Immunohistochemical stainings for TTF-1 and caveolin-1 during pseudoglandular, canalicular, saccular and alveolar periods of developing human lung. 6A. Epithelial cells of small developing airways during pseudoglandular period are strongly positive for TTF-1 (arrows). 6B. Epithelial cells of a developing airway during pseudoglandular period are negative for caveolin-1 (arrow) while the cells surrounding the airway (short arrow) and obvious endothelial cells (arrowheads) are positive for it. 6C. Pretype II cells during the canalicular period are positive for TTF-1 (arrows). 6 D. Pretype II cells during the canalicular period do not show any positivity for caveolin-1 (arrows) while the cells rounding alveoli (short arrows) as well as the endothelial cells (arrowheads) are positively stained. 6E and 6G. Type II pneumocytes are positive for TTF-1 during saccular (6E) and alveolar (6G) periods (arrows). 6F and 6H. Caveolin-1 is positive during saccular (6F) and alveolar (6H) periods in alveolar epithelial cells (arrows) whereas bronchial epithelial cells do not exhibit any posivity (short arrows). Scale bars = 20 μm.