There are five stages in lung organogenesis. In the embryonic period, the tracheal bud forms. The pseudoglandular and canalicular periods are characterized by branching morphogenesis giving rise to the bronchial three while during the saccular and alveolar periods the alveoli form [17] . In investigations on claudin expression during these stages claudins 1,3,4, 5 and 7 can be detected in bronchial epithelium during the pseudoglandular and canalicular periods [17] . Pretype alveolar cells express claudins 3, 4, 5 and 7 but not claudin 1 [17] . In saccular and alveolar periods claudin 5 is lost from alveolar cells but weakly present in bronchial epithelial cells. Claudins 3, 4 and 7 show strong positivity in bronchial and alveolar cells while claudin 1 is negative in alveolar pneumocytes [17] .