Respiratory Embryology

Lung development involves 5 stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar. The inner respiratory epithelium arises from the endoderm, and the cartilage, bronchial muscles, connective tissue, and vasculature all arise from the mesoderm. Starting in the embryonic stage at 4 weeks of development, the lung bud branches off the ventral side of the foregut, forming the esophagus posteriorly and the trachea anteriorly. In the pseudomembranous stage, the trachea undergoes multiple generations of branching, and in the canalicular stage, primitive alveolar structures and capillaries develop. Next, in the saccular stage, gas exchange becomes possible as the capillaries more closely associate with maturing alveoli, and type II pneumocytes have started secreting surfactant. In the alveolar stage, the alveoli continue to grow in number and size and continue to mature until a child is 8 years old.