Weird olfaction: Scientists sniff out 'long lost' map for smell

Over the past century, scientists have mapped several of the body’s sensory systems in microscopic detail, discovering that the cells that process sight, sound and touch are arranged in predictable spatial patterns.

It has been much harder to navigate the complex landscape of the nose, with its enormous array of scent receptors. For years, many scientists believed that these receptors were distributed mostly at random.

Now, two teams of scientists have managed to map the nose of a mouse. Using advanced genetic sequencing and imaging techniques, the researchers found that each of the 1,100 different types of olfactory receptors in the mouse nose occupied a distinct and predictable position, consistent from mouse to mouse.

The findings, published in the journal Cell on Tuesday, represent the first comprehensive, fine-scale maps of odour receptors in the nose. They suggest that topographic maps may be as fundamental to olfaction as they are to other senses. “We have, to some extent, unveiled this long-lost map for smell,” said Sandeep Robert Datta, a neurobiologist at Harvard University.

Scientists have not yet demonstrated that the same sort of map exists in the human nose and do not yet understand why the receptors are arranged in the way that they are. But the research sheds light on how the olfactory system develops and paves the way for a better understanding of an often overlooked sense.

“Having this comprehensive understanding, this broad understanding of the organisation of the main olfactory system is absolutely essential to understand how we process scent,” said Catherine Dulac, a molecular biologist and neuroscientist at Harvard and an author of the other paper.

Topographic maps preserve key sensory information about the world and help the brain process that information more efficiently. In the ear and the auditory cortex, for instance, adjacent cells detect adjacent sound frequencies. In the eye and the visual cortex, neighbouring neurons process information from neighbouring points in the visual field.

But scientists have not been able to detect equivalent maps in the nose, which contains a staggeringly diverse array of olfactory receptors. These receptors are specially shaped proteins that sit on the surface of neurons and bind to complementary odour molecules. Humans have several hundred different types of these receptors, but some species, including mice, have 1,000 or more.

In their study, Datta and his colleagues analysed neurons from mouse noses to determine exactly which genes were active, or expressed, in each individual neuron. It revealed which particular olfactory receptor each neuron was expressing. But the scientists also identified hundreds of additional genes, with a variety of different functions, whose activity seemed to vary from neuron to neuron, depending on which receptor type was present.

Some of these genes were known to be active only in certain regions of the nose, while others were known to help guide neuron development in physical space. The researchers hypothesized that these genes might help determine which receptor any given neuron expressed and that they might do so based on where that neuron was located in physical space.

The New York Times