The formation of granules in the bronchiolar Clara cells of the rat. 1. Electron microscopy

A morphologic study of bronchiolar Clara cells used rat lungs fixed by vascular perfusion or by inflation through the airways. Clara cells contained membrane-limited granules which had a crystalline structure with an 80 Å periodicity when fixed by perfusion. The crystalline structure was infrequently seen after inflation fixation. The smooth endoplasmic reticulum was highly developed in the apex of Clara cells and appeared to be a major site of formation of the characteristic granules. Some nascent granules were also seen in association with the Golgi apparatus. Perfusion fixation retained a bronchiolar lining layer which was denser than the alveolar lining layer in the same sections and which resembled the matrix of Clara cells in opacity. One explanation for the difference between the bronchiolar and alveolar linings may be that the bronchiolar lining is derived in part from the secretions of Clara cells. Images suggesting exocytosis, albeit infrequent, support this suggestion.