[Normal ultrastructure of the small bronchi and bronchioli in man]

Ultrastructural topographic and morphological analysis of the small bronchi and bronchioli was carried out on selected biopsy specimens obtained from 6 operative specimens. The small bronchi had a ciliated cylindrical epithelium identical to that in the large bronchi, apart from a smaller number of goblet cells. Their reticulin layer was thin (1 to 3 microns). The glands in the submucosa were rare whereas there was a rich submucosal vascular network. The lobular bronchioli had a ciliated cylindrical epithelium without goblet cells but possessing rare Clara cells. The epithelium of the terminal bromchioli was characterized by: 1 - The presence of numerous Clara cells, generally grouped in groups of 2 or 3. 2 - The appearance on the opposite side of a satellite artery (this side shows alveoli first) of cells which we interpreted as pre-ciliated cells. These cells possess a range of basal corpuscles under the luminal membrane but do not have cilia. 3 - The appearance of small membranous pneumocytes in islets. The epithelium covering the smooth wall of the respiratory bronchioles still have a gutter of ciliated cells and Clara cells which become thinner towards the periphery. After the secondary respiratory bronchioli, there appears in this gutter a new cell type, the cubical cells. These cells which have poorly differentiated cytoplasmic characteristics, seem to us to be immature bronchial cells, precursors of the Clara cells rather than of the ciliated cells. The remainder of the smooth wall of the respiratory bronchioli is covered with membranous pneumocytes and, in the case of the 3rd order of bronchioli, also by granular pneumocytes. The sub-basal reticulin layer is lacking in the terminal and respiratory bronchioli. The total thickness of the wall becomes considerably thinner, the muscular layer becoming proportionally very thick.