Elastic properties of the bronchial mucosa: epithelial unfolding and stretch in response to airway inflation

Abstract

The bronchial mucosa contributes to elastic properties of the airway wall and may influence the degree of airway expansion during lung inflation. In the deflated lung, folds in the epithelium and associated basement membrane progressively unfold on inflation. Whether the epithelium and basement membrane also distend on lung inflation at physiological pressures is uncertain. We assessed mucosal distensibility from strain-stress curves in mucosal strips and related this to epithelial length and folding. Mucosal strips were prepared from pig bronchi and cycled stepwise from a strain of 0 (their in situ length at 0 transmural pressure) to a strain of 0.5 (50% increase in length). Mucosal stress and epithelial length in situ were calculated from morphometric data in bronchial segments fixed at 5 and 25 cmH2O luminal pressure. Mucosal strips showed nonlinear strain-stress properties, but regions at high and low stress were close to linear. Stresses calculated in bronchial segments at 5 and 25 cmH2O fell in the low-stress region of the strain-stress curve. The epithelium of mucosal strips was deeply folded at low strains (0 -0.15), which in bronchial segments equated to Յ10 cmH2O transmural pressure. Morphometric measurements in mucosal strips at greater strains (0.3-0.4) indicated that epithelial length increased by ϳ10%. Measurements in bronchial segments indicated that epithelial length increased ϳ25% between 5 and 25 cmH2O. Our findings suggest that, at airway pressures Ͻ10 cmH2O, airway expansion is due primarily to epithelial unfolding but at higher pressures the epithelium also distends. mucosal folds; epithelium; airway mechanics THE AIRWAY MUCOSA HAS SEVERAL important roles in airway and lung biology. The mucosa contributes to mechanical properties of the airway wall that may influence the degree of airway expansion produced by lung inflation and the extent to which the airway narrows in response to bronchoconstrictor substances. The mucosa comprises the epithelium (with its basement membrane), lamina propria glands, vascular tissue, and an elastic matrix. The epithelium is a continuous layer of cells, with numerous folds that deepen on lung deflation or airway smooth muscle (ASM) contraction (9, 12). By its position internal to ASM, the mucosa imposes an afterload on ASM that may be substantial enough to restrict ASM shortening and, thus, active airway narrowing in response to bronchoconstrictor substances (5, 12). Elastic properties of the mucosa also contribute to the pressure-volume behavior of the airway wall. On lung inflation, the airway lumen expands and the depth of epithelial folds is progressively reduced, which help accommodate the increased volume of the airway lumen (4, 5, 10).Structural and mechanical properties of the mucosa potentially affect the response of the airway wall to lung inflation, deflation, and ASM contraction.Folding properties of the epithelium relevant to ASM contraction have been widely studied (4 -6, 10, 12). The mechanisms of mucosal expansion during lung inflation are less ...

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