Anatomy of the Olfactory Epithelium

Assists in the Sense of Smell

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The olfactory epithelium is membranous tissue located inside the nasal cavity. It measures about 3 centimeters (cm) square in adults. Containing olfactory receptor cells, it is involved in smell.

Olfactory disorders can range from a mild decrease in smell to a complete loss of smell. A loss of smell can indicate a mild illness or injury or something more serious. A decrease in the ability to smell can be an early indicator of Parkinson’s disease and Alzheimer’s disease.

Olfactory dysfunction is also associated with some psychiatric conditions, such as schizophrenia, mood and anxiety disorders. Also, loss of smell is a key symptom of COVID-19.

Profile of person with short dark hair and brown skin, smelling a melon

JGI/Jamie Grill / Getty Images

Anatomy

The olfactory epithelium lies on the roof of the nasal cavity. In adults, it is situated about 7 cm behind the nostrils. It is part of the nasal septum and the superior turbinate bones.

Three cell types make up the olfactory epithelium: basal, supporting, and olfactory. Olfactory receptor cells have hair-like extensions called cilia.

It used to be thought that the olfactory epithelium developed singularly from the olfactory placode. More recent embryonic origin studies, however, have found that it also develops from neural crest cells.

Anatomical Variations

The nasal epithelium can be affected by congenital (present at birth) conditions. Kallmann syndrome is a genetic disorder in which the hypothalamus and the olfactory neurons do not fully develop. This can result in anosmia (the inability to smell).

Normosmic idiopathic hypogonadotropic hypogonadism (nIHH) involves cases where only the hormone deficiency is present. In those instances, a person’s sense of smell remains. The main symptoms of Kallmann syndrome are delayed puberty and an impaired sense of smell.

Ciliopathies are another genetic disorder that can impair smell. In ciliopathy, the formation of cilia is impaired. When cilia are absent or malformed in the olfactory neurons, odor detection can not occur. 

Function

The olfactory epithelium is part of the olfactory sensory system, whose role is to pass along smell sensations to the brain. It does this by trapping odors that pass across the cilia then sending the information about those odors to the olfactory bulb.  

The olfactory bulb is located in the front of the brain. After the olfactory bulb receives information from the cells in the nasal cavity, it processes the information and passes it to other parts of the brain. 

Associated Conditions


The olfactory epithelium can be damaged and lead to the loss of smell. Damage is most often caused by toxic fumes, physical trauma, blockage in the nasal passage, tumors in the brain, allergies, or infections. Olfactory disorders can be temporary, but in some cases, they are permanent.

The loss of smell is a spectrum, ranging from a distortion (dysomia) to diminished (hyposmia) to the complete loss of smell (anosmia). Loss of smell is not uncommon in those with traumatic brain injury. One study found that 15–35% of those with traumatic brain injury sustained loss of smell.

Since smell and taste are so closely linked, the loss of smell often contributes to a loss of taste. There are some risks to not being able to smell, such as not being able to smell something burning. Lack of smell can contribute to mood disorders like anxiety and depression.

Presbyosmia is an age-related loss of smell. According to a study, up to 39% of those over 80 have olfactory dysfunction. Presbyosmia occurs gradually and is not preventable. It may be related to a loss of nerve endings and mucus produced as people age. Medication use and neurological disorders may also contribute to age-related loss of smell.

Cancer of the nasal cavity is rare. Squamous cell cancer is the most common cause of nasal and paranasal cancers, followed by adenocarcinoma.

Nasal cancer symptoms may include a runny nose, congestion, and a sensation of fullness or tenderness in the nose. Later stages may involve nose bleeds, facial and tooth pain, and eye problems.

Early symptoms of some neurodegenerative disorders, like Parkinson’s disease and Alzheimer’s disease, include a decreased ability to smell. Some psychiatric illnesses, like schizophrenia, mood disorders, and anxiety disorders, are also associated with olfactory dysfunction. Loss of smell is one of the key symptoms of COVID-19. 

Tests

Testing for Kallmann syndrome may include blood tests to check hormone levels. Magnetic resonance imaging (MRI) of the head and nose may be done to check for anatomical abnormalities of the nose, hypothalamus, and pituitary gland. Molecular genetic testing may also be done to identify gene mutations.

Treatment for Kallmann syndrome and nIHH usually involves hormone replacement therapy. Medication to strengthen the bones may also be used, as the absence of puberty hormones can weaken them.

New advances in DNA testing may allow for the diagnosis of ciliopathy. Using RNA sequence analysis, RT-PCR of the RNA may be used to diagnose ciliopathy. Ciliopathy is considered incurable; however, more recent advances in gene therapy could mean treatment options may be available in the future.

Your doctor may order certain scans to diagnose loss of smell, like computed tomography (CT scan), MRI, or an X-ray. Nasal endoscopy may be used to look inside of your nose. A test known as the University of Pennsylvania Smell Identification Test may be done to assess the degree to which your smell is affected.

Treatment of anosmia, dysomia, and hyposmia will depend on what is believed to have contributed to the olfactory dysfunction. Your doctor may prescribe decongestants, antihistamines, or nasal sprays for allergies. If an infection is the cause, antibiotics may be in order.

Smoking is known to impair the ability to smell, so quitting smoking may also help. If there is a blockage in the nasal passage, your doctor may discuss surgically removing a polyp or tumor.

Nasal cancer is diagnosed by an ear, nose, and throat physician or otolaryngologist. Your doctor will first need to get a good look at the tumor. They may do this through certain tests, like a nasal endoscopy, CT scan, MRI, X-ray, or positron emission tomography (PET scan).

To definitively determine if the tumor is malignant (cancerous) or benign (noncancerous), your doctor will take a biopsy of the tumor. Treatment options for nasal cancers include surgery, chemotherapy, radiation therapy, targeted therapy, and palliative care

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