What structures in the respiratory tract produce mucus?

Mucus-Producing Structures in the Respiratory Tract

The primary structures that produce mucus in the respiratory tract are the tracheobronchial submucosal glands and the surface goblet cells, which together form the major sources of respiratory mucins essential for airway protection and clearance. 1, 2

Main Mucus-Producing Structures

Submucosal Glands

• Located in the airway wall wherever there is supportive cartilage (from larynx to small bronchi)
• Occupy approximately one-third of the inner airway wall in adults 2
• Considered the major source of tracheobronchial mucus, especially in hypersecretory conditions 2
• Contain both mucous and serous acini 1
• Serous acini produce important protective substances including:
  • Lactoferrin
  • Antiproteases
  • Lysozyme 1

Goblet Cells

• Distributed throughout the tracheobronchial tree 2
• Surface mucous (goblet) cells undergo hyperplasia in response to irritation 1
• Normally sparse or absent in bronchioles (airways <1mm diameter) 2
• Increase in number during hypersecretory conditions 2
• Their secretions contribute to airflow obstruction when overproduced 2

Mucin Types and Production

Secreted Mucins

• MUC5AC and MUC5B are the primary polymeric mucins in airway mucus 3
• Provide the organizing framework of the airways mucus gel 3
• Major contributors to the viscoelastic properties of mucus 4
• Essential for effective mucociliary clearance 4

Cell-Associated Mucins

• Present on the epithelial surface 4
• Shield the epithelium from pathogens through their extracellular domains 4
• Regulate intracellular signaling through their cytoplasmic regions 4

Normal Mucus Production and Clearance

• Healthy individuals produce approximately 500 mL of mucus daily 1
• Mucus is normally eliminated by mucociliary clearance to the hypopharynx where it is swallowed 1, 5
• The mucociliary clearance system traps and removes foreign particles 5
• Mucus transport rates depend heavily on mucus concentration 6
• Transepithelial fluid transport generates the fluid needed to hydrate mucus 6

Pathological Changes in Mucus Production

• In chronic bronchitis and other hypersecretory conditions:

  • Enlargement of tracheobronchial submucosal glands 1
  • Hyperplasia of tracheobronchial surface mucous (goblet) cells 1
  • Increased acidic mucus proportion in submucosal glands 1
  • Appearance of goblet cells in small airways (mucous metaplasia) 1
  • Reduction in serous acini of submucosal glands 1
  • Decrease in number and length of cilia 1
  • Squamous metaplasia of epithelium 1

• Inflammatory mediators contribute significantly to mucus hypersecretion:

  • Neutrophil elastase
  • Mast cell chymotryptase
  • Interleukins (IL-4, IL-13)
  • Products of arachidonic acid metabolism (15-HETE) 2

Clinical Significance

• Overproduction of mucins is a major factor in morbidity and mortality of chronic airways diseases:

  • Asthma
  • Cystic fibrosis
  • Chronic obstructive pulmonary disease 3

• In disease states, mucus becomes hyperconcentrated (dehydrated) 6

• Formation of adherent mucus plaques and plugs in distal airways leads to:

  • Locally hypoxic conditions
  • Airflow obstruction
  • Inflammation
  • Infection
  • Airway wall damage 6

Understanding the structures responsible for mucus production is essential for developing targeted therapies for conditions characterized by mucus hypersecretion and impaired clearance.