Does airway inflation stretch the bronchial mucosal membrane?

The bronchial mucosa, which consists of the airway epithelium and basement membrane, forms folds along the inner lumen of the airway wall when airway lumen size decreases during bronchoconstriction or at low lung volumes (10, 12). Epithelial folding progressively diminishes as the airway lumen expands during the distension of the lungs (12). Some years ago, James and colleagues (4, 5) described a constant relationship between airway wall area and the airway epithelial perimeter in guinea pig airways that were fixed at different lung volumes with and without bronchoconstriction. On the basis of these observations , they suggested that bronchial epithelial perimeter length was unaffected by lung inflation or bronchoconstriction (4, 5). The constant length of the airway epithelial perimeter under varying conditions of strain and compression was purportedly due to the inextensibility of the basement membrane that subtends the epithelium. The assumption that the epithelial perimeter of the airways does not change under differing conditions of contraction, relaxation, or lung volume has provided a basis for the use of this parameter as a marker for airway size; the epithelial perimeter can be easily determined and used to predict the size of the airway lumen in a relaxed open state. Measurements of airway perimeter have thus provided a convenient shortcut for assessing the degree of airway narrowing and smooth muscle shortening in histologic sections of airways. This approach has been used in the evaluation and modeling of the effects of diverse physiological conditions on airway narrowing and to assess the effects of pathophysiolog-ical conditions such as asthma on the structure of the airway wall (1-3, 7, 11, 13). In this issue of the Journal of Applied Physiology, Noble and colleagues (9) challenge the concept that the bronchial mucosa is not distensible and that the airway epithelial perimeter maintains a constant length over the physiological range of airway transmural pressures. The authors measured the disten-sibility of the bronchial mucosa in strips of airway mucosa dissected from pig bronchi. The mucosal strips were oscillated between fixed levels of strain to obtain stress-strain curves, and the effects of strain on epithelial length, thickness, and folding were determined. Epithelial length was not altered at modest levels of strain, but higher levels of strain resulted in significant increases in epithelial length and a slight reduction in epithelial thickness. To correlate mucosal tissue stress-strain curves with physiological pressures, airway wall stress was calculated in intact pig bronchial …