![]() ![]() It is interesting that the cilia of airway epithelia develop at their tip fine claws with which they can grasp the mucus blanket in the phase of their forward beat, whereas on their return to the upright position they glide past the mucus blanket. 20, 21 In the airway epithelium, the cilia are oriented in such a fashion that their beat is directed outward. 2-10) are motile cell extensions that are known to beat rhythmically in a given direction and at a frequency of about 12 to 20 Hz. 19 The mucus layer is capable of trapping dust particles that are still contained in the air entering the lung. This mucus spreads out as a thin blanket on top of the cilia, which are embedded in a periciliary layer containing a dense network of mucins and mucopolysaccharides tethered to the cilia. If we first have a closer look at the epithelium of larger conducting airways, we see that the lining cells are provided with a tuft of kinocilia at their apical cell face, whereas the secretory cells are goblet cells that produce and discharge to the surface a sticky mucus ( Figs. Transcytosis through mesothelial cells in both directions represents another mechanism involved in pleural fluid homeostasis. Drainage occurs partly via lymphatic stomata in the parietal pleura. Pleural fluid originates from pleural capillaries through microvascular filtration. The volume and composition of the pleural fluid have to be tightly controlled to ensure an efficient mechanical coupling between chest wall and lung. ![]() The total volume of pleural fluid is about 15 to 20 mL, with approximately 1700 cells/mm 3 (75% macrophages, 23% lymphocytes, 1% mesothelial cells). The apical microvilli increase the surface area available, suggesting that pleural mesothelial cells are capable of participating in active transserosal transport of solutes. 1 Both pleural surfaces are lined by a squamous epithelial layer, often called mesothelium (due to its mesodermal origin), whose surface is richly endowed with long microvilli. However, this serosal space is minimal, since the visceral pleura is closely apposed to the parietal pleura, with only a thin film of serous fluid intercalated as a lubricant between the two surfaces. The structural background for this mobility of a healthy lung is the formation, during morphogenesis, of a serosal space that is lined on the interior of the chest wall and on the lung surface by a serosa, the parietal and visceral pleurae, respectively ( Fig. ![]()
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