Development of the lung can be divided into two phases, lung growth (structural development) and lung maturation (functional development). Lung growth can be influenced by a host physical factors. Lung maturation and the achievement of functionality is primarily a biochemical process and is under the control of a number of different hormones. Lung growth proceeds through gestation. There is progressive branching of the airways and finally development of alveolar spaces capable of gas exchange in the last trimester. The surfactant system, composed of phospholipids that decrease surface tension within the alveoli and prevent alveolar collaps during exhalation, develops in the last trimester, and reaches maturity by approximately 36 weeks. Lung growth continues after birth as alveolar number continuous to increase. The end result of the development of the lung is an organ with a tremendously large surface area that is approximately 50-100m2, capable of exchanging oxygen and carbon dioxide across a very thin membrane.
Successful development and function of the lung requires the completion of both physical development, required for the structure of the lung, and biochemical development of the surfactant system, required for the stability of this very large surface area. The two processes clearly are related. Incomplete development of lung structure and premature birth prior to the development of the surfactant system will lead to respiratory compromise or insufficiency in the newborn.
The stages of lung development:
- 15-16 weeks (pseudoglandular period)
Branching has continued to form terminal bronchioles. No respiratory bronchioles or alveoli are present.
- 16-26 weeks (canalicular period)
Each terminal bronchiole divides into 2 or more respiratory bronchioles, which in turn divide into 3-6 alveolar ducts.
- 26 weeks to birth (terminal sac period)
Terminal sacs (primitive alveoli) form, and capillaries establish close contact.
- 8 month to childhood (alveolar period)
Mature alveoly have well-developed epithelial endothelial (capillary) contacts.
There are five phases of structural lung development that occur at progressive times during gestation. The timing of the phases is approximate, with variation between fetuses, and in fact, there is no absolute agreement about the weeks that comprise each phase among various authors and texts.
The embryonic stage is apparent in the 3 weeks old embryo. The lung bud develops from the foregut and in communication with it. Separation of the two lung buds comes about with fusion of the esophagotracheal ridges to form the esophagotracheal septum. When the embryo is 5 weeks old, two primary lung buds are identifiable.Development progresses in the 8 weeks old embryo as the lobar buds subdivide and form the bronchopulmonary segments.
Lung buds are lined by endodermally derived epithelium which differentiates into respiratory epithelium that lines the airways and specialized epithelium that lines the alveoli. The innervation of the lungs is derived from ectoderm, while the mesoderm is origin of pulmonary blood vessels, smooth muscle, cartilage and other connective tissue.