Reactive Mesothelial Cells in the Lymphatic Network of the Serous Membrane in Prolonged Body Fluid Retention: An Autopsy Case Report

Reactive mesothelial cells (RMCs) are of interest for differentiating mesothelioma from benign conditions and have been discussed in cytology and biopsy; however, their behavior in the body remains poorly understood. In this study, we report an autopsy case of an older woman with a long-standing pleural effusion due to cardiac disease, providing insights into the relationship between body cavities, and lymphatic vessels (LVs), mesothelial cells (MCs), and endothelial cells. Cytological examination of pleural effusion revealed RMCs with mild atypia, multinucleation, and intercellular phagocytosis. Immunohistochemistry confirmed the mesothelial origin of these cells. Autopsy findings showed extensive involvement of RMCs in the pleura, diaphragm, peritoneum, lymph vessels, and lymph node sinuses. The visceral pleural submesothelial LVs were dilated, had small openings in the thoracic cavity, and were lined with endothelial and mesothelial cells. Large cavernous LVs with MC clusters were observed on the diaphragm. These structures resembled "stomata" or "lacunae," suggesting a mechanism by which RMCs migrate from the body cavities to the lymphatic network. This study focuses on the RMCs in LVs and shows the contiguity between body cavities and lymphatic networks, providing important insights into the flow of bodily fluids.