Lecture 10: Histology of the Respiratory Tree

• The histology of the respiratory tree includes both conducting and respiratory components of the respiratory system.
• It is important to describe the tissue elements and their functions specific to the components of the respiratory system.
• The specific structural organization of each bronchial tree element is compared
• Also the histological make-up and functional significance of the alveoli and the interalveolar septum.
• Clinical examples can be used to elaborate on the function of the conducting and respiratory branches of the respiratory system.

Respiratory System Overview

• The respiratory system has a conducting portion, and a respiratory portion.
• The conducting portion includes the nasal cavities, nasopharynx and oropharynx, larynx, trachea, bronchi, and bronchioles that end at the terminal bronchioles.
• The respiratory portion includes the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Nasal Cavity

• The nasal cavity consists of the vestibule, respiratory segment, and olfactory segment.
• The vestibule has stratified squamous epithelium and numerous sebaceous glands.
• The respiratory segment of nasal cavity warms, moistens, and filters inspired air.
• The lateral walls are the nasal conchae and is divided by the nasal septum
• The respiratory segment of the nasal cavity has ciliated, pseudostratified columnar epithelium.
• Goblet cells synthesize and secrete mucus.
• Ciliated cells have cilia that project into & move the mucus.
• Brush cells have short, blunt microvilli and are sensory receptors.
• Small granule cells contain secretory granules.
• Basal cells are stem cells.
• The olfactory segment is lined with olfactory mucosa.

Olfactory Segment of Nasal Cavity

• The cell types in the olfactory segment include olfactory, supporting, basal, and brush cells.
• Olfactory cells are bipolar neurons and their cilia project from the apical surface where they synapse on olfactory bulb.
• Supporting cells serve as mechanical and metabolic support.
• Basal cells are stem cells.
• Brush cells are for transduction of general sensation.

Trachea

• The trachea extends from the larynx to the middle of the thorax, and ends once it divides into two main bronchi.
• The tracheal wall consists of mucosa, submucosa, cartilaginous layer, muscular layer, and adventitia.
• The mucosa has ciliated, pseudostratified epithelium.
• A prominent basal lamina/membrane exists between the mucosa and submucosa.
• The submucosa is slightly denser connective tissue.
• The cartilaginous layer contains C-shaped hyaline cartilage.
• The muscular layer contains the trachealis muscle which posteriorly completes the tracheal "ring".
• The adventitia is connective tissue.

Tracheal Epithelium

• Ciliated cells extend through the full thickness of the epithelium and remove mucous and small inhaled particles.
• Mucous (goblet) cells are interspersed among ciliated cells.
• Brush cells are columnar cells with microvilli and act as sensory receptors.
• Small granule cells are a type of hormone-secreting cell.
• Basal cells are stem cells that differentiate into cells of the tracheal epithelium.

Bronchi

• The trachea divides into two main (primary) bronchi.
• Main bronchi divide into lobar (secondary) bronchi.
• Lobar bronchi divide into segmental (tertiary) bronchi.
• Segmental bronchi serve a region of lung parenchyma called the bronchopulmonary segment.
• Bronchopulmonary segments are subdivided into pulmonary lobules which are supplied by bronchioles.
• Bronchioles eventually terminate as terminal bronchioles and respiratory bronchioles.
• The bronchi are identified by cartilage plates and a circular layer of smooth muscle, and its plates are discontinuous and decrease in size distally.
• The wall of the bronchus has 5 layers: mucosa, muscularis, submucosa, cartilage, and adventitia.
• The mucosa is pseudostratified epithelium.
• The muscularis is a continuous layer of smooth muscle that regulates appropriate diameter of the airway.
• The submucosa contains loose connective tissue and numerous glands.
• The cartilage has discontinuous cartilage plates.
• The adventitia is dense connective tissue.

Bronchioles

• Bronchioles supply pulmonary lobules of the bronchopulmonary segments.
• Connective tissue septa divide the lobules.
• Pulmonary lobules are subdivided into pulmonary acini which include the terminal and respiratory bronchiole, and alveoli.
• Cartilage plates and glands are absent from bronchioles.
• Smooth muscle is present and functions for constriction/dilation of the passageways.
• Large bronchioles are initially ciliated, pseudostratified columnar epithelium which becomes simple columnar.
• Small bronchioles contain simple columnar to cuboidal epithelium, and consist of terminal and respiratory bronchioles.
• The small bronchioles possess ciliated epi and Club cells (CC).
• Club cells prevent luminal adhesion if the airway collapses.

Alveoli

• Located at the site of respiratory gas exchange and at the terminal air spaces of the respiratory system.
• Each alveolus is surrounded by a network of capillaries & elastic fibers.
• Each alveolus encompasses alveolar ducts are elongation of the respiratory bronchiole, and alveolar sac which is found at the termination of the alveolar duct.
• The alveolar sac has spaces surrounded by cluster of alveoli and is separated by interalveolar septum & connected by alveolar pores (of Kohn).

Respiratory Portion of the Bronchial Tree

• A photomicrograph shows terminal bronchiole (TB) branching into two respiratory bronchioles (RB).
• The terminal bronchiole does not engage is in respiratory exchange, however the respiratory bronchiole does
• The respiratory bronchiole gives rise alveolar ducts with alveolar sacs at its termination.
• A thin connective tissue layer containing capillaries separates alveoli from one another.

Alveolar Epithelium

• The alveolar epithelium encompasses Type I and II alveolar cells and some brush cells & macrophages.
• Type I cells are thin squamous cells lining most (95%) of the alveolar surface joined to one another and other cells by occluding junctions.
• Type II cells are secretory and cuboidal, cover 5% of the alveolar surface at junctions and secrete phospholipids and proteins to form surfactant.
• Progenitor cells can restore both types of cells.

Surfactant

• Surfactant is a phospholipid produced by Type II alveolar cells that decreases alveolar surface tension at the air-epithelium interface and participates in clearance of foreign materials.
• Surfactant proteins include: protein A that is most abundant and responsible for surfactant homeostasis, protein B for adsorption and spreading of surfactant, protein C for maintenance of thin film layer within the alveoli and protein D as host immunological defenses.

Alveolar Septum

• The site of the air-blood barrier facilitates the diffusion of gases.
• The air-blood barrier has a thin component of surfactant, a type I alveolar cell, and an endothelial cell that shares a fused basal lamina for gas exchange and a thick component that has connective tissue cells and fibers between basal laminae for fluid distribution.

Emphysema

• Emphysema is characterized by permanent enlargement of the air spaces distal to the terminal bronchiole as a result of chronic obstruction of airflow, like narrowing of the bronchioles or destruction of alveolar wall.
• The area of gas exchange is lost in this disease and is caused by chronic inhalation of foreign particles.