There is hope for older people losing their sense of smell, as stem cells can now be used to bring it back.
Up to one in 20 people in Britain have either lost or are losing their sense of smell, often in old age or following viral infections.
According to the Hearing Loss Association of America, approximately 48 million Americans – or one in five people – report some degree of hearing loss.
This can harm quality of life, as smell is linked to taste, and it can leave food flavourless, which is why many elderly people have a lack of appetite and risk malnutrition.
However scientists seeking to reverse a permanently damaged sense of smell have now been successful in mice.
The solution came from a nasal spray given to the animals and containing stem cells which are capable of becoming any cell in the body.
In the nasal cavity, they become nerve cells which pick up smells coming through the nostrils, transmitting the signal to the brain which identifies the aroma as nice or nasty.
When mice with no sense of smell were given the stem cells, then days later exposed to a chemical smelling like the urine of a larger predatory animal, they tried to get away from it, showing their sense of smell had come back.
Researchers, who confirmed the mice could smell again by recording their nasal cell activity, hope the same treatment might work for people in the future using a small tube to dispense the stem cells.
Dr Bradley Goldstein, who led the study from the University of Miami, said: ‘This is the first model of smell loss showing evidence of recovery using a cell-based therapy.
‘It is very important to understand that many questions would need to be worked out before considering this in a human patient. However, it does provide evidence that such an approach warrants further study.’
It has previously been possible to restore a sense of smell in mice using gene therapy.
This switches on a gene important for sniffing out odours which is lost, in mice and in humans, as a result of certain diseases.
But stem cells are hoped to help older people who have lost their sense of smell, by replacing the nerve cells experts believe disappear as we age and which are vital to detect odours.
To test the stem cell solution, the researchers knocked out mice’s sense of smell using the breast cancer drug tamoxifen.
This turns off an important gene, getting rid of small hairs in the mice’s noses called ‘cilia’ which they need to pick up smells.
They gave four mice a nasal spray containing stem cells, which was infused under an anaesthetic over 20 to 30 minutes. Another five mice got a dummy nasal spray which contained no cells.
Three of the mice given the stem cells first indicated they had regained their sense of smell when presented with a chemical similar to the urine of an animal which would eat them, such as a fox, and backing away in fear.
However it was very clear the stem cells had worked when clusters of nerve cells showed up in the noses of the mice in the study, containing newly grown ‘cilia’ hairs for smelling, and growing long strands connecting to the brain to pass on smell signals.
The study also offers hope to people who have lost their sense of smell following illness, head injuries and genetic disorders.
However the authors caution that the same method, while shown to be safe and not to cause cancer, may not work in connecting the human nose and brain.
Dr Goldstein said: ‘We were a bit surprised to find that cells could take hold fairly robustly with a simple nose drop delivery. To be potentially useful in humans, the main hurdle would be to identify a source of cells capable of doing this, becoming the right neurons, and properly connecting to the brain.
‘Regardless, continuing to study the mechanisms in human cells that are required for production of functional olfactory neurons from progenitor cells will be broadly important for moving forward.’
The full findings of the study were published in the journal Stem Cell Reports.