Terminal bronchioles

Terminal bronchioles are the airways produced by the last few divisions of the bronchioles within the bronchial tree before it becomes adapted to respiration. There are about 70,000 within each lung with diameters of around 0.1-0.2mm.

The structure of their walls is similar to that of the bronchi but they are more adapted to respiration:

• mucosa:
  • increasing numbers of Clara cells which come to line the lumen of the bronchiole; Clara cells secrete a surface-active agent similar to surfactant
  • progressively fewer:
    • cuboidal ciliated cells; despite the lack of a mucus layer, cilia are still required to move particles which pass distally into the respiratory tree
    • goblet cells; however, the mucosa may show metaplastic change with increasing numbers of goblet cells in chronic smokers
    • dense core granule cells
    • brush cells
• submucosa: no glands
• muscular layer: prominent spirally-orientated smooth muscle

There are three divisions of the the terminal bronchioles before respiratory bronchioles are first produced.

The bronchiole tree distal to a primary terminal bronchiole is termed a pulmonary lobule.