Immunohistochemical studies were made of the distribution of various cytokeratins (K), Clara cell secretory protein (CC10), surfactant protein A (SP-A) and type VII collagen in normal human airways. Electron microscopic studies were made to identify hemidesmosomes and anchoring fibrils on the basal surfaces of the epithelial cells. K19 was detected in all epithelial cells, and K17 in all basal cells. K14 was coexpressed in a few basal cells, and this coexpression was decreased in distal airways. Two types of basal cells were recognized: one type, found mainly in large airways, was characterized by abundant intermediate filaments and well developed hemidesmosomes and anchoring fibrils. The second type contained few intermediate filaments and poorly developed hemidesmosomes and anchoring fibrils. Reactivity for type VII collagen was found along the basement membrane throughout the airways, but not in alveoli. Clara cells were reactive for CC10 and K17, but not for K14 and SP-A. The bronchiolar cuboidal cells in respiratory bronchioles were positive only for K19. SP-A was present only in type II alveolar epithelial cells. Thus, two types of basal cells are present in airways, and the bronchiolar cuboidal cells appear distinct from these basal cells, Clara cells and type II alveolar epithelial cells.
