Microanatomy of the Lung

Overview

Airway lining
- pseudostratified ciliated columnar cells extend to the bronchioles
  - mucus secretions are swept out of the lungs toward the mouth by ciliated cells
  - smoking causes these cells to undergo metaplasia
    - replaced by stratified squamous
- simple ciliated columnar epithelium extends to the terminal bronchioles
- ciliated cuboidal cells extend to the respiratory bronchioles
- alveolar sac are composed of pneumocytes
  - macrophages clear debris in alveoli
- goblet cells extend to the larger bronchioles but stop before the terminal bronchioles
Type I pneumocytes
- 97% of alveolar surfaces
- line the alveoli
- squamous
- thin for optimal gas diffusion
- form blood-gas barrier with pulmonary capillary endothelial cells
Type II pneumocytes
- 3% of alveolar surfaces
- secrete pulmonary surfactant within lamellar bodies
  - dipalmitoyl phosphatidylcholine
  - lowers the alveolar surface tension
- cuboidal and clustered
- serve as precursors to type I cells and other type II cells
- proliferate during lung damage
Club (Clara) cells
- nonciliated
- columnar with secretory granules
- secrete component of surfactant
- degrade toxins
- act as reserve cells
A lecithin-to-sphingomyelin ratio of > 2.0 in amniotic fluid is indicative of fetal lung maturity