Pseudoglandular, canalicular, saccular and alveolar phase
The pseudoglandular phase spans weeks 5 to 17. During this phase, the lung resembles a tubulo-acinous gland. The entire air-conducting bronchial tree is set up, including the terminal bronchioles. Type II pneumocytes or their precursors already can be observed in the cuboidal epithelium of the future respiratory portion.
The canalicular phase spans weeks 13 to 25. During this time, canaliculi develop from the terminal bronchioli, and later give rise to the part of the lung where the gas exchange occurs. Capillaries grow along the cuboidal epithelium. The acini or respiratory units then develop, which include the respiratory bronchioles and the alveolar ducts. From week 19 onward, the epithelium of the acini initially consists of type II pneumocytes (alveolar epithelial cells of type II) from which also type I pneumocytes (alveolar epithelial cells of type I) develop.
The saccular phase spans week 25 to birth. It is characterized by temporary sac-like enlargements at the distal end of the respiratory bronchioles, called terminal sacs. A dense capillary network appears. The capillaries make very close contact with the respiratory epithelium. From week 28 onward, the production of surfactant by type II pneumocytes commences. The presence of terminal sacs and the production of surfactant are essential for the survival of a premature baby.
The alveolar phase begins at week 29 and continues until the child is aged 8 years. It is marked by an intense increase in length of respiratory bronchioles and alveolar ducts. The alveoli arise through subdivisions of terminal sacs by means of septa of connective tissue as well as through sprouting from distal branches. The number of alveoli increases up to 300-400 million.
Lung fluid is expelled at birth. However, in order for the alveoli to unfold, great resistance must be overcome with the first breath. After the alveoli unfold, the surfactant is released very quickly into the alveolar space. It spreads as a monomolecular layer, and reduces the surface tension. Due to this, the effort necessary for respiration is considerably reduced.