MATURATION OF THE LUNG :
- Psedoglandular stage
The pseudoglandular stage takes place between 5-16 weeks of embryonic development. Conducting airways are formed by progressive branching. Eventually 16-25 generation of primitive airways are formed. Endodermal lung buds undergo branching only if they are exposed to bronchial mesoderm, The rate and extent of branching appear directly proportional to amount of mesenchyme present. All bronchial airways are formed by 16 weeks. After this time, futher growth occurs by elongation and widening of existing airways.
During this stage, the first differentiation of lung epithelium occurs. By 13 weeks cilia appear in the proximal airways. Mesenchyme is necessary for this epithelium differentiation to occur and there is a transition from formation of bronchial epithelial cells (ciliated columnar and goblet cells) to alveolar type II cells. Conversely, the differentiation of lung mesenchyme requires the presence of lung epithelium.
- The canalicular stage
The canalicular stage takes place between 16-26 weeks. At this time the gas exchanging portion of the lung is formed and vascularized. There is a decrease of interstiatial tissue and growth of the capillary network. By 20 weeks, there is differentiation of the type 1 pneumocyte. The type peneumocyte is the primary structural cell of the alveolus, and gas exchange will occur across these very thin, membran-like cells (if the potential alveolar space is considered proximal). At about the same time, there is the appearance of lamellar bodies, also called inclusion bodies, in type II alveolar cells. The lamellar body is the site of surfactant storage, prior to its release into the alveolar space.
- The terminal or saccular stage
The terminal or saccular stage encompasses the period from 26 weeks until term. During this stage, there is a decrease in interstitial tissue, and a thinning of the airspace (alveolar) walls. As this stage progressess, there are recognizable type I and type II cells. The lamellar body of the type II cells are the site of storage of surfactant, which is rich in phosphatidylinositol (phosphatidyl choline and phosphatidyl glyserol in late gestation lungs), and is necessary for alveolar stability. The stability of the lungs at birth correlates with the number of lamellar bodies present. In the absence of surfactant, the lung can maintain alveoli in an open state for only a very short time.
At birth, the air-containing space, later to become the alveolus, has called a primitive saccule. There are approximately 20×106 saccules at birth.
- Post natal or alveolar stage
The saccules continue mature following birth in the post natal or alveolar stage. While these saccules are lined by mature type I cells, the shape of geometry of the saccules doesnt achieve “adult” configuration until approximately 5 weeks after birth. The functioning alveolus is connected to an alveolar duct is lined with type I cells, which are in intimate contact to pulmonary capillaries , contain surfactant produce by type II cells and have pores (pores of Kohn) connecting them to adjacent alveoli. The interstitial capillaries are exposed to two alveoli simultaneously. The air/blood interface consist of type I cell, a very thin basement membrane and the pulmonary capillary endothelium. At functional maturity, there are approximately 300×106 alveoli in the lungs. This number of alveoli appears to be achieved by the age of 8.