Histology of the Lower Respiratory Tract
Air is introduced to the lungs through an interconnected pathway with constantly changing epithelial linings. While the upper respiratory tract acts solely as a conducting portion, the lower respiratory tract serves as both conducting and respiratory portions of the airway. This article continues the review of the epithelial mucosa and supporting cells of the lower respiratory tract (from the larynx to the alveoli of the lungs).
Pseudostratified columnar ciliated epithelium with seromucous glands in its lamina propria (laryngeal mucosa) continues in to the larynx and covers the false vocal fold and the end of the laryngeal ventricle (a depression between the false vocal fold and the true vocal cord). Goblet cells, seromucous glands, lymphatic nodules and adipocytes are observed throughout the lamina propria to the end of the ventricle. The cilia assists in retaining the mucous lining, which helps to reduce damage caused during phonation. The large concentration of lymphatic nodules in the ventricle has led to it being referred to as the laryngeal tonsils.
Passing the lower border of the ventricle, the epithelium changes again to non-keratinized stratified squamous epithelium that covers the true vocal cords. At this point, the lamina propria is avascular, thin, and lacks glands and lymphatic tissue. The additional cell layers replace those lost during the closed phase of vibration. Dense elastic fibers of the vocalis ligament project into the lamina propria and attaches to the vocalis muscle (skeletal muscle). The epithelium again changes to pseudostratified ciliated columnar epithelium with the cricoid cartilage (hyaline) forming the lower border between the larynx and the trachea.
Trachea and Bronchi
The trachea is attached to the cricoid cartilage of the larynx by the cricothyroid membrane and lies anterior and adjacent to the esophagus. It is a mucocartilagenous tube that is completed posteriorly by smooth trachealis muscle. The hyaline cartilage rings prevents the airways from collapsing during inspiration. Along the convexity of the C-shaped rings, the adventitia contains numerous adipocytes, blood vessels and nerves, and blends with the perichondrium of the hyaline cartilage. In the concavity of the cartilaginous rings, the submucosa has numerous seromucous glands, blood vessels, loose connective tissue fibers and scattered lymphatic tissue. The lumen of the trachea has numerous mucosal folds along the posterior wall, where there is no hyaline cartilage. Throughout its length, the lumen is lined by pseudostratified ciliated columnar epithelium with goblet cells (respiratory epithelium).
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.
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.
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.
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 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.
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. 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.
An oedematous larynx can result in obstruction of the airway in young children. The inflammation may be associated with stridor upon inspiration, coughing and hoarseness. This is a relatively common pathology known as croup.
Non neoplastic polyps may appear on the vocal cords in response to inflammation or trauma. This is common in all age groups and affects those who sing or preach loudly (see Glottis). Both viruses and bacteria are notable agents responsible for inflammatory pathogenesis of the lower airway. Symptoms may include coughing, feelings of tightness in the chest, wheezing, shortness of breath and cyanosis. Situations are direr when the patients are infants and children. Notable diseases of the lower respiratory tract include asthma, chronic bronchitis, emphysema, pneumonia, and tuberculosis; all of which, if left untreated, could lead to respiratory failure and death.