Histology of the Respiratory System

Respiratory System Histology

Overview

The respiratory system is functionally divided into:

  1. Conducting Portion: Conveys air from outside the body to the lungs (nose to terminal bronchioles).
  2. Respiratory Portion: Exchange of gases between the air and blood (respiratory bronchioles to alveoli).
  3. Mechanism of Ventilation: Controlled by inspiratory and expiratory movements of the thoracic cage.

Functions of Respiratory System

The respiratory system has several key functions:

  1. Conducting: Transports air.
  2. Conditioning: Warms, moistens, and removes particulate materials from the air.
  3. Gas Exchange: Facilitates the exchange of oxygen and carbon dioxide.
  4. Sense of Smell: Contains olfactory receptors.
  5. Immune Defense

Conditioning of Air

  • Warming
  • Moistening
  • Removal of Particulate Materials: Achieved by mucous and serous secretions and the sweeping movement of cilia (functions of goblet cells and mucus-secreting glands).

Respiratory Epithelium

The respiratory epithelium is composed of five cell types:

  1. Ciliated Cells: 30\% of the total cell population.

    • Columnar cells with cilia that beat in one direction (towards the oropharynx).
    • Loss of cilia activity results in respiratory infections.

  2. Mucous Goblet Cells: 30\% of the total cell population.

    • Secrete mucus that traps particulate matter.
    • Sparse microvilli.

  3. Basal Cells: 30\% of the total cell population.

    • Located at the base of the epithelium and do not reach the lumen.
    • Stem cells for the other cell types.

  4. Brush Cells

    • Columnar cells with short, blunt microvilli on the apical surface.
    • Sensory receptor cells.

  5. Small Granule Cells

    • Resemble basal cells but contain secretory granules.
    • Neuroendocrine function.

The respiratory epithelium is often described as "Pseudostratified ciliated columnar epithelium with goblet cells."

Mucociliary Clearance

  • Secretions from seromucous glands and epithelial goblet cells trap small particles in the inspired air within a thin layer of surface mucous.
  • The fluid contains mucin, water, ions, and antimicrobial substances (IgA, lysozyme).
  • This mucous layer is propelled towards the pharynx by the coordinated movement of the cilia.
  • In the pharynx, most of the mucus is swallowed, and gastric acid destroys any trapped bacteria.
  • Normal mucociliary transit time: 12 - 15 minutes.

Nose and Nasal Cavity

The nasal cavity includes three main areas:

  1. Vestibular Area: Opens to the outside at the anterior nares.
  2. Respiratory Area: Includes nearly all of the septum and lateral walls.
  3. Olfactory Area: Located on the superior concha and adjacent septum.

Vestibular Region

  • Epithelium: Keratinized stratified squamous epithelium associated with hairs (skin epithelium).
  • Lamina Propria: Contains hair follicles, sebaceous, and sweat glands.
  • This epithelium is continuous with that of the external nose and transitions to nonkeratinizing squamous cells and then to pseudostratified ciliated columnar epithelium in the rest of the nasal cavities.

Respiratory Region

  • Occupies nearly all of the septum and lateral walls.
  • The surface area of the lateral walls is increased by shelf-like projections supported by bone called conchae.
  • Epithelium: Pseudostratified ciliated columnar epithelium with goblet cells. Cilia beat backwards towards the pharynx.
  • Lamina Propria: Loose fibroelastic connective tissue (FECT).
    1. Contains mixed seromucous glands (tubulo-alveolar).
    2. Contains a rich cavernous venous plexus, which warms the passing air. This plexus’s high vascularity can be used for the introduction of therapeutic drugs into systemic circulation, avoiding degradation in the GI tract. Upon irritation, the plexus can be distended by blood, reducing airflow.

Olfactory Region

  • Epithelium: Pseudostratified ciliated columnar epithelium without goblet cells (Olfactory epithelium).
    • Composed of olfactory cells, supporting cells, and basal cells.
    • Life span = 1 month.
    • Regenerates after damage; the only neuron replaced during postnatal life.

  1. Olfactory Cells

    • Bipolar neurons, spindle-shaped, with round nuclei.
    • Nuclei are in the middle and deep zone of the epithelium.
    • The apical part extends to the surface with olfactory cilia (non-motile).
    • The basal part forms a long thin axonal process, constituting the olfactory nerve fiber, which extends into the lamina propria and traverses the cribriform plate.
    • Stimulated by gaseous, odoriferous substances dissolved in the secretion of serous glands.

  2. Supporting Cells

    • Nuclei lie close to the lumen.
    • Narrow base, bulky cytoplasm near the lumen.

  3. Basal Cells

    • Progenitor cells differentiate into supporting cells.
    • Dark ovoid nuclei lie close to the basement membrane.
    • Contain few organelles and do not contact the lumen.
  • Bowman's Glands
    • Branched, tubulo-alveolar (mainly serous) glands supply solvents to trap odoriferous substances.
    • Continuous replacement of secretion keeps the receptors ready for new stimuli.
  • Lamina Propria
    • The venous plexus in the lamina propria is continuous with that of the respiratory regions.

Nasopharynx

  • From posterior nares to soft palate.
  • Epithelium: Respiratory epithelium (RE) - pseudostratified ciliated columnar epithelium with goblet cells.
  • Lamina Propria: Contains the nasopharyngeal tonsil and seromucous glands.

Oropharynx

  • From soft palate to epiglottis/hyoid bone.
  • Passage for air, food, and drink.
  • Epithelium: Nonkeratinized stratified squamous epithelium.
  • Lamina Propria: Elastic fibers; palatine and lingual tonsils.

Larynx

  • Respiratory conducting part.
  • Functions in phonation.

Epiglottis

The epiglottis has two surfaces with different types of epithelium:

  1. Lingual Surface (tip of epiglottis): Nonkeratinized stratified squamous epithelium.
  2. Laryngeal Surface: Pseudostratified columnar ciliated epithelium with goblet cells.
  • Lamina Propria: Contains seromucous glands.
  • Cartilage: Elastic cartilage.

Laryngeal Cavity

Consists of two pairs of folds:

  1. False Vocal Cords (Vestibular Folds)

    • Covered by respiratory epithelium.
    • Seromucous glands in the lamina propria.

  2. True Vocal Cords (Vocal Folds)

    • Covered by nonkeratinized stratified squamous epithelium.
    • Contain the vocal ligament and vocalis muscle.
  • The rest of the larynx is covered with pseudostratified ciliated columnar epithelium with goblet cells (RE).
  • Chronic coughing, smoking, or bronchitis can affect the type of epithelium present.

Trachea

Layers

  1. Mucosa
    • Epithelium: Respiratory type resting upon a prominent basal lamina.
    • Lamina Propria: A thin layer of loose connective tissue rich in elastic fibers.
  2. Submucosa: A deeper layer with tubulo-acinar, mixed seromucous glands.
  3. Cartilages: 15-20 C-shaped hyaline cartilage rings; the posterior end is bridged by a fibroelastic membrane and smooth muscle (trachealis muscle).
  4. Adventitia: A loose irregular connective tissue layer surrounding the trachea.

Bronchus

Layers

  1. Mucosa
    • Epithelium: Respiratory epithelium with goblet cells.
    • Lamina Propria: Smooth muscle layer (muscularis mucosae) and elastic fibers.
  2. Submucosa: Contains seromucous glands.
  3. Cartilage: Discontinuous plates of hyaline cartilage around the circumference of the wall.
  4. Adventitia

Types

  • Extrapulmonary Bronchus (Primary Bronchus): Outside the lungs; similar to the trachea but has a smaller diameter and thinner wall; cartilage is an irregular plate of hyaline cartilage around the wall's circumference.
  • Intrapulmonary Bronchus (Secondary and Tertiary Bronchi): In the lungs.

Bronchiole

  • Diameter: 5 mm or less.
  • No glands, no cartilage.
  • Relatively thick layer of smooth muscle.

Structure

  1. Mucosa
    • Epithelium: Gradually changing respiratory epithelium.
      • Reduced epithelium layer (pseudostratified → simple).
      • Epithelium decreases in height (ciliated columnar → simple cuboid in terminal bronchiole).
      • Goblet cells decrease in number and are replaced by Clara cells in the terminal bronchioles.
    • Lamina Propria: Contains elastic fibers.
  2. Submucosa: Contains bands of smooth muscle (major component of the wall) controlled by:
    • Vagus nerve (decreases diameter).
    • Sympathetic nerve (increases diameter).
  3. Adventitia: Network of blood vessels.

Clara Cell (Club Cell)

  • Dome-shaped apical surface, no cilia.
  • Basal nucleus.
  • Abundant rough endoplasmic reticulum (rER) and mitochondria.
  • Dense granules at the apical surface (secretory granules).

Functions:

  • Neutralizing toxins, antibacterial, antioxidant.
  • Producing surfactant proteins coating the surface of the bronchiolar epithelium, preventing airway collapse.
  • Proliferating to regenerate the bronchiolar epithelium and even migrating to replenish alveolar epithelial cells (