Olfactory epithelium

Epithelium olfactorium; regio olfactoria

Also known as: olfactory mucosa, olfactory region, Bowman glands, olfactory glands, sustentacular cells, basal cells

A specialised, yellowish-brown pseudostratified epithelium in the roof of the nasal cavity containing olfactory receptor neurons, supporting (sustentacular) cells and basal stem cells, with mucus-secreting olfactory (Bowman) glands beneath.

Overview

Area≈2.5–5 cm² per side

Receptor neurons≈6 million per side (estimates vary)

Neuron lifespan≈30–60 days

Cilia per olfactory knob≈10–30

The olfactory region occupies the roof of the nasal cavity beneath the cribriform plate of the ethmoid, extending onto the upper nasal septum and the superior nasal concha (and a variable part of the middle concha) — roughly 2.5–5 cm² on each side, patchy and interspersed with respiratory epithelium in adults. It is thicker (≈60–100 µm) than respiratory epithelium and lacks goblet cells. Only a small fraction of inspired air reaches it; sniffing creates eddies that carry odorants upward, and odorants also arrive retronasally from the mouth during eating — the main contributor to "flavour".

The epithelium contains four cell types: (1) olfactory receptor neurons (several million per side) — bipolar neurons whose dendrites end in an olfactory knob bearing 10–30 long non-motile cilia that lie in the mucus and carry the odorant receptors; (2) sustentacular (supporting) cells — tall columnar cells with apical microvilli that insulate neurons, regulate mucus composition and detoxify odorants and xenobiotics (cytochrome P450); (3) globose basal cells — proliferating progenitors that continually replace neurons; and (4) horizontal basal cells — quiescent reserve stem cells activated after injury. Microvillar cells are also present.

In the lamina propria, olfactory (Bowman) glands secrete a watery, serous mucus containing odorant-binding proteins, lysozyme and IgA; this layer dissolves odorants and flushes them away so the receptors are continually refreshed. The lamina propria also contains bundles of unmyelinated olfactory axons ensheathed by olfactory ensheathing cells, which pass through the cribriform plate as ≈20 olfactory fila per side — together forming the olfactory nerve (CN I).

Location

Roof of the nasal cavity under the cribriform plate, upper nasal septum and superior nasal concha.

Parts & subdivisions

Olfactory receptor neurons

Bipolar primary sensory neurons with cilia; axons form the olfactory nerve.

Sustentacular (supporting) cells

Columnar cells with microvilli; support, insulate and detoxify; express ACE2/TMPRSS2 — the target of SARS-CoV-2.

Basal cells

Globose (proliferating progenitors) and horizontal (reserve stem) basal cells; regenerate neurons every ≈30–60 days.

Olfactory (Bowman) glands

Tubuloalveolar serous glands in the lamina propria; secrete the mucus that dissolves odorants; contain odorant-binding proteins.

Olfactory fila

≈20 bundles of unmyelinated axons per side passing through the foramina of the cribriform plate.

Anatomy

Innervation

Special sense: olfactory nerve (CN I). General sensation (touch, pain, irritants such as ammonia and menthol): ophthalmic (V1, anterior ethmoidal nerve) and maxillary (V2) branches of the trigeminal nerve. Secretomotor to Bowman glands: facial nerve parasympathetics via the pterygopalatine ganglion.

Arterial supply

Anterior and posterior ethmoidal arteries (ophthalmic artery) and sphenopalatine artery branches (maxillary artery).

Histology

Pseudostratified columnar epithelium without goblet cells: nuclei of sustentacular cells superficially, receptor neurons in the middle, basal cells deepest. Lamina propria with Bowman glands, axon bundles and venous plexus.

Development

From the nasal (olfactory) placodes of surface ectoderm in week 5; the placodes also give rise to GnRH neurons that migrate into the hypothalamus along olfactory/terminal nerve fibres.

Clinical significance

Post-viral and COVID-19 anosmia

Upper respiratory infections can damage the olfactory epithelium. In COVID-19, sudden anosmia was common because SARS-CoV-2 infects ACE2-expressing sustentacular cells (not the neurons themselves), causing secondary neuronal dysfunction; most recover within weeks to months, aided by olfactory training, but some have persistent loss or parosmia (distorted smell) during regeneration.

Conductive anosmia

Nasal polyps, chronic rhinosinusitis, septal deviation or swelling prevent odorants reaching the olfactory cleft; often improves with corticosteroids or surgery.

Olfactory neuroblastoma (esthesioneuroblastoma)

Rare malignant tumour arising from olfactory epithelium basal cells, presenting with unilateral nasal obstruction, epistaxis and anosmia; may invade through the cribriform plate.