Researchers at the University of Michigan have uncovered a previously unknown biological pathway that explains how certain touch-sensitive hairs trigger the sensation of itch. The findings, made in mouse models, reveal a dedicated sensory system that may eventually help scientists develop better treatments for chronic itching disorders.
“Itch is one of the major symptoms in most chronic skin inflammation patients,” said Bo Duan, associate professor in the Department of Molecular, Cellular, and Developmental Biology. “What we’ve discovered is a pathway that we believe plays a very important role for both acute and chronic itch sensation.”
The researchers identified a previously unknown type of hair in mice called vellus-like hairs, along with a specialized group of touch-sensitive nerve cells connected to them. These hairs resemble the fine, short, light colored vellus hairs that cover much of the human body, commonly known as peach fuzz.
The study, which received support in part from the National Institutes of Health, was published in the journal Neuron.
Hidden Nerve Cells Linked to Chronic Itch
To investigate the role of these neurons, the team studied mice with chronic skin inflammation, a condition comparable to eczema in humans. Mice with the specialized neurons scratched normally in response to itch. However, animals lacking those neurons, or in which the neurons had been switched off, showed a dramatic reduction in scratching behavior.
Current treatments work reasonably well for chemical itch caused by irritants such as mosquito bites or poison ivy. They are far less effective against the persistent itch associated with chronic skin inflammation. According to Duan, the newly identified “mechanical itch” pathway may provide an entirely new target for future therapies.
“We need a new pathway to target if we want to treat chronic itch,” Duan said. “And our research suggests that this population of neurons could be a target in the future. We have ongoing projects looking at this.”
Although the researchers cannot directly test for the same pathway in people, several lines of evidence suggest humans may possess a similar system. For example, humans carry the genes needed to produce these specialized touch-sensitive neurons.
The team also identified proteins in mice that carry itch signals from the hairs to the spinal cord through these neurons. When human neurons grown in laboratory cultures were exposed to the same proteins, they responded in similar ways.
“Our study indicates that humans may have this same kind of mechanism to transmit mechanical itch,” Duan said. “It also reveals that the body has a dedicated system for this type of sensation.”
Why Peach Fuzz Can Make You Itch
One of Duan’s favorite classroom demonstrations helps illustrate the phenomenon.
Roll one corner of a tissue into a long, fine point and gently brush it across the tiny hairs around your lips. If you lightly touch the fine vellus hairs instead of the thicker terminal hairs, you may suddenly feel an itch.
“Humans and animals experience this kind of itch, but no one knew the molecular and cellular mechanisms behind it,” Duan said.
The new findings identify the sensory pathway connecting these specialized hairs to the nervous system. Combined with the team’s earlier research, the work provides a clearer picture of how mechanical itch signals travel through the body.
Solving a Century-Old Mystery
Scientists first described the unusual vellus-like hairs found on mice more than 100 years ago. These hairs are especially common behind the ears, beneath the lips, and near the base of the paws. Despite their early discovery, they have received relatively little attention from sensory researchers.
Because there were no established methods for studying this type of itch in mice, Duan’s team had to develop its own experimental approach.
“A mouse can’t say that it’s itchy,” Duan said. “But it will scratch.”
The researchers gently stimulated the animals’ vellus-like hairs using a small loop of thread to produce mechanical itch. After identifying the neurons responsible for the response, they genetically modified those cells so they could be activated with blue light. Simply shining blue light onto the mice triggered the same scratching behavior seen during mechanical stimulation, providing strong evidence that these neurons directly produce the itch sensation.
Why We Are Not Constantly Itchy
Peach fuzz and similar hairs are especially abundant around the mouths and ears of both humans and mice. Duan believes these hairs may have evolved as an early warning system that alerts mammals when insects or parasites come into contact with sensitive areas of the body.
Even though humans are covered with vellus hair (with some notable exceptions like the palms of our hands), we are not constantly scratching. Previous work from Duan’s laboratory offers one possible explanation. The spinal cord contains “gating” circuits that normally suppress mechanical itch signals, allowing them to pass through only under specific conditions.
Understanding how this hidden sensory system works could ultimately help researchers design new treatments for chronic itch, particularly for patients with inflammatory skin diseases whose symptoms remain difficult to control using existing medications.