For decades, scientists understood how the eyes, ears, and skin send information to the brain. Smell was the exception. Now, for the first time, researchers have mapped exactly how smell receptors are arranged inside the nose, and the answer upends what the field assumed.
A study published April 28 in the journal Cell, led by researchers at Harvard Medical School, reveals that the neurons carrying smell receptors in mice are not distributed randomly across the nasal cavity. Instead, they form neat, overlapping horizontal stripes running from top to bottom, organized by receptor type. The structure is invisible to the naked eye but precise at the cellular level.
"Our results bring order to a system that was previously thought to lack order, which changes conceptually how we think this works," said Sandeep Robert Datta, professor of neurobiology at Harvard's Blavatnik Institute and senior author of the study.
The complexity of the olfactory system is part of why it took so long to crack. Mice have roughly 20 million olfactory neurons, each expressing one of more than a thousand distinct receptor types. Human color vision, by comparison, relies on just three main receptor types. Each smell receptor detects a specific set of odor molecules, making the system far more intricate than any other sensory system scientists have mapped. Researchers first identified smell receptors in 1991 and spent the following decades searching for any underlying pattern in how they were arranged. Earlier studies suggested there might be only broad, loosely defined zones, not the fine structure now revealed.
The new work goes further than just the nose. The researchers also found that the organized map in the nasal cavity aligns with corresponding maps in the olfactory bulb, the region of the brain that processes smell. That connection reveals a coordinated architecture that runs from the receptor level all the way into neural circuits, something scientists had not previously been able to demonstrate.
"Olfaction has been the one exception; it's the sense that has been missing a map for the longest time," Datta said.
The implications reach beyond basic biology. Smell loss is a significant medical problem. It affects people with Parkinson's disease, Alzheimer's disease, and long COVID, and it was one of the most widely reported symptoms of early COVID-19 infection. Understanding the structural organization of smell receptors could open new paths toward restoring lost function.
To build the map, the team used a technique that allowed them to trace millions of individual neurons and identify which receptor type each one expressed, then plot their locations within the tissue. The resulting picture showed the striped pattern clearly for the first time.
The study does not yet answer every question about how smell works. But it establishes a framework the field has lacked for more than 30 years and gives future researchers a detailed map to work from.
