Chapter 4 - Development of Alveoli

Abstract

This chapter discusses the processes involved in the formation of alveoli in the developing lung. Alveoli are tiny, thin-walled sacs that facilitate the exchange of gases between capillaries in the alveolar wall and air brought into the lung during inspiration. In humans, the formation of pulmonary alveoli begins during late gestation and continues after birth. In other species, alveolarization is either predominantly postnatal or predominantly prenatal. Alveolarization is a relatively late event in lung development, occurring after branching morphogenesis has laid down the conducting airway system. However, some of the regulatory factors that control alveolarization are also involved in lung bud formation and branching morphogenesis. Shortly after birth, in both rats and humans, the surface area of the air-blood interface begins to increase markedly as the terminal saccules become alveolar ducts and these in turn give rise to alveolar sacs and the alveoli. Alveolar formation in the developing lung has been divided into several phases: the first is termed secondary septation, which is characterized by the formation of new alveolar septa by septation of the terminal sacs. The second phase of alveolarization is marked by further lengthening and thinning of the secondary septae, primarily via the loss of interstitial mesenchymal cells and extensive capillary remodeling.