Anchoring fibrils in the normal canine respiratory system.

Electron microscopic study of the normal canine respiratory tree disclosed the presence of anchoring fibrils, a distinct class of fibrils of extracellular connective tissue, in association with the following types of cells: (1) basal cells and special type cells of trachea and bronchi; (2) ciliated cells and basal cells of bronchioles; (3) ductal cells, secretory cells, and myoepithelial cells of tracheobronchial glands. Anchoring fibrils in the normal respiratory system measured up to 6,000 A in length and from 170 to 400 A in thickness, and had a banding pattern that differed from that of collagen fibrils and connective tissue microfibrils. They formed arcs, the ends of which inserted into the basal lamina underlying the basal portions of the cells, often in the vicinity of hemidesmosomes. Anchoring fibrils decreased in number and size in the more distal portions of the respiratory tree, and were not found in alveolar septums. Anchoring fibrils in lung appeared similar to those described in other organs, but were often small and inconspicuous. The function of these structures is to reinforce the attachment of the epithelial basal lamina to the underlying connective tissues.