Pores of Kohn

At the terminal point of the airways, the terminal bronchiole becomes the acinus, or the respiratory unit, of the lung. The acini vary in size but in general contain several generations of respiratory bronchioles (bronchioles that have alveolar sacs directly opening to their lumen), which lead to alveolar ducts and ultimately alveolar sacs. Connective tissue lines the airways and forms septa that separate acinus from acinus. Collateral ventilation can occur, however, between acini through the pores (the pores of Kohn) that are located in the adjacent alveolar walls. 

Pores of Kohn are collateral connections between air spaces through which infections can spread. 

All 48 alveolar macrophages that were seen in the serially sectioned human alveoli fixed in the inflated state were found within or bordering on alveolar junction zones (Parra et al. 1986). A computer-reconstruction showed the predilection of alveolar macrophages and type II pneumocytes located in septal jxmction zones. Superposing alveolar macrophages and type II cells in such a reconstruction obliterated almost all gaps in the basement membranes, except for those produced by pores of Kohn when these were free of cells. 

Figure 3 Scanning electron micrograph of a rat lung following fixation with a nonaqueous osmium-fluorocarbon mixture. Surface details of the alveoli, for example microvilli on type II cells or pores of Kohn, are not seen beeause of the preservation of the surfaetant and assoeiated hypophase as a eontinuous film (magnification X 1000).

Figure 4 Scanning electron micrograph of alveolar region from rat fixed with aqueous glutaraldehyde With this preparation the surfactant film is lost and details of the epithelial cells and pores of Kohn can be seen (magnification X 1000).

The film appears continuous over the alveolar surface and covers the pores of Kohn (74 Figs. 3 and 4). By transmission electron microscopy (TEM) the surface film is seen to be multilayered (Fig. 5), consistent with the concept of a surfactant reservoir. The mechanism of formation of this multilayered film and the ways in which respiratory forces interact with it are not understood. One model proposed by Ries and Swift (79) is shown in Figure 6. 

Thommes, M., Kohn, R., and Froba, M. (2000). Systematic sorption studies on surface and pore size characteristics of different MCM-48 sihca materials. Stud. Surf. Sci. Catal. Characterisation Porous Solids V, 128, 259-68. 

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