Development of Alveoli

The alveoli begin development approximately 4 to 6 months into pre-natal development, when the respiratory bronchioles form through the branching of both bronchi and the bronchioles (1). This stage, called the canalicular stage, sees the formation of what is to become the air-blood barrier, as well as the differentiation of the columnar pulmonary epithelial cells into Type II and Type I cells (1). The synthesis of surfactant also begins late in this stage of development, and during a brief period both before and after birth (approximately 1 month), a sharp increase in the number of alveolar macrophages is also exhibited (1).

Over the course of the period from 6 months to birth, the alveolar ducts begin branching, and continue to do so for about 8 years after the birth of the child (1). The primary septa, which are located between air spaces within the lung, are comprised of a layer of connective tissue which has on both sides, a layer of capillaries. In the several days following birth, one of these capillary beds gives rise to a secondary septa through the process of upfolding, and it will be these septa that, with the formation of elastic tissue, give rise to the alveoli of the lung (1).

Figure 2: Composition of the Alveoli (Retrieved from

http://faculty.ksu.edu.sa/75719/Pictures%20Library/Respiratory%20system/Alveolus.jpg)