Physical, endocrine and growth factors in lung development

Before birth, the lung is filled with liquid and plays no role in gas exchange. But when the umbilical cord is cut, the lung must be sufficiently developed to take over the role of gas exchange, a role that it is has never played before. Therefore, to survive the transition from prenatal to postnatal life, humans, like other mammals, must have a functionally mature respiratory system by the time of birth. The lung must develop an intricate branched structure for the conduction of gases to and from the gas-exchanging regions of the lung. The gas exchanging regions of the lung must develop a large surface area that is closely apposed to a rich vascular network to enable efficient exchange of gases. The tissue separating the air spaces from the vascular network must be thin enough for rapid gas exchange, elastic enough to allow ease of expansion, yet strong enough to prevent damage to the delicate tissue structures. The cell types lining the gas-exchanging region of the lung must have differentiated into type I alveolar epithelial cells (AECs) that have long, thin cytoplasmic extensions that reduce the distance for gas exchange, and type-II AECs that produce and secrete surfactant that reduces surface tension at the air-liquid interface. The lungs of most neonates born at term take over the role of gas exchange smoothly and effectively. However, infants that are born preterm, before the lungs have had sufficient time to develop, or who have suffered some compromise to their lung development, may suffer respiratory insufficiency at birth; this is a major cause of neonatal morbidity and mortality and can lead to long-term deficits in respiratory health. Because appropriate lung development during fetal life is essential for the successful transition to extra-uterine life, it is important to understand the factors that control prenatal lung development. The factors that regulate lung development include physical factors, circulating factors, and locally produced factors.