[The pores of Kohn in pulmonary alveoli]

The pores of Kohn are apertures in the alveolar septum, which allow the communication of two adjacent alveoli. Their existence has been most disputed, but was lastly supported in man and adult mammalians by observations from electron microscopy. Located between the meshes of the capillary network, these alveolar apertures are circular or oval, and are bordered by extensions of type I pneumocytes. Type II pneumocytes are often located close to the pores of Kohn. The presence of two near apertures gives a "double-barrelled gun" aspect, frequently observed in the old animal. The specific connections of the pore of Kohn with the septal components, particularly by the myofibroblasts, are analysed. The quantification of the pores of Kohn in various species shows great variations according to the fixation technique, animals age and site of alveoli in the lung and the lobules. The size of these apertures also depends on species, varying from 0.8 to 15 micrograms. Histogenesis of these apertures is still poorly understood and various processes can be evoked. Ventilation, which differs according to the site of the alveoli in the lung, is a determinant factor, proved by the great number of pores of Kohn in the hyperventilated areas. The role of the macrophages (synthesis and excretion of proteases and lipases) is also discussed. The formation of pores of Kohn is linked with renewing and dehiscence of alveolar epithelial cells. The role of alveolar pores in collateral ventilation seems relatively weak in the physiological state, whereas it is certain in lung pathology, contributing to the propagation of bacterial infections in the adult and to changes of local ventilatory conditions in atelectasis and asthmatic bronchoconstriction. The alveolar apertures can explain the specific propagation of some epitheliotropical peripheral cancers (bronchiolo-alveolar carcinoma). The pore of Kohn is one of the first signs of parenchymatous senescence of the pulmonary alveolus; its part in the appearance and extension of emphysematous disorders is also discussed.