The distribution and surface ultrastructure of airway epithelial cells in the rat lung: a scanning electron microscopic study.

The fine structure and distribution of the epithelial cells of the airway in the rat were studied continuously from the trachea to terminal bronchioles by scanning electron microscopy (SEM). The airway could be divided into three different regions according to cell population: 1) the trachea and extrapulmonary bronchi; 2) intrapulmonary bronchi (larger than 500 microns in caliber); and 3) bronchioles (smaller than 500 microns in caliber). From their surface structures, the epithelial cells could be classified into ciliated and non-ciliated cells, the latter including brush cells, Clara cells and other non-ciliated (secretory) cells. 1. Ciliated cells. The cilia are longer, thicker and more numerous in the trachea; they decrease in length, thickness and number toward the periphery. 2. Brush cells. They possess thin microvilli (0.2 micron in thickness) in the trachea and extrapulmonary bronchi, with a rounded end. In the bronchioles they possess thick microvilli (0.3 micron in thickness) abruptly ending in a right angle edge. The brush cells are distributed sparsely but rather uniformly, and apt to be grouped in two or more cells. 3. Clara cells. Their apical cytoplasm shows a domed or papillary swelling and possesses a few microvilli. The Clara cells are distributed in the bronchioles and can already be found some distance proximal to the bronchial furcations into bronchioles. 4. Other non-ciliated (secretory) cells. Their apical cytoplasm seems to contain secretory granules immediately beneath the cell surface. They often gather in groups in the trachea and extrapulmonary bronchi, tending to form large areas corresponding to sites supported by tracheal or bronchial cartilage. There were found several orifices in tracheal or bronchial glands whose long axes paralleled the tracheal or bronchial axes. Dome-shaped elevations sometimes appear near the branching points of the intrapulmonary bronchi. There were regarded as bronchus-associated lymphoid tissue (BALT).