Colocalization of cytokeratin 18 and villin in type III alveolar cells (brush cells) of the rat lung.

Alveoli of the rat lung are lined by three different cell types, the flat type I cells and the cuboidal type II and type III cells. Type III cells differ from type II cells by the presence of an apical tuft of microvilli and the absence of lamellar type secretory granules. In the present study we show by double immunolabelling that type III cells of the rat lung can be identified at the light- and electron microscope level by antibodies against both cytokeratin 18 and the actin-crosslinking protein villin. At the ultrastructural level, microvilli and their rootlets in the apical cytoplasm were labelled by the anti-villin antibodies, whereas a monoclonal antibody against cytokeratin 18 (Ks18.04) labelled bundles of intermediate filaments. In conclusion, antibodies against villin and certain monoclonal antibodies specific for cytokeratin 18 can be used as tools for selective visualization of type III cells in the rat lung.