Histology of the Upper Respiratory Tract
The respiratory tract is the pathway through which much needed oxygen enters the body. It begins with the nose, passes through the pharynx, the larynx, trachea, bronchi, bronchioles and ends in the alveoli. The airway is divided into two segments: a conducting segment (from the nostrils to the terminal bronchiole) and a respiratory segment (from the respiratory bronchioles to the alveoli). Along the respiratory pathway, the epithelial lining changes to accommodate different functions. This article reviews changes in the epithelia and supporting cells of the upper respiratory tract (from the nasal cavity to the pharynx).
The nose, as the primary mode of entry of air into the airway, has both respiratory and olfactory functionality. In its respiratory capacity, it modifies the air so that gaseous exchange will occur more efficiently in the lungs. While in its olfactory capacity, it detects various odors and transmits those impulses to the brain for interpretation. Entering the nares, the nasal vestibule is lined by keratinized stratified squamous epithelium – a continuation of the cutaneous lining from the external nose. It is also equipped with modified hairs, called vibrissae that filter out larger particles from inspired air. The membrane transitions from keratinized stratified squamous epithelium to pseudostratified columnar ciliated epithelium with goblet cells (also called respiratory epithelium) at the point of the limen nasi. The respiratory epithelium covers the floor, medial and lateral walls (just below the superior concha) of the nasal cavity to the choana (posterior boundary of the nasal cavity). Additionally, there are seromucous glands dispersed throughout the mucous membranes. Their secretions aid in respiration by moistening the inspired air and trapping unwanted particles. The trapped particles are propelled by the cilia to the pharynx where they can be expelled orally, or swallowed and digested.
The roof of the nasal cavity in the region of the cribriform plate of the ethmoid bone, the superior concha and the superior aspect of the nasal septum (the perpendicular plate of the ethmoid bone) are covered with pseudostratified columnar epithelium without goblet cells and motile ciliae (olfactory epithelium). There are several cell types found in the epithelium that make olfaction possible. Air is first directed towards the olfactory epithelium by the turbinates (bones in the conchae that support the mucosa). In the lamina propria, Bowman’s glands (also called olfactory glands) produce serous secretions that dissolve odiferous particles so that they can interact with the olfactory cilia. The olfactory cilia are short hair-like projections that extend into the mucous lining to detect and transmit odors through the olfactory nerve cells. Olfactory nerve cells are bipolar neurons span the thickness of the epithelium. The impulses from the olfactory cilia are transmitted by nerve fibers from the olfactory cells that travel through the cribriform plate of the ethmoid bone. The afferent fibers then enter the cranial cavity and synapse with mitral cells in the olfactory bulb (CN I).
Although it is rather abrupt, the transition from respiratory to olfactory epithelium can be noted grossly by the change in colour (from pink respiratory to yellow olfactory epithelium) and histologically by the change in cell types and morphology. The columnar cells in the olfactory epithelium are generally taller than those in the respiratory epithelium. Sustentacular (supportive) cells distributed throughout the epithelium are interspersed with olfactory nerve cells and basal cells proximal to the cribriform plate of the ethmoid bone. The cell shapes are hard to distinguish; so the position and shape of the nuclei are used to distinguish the cell types. The nuclei of the basal cells are spherical and proximal to the cribriform plate of the ethmoid bone, while the nuclei of sustentacular cells are more elongated and distal to the cribriform plate of the ethmoid bone. The nuclei of the olfactory nerve cells are seen between those of the basal and sustentacular cells. It should also be noted that the respiratory portion of the nasal cavity is more vascularized than its olfactory portion. The mucosa of the nasal sinuses is also respiratory epithelium. The only difference is that the epithelium is thinner and has fewer goblet cells and serous and mucous glands. The nasal sinuses are typically devoid of lymphoid tissue.
Pharynx and Epiglottis
The epithelia of the pharyngeal portion of the conducting zone changes with respect to each pharyngeal segment. In the nasopharynx, the epithelium is continuous with that of the nasal cavity. The ciliae here continues to wharf foreign particles through the pharynx to be swallowed. In the oropharynx and laryngopharynx, the epithelium transitions to non-keratinized stratified squamous epithelium. This durable epithelium is better suited to accommodate friction associated with swallowing food. Additionally, lymphatic aggregates (distributed throughout the mucosa) act as a first contact point for the immune system to sort through particles entering the body (see Waldeyer’s Ring).
The epiglottis is a cartilaginous structure located cranial to the larynx. It projects posteriosuperiorly to separate the pharynx from the larynx and prevents food from entering the lower airway during swallowing. The mucosa of its lingual surface (and half of its laryngeal surface) is continuous with that of the laryngopharynx (lingual mucosa). The other half of its laryngeal surface is lined by pseudostratified ciliated columnar epithelium with seromucous glands distributed throughout its mucous lining (laryngeal mucosa). Elastic cartilage in the center of the epiglottis provides scaffolding for the epithelia. Both surfaces of the epiglottis are equipped with lymphatic nodules and taste buds.
Inflammation of the nasal mucosa is referred to as rhinitis. The inflammatory process may be of viral or allergenic aetiology. Hypersecretion of mucous will manifest as rhinorrhoea (runny nose). In the case of allergic rhinitis, symptomaticly can be chronic or seasonal, depending on the allergens that initiate the inflammatory process. Inflammation restricted to the mucous membrane of the paranasal sinuses is specifically referred to as sinusitis. It results from bacterial proliferation subsequent to obstruction of the ostium that drains the sinus.
The pharynx is also susceptible to inflammation (called pharyngitis). Also associated with pharyngitis, are cases of tonsillitis. These inflammatory processes can be cause by communicable bacterial infections as well as non-infectious entities. However, streptococcus A bacteria are the primary agents implicated in these pathogenesis. Viruses, such as the respiratory syncytial virus, influenza virus and rhinovirus may also cause generalized nasopharyngeal inflammation.
Epiglottitis is a more serious inflammatory condition usually caused by a type B Haemophilus influenzae infection. The infection usually occurs in young children and infants. Consequent to the swelling of the epiglottis, the airway can be obstructed. Loud inspiratory wheezing is a classic symptom of this pathology. Appearance of cyanosis indicates the need for a tracheostomy.