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Lung Epithelium: Functions & Importance for Respiratory Health
The respiratory epithelium is a pseudostratified ciliated columnar epithelium (trachea/bronchi) transitioning to simple cuboidal (bronchioles) and simple squamous (alveoli).
Key Functions
- Barrier defense: tight junctions, mucociliary clearance (cilia beat 12–15 Hz), antimicrobial peptides (defensins, lysozyme, lactoferrin), secretory IgA.
- Gas exchange: alveolar type I cells (95% surface) — ultra-thin for diffusion; type II cells secrete surfactant (DPPC) — reduces surface tension.
- Immune regulation: dendritic cells sample antigen; club cells (Clara) produce anti-inflammatory proteins (CC10); neuroendocrine cells sense hypoxia.
- Fluid homeostasis: ENaC/CFTR regulate airway surface liquid depth (critical for ciliary beat).
- Repair/regeneration: basal stem cells (trachea/bronchi), type II → type I (alveoli), club cells (bronchioles).
Disease Relevance
- Cystic fibrosis: CFTR mutation → dehydrated mucus → infection/inflammation.
- Asthma: epithelial fragility, Th2 inflammation, remodeling.
- COPD: squamous metaplasia, cilia loss, goblet cell hyperplasia.
- IPF: aberrant type II → fibroblast activation.
- Lung cancer: origin from basal/club/type II cells.
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