The distal airways, defined anatomically as the region of the respiratory system including the terminal bronchioles through alveoli, were characterized in the guinea pig by means of light microscopy and by both scanning and transmission electron microscopy.

The epithelium of the terminal bronchioles was comprised of two cell types. Ciliated cells were cuboidal and contained long thin microvilli, ellipsoid mitochondria, and both rough and smooth endoplasmic reticulum. Nonciliated (Clara) cells were dome-shaped and usually protruded into the bronchiolar lumen. Numerous large mitochondria, granules of varying density, and crystalloid inclusions were notable in nonciliated cells.

Respiratory bronchioles were characterized by a smooth-surfaced, low cuboidal epithelium. The cells in this region contained the crystalloid material found in terminal bronchioles, numerous large mitochondria, lysosomelike inclusions, and unusual tubular structures arranged in a matrix.

The epithelium became progressively squamous toward the alveolar duct, where transition from bronchiolar cells to pneumocytes occurred. Transitional zones consisted of cells which, in addition to the above-mentioned structures, contained inclusions with internal laminations. These inclusions were structurally similar to the lamellar bodies observed in typical type II pneumocytes of the alveoli.

The epithelium of both the alveolar ducts and alveoli was composed of type I and type II pneumocytes. Classical type I pneumocytes were squamous and very similar in cytoplasmic characteristics to the endothelial cells of the adjacent capillaries. Type II pneumocytes were characterized by the presence of lamellar bodies and numerous mitochondria.