Normobaric hypoxia induces mild damage to epithelium of terminal bronchioles in rabbits (ultrastructural study)

We studied the ultrastructure of the epithelium of terminal bronchioles in rabbits exposed for 96 hours to hypoxia (10% O2) in isobaric hypoxic chamber. Rabbits of the first control group (treated controls) were exposed for the same time in the same hypoxic chamber with atmosphere regulated at 21% O2. In both groups, the temperature in the chamber was 23°C and humidity 100% during the whole experiment. The second control group (untreated controls) was kept under standard conditions. The target cells for the effect of high temperature, humidity and normobaric hypoxia were the secretory elements. Both in hypoxic animals and treated controls, isolated Clara cells revealed signs of pathological alteration. More degenerative changes of Clara cells and marks of their compensatory proliferation were ascertained after exposure to hypoxia. Electron dense secretory granules were observed in most Clara cells. This finding reflected the initial stage of their secretory product formation. The secretory granules were usually stored in cytoplasm; morphological signs of their evacuation were found only exceptionally. The ciliated cells were less damaged than the secretory ones. On their apical surfaces, formation of cytoplasmic protrusions, which sometimes led to degeneration of free cilia, was ascertained. The damage of the epithelium and mild secretory stimulation of its secretory elements could play a role in the lung injury caused by the subacute normobaric hypoxia.