Airway muscle stereology: implications for increased shortening in asthma

Increased airway smooth muscle, resulting from either hyperplasia or hypertrophy, has been implicated as a cause of excessive bronchoconstriction in asthma despite the many methodologic limitations of studies to date. Our recent failure to demonstrate increased muscle volume in an asthmatic airway preparation having 3-fold greater shortening than nonasthmatic controls prompted us to reassess the quantity of muscle in asthmatic versus nonasthmatic airways. Smooth muscle was quantified in axially sectioned, 2nd- to 4th-generation bronchi, using standardized stereologic methods on high-magnification images of cross-sectional airway smooth muscle profiles in tissues from five asthmatic subjects and five nonasthmatic smokers. When data were normalized by total cross-sectional tissue area, no differences between the two groups (asthmatic versus nonasthmatic) were detected for the proportion of smooth muscle (3.45 +/- 0.81% versus 2.74 +/- 0.76%), extracellular matrix between muscle cells (1.65 +/- 0.46% versus 1.06 +/- 0.25%), or connective tissue within smooth muscle bundles (1.65 +/- 0.34% versus 1.53 +/- 0.59%). These methodologies for evaluating cross-sectional airway muscle in axial airway sections at high resolution provide no evidence of increased airway smooth muscle in asthmatic large airways, and suggest that differences in mechanical responses of asthmatic airways cannot be explained solely by the amount of smooth muscle.