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Notes: Background An increase in types III and V collagen below the bronchial epilhelium of the airway is known to be responsible for apparent thickening of the reticular basement membrane (RBM) and is recognized as a characteristic feature of asthma. Wide variations exist in reported ranges of the thickness of the RBM in both asthmatic and control airways. The description of collagen deposition has been confined to the RBM with no previous reports of increased collagen deeper within the bronchial wall (i.e. submucosa), although inflammation is known to extend to deeper levels.Objective The objective of this study was to compare different methods used to measure RBM thickness and to examine collagen deposition in deeper asthtnatic bronchial submucosa.Methods We examined bronchial mucosal biopsies from 15 asthmatic and 13 normal healthy controls to measure RBM thickness by light and electron microscopic methods on resin sections and to estimate the extent of collagen deposition in the submucosa by immunochemical staining with monoclonal antibodies to collagen subtypes on paraffin sections.Resutts Simple staining with toluidine blue on resin sections coupled with light microscopic evaluation was as accurate and less expensive than electron microscopic methods in the measurement of the thickness of RBM (mean ± sd, light tnicroscopy: asthma vs control. 6.01 ± 2.09 vs 3.19 ± 0.55 μm,P 〈 0.001; electron microscopy: asthma vs control. 6.09 ± 2.19 vs 2.85 ± 0.86 μm, P 〈 0.01). Using monoclonal antibodies to collagen subtypes and image analysis methods, we identified increased collagen in the submucosa in asthma (type III collagen, asthma vs control: 62 ± 7 vs 51 ± 9%,P 〈 0.05, type V collagen, asthma vs control: 60 ± 6 vs 52 ± 9%, P 〈 0.05).Conctusions This study has demonstrated that the RBM may be reliably measured by light microscopic methods. This study shows for the first time that the submucosa of the asthmatic airway contains significantly more collagen than nonnal controls, giving rise to the possibility that airway scar formation may have greater functional implications than has been previously believed from consideration of the RBM alone.