Cellular and connective tissue changes in alveolar septal walls in emphysema

Emphysema is commonly defined as enlargement of airspaces distal to termina l bronchioles accompanied by destruction of alveolar walls, but without obv ious fibrosis. Morphometric techniques were used to correlate changes in co mponents of the alveolar septa surrounding enlarged airspaces in human emph ysema with the mean linear intercept (Lm) of those airspaces. Alveolar and capillary surface density decreased with increased Lm, but the ratio of the se surface densities to each other remained close to normal for mild to mod erate increases in Lm. This suggests that the decreased gas exchange observ ed in emphysema is initiated by a total loss of septa and not by selective pathological changes of the microvasculature. Increases in septal wall thic kness directly correlated with increases in Lm. For the mild to moderate em physema lesions included in this study, an increase of 100% in Lm correlate d with a 130% increase in the relative volume of the alveolar septal inters titium. Significant increases occurred in both elastin (0.14 to 0.56 mu m(3 )/mu m(2) basement membrane [BM]) and collagen (0.49 to 1.63 mu m(3)/mu m(2 ) BM). The increase in elastin and collagen raises the possibility of a rem odeling process in the connective matrix in alveolar walls. Whether or not the new connective tissue represents a disordered, nonfunctional regional r esponse needs to be determined.