Histology

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