Sources of Squamous Mucosa in the Respiratory Tract
True stratified squamous epithelium in the respiratory tract is normally found in the larynx (particularly the vocal cords and mechanically exposed areas), the oropharynx, and the proximal portions including the epiglottis, while the remainder of the tracheobronchial tree is lined by pseudostratified ciliated columnar (respiratory) epithelium.
Normal Anatomic Distribution
The respiratory tract exhibits distinct epithelial patterns based on mechanical stress and functional requirements:
Upper Airway Regions with Stratified Squamous Epithelium
Larynx: The mechanically exposed areas, including the true vocal cords and portions of the false vocal cords, are covered by stratified squamous nonkeratinized epithelium 1. This represents the primary location of true squamous epithelium in the healthy respiratory tract.
Pharyngolaryngeal region: Recent single-cell analysis identifies a distinct Tmprss11a+ pharyngolaryngeal compartment composed of stratified squamous epithelium with a specific Keratin gene expression code 2.
Vocal folds: These are specifically covered by a low squamous or "squamoid" epithelium 3.
Regions WITHOUT True Squamous Epithelium
Tracheobronchial tree: The trachea and bronchi are lined by pseudostratified ciliated columnar epithelium (respiratory epithelium), characterized by an Nkx2-1+ signature 2, 4. This is not true stratified squamous epithelium despite sometimes being called "respiratory epithelium."
Nasal passages: Predominantly respiratory epithelium with distinct gene expression (MUC4, PI3, SIX3) compared to tracheobronchial regions 4.
Critical Clinical Distinction: Metaplasia vs. Normal Epithelium
Important caveat: Finding squamous epithelium in the central airways (trachea/bronchi) represents squamous metaplasia, not normal anatomy. This is a pathologic finding that represents:
A precursor lesion pathway to squamous cell carcinoma, progressing through hyperplasia → metaplasia → dysplasia → carcinoma in situ → invasive cancer 5
A response to chronic irritation, particularly smoking exposure, through "field cancerization" where multiple foci of metaplastic lesions develop throughout the respiratory epithelium 5
An adaptive but abnormal change that requires clinical attention and surveillance 6
Practical Implications
When encountering squamous epithelium on bronchoscopy or biopsy:
- In the larynx/vocal cords: This is normal anatomy
- In the trachea or bronchi: This represents metaplasia and warrants evaluation for dysplasia and increased lung cancer risk, particularly in smokers 7
The distinction matters because squamous cell carcinomas tend to arise centrally and are associated with bronchial squamous metaplasia and dysplasia, typically in cigarette smokers 7.