Role of human visceral pleura in pleural fluid turnover: Need for morphological evidence of lymphatic stomata

Abstract

To the Editor: We read with great interest the article by Gao and his colleagues1 proposing a new schema and hypothesis regarding pleural fluid turnover. In the proposed hypothesis, the visceral pleura possesses a more active role in humans than what was believed. The authors support the notion that in humans the pulmonary lymphatic vessels originate directly from the mesothelium of the visceral pleura and that it has an effect on both infiltration and reabsorption of pleural fluid in human beings. However, there are some points that are worth commenting. Should the lymphatic vessels originate from the visceral mesothelium, they would form stoma-like projections on the pleura cavity, as implied by the authors. According to the study of Leak and Rahil2 and the study of Nakatani et al,3 the morphology of a typical orifice of a lymphatic stoma under the scanning electron microscopy (SEM) consists of two margins: an outer microvillous margin formed by a perimeter of cuboidal mesothelial cells and an inner, relatively microvillous-free margin, formed by lymphatic endothelial cells. A study on the existence of lymphatic stomata in humans failed to demonstrate any stomas in specimens of 30 patients.4 Peng et al5 found stomas only in 2 of 9 human specimens. Additionally, Li6 demonstrated stomas only in human diaphragmatic parietal pleura but none in both the thoracic parietal pleura and in the visceral pleura in this study. Another study on the sheep pleura, which is thick and morphologically resembles the human pleura,7 revealed that the pleura mesothelium was composed of two types of mesothelial cells: flattened cells with well developed tight junctions and cuboidal cells with lesser tight junctions. The visceral pleura consists mainly of flat cells, indicating a tighter mesothelium. The parietal pleura consists of both flattened and cuboidal mesothelial cells as mentioned above around the orifice of a stoma. The lower permeability of the visceral sheep pleura compared to the parietal was also verified by our research group by means of electrophysiology.8 The absence of many cuboidal cells in the visceral pleura and the finding of a tighter mesothelium other than the parietal pleura, do not favor the existence of visceral lymphatic stomata. It will be of great interest if the authors could provide any figures of SEM. Should these figures depict stomas, the theory proposed by Gao and colleagues will be strengthened. Nevertheless, further experimental evaluation of this interesting new theory is imperative. If it is proven to be valid, research into the pleural fluid turnover will be redirected.