Chapter 2 The panorama of different faces of mesothelial cells
Pathologic processes affecting a serosal cavity, such as inflammation, neoplasia, and trauma, lead to reactive changes of the extremely sensitive mesothelial cells that line the serosal surfaces. The mesothelial cells react to the altered environment by hypertrophy and proliferation, resulting in a wide spectrum of morphologic appearances (Tables 2.1, 2.2, 2.3, 3.1).1–6 These changes may disappear completely with the resolution of the underlying pathologic process.
Table 2.1 Cytomorphology of mesothelial cells—the spectrum
Table 2.2 Overlapping malignant cytomorphology of reactive mesothelial cells
Table 2.3 Features that make effusion cytology more elusive*
* These features are frequent with the possible pitfall of misinterpreting reactive effusions as malignant.
In general, benign effusions are more common than malignant effusions even in tertiary care institutions. Cytologic evaluation is more useful for malignant effusions, which usually show diagnostic malignant cells. However, some benign effusions may also show diagnostic morphologic features consistent with a particular non-neoplastic pathologic process (see Chapter 6).
REACTIVE MESOTHELIAL CELLS
The mesothelial cells are one of the most versatile cells of the human body, with an extremely variable and wide morphologic spectrum. The mesothelial cells associated with a variety of processes demonstrate variable morphologic appearances. They are usually referred to as ‘reactive mesothelial cells’. Many other terms have been used to label these benign cells: reactive mesothelial proliferation, reactive mesothelial hyperplasia, irritated mesothelial cells, activated mesothelial cells, hyperplastic mesothelial cells, hypertrophic mesothelial cells, proliferative mesothelial cells, and atypical mesothelial cells.7 General agreement on the definition and use of these terms is lacking.1,4,5 However, some of them, including atypical mesothelial cells, should be avoided. In our laboratory, we generally use the term ‘reactive mesothelial cells’, which is how they will be referred to in this book.
Typically, the reactive mesothelial cells range from 15 to 30 mm (but may be up to 50 mm) in diameter. They have enlarged nuclei, with some variation in their sizes and shapes. Nucleoli are usually conspicuous. The amount of cytoplasm is variable (Figures 2.1, 2.2, 2.3, 2.5, 2.6, 2.7). These cells are frequently binucleated or multinucleated (see Figure 1.7). Cohesive clusters of mesothelial cells with or without a papillary configuration may be present. Some cells have scanty cytoplasm with slightly hyperchromatic enlarged nuclei with prominent nucleoli (see Figures 2.6, 2.7). 
This reflects the astonishingly wide morphologic spectrum of reactive mesothelial cells, which may extensively overlap with the morphologic spectrum of malignant cells (see Tables 2.1, 2.2, 2.3, 3.1, Figures 2.2, 2.7).
Figure 2.1 Mesothelial cells with central to slightly eccentric nuclei (ascitic fluid). The cytoplasm shows a two-zone staining pattern. For additional variations see also Figures 2.2, 2.3, 2.5. [a–c, DQ-stained Cytospin smear (a–c, 100× zoomed).]
Figure 2.2 Panorama of mesothelial cells (ascitic fluid). Central to near central nuclei. Rare mesothelial cells may show eccentric nuclei touching the cell membrane, but usually there is a narrow rim of cytoplasm separating the nucleus from the cell border (arrowheads) (see also Figures 2.1, 2.3, 2.5). [a–x, DQ-stained Cytospin smears (a–x, 100× zoomed).]
Figure 2.3 Mesothelial cells with eccentric nuclei (ascitic fluid) (see also Figures 2.2g,i,o,p,r,t,u,v,w). Careful scrutiny usually shows a narrow rim of cytoplasm separating the nucleus from the cell border (arrows) (see also Figures 2.1, 2.2, 2.5). [i,o, DQ-stained Cytospin smears (i,o, 100× zoomed).]
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