[Cytodiagnosis of pleural exudates with electron microscopy. I. Activated pleural mesothelium].

Detailed examination of 48 pleural exudations showed that a sequence of morphological changes [desquamation, proliferation, activation, degeneration] was a uniform response of mesothelium to various pathological stimuli. Desquamated mesothelial cells grow round and bigger, their nuclei increase, multiply, become irregular and get a coarse chromatin pattern. Their cytoplasm is less often homogeneous, a bit basophil and mostly vacuolized to some extent; fusion of vacuoles can lead to a signet-ring appearance. Provided that highly activated mesothelial cells phagocytize foreign corpuscular material, they cannot be distinguished from the macrophages. Their cytoplasmic organelles multiply progressively. More frequent lysosomes occur sometimes with dense lamellated myelin-like bodies. Glycogen particles increase in number substantially in peripheric cytoplasmic zone. Tonofilaments are very conspicuous, arranged round the nucleus, but disappear during progressive mesothelial activation and degradation. Numerous microvilli are gradually reduced in number, change into voluminous plasmatic blebs, and finally, mesothelial surface grow irregular or smooth. In dubious cases, electron microscopy [transmissive and scanning] gives more precise features of organelles and surface structures in activated mesothelial cells and helps to differentiate them from other exudation cells especially the neoplastic ones.