Respiratory tract epithelialm

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Here is a comprehensive overview of the epithelium of the respiratory tract, from nose to alveoli, based on Histology: A Text and Atlas and Junqueira's Basic Histology.

Respiratory Tract Epithelium

The lining epithelium changes progressively from the nasal vestibule to the alveoli, transitioning from stratified → pseudostratified → simple → squamous as the airways narrow.

1. Nasal Vestibule

Stratified squamous epithelium — a direct continuation of facial skin
Contains vibrissae (hairs) that trap large particulate matter
Sebaceous glands assist in particle entrapment
Posteriorly transitions into pseudostratified epithelium

2. Respiratory Region (Nasal Cavity, Trachea, Bronchi)

The dominant epithelium of the conducting airways is pseudostratified ciliated columnar epithelium (also called "respiratory epithelium"). It lines the nasal cavity, nasopharynx, larynx (most of it), trachea, bronchi, and larger bronchioles.

Five cell types make up this epithelium:

Cell Type	Features	% of Cells
Ciliated columnar cells	~250 cilia/cell; extend full thickness; mucociliary escalator	~70% (most abundant)
Goblet cells	Mucus-secreting; interspersed among ciliated cells; increase during chronic irritation	5–15%
Brush cells	Blunt microvilli; chemosensory/general sensory; synaptic contact with afferent nerve endings	Few
Small granule cells (Kulchitsky cells)	Secretory granules; endocrine cells of diffuse neuroendocrine system (DNES)	Few
Basal cells	On basement membrane; do not reach surface; stem cells for all other types	Variable

Electron micrograph of human trachea — showing ciliated cells, goblet cells, and basal cells (×1,800):

Tracheal epithelium EM showing ciliated cells, goblet cells, and basal cells

H&E photomicrograph of tracheal wall — epithelium (Ep) with goblet cells (G), prominent basement membrane (BM), lamina propria (LP), and submucosa (SM) with seromucous glands (×400):

Tracheal wall H&E showing epithelium, lamina propria, submucosa

Key functional point: The cilia beat in coordinated waves, forming the mucociliary escalator — sweeping the mucous blanket (with trapped particles) upward toward the pharynx for clearance.

3. Terminal Bronchioles

As the airways narrow (<1 mm diameter), the epithelium transitions:

Simple columnar ciliated → simple cuboidal epithelium
Goblet cells disappear
Club cells (formerly Clara cells) become the dominant non-ciliated cell type
- Dome-shaped, non-ciliated, no mucus granules
- Secrete surface-active agents (surfactant-like lipoproteins) to aid airway expansion
- Also have detoxifying enzyme activity (smooth ER-rich)

Diagram of terminal bronchiolar epithelium showing brush cell, club cell, small granule cell, and ciliated cell with their ultrastructural features:

Diagram of terminal bronchiolar epithelium cell types

4. Respiratory Bronchioles

Wall alternates between thick regions (club cells + smooth muscle) and thin regions (alveolar wall-like, for gas exchange)
Scattered alveoli interrupt the wall — this is where gas exchange begins

5. Alveoli (Alveolar Ducts → Alveolar Sacs → Alveoli)

The alveolar epithelium consists of two cell types (pneumocytes):

Cell	Type	Features
Type I alveolar cells (type I pneumocytes)	Extremely thin, squamous	Cover ~95% of alveolar surface; form the air-blood barrier
Type II alveolar cells (type II pneumocytes)	Cuboidal; lamellar bodies	Secrete surfactant (reduces surface tension); serve as progenitors for type I cells; innate immune role

The interalveolar septum — the site of the air-blood barrier — consists of:

Alveolar epithelium + basal lamina
Capillary endothelium + basal lamina
± Interstitial connective tissue (elastic and reticular fibers)

Summary: Epithelial Transitions Along the Airway

Nasal vestibule     → Stratified squamous
Nasal cavity/trachea/bronchi → Pseudostratified ciliated columnar (respiratory epithelium)
Bronchioles         → Simple columnar/cuboidal ciliated + club cells
Terminal bronchioles → Mostly club cells (simple cuboidal)
Respiratory bronchioles → Cuboidal (club cells) + interrupted by alveoli
Alveolar ducts/sacs → Squamous (type I) + cuboidal (type II) pneumocytes

Olfactory Epithelium (roof of nasal cavity)

A special region: pseudostratified but contains:

Bipolar olfactory neurons (chemosensory)
Supporting (sustentacular) cells
Basal stem cells

Sources: Histology: A Text and Atlas with Correlated Cell and Molecular Biology (Ross & Pawlina), 9th ed.; Junqueira's Basic Histology, 17th ed.

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