Histology of the upper respiratory tract

Nasal vestibule

Entering the nares, or nostrils, 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 a point known as the limen nasi.

Floor and walls of nasal cavity

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.

Roof of nasal cavity

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 (composed of the perpendicular plate of the ethmoid bone) are covered with pseudostratified columnar epithelium without goblet cells and motile ciliae (olfactory epithelium).

It should also be noted that the olfactory portion of the nasal cavity is less vascularized than the lower respiratory portion.

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.

Cell types

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).

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.
• 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.

The mucosa of the paranasal 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 paranasal sinuses are typically devoid of lymphoid tissue.

Pharynx

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).

Epiglottis

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 the lingual surface of the epiglottis (as well as half of the laryngeal surface) is continuous with that of the laryngopharynx. It is lined with stratified squamous non keratinized epithelium (lingual mucosa); the other half of its laryngeal surface is lined by pseudostratified ciliated columnar epithelium. Deep within the lamina propria of the mucosa are seromucous glands.

Elastic cartilage in the center of the epiglottis provides scaffolding for the overlying mucosa. Both surfaces of the epiglottis are equipped with diffuse lymphoid tissue and taste buds.