New research suggests that an individual's unique sense of smell is the result of genes interacting with experience. A study of genetically similar mice shows that while genes may decide the types of odor-detecting cells that an animal has in its nose, its life history influences the numbers of different cell types, giving each animal a unique perception of smell.
The study - which is published in the journal eLIFE - is the work of the Wellcome Trust Sanger Institute in Cambridge in the United Kingdom, and other research centers in the United States and Brazil.
Senior author Dr. Darren Logan, of the Wellcome Trust Sanger Institute, explains that olfactory neurons - the nerve cells in the nose that detect odors - are "highly connected" to neurons in the brain, and that studying them can help us to understand how neurons develop. He adds:
"We have shown that each individual has a very different combination of possible olfactory neurons, driven by genetics. In this study we also show that, with experience of different smells, these combinations of neurons change, so both genetics and environment interplay to give every individual a unique sense of smell."
The olfactory organ in the nose of an animal contains neurons, or nerve cells, that identify odors and send the information to the brain. The cells contain special proteins, or receptors, that react to specific chemicals in the odor.
In their paper, the researchers describe how there are "hundreds to thousands" of different types of olfactory receptors in the nose of an animal, allowing the animal to identify a huge range of different smells.
Genes provide some, but not all, of the answers
Olfactory receptors, like all proteins, are encoded by genes, which can vary widely among different individuals. This may partly explain why an odor is pleasant to one person and odious to another.
However, the authors note that this genetic explanation is not sufficient to account for why some people have a more sensitive sense of smell than others.
Could it be, they ask, that an individual's sense of smell relies not only on the types of smell receptors in their olfactory organ, but also on the relative numbers of each type?
To answer this question, there is a need for technology that can accurately count the many millions of smell receptors in the noses of mammals, something that had not been possible before, say the researchers.
For the study, the team developed and validated an RNA sequencing-based method to accurately count the number of each type of smell receptor in a mouse's olfactory organ, which comprises 10 million nerve cells, each of which has one receptor that is coded by a "gene from a pool of over 1,000."
The researchers used two groups of mice for their investigation. In one group, the animals had the same genetic makeup, but they were raised in different environments. In the other group, the mice were genetically dissimilar but were raised in the same environment.
Environment affects numbers of smell receptors
From the RNA sequencing, the team established that an animal's genetic makeup decided which receptors were present in its olfactory organ.
However, just as importantly, the researchers found that the environment an animal was raised in had a significant effect on the relative numbers of the different types of smell receptors and the animal's ability to identify different smells.
Part of the experiment involved putting different smells in the mice's drinking water for several weeks and then examining how this affected their olfactory organs.
The researchers found that some types of smell receptor increased in number while others reduced, after exposure to a particular smell. The changes were directly related to recognizing the smells, they note, showing that just smelling an odor can alter the arrangement of receptors in the mouse's olfactory organ.
"We found the cellular and molecular construction of the olfactory tissue at a given moment is prepared not only by the organism's genes but also by its life history."
Prof. Fabio Papes, study co-author
The researchers suggest that their findings could have implications for personalized medicine. For example, if it is the case that a person's experiences influence the development of their sense organs, then treatments devised from genetic information alone may not always have the desired response.