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Sense of Smell is Unique to Each Person's Genes and Experience

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A study published in the journal eLIFE has suggested that an individual's unique sense of smell is a result of their genes combined with personal life experience. Senior author of the study Dr. Darren Logan of the Wellcome Trust Sanger Institute explains that olfactory neurons (the nerve cells in the nose which detect odors) are "highly connected" to neurons in the brain. "We have shown that each individual has a very different combination of possible olfactory neurons, driven by genetics," says Dr. Logan. "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." When an animal uses its nose to smell, the olfactory organ inside it (which contains neurons or nerve cells) identifies any odors it comes into contact with, and then transmits that information to the brain. These cells contain special proteins or receptors, which then react to specific chemicals in the odor, and thus transmitting to the animal's brain what that scent is. Animals have "hundreds to thousands" of different types of olfactory receptors in their noses according to the researchers, and this allows them to identify a wide variety of scents. The researchers are questioning whether or not an individual's sense of smell relies not only on the types of scent receptors in their olfactory organs, but also on the relative numbers of each type. For their study, the team created an RNA sequencing-based method which accurately measures the number of each type of scent receptor in a mouse's olfactory organ (which contains 10 million nerve cells). Using two groups of mice for their investigation, the researchers set to work. One group of mice had the same genetic makeup, but were raised in different environments. In the other group, the mice were genetically dissimilar but were raised in the same environment. From the RNA sequencing, the team established that an animal's genetic makeup decided which receptors were present in its olfactory organ. The researchers also discovered that the environment an animal was raised in had a noticeable impact on the animal's ability to identify different smells. To validate this, part of the experiment involved placing different scents in the mice's drinking water for a number of weeks and then examining how this affected their olfactory organs. The researchers found that some types of scent receptors increased in number while others reduced, after exposure to a particular smell. These changes were directly related to recognizing the smells, showing that just smelling an odor could alter the arrangement of receptors in the mouse's olfactory organ. Co-author of the study Prof. Fabio Papes stated that, "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."  
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