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Conducting Airway Histology

  • The "Conducting Airways" are defined as those sections of the respiratory tract which do not directly participate in gas exchange and instead simply allow bulk flow of air to areas which are responsible for gas exchange. The conducting airways of the lower respiratory tract begin with the trachea which subsequently divides into two main bronchi that in turn branch into several lobar bronchi and so on until the terminal bronchiole which is considered the final, purely conducting segment of the respiratory tract. The histological architecture of all the conducting airways is roughly the same although some layers may not be present in all segments.
Basic Components
  • Respiratory Epithelium:
    • The Respiratory is composed of a layer of respiratory epithelial cells which begin as a ciliated pseudostratified columnar epithelium in the trachea and slowly transition to that of a non-ciliated simple cuboidal epithelium in the terminal bronchioles.
    • Goblet Cells: Are mucin-secreting cells scattered throughout the respiratory epithelium. They most densely populate the trachea and thin out as airways branch, disappearing by the terminal bronchiole
    • Kulchitsky Cells: Are a neuroendocrine cell similar to enterochromaffin cells that are scattered throughout the epithelium and secrete bronchoactive hormones
  • Respiratory Lamina Propria:
    • Is a band of vascular fibro-elastic tissue that supports the respiratory epithelium. The lamina propria also contains aggregates of MALT which support the development and secretion of IgA-secreting B-cells that help protect infection of the respiratory system.
  • Respiratory Smooth Muscle Layer:
    • A band of smooth muscle cells, forming the bronchial smooth muscle, lies deep to the lamina propria. While absent from the trachea, the layer gradually increases in thickness as airways branch. This band of smooth muscle is used to regulate the diameter of the airways, which consequently modulates their resistance to airflow.
  • Respiratory Submucosa
    • The Submucosa is a loose layer which largely contains serous and mucous glands. This layer is most prominent in the trachea and largely disappears by a few branches after the lobar bronchi.
  • Cartilage
    • A layer of cartilage may invest the submucosa and helps maintain patency of the airways during the breathing cycle. The cartilage layer is most prominent in the trachea and largely disappears a few branches after the lobar bronchi.