The study, published in the journal Cell, showed that olfactory receptors are not randomly distributed within the lining of the nose as previously thought, but are very precisely organized within narrow and specific ranges, resembling a "sensory map" that helps the brain interpret smells.
Odors are detected by olfactory sensory neurons located in the nasal cavity, where each neuron expresses only one of 1,172 different receptors encoded in the mouse's DNA, and each receptor is specialized for recognizing a particular type of odor.
Unlike senses like hearing, sight, and touch, which have long relied on clear sensory maps, scientists didn't believe that the sense of smell worked in the same way. But in recent years, new techniques have allowed scientists to examine some 5.5 million neurons in more than 300 mice, providing a deeper understanding of the genes active within nasal cells.
Dr. Sandeep Datta, a neurobiologist at Harvard Medical School and the lead author of the study, explained that the "single-cell sequencing" technique enabled his team to examine each mature olfactory neuron individually and identify the receptor it expresses, while the "spatial transcriptomics" technique helped to pinpoint the exact location of each receptor inside the nose.
Using this data, scientists mapped more than 1,100 olfactory receptors, revealing the existence of "a thousand separate bands" of odor receptors, overlapping with each other but highly organized, according to Data.
The team also found that the nerve cells carrying the same receptor inside the nose send their signals to the same point inside the olfactory bulb, the brain's smell processing center, meaning that the map of the nose directly matches the map of the brain.
Data said the discovery reveals an astonishing level of complexity within the lining of the nose, noting that mice have about 20 million olfactory neurons expressing more than a thousand types of olfactory receptors, compared to only three main types of color vision receptors.
Another striking finding was that the locations of these receptors were almost identical in all the laboratory mice included in the study, indicating a precise and consistent system in their organization.
Scientists have also identified a molecule known as retinoic acid (RA), which is believed to play a key role in directing each nerve cell to express the appropriate receptor based on its location within the nose. Altering the levels of this molecule led to a shift in receptor positions, further supporting the hypothesis of its regulatory role.
Scientists are now working to understand why these receptors are arranged in this specific way, as well as to determine whether the same principles apply to humans.
Data pointed out that the human olfactory system is very similar to that of mice, although humans have fewer odor receptors. He explained that understanding this mechanism may help in the future to develop treatments for loss of smell and its associated health and psychological effects, such as an increased risk of depression.
