It’s not common that researchers discover what could be an entirely new part of the human body. But a team in Sweden claims to have uncovered an intricate network of cells underneath skin that helps process certain kinds of pain. The find could broaden our conceptions of how we feel pain, as well as how to relieve it.
Typically, it’s thought that we perceive harmful sensations on our skin entirely through the very sensitive endings of certain nerve cells. These nerve cells aren’t coated by a protective layer of myelin, as other types are. Nerve cells are kept alive by and connected to other cells called glia. Outside of the central nervous system, one of the two major types of glia are called Schwann cells.
The authors of the new study, published Thursday in Science, say they were studying these helper cells near the skin’s surface in the lab when they came across something strange — some of the Schwann cells seemed to form an extensive “mesh-like network” with their nerve cells, differently than how they interact with nerve cells elsewhere. When they ran further experiments with mice, they found evidence that these Schwann cells play a direct, added role in pain perception, or nociception.
One experiment, for instance, involved breeding mice with these cells in their paws that could be activated when the mice were exposed to light. Once the light came on, the mice seemed to behave like they were in pain, such as by licking themselves or guarding their paws. Later experiments found that these cells — since dubbed nociceptive Schwann cells by the team — respond to mechanical pain, like being pricked or hit by something, but not to cold or heat.
Because these cells are spread throughout the skin as an intricately connected system, the authors argue that the system should be considered an organ.
“Our study shows that sensitivity to pain does not occur only in the skin’s nerve [fibres], but also in this recently discovered pain-sensitive organ,” said senior study author Patrik Ernfors, a pain researcher at Sweden’s Karolinska Institute, in a release from the university.
Ernfors and his team aren’t the first to claim discovery of a new organ recently. Last year, researchers in the U.S. argued that a fluid-filled network they called the interstitium — also found under the skin, coincidentally, as well as surrounding other internal organs — should be considered an organ.
But it often takes years before more authoritative sources like medical textbooks decide to agree on the classification (indeed, some other researchers quickly argued back that the interstitium isn’t distinctive or confirmed enough to be enshrined as an organ).
So, as with most things in science, there needs to be a lot more work done to study these cells and how they function. Mice are invaluable models for studying the origins of pain in people, for instance, but we know next to nothing about how these cells actually operate in the human body. All of the experiments detailed in the study only involved mice, so it’s possible this system doesn’t exist or function the same way in humans.
“We have not studied humans yet. However, considering that all previously known sensory organs found in mouse also exist in humans, it is possible if not likely that it does exist also in the human skin,” Ernfors told Gizmodo in an email.
Still, given how difficult it’s been to effectively treat chronic pain, any new potential lead is exciting and worth keeping an eye on.
“Mechanical allodynia (allodynia = feeling pain from non-painful mechanical stimuli such as when putting on a shirt or other types of skin touching) is often associated with neuropathy,” said Ernfors, noting that as many as 10 per cent of people in the U.S. and Europe may suffer from this sort of pain. “The mechanisms that causes mechanical allodynia are not known. Because these cells are important for sensing mechanical (pressure) pain they could also be involved in mechanical allodynia.”
The team next plans to study in depth how these cells are actually activated by pain, which would involve finding the proteins on their surface that respond to mechanical stimulation. They also plan to look into the role they might play in chronic pain using animal models.