An ultrastructural study of pulmonary bronchiolar and alveolar epithelium in sheep.

The cellular population lining the bronchiolar and alveolar epithelium of the lower respiratory tract of three clinically healthy, purebred Suffolk lambs of both sexes and 3-4 months old was studied using a transmission-electron microscope. The epithelium of primary and secondary bronchioles consisted of four cell types: basal cells, intermediate cells, ciliated cells, and non-ciliated (Clara) cells, whereas the epithelium of terminal and respiratory bronchioles consisted of only two kind of cells: ciliated and Clara cells. The alveolar wall, including alveolar ducts and saccules, was covered by type I and type II pneumocytes. In addition, the presence of pulmonary intravascular macrophages was a constant in most of the alveolar capillaries. The morphologic characteristics of all these cells are described in detail and discussed.