Extracellular lining of bronchioles after perfusion‐fixation of rat lungs for electron microscopy

Rat lungs were fixed by perfusion of fixatives through pulmonary vessels which resulted in good fixation of bronchioles. This technique allows the preservation of a hitherto not described extracellular lining of the bronchiolar surface, which by conventional fixation (immersion or instillation of the fixatives into the airways) is washed out. The cilia appear to be embedded in an extra‐cellular, amorphous material. This material is often lined at its surface by a strongly osmiophilic layer. Tubular myelin figures can also be identified. Comparison with recent physiological and biochemical work, which establishes the presence of surface‐active material in the airways, strengthens the conclusion that this layer is related to bronchiolar “surfactant” fixed in situ. A mixed origin of this material from the Clara cells and from the alveolar surface lining layer is suggested.