Olfactory Epithelium

The olfactory epithelium is a specialized sensory tissue responsible for the detection of odors. It contains olfactory receptor neurons and is located in the roof of the nasal cavity. This epithelium is part of the peripheral olfactory system and initiates the sense of smell by transducing chemical signals into neural impulses.

Location

The olfactory epithelium is located in the upper portion of the nasal cavity, lining the:

• Roof of the nasal cavity
• Upper part of the nasal septum
• Superior nasal concha (part of the ethmoid bone)

This region corresponds to a relatively small surface area, approximately 2–5 cm² in humans.

Structure

The olfactory epithelium is a pseudostratified columnar epithelium composed of five primary cell types:

• Olfactory receptor neurons: Bipolar neurons with cilia that detect odorant molecules. Their axons pass through the cribriform plate to synapse in the olfactory bulb.
• Supporting (sustentacular) cells: Provide metabolic and physical support to olfactory neurons and help detoxify inhaled chemicals.
• Basal cells: Stem cells located at the base of the epithelium; they regenerate olfactory neurons and supporting cells.
• Brush cells: Rarely present, may play a sensory or regulatory role.
• Bowman’s glands: Serous glands in the lamina propria that produce mucus to dissolve odorants and clear the epithelium.

Function

The primary function of the olfactory epithelium is to detect odorant molecules and convert them into neural signals. The process includes:

• Odorant molecules are inhaled and dissolve in mucus covering the epithelium.
• They bind to G-protein coupled receptors on the cilia of olfactory neurons.
• Signal transduction occurs, leading to depolarization of the olfactory neuron.
• Action potentials are sent via axons of olfactory neurons through the cribriform plate to the olfactory bulb.

Olfactory Pathway

Once the signal reaches the olfactory bulb, it is processed and relayed to various brain regions, including:

• Primary olfactory cortex in the temporal lobe
• Orbitofrontal cortex for conscious perception of smell
• Amygdala and hippocampus for emotional and memory-related responses to odor

Histology

Under the microscope, the olfactory epithelium shows:

• A pseudostratified columnar epithelium
• Apical cilia extending from olfactory neurons into the mucus
• Basal cells along the basement membrane
• Lamina propria containing Bowman’s glands and unmyelinated axons

Regeneration

One of the unique features of the olfactory epithelium is its ability to regenerate. Olfactory neurons have a lifespan of 30–60 days and are continuously replaced by basal cell division and differentiation.

Blood Supply

The olfactory epithelium receives its blood supply from branches of the ophthalmic and maxillary arteries:

• Anterior and posterior ethmoidal arteries (branches of the ophthalmic artery)
• Sphenopalatine artery (branch of the maxillary artery)

Nerve Supply

The olfactory epithelium is innervated by:

• Olfactory nerve (CN I): Carries sensory input from receptor neurons to the olfactory bulb
• Trigeminal nerve (CN V1): Provides general sensation to the nasal cavity, including pain from irritants

Clinical Significance

Anosmia

Loss of smell (anosmia) can result from damage to the olfactory epithelium, olfactory nerve, or bulb. Causes include viral infections, trauma (cribriform plate fracture), aging, or neurodegenerative diseases such as Parkinson's and Alzheimer's.

Hyposmia and Dysosmia

Reduced or distorted smell perception can occur due to chronic rhinitis, sinus infections, or chemical exposure damaging the olfactory mucosa.

COVID-19 and Olfactory Loss

Temporary loss of smell is a common early symptom of COVID-19, likely due to viral-induced inflammation and damage to supporting cells in the olfactory epithelium.

Development

The olfactory epithelium develops from the olfactory placode, an ectodermal thickening in the developing nasal region. This placode invaginates to form the olfactory pit and eventually the olfactory mucosa.

Imaging and Assessment

While imaging (MRI) may be used to assess the olfactory bulb and tract, clinical testing of olfaction typically involves identifying a series of standard odors to evaluate olfactory function. Biopsy of the olfactory epithelium may be used in research or in rare diagnostic settings.