Developmental changes in alveolar epithelial permeability

The pattern of developmental changes in alveolar epithelial permeability is not clear. The prevailing view prior to 1982, based on extensive lamb studies by the London group, was that the alveolar epithelium shows selectivity toward lipid-insoluble small molecules, and that the degree of selectivity (measured by estimated pore radius) is essentially constant from fetal to adult animals with the exception of the period immediately following birth. Our studies on isolated, perfused rabbit lungs, on the other hand, suggest that the barrier shows no selectivity for these molecules, and that this is the case from prenatal to adult animals. Also, however, the absolute permeability of the epithelium does decrease as the animals mature during this period. We are thus left with divergent views to resolve, not only with regard to developmental changes, but also concerning the fundamental mechanisms of solute transport across the alveolar epithelium.