The proximal border of the human respiratory unit, as shown by scanning and transmission electron microscopy and light microscopical cytochemistry

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Abstract
The epithelial cell types present in respiratory (= distal alveolarized) and terminal (= distal nonalveolarized) bronchioles in adult human lung were characterized with scanning and transmission electron microscopy (SEM, TEM) and light microscopic cytochemistry, using specific antibodies against surfactant protein SP‐A and mucins, and Alcian blue/periodic acid‐Schiff (AB/PAS) staining.In the respiratory bronchiole, two epithelial cell populations share the same basal lamina: one pseudostratified columnar with ciliated, secretory, and basal cells and the other predominantly simple cuboid with some interspersed flat (type I) cells. The columnar secretory cells show the ultrastructure of mucous cells. Light microscopically, they react with mucin antibodies and contain primarily periodatereactive acid mucins. The mucous cells are the distal secretory cells described by Clara (1937). The cuboid cells are identified as type II (precursor) cells based on ultrastructural criteria for embryonic type II cells (Ten Have‐Opbroek et al., 1988a, 1990a), including a cuboid cell shape, a large and roundish nucleus, rough and smooth endoplasmic reticulum (ER), osmiophilic multivesicular bodies, and dense bodies. These dense bodies in turn frequently exhibit—like those in embryonic type II cells—internal vesicles or lamellae, variability in size and shape, a specific relationship to ER and a widespread cytoplasmic distribution. Finally, the cuboid cells show a cytoplasmic staining pattern for SP‐A. The terminal bronchiole is lined by the columnar cell population.In the respiratory bronchiole, the columnar (bronchial) and cuboid (alveolar) cell populations occupy distinctly different zones (pulmonary artery zone versus remaining wall). The alveolar part of the respiratory bronchiole (called alveolar tubule) defines the proximal border of a true respiratory unit.

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