Thin alveolar epithelial partitions across connective tissue gaps in the alveolar wall of the human lung: ultrastructural observations.
We describe sharply localized attenuated areas in the alveolar walls of 17 of 25 human specimens. In these areas the connective tissue scaffolding of the alveolar septum was interrupted, and the thickness of the alveolar wall was reduced to that of a single or double layer of alveolar epithelium, devoid of basal laminae. Serial sections proved that most such gaps were bridged by a thin partition of intact epithelium continuous with Type I cells at the borders of the gap. These areas were observed infrequently during a systematic ultrastructural survey of specimens from grossly normal areas of lung tissue obtained at surgery or at autopsy. The mean diameter of 25 gaps bridged by a double layer of epithelium was 4.1 +/- 2.0 micrometers, whereas that of 49 gaps bridged by a single layer of epithelium was 5.0 +/- 1.9 micrometers. This contrasted with the mean diameter of 329 interalveolar pores, which was 6.5 +/- 2.4 micrometers (p less than 0.01). The number of profiles of epithelial partitions per centimeter of section profiles of interalveolar septums examined correlated directly with the number of pore profiles (p less than 0.003). This correlation, and the stepwise increase in size of gaps beginning with those bridged by 2 layers of epithelium and moving to those with only a single layer and finally to interalveolar pores, suggests that the epithelium-covered gaps are a transitional stage in the development of pores. This hypothesis is strengthened by the rare occurrence of epithelium-covered gaps in which the epithelial partition is itself interrupted.
D000328:Adult; D000368:Aged; D003238:Connective Tissue / Q000648:ultrastructure*; D004848:Epithelium / Q000648:ultrastructure; D006801:Humans; D008171:Lung Diseases / Q000473:pathology; D008297:Male; D008875:Middle Aged; D008953:Models, Anatomic; D011650:Pulmonary Alveoli / Q000648:ultrastructure*