Last reviewed 01/2018

The bronchi of the respiratory tree are characterised by a similar histological structure to the trachea with which they are continuous; typically, there are four layers from internal to external:

  • mucosa:
    • pseudostratified ciliated columnar epithelium; mucociliary escalator moves particulate matter in a layer of mucus superiorly towards pharynx
    • in the smallest bronchi, the epithelium changes to simple cuboidal
    • goblet cells; role in mucus production. Diminish in number distally - absent by the smallest bronchi.
    • basal cells; near the basal lamina, role as progenitor of other cells
    • brush cell or dense core granule cell; closely associated with neurones and may have role in both sensation and local hormonal release
  • submucosa:
    • rich in elastic fibres
    • mixed seromucus glands which secrete onto the mucosa through cilia-lined ducts; decrease in number distally and absent by the level of the bronchioles.
    • increasing numbers of spirally-orientated smooth muscle fibres around the bronchi as it passes distally
  • cartilage and smooth muscle layer:
    • unlike the trachea, the rings of hyaline cartilage fuse with each other at irregular points
    • subsegmental bronchioles have the cartilage arranged as widely-spaced plates
    • by the division into bronchioles, the cartilage has disappeared
    • there is no equivalent of the trachealis muscle
    • smooth muscle spiraling around the bronchi becomes more evident with increasing distance down the tracheal tree
  • adventitia: disappears by the later divisions of bronchi as the outer walls are now surrounded by lung parenchyma