Histology of the lower respiratory tract

Primary bronchi

After about 10 – 15 cm, the trachea bifurcates at the carina to form a left and a right primary bronchus. The bronchi are also kept patent by C-shaped rings of hyaline cartilage and their lumens are also lined with respiratory epithelium.

Secondary bronchi

The primary bronchi enter the lungs and further divide into secondary (intrapulmonary) bronchi. These smaller bronchi are kept open by plates of hyaline cartilage, instead of the C-shaped rings observed in the trachea and primary bronchi. The lining of respiratory epithelium continues from the primary bronchi into the lumen of the secondary intrapulmonary bronchi.

The lamina propria is circumscribed by a thin layer of smooth muscle that also separates the former from the submucosal layer (containing seromucous glands). The presence of the muscular layer causes significant mucosal folding along the lumen of the intrapulmonary bronchi.

Tertiary bronchi

Each secondary bronchus then divides into a tertiary bronchus, with a smaller lumen. The hyaline plates are still present, but are smaller and further apart than those around the secondary bronchi. The mucosa of the lumen also demonstrates folding due to the smooth muscles in the wall of the bronchus.

Seromucous glands and smaller arteries and veins can also be observed in the submucosa and surrounding connective tissue, respectively. At this level, alveoli are now visible around the bronchi, but they do not communicate with them.

Terminal bronchioles

The tertiary bronchi continue to divide into smaller tubular structures called terminal bronchioles. The epithelial lining of the lumen of the bronchioles differ from that of the bronchi. Simple columnar ciliated cells now line the numerous mucosal folds.

The sub-epithelial glands and hyaline cartilaginous plates are absent in this region of the airway. Structural support is instead provided by fibrous connective tissue along with more prominent smooth muscle layers.

Also present in the terminal bronchioles are exocrine bronchiolar cells or club cells (formerly known as Clara cells). These non-ciliated cuboidal cells contribute to the production of surfactant (reduces tension in the narrow lumen of the respiratory portion of the airway), detoxifies air and may differentiate into bronchial epithelial cells to replace older cells.

Respiratory bronchioles

Terminal bronchioles then branch to form respiratory bronchioles. This marks the transition point from the conducting to the respiratory portion of the respiratory system. The epithelium here is simple cuboidal that may be ciliated proximally, but devoid of cilia distally. The smooth muscle layer is thinner here than in the conducting portion.

Alveoli

A marked feature of the respiratory portion of the airway is gaseous exchange. The thin walled respiratory bronchioles branch into alveolar ducts with alveolar sacs at the end; whose simple squamous alveolar cells (type I pneumocytes) provides an ideal environment for gases to pass to and from adjacent capillary beds.

Type II pneumocytes are larger granular cuboidal cells that are found along points of alveolar intersection. They (along with clara cells) are responsible for the production of surfactant. Type II pneumocytes also act as progenitor cells for both types of pneumocytes. There are also dust cells (alveolar macrophages) of monocytic origin that carry out a phagocytic role.