Hidden way for us to feel touch uncovered by Imperial researchers
[Nov. 8, 2023: Staff Writer, The Brighter Side of News]
For years, scientists believed that the sensation of touch was primarily mediated by nerve endings residing within the skin. (CREDIT: Creative Commons)
In the realm of sensory perception, touch has always been a fascinating subject of study. For years, scientists believed that the sensation of touch was primarily mediated by nerve endings residing within the skin. However, groundbreaking research from Imperial College London has shattered this long-held belief.
This new study reveals that not only are skin and its surrounding hair follicles involved in touch detection, but the cells within these hair follicles themselves play an unexpected role in this sensory experience.
The research, led by Dr. Claire Higgins, a prominent figure in Imperial's Department of Bioengineering, unveils a previously unknown facet of touch perception. Dr. Higgins and her team have discovered that the cells encapsulating hair follicles—those structures that encompass the hair fiber—are also equipped to sense touch.
Furthermore, these hair follicle cells were found to release two critical neurotransmitters, histamine and serotonin, in response to tactile stimulation. These findings hold the promise of shedding light on the role of histamine in inflammatory skin disorders like eczema.
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Dr. Higgins commented on the surprising discovery, stating, "This is a surprising finding as we don't yet know why hair follicle cells have this role in processing light touch. Since the follicle contains many sensory nerve endings, we now want to determine if the hair follicle is activating specific types of sensory nerves for an unknown but unique mechanism."
Our ability to perceive touch is a complex process, involving various mechanisms working in harmony. Traditionally, researchers believed that sensory nerve endings located within the skin were the primary detectors of tactile stimuli, relaying signals to the brain. Additionally, hair follicles, densely innervated and capable of detecting hair fiber movement, were also implicated in touch perception. Furthermore, a specialized class of sensory nerves known as C-LTMRs, found exclusively in hairy skin, were responsible for processing emotional or "feel-good" touches.
However, the recent research conducted by Imperial College London has uncovered a novel process occurring within hair follicles. The study, which leveraged single-cell RNA sequencing data of human skin and hair follicles, revealed that hair follicle cells possessed a higher percentage of touch-sensitive receptors compared to their counterparts in the skin.
Hair follicle (purple) surrounded by sensory nerves (fluorescent green). (CREDIT: Julia Agramunt)
To delve deeper into this phenomenon, the researchers initiated co-cultures of human hair follicle cells and sensory nerves, subjecting the hair follicle cells to mechanical stimulation. Astonishingly, this stimulation resulted in the activation of nearby sensory nerves. The next logical step was to investigate the signaling mechanism employed by hair follicle cells to communicate with sensory nerves.
To accomplish this, the research team adapted a technique called fast scan cyclic voltammetry, which allowed them to analyze cells in culture. Their experiments showed that hair follicle cells responded to touch by releasing the neurotransmitters serotonin and histamine. When the researchers blocked the receptors for these neurotransmitters on sensory neurons, the neurons ceased to respond to the hair follicle cell stimulation.
Researchers have uncovered an unknown mechanism for us to experience touch. (CREDIT: Creative Commons)
Similarly, when synaptic vesicle production by hair follicle cells was inhibited, they were no longer able to signal to the sensory nerves. This evidence led the researchers to conclude that, in response to touch, hair follicle cells release neurotransmitters that activate nearby sensory neurons.
Intriguingly, the researchers also conducted parallel experiments using skin cells instead of hair follicle cells. These skin cells responded to light touch by releasing histamine but did not release serotonin. Dr. Higgins highlighted the significance of this observation, saying, "This is interesting as histamine in the skin contributes to inflammatory skin conditions such as eczema, and it has always been presumed that immune cells called mast cells release all the histamine. Our work uncovers a new role for skin cells in the release of histamine, with potential applications for eczema research."
Human occipital scalp hair follicles are innervated by several LTMRs and an HTMR. Example of a human follicular unit from scalp skin composed of terminal hairs before (left) and after (right) clearing process. (CREDIT: Science Advances)
Dr. Parastoo Hashemi, a co-author of the paper and an expert in fast scan cyclic voltammetry, provided insight into the technique's contribution to their research. She remarked, "Fast scan cyclic voltammetry allowed us to see the effect of touch on these cells in real time. It's an exciting finding as it opens up so many more questions for these cells: why do they have this role, and what else can we learn from them about how our skin senses touch?"
While these findings are undoubtedly groundbreaking, it's essential to remember that the research was conducted in cell cultures. To confirm the validity of these discoveries, the next logical step is to replicate the experiments in living organisms. Additionally, the researchers are keen to explore whether hair follicles activate specific types of sensory nerves. Given that C-LTMRs are exclusive to hairy skin, they are particularly interested in uncovering any unique signaling mechanisms the hair follicles may employ to communicate with these specialized nerves.
ORS cells in vitro respond to mechanical stimulus in vitro. Heatmap depicted coexpression of scRNA-seq data of human scalp hair follicles showing that 5.4% of K15 cells also express TRPC1. (CREDIT: Science Advances)
The implications of this research extend beyond mere scientific curiosity. Understanding the intricate processes by which our bodies perceive touch can have far-reaching implications for various fields, from neuroscience to dermatology. The revelation that hair follicle cells play a role in touch perception opens up a new frontier of exploration, one that may eventually yield insights into the treatment of inflammatory skin conditions and possibly other sensory disorders.
The age-old assumption that touch is solely the domain of nerve endings in the skin has been challenged by the groundbreaking work of Dr. Claire Higgins and her team at Imperial College London. Their discovery that hair follicle cells actively participate in touch detection and communication with sensory nerves has broad implications for our understanding of sensory perception. As research in this field advances, it holds the promise of unraveling the mysteries of touch and potentially revolutionizing our approach to conditions like eczema and other sensory-related disorders.
For more science news stories check out our New Discoveries section at The Brighter Side of News.
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