Objectives: The aim of this study was to clarify the dynamics of lymphatic drainage of the pleural cavity to understand the mechanism of malignant pleural effusion.
Methods: We injected carbon particles into the pleural cavity of monkeys subjected to general anesthesia. We then observed the parietal pleura with a video-assisted thoracoscope and scanning and transmission electron microscopes to examine the regions of the parietal pleura where the carbon particles had been absorbed.
Results: The video-assisted thoracoscope showed that the carbon particles had gone directly to the costal, mediastinal, and diaphragmatic pleura by 10 to 15 minutes after injection. From the scanning and transmission electron microscopes, we found that the parietal pleura in the costal and mediastinal regions consisted of 3 elements: a layer of small mesothelial cells, the macula cribriformis, and lymphatic lacunae. Stomata (3-5 microm in diameter) were found between the small mesothelial cells. The macula cribriformis was composed of densely packed collagen fibrils and had many foramina (3-10 microm in diameter). Intrapleurally injected carbon particles were carried into the lymphatic lacunae via the stomata and vesicles of the mesothelial cells and the foramina of the macula cribriformis. The lymphatic lacunae filled with carbon particles were richly distributed in both the anterior costal pleura and the mediastinal pleura.
Conclusion: We suggest that the mesothelial stomata and the macula cribriformis are structures essential to the absorption of macromolecules and cellular elements from the pleural cavity into the lymphatic system.