Tender touches come courtesy of skin cells with an unromantic name: Merkel cells.
Scientists have debated the role of Merkel cells in the skin since they were first discovered in 1875. Now a study in the June 19 Science, led by researchers at the Baylor College of Medicine in Houston, shows that the cells are essential for transmitting the sense of light touch, the kind of touch that helps distinguish between velvet and Velcro.
Though researchers know a lot about Merkel cells, no one has been able to say with certainty what they do. The cells are scattered sparsely around the body in clusters of 10 to 20, and only about 0.1 percent of skin cells are Merkel cells, says neuroscientist Ellen Lumpkin, a leader of the study along with Huda Zoghbi. The cells are more prevalent in touch-sensitive areas such as fingertips and lips, and have characteristics that could make them touch receptor cells, similar to hair cells in the ear that sense sound. A cluster of Merkel cells forms connections with a single sensory neuron that is found in skin structures called touch domes, and Merkel cells make a chemical called glutamate which can be used to communicate with nerves.
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Researchers have previously tried to deduce Merkel cells’ functions by destroying the cells with either chemicals or light and seeing if the sense of touch is disrupted. But those techniques also damage nerves and surrounding skin cells, says Cheryl Stucky, a sensory neurophysiologist at the Medical College of Wisconsin in Milwaukee.
“The value of this study is that it uses a very elegant genetics approach to selectively destroy the Merkel cells but preserve other cell types,” Stucky says.
Lumpkin, Zoghbi and their colleagues used a molecular trick to remove a gene called ATOH1 from all of the skin cells between the necks and the toes of mice. The protein the gene encodes is a regulatory molecule that helps cells become either neurons in the hindbrain, hair cells in the inner ear or Merkel cells. Without the gene, the team found, the mice didn’t develop Merkel cells anywhere below their necks, but the mice still had the cells on their heads (where ATOH1 wasn’t removed).
Without Merkel cells, neurons still form in the touch domes but have more branches than when Merkel cells are present, the team found. But recordings of the neurons’ activity showed that they no longer respond to touch without Merkel cells, indicating that the skin cells are necessary to feel touches. Neurons in the skin still responded to stretching, showing that Merkel cells aren’t involved in sensing that sort of stimulus.
These results could mean that Merkel cells are indeed touch receptor cells. “The pieces are all in place for that to be the case,” Stucky says, “but the question is, ‘is it fast enough?’” Chemical communications, even with fast-acting glutamate, may not be swift enough to transmit details about texture and shape contained in touch information.
Another possibility is that Merkel cells could modulate the firing rate or the activity pattern of neurons so that they can send a touch message, Lumpkin says. More research is needed to determine the cells’ exact role.
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Audio Credits: Audio files courtesy of E.A. Lumpkin and S.A. Wellnitz