Fig. 2From: Developmental genetics of the COPD lungFive stages of lung development. Stages of lung development in humans: Diagrammatic timeline of the developmental organisation of the mammalian respiratory system. At the embryonic stage, the major airways are formed. During the canalicular stage epithelial differentiation occurs and the air-blood barrier is formed. In the saccular stage of lung development, air spaces expand and finally at the alveolar stage, secondary septation occurs. Reprinted from [102] with permission from Elsevier, copyright (2007).Back to article page