Background information
Wintertime: ‘tis the season for runny noses and RSV
by Tanja Restin
Skin, gut, heart: which parts of your body have a sense of smell?
6/12/2023
Translation: Jessica Johnson-Ferguson
Sperm like the scent of lily of the valley. If there’s a smell of sandalwood in the air, you can stop hair falling out and help wounds heal faster. Today’s research has revealed a lot about our organs’ favourite smells. So what do olfactory receptors actually do in a healthy body, and how can they help diagnose and treat diseases?
Your nose is equipped with olfactory receptors. Kind of obvious, right, or you wouldn’t have a sense of smell. For a long time, scientists assumed that this was the only place where this type of sensor could be found. After all, the nose is responsible for smelling. So why would other organs need olfactory receptors?
Turns out, matters are quite different. Every single cell in our body has olfactory receptors, be it in the gut, the heart or the lungs. They’re simply everywhere. And yet, for the most of them, we don’t know which scents they react to or what purpose they even serve.
How does the nose work?
But let’s start from the top. How does smelling work in the first place? When a scent reaches your nose, the scent molecules dock onto the receptors in the olfactory sensory cells. These are located in the olfactory mucosa at the nasal septum. The signal is then transmitted to the limbic system of the brain. That’s the area responsible for your emotions. This is the reason why odours are often strongly associated with feelings and remain in your memory for a long time.
Your nose is always busy. Even when you’re sleeping, it transmits information to the brain with every breath you take. And this is something that starts even before you're born. The nose is already active in the womb and processes everything the expectant mother smells or eats.
The 30 million olfactory cells in your nose renew themselves every four weeks. Stem cells ensure that this process lasts a lifetime.
Why are there olfactory receptors outside the nose?
Natural fragrances as well as perfumes are always a composition of different scent molecules. In total, there are around 350 different olfactory receptors, each of which reacts to very specific scent molecules. For example, one type of receptor will react to vanillin, another to musk. The various receptor types cover the entire world of fragrances.
When a scent molecule docks onto the corresponding receptor, the signal is first amplified before it passes the information on to the brain. In the brain, the individual scent information (i.e. a combination of receptors that's emitted a signal) is reassembled, and the specific combination recognised and stored as a scent.
So far, however, which receptor reacts to which scent is only known in about 10 per cent of cases. That’s because researching this is quite challenging. Why? Because the receptor type needs to be isolated first and then reproduced in the laboratory. Once that’s been achieved, the right odour needs to be found to activate it. It’s laborious, time-consuming and therefore expensive research work. Known receptors include those for vanillin, violet, lemon, banana and musk.
Bile, lungs, liver: smell receptors throughout the body
There are olfactory receptors in all cells of our body. From the liver and lungs to the kidneys, gall bladder, stomach and intestines and even in the heart – they’re absolutely everywhere. Each tissue always has its own, specific receptor pattern – a combination of different olfactory receptor types. Which is not to say that these organs can actually smell. This is because the information isn’t transmitted to the brain as an olfactory signal. And yet, the body reacts to these odour molecules.
What do the olfactory receptors do?
They’re important for regular metabolic processes in the body. Take the gut, for example. It has over 20 types of olfactory receptors that analyse the food you eat. It's known that some receptors react specifically to spices such as caraway or cloves by triggering the release of serotonin and thereby increasing intestinal activity. When you drink bitters, for example, you activate your digestion via these receptors in the intestine.
The olfactory receptors in the lungs in turn analyse the air that you breathe. Let’s say the muscles around the bronchi contract due to an allergic reaction and the lungs can no longer fully expand. This leads to shortness of breath. A specific type of receptor (OR2AG1) that reacts to a substance called amyl butyrate could prove useful in this scenario. If you breathe in this apricot-like fragrance, your muscles relax and your bronchial tubes expand again.
Sperm like the scent of lily of the valley
The first olfactory receptors discovered outside of the nose were found in sperm cells. In line with this, there are 15 scents in the female genital tract that can dock onto sperm receptors. They control the swimming speed and direction of the sperm so that they find their way to the egg.
Wounds, skin and hair like to «sniff» sandalwood
There are also olfactory receptors in your skin that are activated by certain aromas. In fact, the skin has over 20 different receptor types. The function and activating fragrance is not yet known for some of them. To avoid confusing or overwhelming these receptors, you should therefore never spray perfume directly on your skin, but rather on your clothes or hair.
What is already known, however, is that one type of receptor reacts to sandalwood by causing the skin cells to divide and move more quickly. This makes sandalwood a substance that supports wound healing. But it also has another function: sandalwood prolongs hair growth. In other words, hair doesn’t die off as quickly and premature hair loss is prevented.
Diagnosing and fighting cancer via olfactory receptors
Olfactory receptors play an important role. Not only in healthy organs, but also in diseased tissues. There may be a change in the number of receptors or the appearance of other receptor types when cells become diseased. In many cases, the underlying mechanisms are not (yet) known. But what is known so far is promising in that, one day, we might be able to use olfactory receptors for diagnosis and treatment.
For example, it has already been shown that leukaemia cells differentiate into red blood cells through the stimulation of certain olfactory receptors. Or that they can be used to inhibit the growth of lung carcinomas, prostate or colon cancer cells.
A specific type of receptor can also be used to diagnose bladder cancer, as these specific olfactory receptors shouldn’t be present in a healthy body. They are, however, found extremely frequently in bladder cancer cells. In this case, a high concentration of this type of receptor can be detected in the urine and can be used to diagnose bladder cancer.
Wide field of research
Even though research on the subject is gathering pace, there are still many questions that haven’t yet been answered. We still don’t know which scent molecules the majority of olfactory receptors react to. However, if we succeed in recreating the receptors in the laboratory and identifying the appropriate scent, this will open up new possibilities for the diagnosis and treatment of diseases in the future.
Sources: The information for this article is based on the review article «Human Olfactory Receptors: Novel Cellular Functions Outside of the Nose» and a highly recommended lecture (in German) by Prof. Hatt, who holds the Chair of Cell Physiology at the Ruhr University Bochum
Header image: Cottonbro/pexels
Science editor and biologist. I love animals and am fascinated by plants, their abilities and everything you can do with them. That's why my favourite place is always outside - somewhere in nature, preferably in my wild garden.
