The touch dome in human skin is supplied by different types of nerve fibers

Skin Innervation

All layers and appendages of the skin are densely innervated by afferent and efferent neurons providing sensory information and controlling skin perfusion and sweating. In mice, neuronal functions have been comprehensively linked to unique single-cell expression patterns and to characteristic arborization of nerve endings in skin and dorsal horn, whereas for humans, specific molecular markers for functional classes of afferent neurons are still lacking. Moreover, bidirectional communication between sensory neurons and local skin cells has become of particular interest, resulting in a broader physiological understanding of sensory function but also of trophic functions and immunomodulation in disease states.

C. elegans touch receptor neurons direct mechanosensory complex organization via repurposing conserved basal lamina proteins

The sense of touch is conferred by the conjoint function of somatosensory neurons and skin cells. These cells meet across a gap filled by a basal lamina, an ancient structure found in metazoans. Using Caenorhabditis elegans, we investigate the composition and ultrastructure of the extracellular matrix at the epidermis and touch receptor neuron (TRN) interface. We show that membrane-matrix complexes containing laminin, nidogen, and the MEC-4 mechano-electrical transduction channel reside at this interface and are central to proper touch sensation. Interestingly, the dimensions and spacing of these complexes correspond with the discontinuous beam-like extracellular matrix structures observed in serial-section transmission electron micrographs. These complexes fail to coalesce in touch-insensitive extracellular matrix mutants and in dissociated neurons. Loss of nidogen reduces the density of mechanoreceptor complexes and the amplitude of the touch-evoked currents they carry. Thus, neuron-epithelium cell interfaces are instrumental in mechanosensory complex assembly and function. Unlike the basal lamina ensheathing the pharynx and body wall muscle, nidogen recruitment to the puncta along TRNs is not dependent upon laminin binding. MEC-4, but not laminin or nidogen, is destabilized by point mutations in the C-terminal Kunitz domain of the extracellular matrix component, MEC-1. These findings imply that somatosensory neurons secrete proteins that actively repurpose the basal lamina to generate special-purpose mechanosensory complexes responsible for vibrotactile sensing.

Preliminary evidence that Merkel cells exert chemosensory functions in human epidermis

The mechanotransduction of light-touch sensory stimuli is considered to be the main physiological function of epidermal Merkel cells (MCs). Recently, however, MCs have been demonstrated to be also thermo-sensitive, suggesting that their role in skin physiologically extends well beyond mechanosensation. Here, we demonstrate that in healthy human skin epidermal MCs express functional olfactory receptors, namely OR2AT4, just like neighbouring keratinocytes. Selective stimulation of OR2AT4 by topical application of the synthetic odorant, Sandalore®, significantly increased Piccolo protein expression in MCs, as assessed by quantitative immunohistomorphometry, indicating increased vesicle trafficking and recycling, and significantly reduced nerve growth factor (NGF) immunoreactivity within MCs, possibly indicating increased neurotrophin release upon OR2AT4 activation. Live-cell imaging showed that Sandalore® rapidly induces a loss of FFN206-dependent fluorescence in MCs, suggesting OR2AT4-dependent MC depolarization and subsequent vesicle secretion. Yet, in contrast to keratinocytes, OR2AT4 stimulation by Sandalore® altered neither the number nor the proliferation status of MCs. These preliminary ex vivo findings demonstrate that epidermal MCs also exert OR-dependent chemosensory functions in human skin, and invite one to explore whether these newly identified properties are dysregulated in selected skin disorders, for example, in pruritic dermatoses, and if these novel MC functions can be therapeutically targeted to maintain/promote skin health.

Anatomical contacts between sensory neurons and epidermal cells: an unrecognized anatomical network for neuro-immuno-cutaneous crosstalk

Sensory neurons innervating the skin are conventionally thought to be the sole transducers of touch, temperature, pain and itch. However, recent studies have shown that keratinocytes - like Merkel cells - act as sensory transducers, whether for innocuous or noxious mechanical, thermal or chemical stimuli, and communicate with intraepidermal free nerve endings via chemical synaptic contacts. This paradigm shift leads to consideration of the whole epidermis as a sensory epithelium. Sensory neurons additionally function as an efferent system. Through the release of neuropeptides in intimate neuroepidermal contact areas, they contribute to epidermal homeostasis and to the pathogenesis of inflammatory skin diseases. To counteract the dogma regarding neurocutaneous interactions, seen exclusively from the perspective of soluble and spreading mediators, this review highlights the essential contribution of the unrecognized anatomical contacts between sensory neurons and epidermal cells (keratinocytes, melanocytes, Langerhans cells and Merkel cells), which take part in the reciprocal dialogue between the skin, nervous system and immune system.

Do Merkel complexes initiate mechanical itch?

Itch is a common sensation which is amenable to disabling patients' life under pathological and chronic conditions. Shared assertion easily limits itch to chemical itch, without considering mechanical itch and alloknesis, its pathological counterpart. However, in recent years, our understanding of the mechanical itch pathway, particularly in the central nervous system, has been enhanced. In addition, Merkel complexes, conventionally considered as tactile end organs only responsible for light touch perception due to Piezo2 expressed by both Merkel cells and SA1 Aβ-fibres - low threshold mechanical receptors (LTMRs) -, have recently been identified as modulators of mechanical itch. However, the tactile end organs responsible for initiating mechanical itch remain unexplored. The consensus is that some LTMRs, either SA1 Aβ- or A∂- and C-, are cutaneous initiators of mechanical itch, even though they are not self-sufficient to finely detect and encode light mechanical stimuli into sensory perceptions, which depend on the entire hosting tactile end organ. Consequently, to enlighten our understanding of mechanical itch initiation, this article discusses the opportunity to consider Merkel complexes as potential tactile end organs responsible for initiating mechanical itch, under both healthy and pathological conditions. Their unsuspected modulatory abilities indeed show that they are tuned to detect and encode light mechanical stimuli leading to mechanical itch, especially as they host not only SA1 Aβ-LTMRs but also A∂- and C-fibres.

Merkel Cells Are Multimodal Sensory Cells: A Review of Study Methods

Merkel cells (MCs) are rare multimodal epidermal sensory cells. Due to their interactions with slowly adapting type 1 (SA1) Aβ low-threshold mechanoreceptor (Aβ-LTMRs) afferents neurons to form Merkel complexes, they are considered to be part of the main tactile terminal organ involved in the light touch sensation. This function has been explored over time by ex vivo, in vivo, in vitro, and in silico approaches. Ex vivo studies have made it possible to characterize the topography, morphology, and cellular environment of these cells. The interactions of MCs with surrounding cells continue to be studied by ex vivo but also in vitro approaches. Indeed, in vitro models have improved the understanding of communication of MCs with other cells present in the skin at the cellular and molecular levels. As for in vivo methods, the sensory role of MC complexes can be demonstrated by observing physiological or pathological behavior after genetic modification in mouse models. In silico models are emerging and aim to elucidate the sensory coding mechanisms of these complexes. The different methods to study MC complexes presented in this review may allow the investigation of their involvement in other physiological and pathophysiological mechanisms, despite the difficulties in exploring these cells, in particular due to their rarity.

The analgesic power of pleasant touch in individuals with chronic pain: Recent findings and new insights

This mini-review covers recent works on the study of pleasant touch in patients with chronic pain (CP) and its potential use as a treatment. While experiments have demonstrated that pleasant touch, through the activation of CT-afferents and the brain regions involved in its affective value, might reduce the unpleasantness and intensity of induced pain, the interaction between pleasant touch and CP remains under-examined. Some experiments show that CP might disrupt the positive aspects of receiving pleasant touch, while in other studies the perception of pleasantness is preserved. Moreover, only a few attempts have been made to test whether touch can have a modulatory effect on CP, but these results also remain inconclusive. Indeed, while one recent study demonstrated that CT-touch can diminish CP after a short stimulation, another study suggested that pleasant touch might not be sufficient. Future studies should further investigate the psychological and neural interplay between pleasant touch and CP. In the conclusion of this mini-review, we propose a new tool we have recently developed using immersive virtual reality (IVR).

Miswiring of Merkel cell and pruriceptive C fiber drives the itch-scratch cycle

Itch sensation provokes the scratch reflex to protect us from harmful stimuli in the skin. Although scratching transiently relieves acute itch through activation of mechanoreceptors, it propagates the vicious itch-scratch cycle in chronic itch by further aggravating itch over time. Although well recognized clinically, the peripheral mechanisms underlying the itch-scratch cycle remain poorly understood. Here, we show that mechanical stimulation of the skin results in activation of the Piezo2 channels on Merkel cells that pathologically promotes spontaneous itch in experimental dry skin. Three-dimensional reconstruction and immunoelectron microscopy revealed structural alteration of MRGPRA3⁺ pruriceptor nerve endings directed toward Merkel cells in the setting of dry skin. Our results uncover a functional miswiring mechanism under pathologic conditions, resulting in touch receptors triggering the firing of pruriceptors in the skin to drive the itch-scratch cycle.

Artificial fast-adapting mechanoreceptor based on carbon nanotube percolating network

Most biological sensors preferentially encode changes in a stimulus rather than the steady components. However, intrinsically phasic artificial mechanoreceptors have not yet been described. We constructed a phasic mechanoreceptor by encapsulating carbon nanotube film in a viscoelastic matrix supported by a rigid substrate. When stimulated by a spherical indenter the sensor response resembled the response of fast-adapting mammalian mechanoreceptors. We modelled these sensors from the properties of percolating conductive networks combined with nonlinear contact mechanics and discussed the implications of this finding.

The Fungiform Papilla Is a Complex, Multimodal, Oral Sensory Organ

When solid or liquid stimuli contact the tongue tip during eating, the sensations of taste, touch and temperature are immediately evoked, and tongue function relies on these simultaneous multimodal responses. We focus on the fungiform papilla of the anterior tongue as a complex organ for taste, tactile and thermal modalities, all via chorda tympani nerve innervation from the geniculate ganglion. Rather than a review, our aim is to revise the classic archetype of the fungiform as predominantly a taste bud residence only and instead emphasize an amended concept of the papilla as a multimodal organ. Neurophysiological maps of fungiform papillae in functional receptive fields demonstrate responses to chemical, stroking and cold lingual stimuli. Roles are predicted for elaborate extragemmal nerve endings in tactile and temperature sensations, and potential functions for keratinocytes in noncanonical sensory signaling. The fungiform papilla is presented as a polymodal lingual organ, not solely a gustatory papilla.

The morphology, size and density of the touch dome in human hairy skin by scanning electron microscopy

The touch domes of mammalian hairy skin are mechanoreceptors characterized by the accumulation of Merkel cell-neurite complexes at the epidermal base. In this study, we examined the shape, size, and density of the touch dome of human skin of the forearm and the abdomen through scanning electron microscopy (SEM). Human skin samples were obtained from donated bodies, as well as a patient who underwent biopsy. Skin pieces were treated with a KOH-collagenase method for the separation of the epidermis from the dermis. The basal surface of the separated epidermis was then observed using SEM. The touch dome was clearly determined as a concave area bordered by a thick epidermal ridge, where neural components accumulated. The touch dome was rather independent from hair follicles, although they were sometimes located beside the touch dome. The average size and density of the touch dome were 0.06 mm2 and 3.82 cm2 in the forearm, and 0.10 mm2 and 1.30 cm2 in the abdomen, respectively. Our results suggested that the regional difference in size and density of the touch dome might be related to the sensation's sensitivity as touch spots in human hairy skin.

The cellular basis of mechanosensory Merkel-cell innervation during development

Touch sensation is initiated by mechanosensory neurons that innervate distinct skin structures; however, little is known about how these neurons are patterned during mammalian skin development. We explored the cellular basis of touch-receptor patterning in mouse touch domes, which contain mechanosensory Merkel cell-neurite complexes and abut primary hair follicles. At embryonic stage 16.5 (E16.5), touch domes emerge as patches of Merkel cells and keratinocytes clustered with a previously unsuspected population of Bmp4-expressing dermal cells. Epidermal Noggin overexpression at E14.5 disrupted touch-dome formation but not hair-follicle specification, demonstrating a temporally distinct requirement for BMP signaling in placode-derived structures. Surprisingly, two neuronal populations preferentially targeted touch domes during development but only one persisted in mature touch domes. Finally, Keratin-17-expressing keratinocytes but not Merkel cells were necessary to establish innervation patterns during development. These findings identify key cell types and signaling pathways required for targeting Merkel-cell afferents to discrete mechanosensory compartments.

Merkel Cells Activate Sensory Neural Pathways through Adrenergic Synapses

Epithelial-neuronal signaling is essential for sensory encoding in touch, itch, and nociception; however, little is known about the release mechanisms and neurotransmitter receptors through which skin cells govern neuronal excitability. Merkel cells are mechanosensory epidermal cells that have long been proposed to activate neuronal afferents through chemical synaptic transmission. We employed a set of classical criteria for chemical neurotransmission as a framework to test this hypothesis. RNA sequencing of adult mouse Merkel cells demonstrated that they express presynaptic molecules and biosynthetic machinery for adrenergic transmission. Moreover, live-cell imaging directly demonstrated that Merkel cells mediate activity- and VMAT-dependent release of fluorescent catecholamine neurotransmitter analogs. Touch-evoked firing in Merkel-cell afferents was inhibited either by pre-synaptic silencing of SNARE-mediated vesicle release from Merkel cells or by neuronal deletion of β₂-adrenergic receptors. Together, these results identify both pre- and postsynaptic mechanisms through which Merkel cells excite mechanosensory afferents to encode gentle touch. VIDEO ABSTRACT.

Quantitative and qualitative normative dataset for intraepidermal nerve fibers using skin biopsy

BACKGROUND

Skin biopsy is the most relevant tool to diagnose small-fiber neuropathy. A well-documented normal dataset for intraepidermal nerve fiber in the distal leg is required to improve its diagnostic value.

METHODS

Three hundred healthy subjects were enrolled in the study, after clinical and biological screening to exclude neurological and systemic pathologies. A distal leg biopsy was taken and intraepidermal nerve fiber density after protein gene product-9.5 immunocytochemistry with brightfield microscopy was determined. Morphological variations of intraepidermal nerve fibers, previously described in small-fiber neuropathies, were analyzed. One hundred biopsies were also analyzed at the ultrastructural level.

FINDINGS

The median number of fibers was lower in men compared to women and decreased with age. Using statistical modeling taking into account age and gender, we calculated the 5th percentile of intraepidermal nerve fiber density as follows: 7.6156-0.0769 x age (years) + 1.5506 x gender (woman = 1; man = 0). We observed a low frequency of large swellings or horizontal branchings but an increasing frequency of small swellings of intraepidermal nerve fibers and irregular distribution along the dermal-epidermal junction with age. Axonal diameter of unmyelinated fibers of the papillary dermis did not vary with age or gender. Ultrastructural analysis also showed that fiber endings in close apposition to Merkel cells should not be mistaken for small-fiber swellings.

CONCLUSIONS

Our dataset allows accurate calculation of the normal density of intraepidermal nerve fibers for each year of age and provides original morphological observations that improve the diagnostic value of skin biopsy in the distal leg for small-fiber neuropathy.

Developing a sense of touch

The sensation of touch is mediated by mechanosensory neurons that are embedded in skin and relay signals from the periphery to the central nervous system. During embryogenesis, axons elongate from these neurons to make contact with the developing skin. Concurrently, the epithelium of skin transforms from a homogeneous tissue into a heterogeneous organ that is made up of distinct layers and microdomains. Throughout this process, each neuronal terminal must form connections with an appropriate skin region to serve its function. This Review presents current knowledge of the development of the sensory microdomains in mammalian skin and the mechanosensory neurons that innervate them.

Schwann Cells in the Ventral Dermis Do Not Derive from Myf5-Expressing Precursors

The embryonic origin of lineage precursors of the trunk dermis is somewhat controversial. Precursor cells traced by Myf5 and Twist2 (Dermo1) promoter activation (i.e., cells of presumed dermomyotomal lineage) have been reported to generate Schwann cells. On the other hand, abundant data demonstrate that dermal Schwann cells derive from the neural crest. This is relevant because dermal precursors give rise to neural lineages, and multilineage differentiation potential qualifies them as adult stem cells. However, it is currently unclear whether neural lineages arise from dedifferentiated Schwann cells instead of mesodermally derived dermal precursor cells. To clarify these discrepancies, we traced SOX2⁺ adult dermal precursor cells by two independent Myf5 lineage tracing strains. We demonstrate that dermal Schwann cells do not belong to the Myf5⁺ cell lineage, indicating that previous tracing data reflected aberrant cre recombinase expression and that bona fide Myf5⁺ dermal precursors cannot transdifferentiate to neural lineages in physiological conditions.

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Adult Myf5-cre^Sor mice aberrantly trace dermal Schwann cells (dSCs)

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Dedifferentiated, SOX2⁺ dSCs are the neural-competent precursors in the dermis

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These findings cast doubt on the multipotency of adult skin-derived precursors

Immunohistochemical investigation of nerve fiber presence and morphology in elderly cervical spine meniscoids

BACKGROUND CONTEXT

Innervation of anatomical structures is fundamental to their capacity to generate nociceptive impulses. Cervical spine meniscoids are hypothesized to be contributors to neck pain; however, their innervation is not comprehensively understood.

PURPOSE

This study aimed to examine the presence and morphology of nerve fibers within cervical spine meniscoids and adjacent joint capsules.

STUDY DESIGN

This is a cross-sectional study.

PATIENT SAMPLE

The sample consists of cervical hemispines of 12 embalmed cadavers (mean [standard deviation] age 82.9 [6.5] years, six female, six left). Either the right or the left half of the cervical spine (hemispine) of each cadaver was included in the sample. So six left sides and six right sides of the cadaver cervical spines made up the 12 hemispines that formed the sample.

METHODS

Cervical spine meniscoids and adjacent joint capsules were excised from lateral atlantoaxial and cervical zygapophyseal (C2-C3 to C6-C7) joints (n=67), then paraffin embedded. Meniscoids were sectioned sagittally (5 µm), slide mounted, and immunohistochemistry was performed using primary antibodies to neurofilament heavy (NF-H) and pan-neurofilament (Pan-NF) to identify nerve tissue. The study was supported by institutional graduate student funding. The authors have no conflicts of interest to declare.

RESULTS

Seventy-seven meniscoids (23 lateral atlantoaxial, 54 cervical zygapophyseal) were extracted and processed (154 sections in total). Sixty-four individual nerve fiber bundles were identified (26 NF-H positive, 38 Pan-NF positive) from 14 meniscoids. Nerves immunoreactive to both NF-H and Pan-NF were identified in 13 of 77 meniscoids (10 of 14 lateral atlantoaxial joint) from 11 joints (eight cadavers). Nerves were always located in joint capsules except three exclusively Pan-NF immunoreactive nerve fiber bundles from two adipose meniscoids.

CONCLUSIONS

The low nerve prevalence in elderly cervical spine meniscoids, with nerves only found in two adipose type meniscoids, suggests these structures may play a minimal role in cervical nociception generation in this demographic. The joint capsules, which were more frequently innervated, appear to be more likely generators of nociception in the elderly. Joint capsule nerves were mostly NF-H positive, indicating potential Aδ-fiber presence.

Cutaneous Surgical Denervation: A Method for Testing the Requirement for Nerves in Mouse Models of Skin Disease

Cutaneous somatosensory nerves function to detect diverse stimuli that act upon the skin. In addition to their established sensory roles, recent studies have suggested that nerves may also modulate skin disorders including atopic dermatitis, psoriasis and cancer. Here, we describe protocols for testing the requirement for nerves in maintaining a cutaneous mechanosensory organ, the touch dome (TD). Specifically, we discuss methods for genetically labeling, harvesting and visualizing TDs by whole-mount staining, and for performing unilateral surgical denervation on mouse dorsal back skin. Together, these approaches can be used to directly compare TD morphology and gene expression in denervated as well as sham-operated skin from the same animal. These methods can also be readily adapted to examine the requirement for nerves in mouse models of skin pathology. Finally, the ability to repeatedly sample the skin provides an opportunity to monitor disease progression at different stages and times after initiation.

Merkel cells and neurons keep in touch

The Merkel cell-neurite complex is a unique vertebrate touch receptor comprising two distinct cell types in the skin. Its presence in touch-sensitive skin areas was recognized more than a century ago, but the functions of each cell type in sensory transduction have been unclear. Three recent studies demonstrate that Merkel cells are mechanosensitive cells that function in touch transduction via Piezo2. One study concludes that Merkel cells, rather than sensory neurons, are principal sites of mechanotransduction, whereas two other studies report that both Merkel cells and neurons encode mechanical inputs. Together, these studies settle a long-standing debate on whether or not Merkel cells are mechanosensory cells, and enable future investigations of how these skin cells communicate with neurons.

Mechanotransduction in epidermal Merkel cells

The cellular and molecular basis of vertebrate touch reception remains least understood among the traditional five senses. Somatosensory afferents that innervate the skin encode distinct tactile qualities, such as flutter, slip, and pressure. Gentle touch is thought to be transduced by somatosensory afferents whose tactile end organs selectively filter mechanical stimuli. These tactile end organs comprise afferent terminals in association with non-neuronal cell types such as Merkel cells, keratinocytes, and Schwann cells. An open question is whether these non-neuronal cells serve primarily as passive mechanical filters or whether they actively participate in mechanosensory transduction. This question has been most extensively studied in Merkel cells, which are epidermal cells that complex with sensory afferents in regions of high tactile acuity such as fingertips, whisker follicles, and touch domes. Merkel cell-neurite complexes mediate slowly adapting type I (SAI) responses, which encode sustained pressure and represent object features with high fidelity. How Merkel cells contribute to unique SAI firing patterns has been debated for decades; however, three recent studies in rodent models provide some direct answers. First, whole-cell recordings demonstrate that Merkel cells are touch-sensitive cells with fast, mechanically activated currents that require Piezo2. Second, optogenetics and intact recordings show that Merkel cells mediate sustained SAI firing. Finally, loss-of-function studies in transgenic mouse models reveal that SAI afferents are also touch sensitive. Together, these studies identify molecular mechanisms of mechanotransduction in Merkel cells, reveal unexpected functions for these cells in touch, and support a revised, two-receptor site model of mechanosensory transduction.

Diversification and specialization of touch receptors in skin

Our skin is the furthest outpost of the nervous system and a primary sensor for harmful and innocuous external stimuli. As a multifunctional sensory organ, the skin manifests a diverse and highly specialized array of mechanosensitive neurons with complex terminals, or end organs, which are able to discriminate different sensory stimuli and encode this information for appropriate central processing. Historically, the basis for this diversity of sensory specializations has been poorly understood. In addition, the relationship between cutaneous mechanosensory afferents and resident skin cells, including keratinocytes, Merkel cells, and Schwann cells, during the development and function of tactile receptors has been poorly defined. In this article, we will discuss conserved tactile end organs in the epidermis and hair follicles, with a focus on recent advances in our understanding that have emerged from studies of mouse hairy skin.

Plasma membrane stretch activates transient receptor potential vanilloid and ankyrin channels in Merkel cells from hamster buccal mucosa

Merkel cells (MCs) have been proposed to form a part of the MC-neurite complex with sensory neurons. Many transient receptor potential (TRP) channels have been identified in mammals; however, the activation properties of these channels in oral mucosal MCs remain to be clarified. We investigated the biophysical and pharmacological properties of TRP vanilloid (TRPV)-1, TRPV2, TRPV4, TRP ankyrin (TRPA)-1, and TRP melastatin (TRPM)-8 channels, which are sensitive to osmotic and mechanical stimuli by measurement of intracellular free Ca(2+) concentration ([Ca(2+)]i) using fura-2. We also analyzed their localization patterns through immunofluorescence. MCs showed immunoreaction for TRPV1, TRPV2, TRPV4, TRPA1, and TRPM8 channels. In the presence of extracellular Ca(2+), the hypotonic test solution evoked Ca(2+) influx. The [Ca(2+)]i increases were inhibited by TRPV1, TRPV2, TRPV4, or TRPA1 channel antagonists, but not by the TRPM8 channel antagonist. Application of TRPV1, TRPV2, TRPV4, TRPA1, or TRPM8 channel selective agonists elicited transient increases in [Ca(2+)]i only in the presence of extracellular Ca(2+). The results indicate that membrane stretching in MCs activates TRPV1, TRPV2, TRPV4, and TRPA1 channels, that it may be involved in synaptic transmission to sensory neurons, and that MCs could contribute to the mechanosensory transduction sequence.

Computation identifies structural features that govern neuronal firing properties in slowly adapting touch receptors

Touch is encoded by cutaneous sensory neurons with diverse morphologies and physiological outputs. How neuronal architecture influences response properties is unknown. To elucidate the origin of firing patterns in branched mechanoreceptors, we combined neuroanatomy, electrophysiology and computation to analyze mouse slowly adapting type I (SAI) afferents. These vertebrate touch receptors, which innervate Merkel cells, encode shape and texture. SAI afferents displayed a high degree of variability in touch-evoked firing and peripheral anatomy. The functional consequence of differences in anatomical architecture was tested by constructing network models representing sequential steps of mechanosensory encoding: skin displacement at touch receptors, mechanotransduction and action-potential initiation. A systematic survey of arbor configurations predicted that the arrangement of mechanotransduction sites at heminodes is a key structural feature that accounts in part for an afferent’s firing properties. These findings identify an anatomical correlate and plausible mechanism to explain the driver effect first described by Adrian and Zotterman.

DOI:http://dx.doi.org/10.7554/eLife.01488.001

Sensory receptors in the skin supply us with information about objects in the world around us, including their shape and texture. These receptors also detect pressure, temperature, and pain, enabling us to respond appropriately to stimuli that could be potentially harmful.

The activation of a touch receptor—for example, due to the movement of a hair—causes ions to flow into the cell, changing the electric charge inside it. When the charge exceeds a threshold value, the cell fires action potentials, which travel along its axon to the central nervous system. The patterns of these action potentials from a population of touch receptors carry all the information about a touch stimulus to the brain. Different types of sensory receptors have unique anatomical structures and distinct signaling patterns; however, little is known about how the structures of sensory receptors influence action potential firing.

Now Lesniak and Marshall et al. have revealed that structure determines function in a type of mammalian touch receptor called the slowly adapting type I receptor, which is concentrated in fingertips and other areas of high tactile acuity. With the aid of high-resolution microscopy, the complex branching structure of the receptor and its network of nerve endings were mapped in three dimensions. Experiments revealed highly variable structures and firing patterns between individual touch receptors, and computational modeling showed that changing either the number or the arrangement of receptor endings influenced the neuron’s firing properties.

This is the first computational model that captures touch encoding by combining skin properties, sensory transduction, and spike initiation. As well as providing new information on how structure permits function, this work opens up new possibilities for exploring how the skin maintains its sensory capabilities during routine maintenance and after injury.

DOI:http://dx.doi.org/10.7554/eLife.01488.002

Increased expression of cell adhesion molecule 1 by mast cells as a cause of enhanced nerve-mast cell interaction in a hapten-induced mouse model of atopic dermatitis

BACKGROUND

Neuroimmunological disorders are involved in the pathogenesis of atopic dermatitis (AD), partly through enhanced sensory nerve-skin mast cell interaction. Cell adhesion molecule 1 (CADM1) is a mast-cell adhesion molecule that mediates the adhesion to, and communication with, sympathetic nerves.

OBJECTIVES

To investigate the role of mast cell CADM1 in the pathogenesis of AD, CADM1 expression levels by comparing between lesional and nonlesional skin mast cells of an AD mouse model, which was developed by repeated application of trinitrochlorobenzene, and to examine, in cocultures, how the alterations in CADM1 detected in lesional mast cells might affect the sensory nerve-mast cell interaction.

METHODS

AD-like lesional and nonlesional skin mast cells were collected separately by laser capture microdissection. CADM1 expression was examined by reverse transcription-polymerase chain reaction and CADM1 immunohistochemistry. In cocultures, adhesion between dorsal root ganglion (DRG) neurites and IC2 mast cells was analysed by loading a femtosecond laser-induced impulsive force on neurite-attendant IC2 cells, while cellular communication was monitored as the IC2 cellular response ([Ca(2+)]i increase) after nerve-specific stimulant-induced DRG activation.

RESULTS

AD-like lesional mast cells expressed three-fold more CADM1 transcripts than nonlesional cells. This was supported at the protein level, shown by immunohistochemistry. In coculture, CADM1 overexpression in IC2 cells strengthened DRG neurite-IC2 cell adhesion and doubled the population of IC2 cells responding to DRG activation. A function-blocking anti-CADM1 antibody abolished these effects in a dose-dependent manner.

CONCLUSIONS

Increased expression of CADM1 in mast cells appeared to be a cause of enhanced sensory nerve-mast cell interaction in a hapten-induced mouse model of AD.

The touch dome defines an epidermal niche specialized for mechanosensory signaling

In mammalian skin, Merkel cells are mechanoreceptor cells that are required for the perception of gentle touch. Recent evidence indicates that mature Merkel cells descend from the proliferative layer of skin epidermis; however, the stem cell niche for Merkel cell homeostasis has not been reported. Here, we provide genetic evidence for maintenance of mature Merkel cells during homeostasis by Krt17+ stem cells located in epidermal touch domes of hairy skin and in the tips of the rete ridges of glabrous skin. Lineage tracing analysis indicated that the entire pool of mature Merkel cells is turned over every 7-8 weeks in the adult epidermis and that Krt17+ stem cells also maintain squamous differentiation in the touch dome and in glabrous skin. Finally, selective genetic ablation of Krt17+ touch-dome keratinocytes indicates that these cells, and not mature Merkel cells, are primarily responsible for maintaining innervation of the Merkel cell-neurite complex.

Touch sense: functional organization and molecular determinants of mechanosensitive receptors

Cutaneous mechanoreceptors are localized in the various layers of the skin where they detect a wide range of mechanical stimuli, including light brush, stretch, vibration and noxious pressure. This variety of stimuli is matched by a diverse array of specialized mechanoreceptors that respond to cutaneous deformation in a specific way and relay these stimuli to higher brain structures. Studies across mechanoreceptors and genetically tractable sensory nerve endings are beginning to uncover touch sensation mechanisms. Work in this field has provided researchers with a more thorough understanding of the circuit organization underlying the perception of touch. Novel ion channels have emerged as candidates for transduction molecules and properties of mechanically gated currents improved our understanding of the mechanisms of adaptation to tactile stimuli. This review highlights the progress made in characterizing functional properties of mechanoreceptors in hairy and glabrous skin and ion channels that detect mechanical inputs and shape mechanoreceptor adaptation.

Spatiotemporal dynamics of re-innervation and hyperinnervation patterns by uninjured CGRP fibers in the rat foot sole epidermis after nerve injury

The epidermis is innervated by fine nerve endings that are important in mediating nociceptive stimuli. However, their precise role in neuropathic pain is still controversial. Here, we have studied the role of epidermal peptidergic nociceptive fibers that are located adjacent to injured fibers in a rat model of neuropathic pain. Using the Spared Nerve Injury (SNI) model, which involves complete transections of the tibial and common peroneal nerve while sparing the sural and saphenous branches, mechanical hypersensitivity was induced of the uninjured lateral (sural) and medial (saphenous) area of the foot sole. At different time points, a complete foot sole biopsy was taken from the injured paw and processed for Calcitonin Gene-Related Peptide (CGRP) immunohistochemistry. Subsequently, a novel 2D-reconstruction model depicting the density of CGRP fibers was made to evaluate the course of denervation and re-innervation by uninjured CGRP fibers. The results show an increased density of uninjured CGRP-IR epidermal fibers on the lateral and medial side after a SNI procedure at 5 and 10 weeks. Furthermore, although in control animals the density of epidermal CGRP-IR fibers in the footpads was lower compared to the surrounding skin of the foot, 10 weeks after the SNI procedure, the initially denervated footpads displayed a hyper-innervation. These data support the idea that uninjured fibers may play a considerable role in development and maintenance of neuropathic pain and that it is important to take larger biopsies to test the relationship between innervation of injured and uninjured nerve areas.

Do acupuncture meridians exist? Correlation with referred itch (mitempfindung) stimulus and referral points

OBJECTIVE

To describe the relationship between referred itch (mitempfindung) stimulus and referral points and acupuncture meridians, noting that the neuroanatomical mechanism of mitempfindung has never previously been satisfactorily explained.

METHODS

Analysis of clinical findings in the author as well as subjects in four previously described studies, comparing proportions in each of five groups.

RESULTS

Ninety-two per cent (range 85-94%) of mitempfindung point pairs (stimulus and referral) aligned to a recognised acupuncture meridian with no statistical difference (p<0.05) between the five data sources.

CONCLUSION

While previous authors have speculated on an association between mitempfindung and acupuncture, this is the first description of a relationship between stimulus and referral points and acupuncture meridians. The author suggests that the transmission of mitempfindung along acupuncture meridians may involve a series of C-fibre-Merkel cell relays, with the final referred itch sensation caused by substance P release triggering mast cell degranulation.

Identification of epidermal progenitors for the Merkel cell lineage

Epithelial stem cells in adult mammalian skin are known to maintain epidermal, follicular and sebaceous lineages during homeostasis. Recently, Merkel cell mechanoreceptors were identified as a fourth lineage derived from the proliferative layer of murine skin epithelium; however, the location of the stem or progenitor population for Merkel cells remains unknown. Here, we have identified a previously undescribed population of epidermal progenitors that reside in the touch domes of hairy skin, termed touch dome progenitor cells (TDPCs). TDPCs are epithelial keratinocytes and are distinguished by their unique co-expression of α6 integrin, Sca1 and CD200 surface proteins. TDPCs exhibit bipotent progenitor behavior as they give rise to both squamous and neuroendocrine epidermal lineages, whereas the remainder of the α6(+) Sca1(+) CD200(-) epidermis does not give rise to Merkel cells. Finally, TDPCs possess a unique transcript profile that appears to be enforced by the juxtaposition of TDPCs with Merkel cells within the touch dome niche.

Ultra-low-level laser therapy

A growing number of laboratory and clinical studies over the past 10 years have shown that low-level laser stimulation (633 or 670 nm) at extremely low power densities (about 0.15 mW/cm(2)), when administered through a particular emission mode, is capable of eliciting significant biological effects. Studies on cell cultures and animal models as well as clinical trials give support to a novel therapeutic modality, which may be referred to as ultra low level laser therapy (ULLLT). In cultured neural cells, pulsed irradiation (670 nm, 0.45 mJ/cm(2)) has shown to stimulate NGF-induced neurite elongation and to protect cells against oxidative stress. In rats, anti-edema and anti-hyperalgesia effects following ULLL irradiation were found. Clinical studies have reported beneficial effects (also revealed through sonography) in the treatment of musculoskeletal disorders. The present paper reviews the existing experimental evidence available on ULLLT. Furthermore, the puzzling issue of the biophysical mechanisms that lie at the basis of the method is explored and some hypotheses are proposed. Besides presenting the state-of-the-art about this novel photobiostimulation therapy, the present paper aims to open up an interdisciplinary discussion and stimulate new research on this subject.

Skin and hair follicle innervation in experimental models: a guide for the exact and reproducible evaluation of neuronal plasticity

The remodelling of skin innervation is an instructive example of neuronal plasticity in the peripheral nervous system. Cutaneous innervation displays dramatic plasticity during morphogenesis, adult remodelling, skin diseases and after skin nerve lesions. To recognize even subtle changes or abnormalities of cutaneous innervation under different experimental conditions, it is critically important to use a quantitative approach. Here, we introduce a simple, fast and reproducible quantitative method based on immunofluorescence histochemistry for the exact quantification of peripheral nerve fibres. Computer-generated schematic representations of cutaneous innervation in defined skin compartments are presented with the aim of standardizing reports on gene and protein expression patterns. This guide should become a useful tool when screening new mouse mutants, disease models affecting innervation or mice treated with pharmaceuticals for discrete morphologic abnormalities of skin innervation in a highly reproducible and quantifiable manner. Moreover, this method can be easily transferred to other densely innervated peripheral organs.

Skin biopsy in the management of peripheral neuropathy

Skin biopsy has been widely used in recent years for the investigation of small-calibre sensory nerves, including somatic unmyelinated intraepidermal nerve fibres, dermal myelinated nerve fibres, and autonomic nerve fibres in peripheral neuropathies, with different techniques for tissue processing and nerve fibre assessment. Here, we review the techniques for skin biopsy, the processing and assessment of the biopsy sample, their possible uses in different types of peripheral neuropathy, and their use in the follow-up of patients and in clinical trials. We also review the association between morphological measures of skin innervation and function and the limits of this method in the aetiological classification of peripheral neuropathies.

Merkel cells

Merkel cells are post-mitotic cells scattered throughout the epidermis of vertebrates. They are particularly interesting because of the close connections that they develop with sensory nerve endings and the number of peptides they can secrete. These features suggest that they may make an important contribution to skin homeostasis and cutaneous nerve development. However, these cells remain mysterious because they are difficult to study. They have not been successfully cultured and cannot be isolated, severely hampering molecular biology and functional analysis. Merkel cells probably originate in the neural crest of avians and mammalians, and their "spontaneous" appearance in the epidermis may be caused by a neuron-independent epidermal differentiation process. Their functions are still unclear: they take part in mechanoreception or at least interact with neurons, but little is known about their interactions with other epidermal cells. This review provides a new look at these least-known cells of the skin. The numerous peptides they synthesize and release may allow them to communicate with many cells other than neurons, and it is plausible that Merkel cells play a key role in skin physiology and physiopathology.

Cutaneous innervation: Form and function

It is useful for dermatologists to know about the innervation of the skin because dysfunction of cutaneous neurons can cause symptoms--such as itching, pain, and paresthesias--that are evaluated by dermatologists. We review the innervation of the skin and update readers about recent neuroscientific discoveries.

Evidence for autonomic responses to pungent chemical stimuli applied to the skin vs inhaled via the nasal cavity

AIM OF THE STUDY

Chemical stimuli can produce different sensations from tingling to pain. Responses to chemical stimuli differ in relation to many factors. This study aims at examining two factors, i.e. the localization of their application and the implied sensory modality.

METHODS

Twenty volunteer students participated in the experiment. Skin conductance responses (SCRs) were recorded to compare the autonomic nervous activation to maximal stimulation with the same irritant stimulus (mustard oil (MO) diluted at 25%) applied to the skin or delivered into the nasal cavity.

RESULTS

The SCRs obtained following maximal nasal stimulation with MO were significantly different from those obtained following maximal skin stimulation. Application of MO on skin elicited higher SCR amplitude as well as a longer rising time and latency when compared to nasal stimulation.

CONCLUSIONS

These findings indicate that the autonomic nervous activation following maximal chemical irritation differ in temporal, amplitude and shape response characteristics. These results are discussed in terms of cutaneous and nasal innervation's.