adenomatoid tumor, lymphangioma, hemangioma and mesothelioma. Cancer 25:171-175, 1970 26. Salazar 14. Kanbour A, Burgess F: Ultraslructurc and observations on the histogenesis of mesotheliomas "adeno matoid tumors" of the female genital tract. Cancer 29:141 152, 1972 27. Mackey B. Bennington J I, Skoglund RW: The adeno matoid tumor Fine structural evidence for a mesothclial origin. Cancer 27-109--115, 1971 28. Stout AP- Mesotheliomas of the pleura and perito neum. J Tenn Med Assoc 44:409-411, 1951 29. Kannerstein M, McCaughey E, Churg J, et al: A critique of the criteria for the diagnosis of diffuse malignant mesothelioma. Mt Sinai J Med 44:485-494, 1977 30. Kannerstein M, Churg J: Peritoneal mesothelioma. Hum Pathol 8:83-94, 1977 31. Roberts GH: Distant visceral metastases in pleural mesotheliomas. Br J Dis Chest 70:246-250. 1976 32. Ratzcr ER, Pool JL, Melamed MR: Pleural mesothe liomas: Clinical experience with 37 patients. Am J Roent genol 99-863-880. 1967 33. Meyer K, Chaffee E: Hyaluronic acid in the pleural fluid associated with a malignant tumor involving the pleura and peritoneum. J Biol Chem 133:83-91, 1940 34. Kannerstein M, Churg J. Magncr D: Histochemistry in the diagnosis of malignant mesothelioma. Ann Clin Lab Sci 3:207-211. 1973 35. Wagner JC, Munday DE. Harington JS: Histochcmical demonstration of hyaluronic acid in pleural mesothelio mas. J Pathol Baci 84:73-78, 1962 36. Harington JS. Wagner JC. Smith M: The detection of hyaluronic acid in pleural fluids of cases with diffuse pleural mesotheliomas. Br J Exp Med 44:81-83, 1963 37 Arai H. Kang K-Y, Sato H, et al: Significance of the quantification and demonstration of hyaluronic acid in tissue specimens for the diagnosis of pleural mesothelioma. Am Rev Rcsp Dis 120:529-532, 1979 38, Matzel W, Schubert G: Hyaluronic acid m pleural fluids: An additional parameter for clinical diagnosis on diffuse mesotheliomas. Arch Gesehwulstforsch 49:146-156, 1976 39, Boersma A. Degand P. Biserle G Hyaluronic acid analysis and the diagnosis of pleural mesothelioma. Bull Eur Phystopathol Resp 16 4| 45. 1980 40, Castor CW. Naylor B- Acid mucopolysaccharide composition of serious effusion: study of 100 patients with neoplastic and non-neoplastic conditions. Cancer 20.462 466.196" 41 Rasmussen KN`. Farber Vc Hyaluronic acid in 247 pleural fluids Scand J Rcspir Dis 48:366 371, 1967 42. Whitaker D. Shilkin KB; The cytology of malignant mesothelioma in Western Australia. Acta Cytol 22:67 70, 1978 43 Klempniunn S Cy tology of diffuse pleural mesothc- 282 lioma. Cancer 15:691-695, 1962 44 Naylor B: The exfoliative etiology of diffuse malig nant mesothelioma. J Pathol Bact 86:293-298, 1963 45. Tao L: The cytopathology of mesothelioma. Acta Cytol 2323:209-213. 1979 46. Roberts GH, Campbell GM: Exfoliative cytology of diffuse mesothelioma. J Clin Pathol 25:577-782, 1972 47. Kobayashi R. Takedo S, Yamamoto T, et al: Cyto logic detection of malignant mesothelioma of the pericar dium. Acta Cytol 22:344-349. 1978 48. Suzuki Y, Kannerstein M, Churg J: Electron micros copy of normal hyperplastic and neoplastic mesothelium, Davis W (cd): Biological effects of asbestos. WHO, Interna tional Agency for Research on Cancer, 1973, pp 74-79 49. Lcgrand M, Parientc R: Diagnostic des tumoeurs pleurales par I'edudc dcs Icquides pleuraux au microscope elcctromquc. Rev Fr Mai Resp 3:983-994, 1975 50. Wange NS, Huang SN, Gold P: Carcinoembryonic antigen (CEA)-like material in mesothelioma and other lung cancers. Lab Invest 4:291, 1979 51. Singh G, Dekker A, Whiteside TL: Anli-mcsothclial cell scrum: A diagnostic reagent for malignant mesothelio ma. Fed Proc 38:912, 1979 52. Echevarria RA. Areans VM: Ullrasiructural evidence of secretory differentiation in a malignant pleural mesothe lioma. Cancer 22:323-332, 1968 53. Wang NG: Electron microscopy in the diagnosis of pleural mesotheliomas. Cancer 31:1046-1054, 1973 54. Kay S, Silvcrberg SG: Ultrastructural studies of a malignant fibrous mesothelioma of the pleura. Arch Pathol 92:449-455. 1971 55 Davis JMG: Ultrastructurcs of human mesothelio mas. J Natl Cancer Inst 52:1715-1723, 1974 56. Verley JM: Diagnostic anatomo-palhologique dcs mesotheliomas pleuraux. Minerva Chir 31,300-306, 1976 57 Stocbner P. MiccKG,Sengel A.etal: Notions d'ullrastructure pleuralc. II. Les mesotheliomas. Pressc Med 24:1403-1408, 1970 58. Dionne GP. Wang N-S. A scanning electron micro scopic study of dilfusc mesothelioma and some lung carci nomas. Lab Invest 36 356, 1977 59 Gondos B. Lai CE, King EB Distinction between atypical mesothclial cells and malignant cells by scanning electron microscopy Acta Cytol 23:321 326.1979 60 Stanton MF. Blackwell R. Miller E' Experimental YASUNOSUKE SUZUKI pulmonary carcinogenesis with asbestos. Am Industr Hyg Assoc J 30:236-244, 1969 61 Smith WE, Miller L, Elsasscr RE: Tests for carcino genicity of asbestos. Ann NY Acad Sci 132:452-588, 1965 62. Wagner JC, Berry G: Mesothelimas in rats following inocculation with asbestos. Br J Cancer 23:567-581, 1969 63. Stanton MF, Wrench C: Mechanisms of mesothe lioma induction with asbestos and fibrous glass. J Natl Cancer Inst 48:797-821, 1972 64. Wagner JC, Berry G, Skidmore J W, et al: The effects of the inhalation of asbestos in rats. Br J Cancer 29:252-269, 1974 65. Baris Yl, Artlinors M, Sahin AA: Environmental mesothelioma in Turkey. Ann NY Acad Sci 330:423-432, 1979 66. Scheur E, Huth F, Roll F. Studies on the morphology of experimental tumors induced in rats by intraperitoneal injection of asbestos dusts. Arch Geschwustforsch 41:121 -- 136,1973 ' 67. McClure HM, Graham CE: Malignant uterine meso theliomas in squirrel monkeys following diethyl-stilbestrol administration. Lab Anim Sci 23:493-498, 1973 68. Pelfrene A, Garcia H: Histology of chemically- induced mesotheliomas in MRC-Wistar rats. Tumori 61:509-516, 1975 69. Grcenblatt M, Likinsky W: Nitrosamine studies: Neoplasms of liver and genital mesothelium in nitrosopyrrol- idine-treated MRC rats. J Nall Cancer Inst 48:1687-1696, 1972 ` ' 70. Sanders CL Jr: Production of abdominal mesothelio mas in rats with 239 PuOj and chrysolite asbestos. Health Phys 23:413, 1972 71. Stout AP, Murray MR: Localized pleural mesothelio ma, investigations of its characteristics and histogenesis. Arch Pathol 34:951-964, 1942 72. Sano ME, Weiss E, Gault EC: Pleural mesothelioma, further evidence of its histogenesis. J Thorac Surg 19:783 788, 1950 73. Chahinian AP, Bcranck JT, Suzuki Y, et al: Trans plantation of human malignant mesothelioma into nude mice. Cancer Res 40:181 - 185, 1980 74. Suzuki Y, Churg J, Kannerstein M: Ultrastructure of human malignant dilfusc mesothelioma. Am J Pathol 85:241-262, 1976 75. Suzuki Y, Rohl AN, Langcr AM, et al: Mesothelioma following intraperitoneal administration of zeolite. Fed Proc 39:640. 1980