Airway morphology: epithelium/basement membrane.

Morphologic studies have given important information about changes in the airways underlying the functional abnormalities in asthma. Recent biopsy studies have revealed that in asthma an airway inflammatory process is present even at a clinically early stage of the disease. This inflammatory response shows a particular cellular picture with concomitant vascular changes. It is widely thought that dramatic epithelial changes such as epithelial shedding are major mechanisms in the pathogenesis of asthma and bronchial hyper-responsiveness. Because of difficulties in obtaining proper biopsy specimens, it has not yet been established to what extent epithelial shedding really occurs in preterminal stages of asthma. Perhaps a more important aspect than epithelial shedding is a more continuous replacement of the ciliated epithelium by another type of epithelium in asthmatic patients' airways during the early disease stage. The airway epithelial cells, their surface receptors, and the underlying basement membrane seem to play an important role in maintaining the structural integrity of the epithelium and establishing functional permeability barriers. Disturbances in the structure may lead to events recognized as tissue damage and to the development of inflammation in asthma. We need to learn more about the adhesion mechanisms in asthma, both at the receptor and the ligand sites, to better assess the role of structural epithelial changes in this disease.