PPT Slide
Two cell types form the lining epithelium in an alveolus: simple squamous Type I cell and cuboidal Type II surfactant producing cell that has a large, pale staining cytoplasm. The most efficient gas exchange occurs across the extremely attenuated and virtually invisible cytoplasm of the Type I cell. The endothelial cells lining the abundant alveolar capillaries are also difficult to visualize in light micrographs. Macrophages are present within the wall of the alveolus and along the surface of the alveolus where they are more readily recognized.