Mammalian cutaneous mechanoreceptors

Deep Hair Phenomics: Implications in Endocrinology, Development, and Aging

Hair quality is an important indicator of health in humans and other animals. Current approaches to assess hair quality are generally nonquantitative or are low throughput owing to technical limitations of splitting hairs. We developed a deep learning-based computer vision approach for the high-throughput quantification of individual hair fibers at a high resolution. Our innovative computer vision tool can distinguish and extract overlapping fibers for quantification of multivariate features, including length, width, and color, to generate single-hair phenomes of diverse conditions across the lifespan of mice. Using our tool, we explored the effects of hormone signaling, genetic modifications, and aging on hair follicle output. Our analyses revealed hair phenotypes resultant of endocrinological, developmental, and aging-related alterations in the fur coats of mice. These results demonstrate the efficacy of our deep hair phenomics tool for characterizing factors that modulate the hair follicle and developing, to our knowledge, previously unreported diagnostic methods for detecting disease through the hair fiber. Finally, we have generated a searchable, interactive web tool for the exploration of our hair fiber data at skinregeneration.org.

Human-Inspired Tactile Perception System for Real-Time and Multimodal Detection of Tactile Stimuli

A human can intuitively perceive and comprehend complicated tactile information because the cutaneous receptors distributed in the fingertip skin receive different tactile stimuli simultaneously and the tactile signals are immediately transmitted to the brain. Although many research groups have attempted to mimic the structure and function of human skin, it remains a challenge to implement human-like tactile perception process inside one system. In this study, we developed a real-time and multimodal tactile system that mimics the function of cutaneous receptors and the transduction of tactile stimuli from receptors to the brain, by using multiple sensors, a signal processing and transmission circuit module, and a signal analysis module. The proposed system is capable of simultaneously acquiring four types of decoupled tactile information with a compact system, thereby enabling differentiation between various tactile stimuli, texture characteristics, and consecutive complex motions. This skin-like three-dimensional integrated design provides further opportunities in multimodal tactile sensing systems.

Correlations among Firing Rates of Tactile, Thermal, Gustatory, Olfactory, and Auditory Sensations Mimicked by Artificial Hybrid Fluid (HF) Rubber Mechanoreceptors

In order to advance the development of sensors fabricated with monofunctional sensation systems capable of a versatile response to tactile, thermal, gustatory, olfactory, and auditory sensations, mechanoreceptors fabricated as a single platform with an electric circuit require investigation. In addition, it is essential to resolve the complicated structure of the sensor. In order to realize the single platform, our proposed hybrid fluid (HF) rubber mechanoreceptors of free nerve endings, Merkel cells, Krause end bulbs, Meissner corpuscles, Ruffini endings, and Pacinian corpuscles mimicking the bio-inspired five senses are useful enough to facilitate the fabrication process for the resolution of the complicated structure. This study used electrochemical impedance spectroscopy (EIS) to elucidate the intrinsic structure of the single platform and the physical mechanisms of the firing rate such as slow adaption (SA) and fast adaption (FA), which were induced from the structure and involved the capacitance, inductance, reactance, etc. of the HF rubber mechanoreceptors. In addition, the relations among the firing rates of the various sensations were clarified. The adaption of the firing rate in the thermal sensation is the opposite of that in the tactile sensation. The firing rates in the gustation, olfaction, and auditory sensations at frequencies of less than 1 kHz have the same adaption as in the tactile sensation. The present findings are useful not only in the field of neurophysiology, to research the biochemical reactions of neurons and brain perceptions of stimuli, but also in the field of sensors, to advance salient developments in sensors mimicking bio-inspired sensations.

Estimation of Fast and Slow Adaptions in the Tactile Sensation of Mechanoreceptors Mimicked by Hybrid Fluid (HF) Rubber with Equivalent Electric Circuits and Properties

In order to advance engineering applications of robotics such as wearable health-monitoring devices, humanoid robots, etc., it is essential to investigate the tactile sensations of artificial haptic sensors mimicking bioinspired human cutaneous mechanoreceptors such as free nerve endings, Merkel's cells, Krause end bulbs, Meissner corpuscles, Ruffini endings, and Pacinian corpuscles. The generated receptor's potential response to extraneous stimuli, categorized as slow adaption (SA) or fast adaption (FA), is particularly significant as a typical property. The present study addressed the estimation of SA and FA by utilizing morphologically fabricated mechanoreceptors made of our proposed magnetically responsive intelligent fluid, hybrid fluid (HF), and by applying our proposed electrolytic polymerization. Electric circuit models of the mechanoreceptors were generated using experimental data on capacitance and inductance on the basis of the electric characteristics of impedance. The present results regarding equivalent firing rates based on FA and SA are consistent with the FA and SA findings of vital mechanoreceptors by biomedical analysis. The present investigative process is useful to clarify the time of response to a force on the fabricated artificial mechanoreceptor.

Morphological Configuration of Sensory Biomedical Receptors Based on Structures Integrated by Electric Circuits and Utilizing Magnetic-Responsive Hybrid Fluid (HF)

Biomedical receptors such as cutaneous receptors or intelligent cells with tactile, auditory, gustatory, and olfactory sensations function in the five senses of the human body. Investigations focusing on the configuration of such receptors are useful in the fields of robotics and sensors in the food industry, among others, which involve artificial organs or sensory machines. In the present study, we aimed to produce the receptors for four senses (excepting vision) by morphologically mimicking virtual human ones. The mimicked receptors were categorized into eight types of configured structure. Our proposed magnetic-responsive hybrid fluid (HF) in elastic and soft rubber and proposed electrolytic polymerization technique gave the solidified HF rubber electric characteristics of piezoelectricity and piezo-capacity, among others. On the basis of these electric characteristics, the mimicked receptors were configured in various types of electric circuits. Through experimental estimation of mechanical force, vibration, thermal, auditory, gustatory, and olfactory responses of each receptor, the optimum function of each was specified by comparison with the actual sensations of the receptors. The effect of hairs fabricated in the receptors was also clarified to viably reproduce the distinctive functions of these sensations.

Modelling the effects of age-related morphological and mechanical skin changes on the stimulation of tactile mechanoreceptors

Our sense of fine touch deteriorates as we age, a phenomenon typically associated with neurological changes to the skin. However, geometric and material changes to the skin may also play an important role on tactile perception and have not been studied in detail. Here, a finite element model is utilised to assess the extent to which age-related structural changes to the skin influence the tactile stimuli experienced by the mechanoreceptors. A numerical, hyperelastic, four-layered skin model was developed to simulate sliding of the finger against a rigid surface. The strain, deviatoric stress and strain energy density were recorded at the sites of the Merkel and Meissner receptors, whilst parameters of the model were systematically varied to simulate age-related geometric and material skin changes. The simulations comprise changes in skin layer stiffness, flattening of the dermal-epidermal junction and thinning of the dermis. It was found that the stiffness of the skin layers has a substantial effect on the stimulus magnitudes recorded at mechanoreceptors. Additionally, reducing the thickness of the dermis has a substantial effect on the Merkel disc whilst the Meissner corpuscle is particularly affected by flattening of the dermal epidermal junction. In order to represent aged skin, a model comprising a combination of ageing manifestations revealed a decrease in stimulus magnitudes at both mechanoreceptor sites. The result from the combined model differed from the sum of effects of the individually tested ageing manifestations, indicating that the individual effects of ageing cannot be linearly superimposed. Each manifestation of ageing results in a decreased stimulation intensity at the Meissner Corpuscle site, suggesting that ageing reduces the proportion of stimuli meeting the receptor amplitude detection threshold. This model therefore offers an additional biomechanical explanation for tactile perceptive degradation amongst the elderly. Applications of the developed model are in the evaluation of cosmetics products aimed at mitigating the effects of ageing, e.g. through skin hydration and administration of antioxidants, as well as in the design of products with improved tactile sensation, e.g. through the optimisation of materials and surface textures.

The cellular and molecular basis of direction selectivity of Aδ-LTMRs

The perception of touch, including the direction of stimulus movement across the skin, begins with activation of low-threshold mechanosensory neurons (LTMRs) that innervate the skin. Here, we show that murine Aδ-LTMRs are preferentially tuned to deflection of body hairs in the caudal-to-rostral direction. This tuning property is explained by the finding that Aδ-LTMR lanceolate endings around hair follicles are polarized; they are concentrated on the caudal (downward) side of each hair follicle. The neurotrophic factor BDNF is synthesized in epithelial cells on the caudal, but not rostral, side of hair follicles, in close proximity to Aδ-LTMR lanceolate endings, which express TrkB. Moreover, ablation of BDNF in hair follicle epithelial cells disrupts polarization of Aδ-LTMR lanceolate endings and results in randomization of Aδ-LTMR responses to hair deflection. Thus, BDNF-TrkB signaling directs polarization of Aδ-LTMR lanceolate endings, which underlies direction-selective responsiveness of Aδ-LTMRs to hair deflection.

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.

Response properties of temporomandibular joint mechanosensitive neurons in the trigeminal sensory complex of the rabbit

The neurophysiological properties of neurons sensitive to TMJ movement (TMJ neurons) in the trigeminal sensory complex (Vcomp) during passive movement of the isolated condyle were examined in 46 rabbits. Discharges of TMJ neurons from the rostral part of the Vcomp were recorded with a microelectrode when the isolated condyle was moved manually and with a computer-regulated mechanostimulator. A total of 443 neurons responding to mechanical stimulation of the face and oral cavity were recorded from the brainstem. Twenty-one TMJ neurons were detected rostrocaudally from the dorsal part of the trigeminal principal sensory nucleus (NVsnpr), subnucleus oralis of the trigeminal spinal nucleus, and reticular formation surrounding the trigeminal motor nucleus. Most of the TMJ neurons were located in the dorso-rostral part of the NVsnpr. Of the TMJ units recorded, 90 % were slowly adapting and 26 % had an accompanying resting discharge. The majority (86 %) of the TMJ units responded to the movement of the isolated condyle in the anterior and/or ventral directions, and half were sensitive to the condyle movement in a single direction. The discharge frequencies of TMJ units increased as the condyle displacement and constant velocity (5 mm/s) increased within a 5-mm anterior displacement of the isolated condyle. Based on these results, we conclude that sensory information is processed by TMJ neurons encoding at least joint position and displacement in the physiological range of mandibular displacement.

Texture-induced vibrations in the forearm during tactile exploration

Humans can detect and discriminate between fine variations of surface roughness using active touch. It is hitherto believed that roughness perception is mediated mostly by cutaneous and subcutaneous afferents located in the fingertips. However, recent findings have shown that following abolishment of cutaneous afferences resulting from trauma or pharmacological intervention, the ability of subjects to discriminate between textures roughness was not significantly altered. These findings suggest that the somatosensory system is able to collect textural information from other sources than fingertip afference. It follows that signals resulting of the interaction of a finger with a rough surface must be transmitted to stimulate receptor populations in regions far away from the contact. This transmission was characterized by measuring in the wrist vibrations originating at the fingertip and thus propagating through the finger, the hand and the wrist during active exploration of textured surfaces. The spectral analysis of the vibrations taking place in the forearm tissues revealed regularities that were correlated with the scanned surface and the speed of exploration. In the case of periodic textures, the vibration signal contained a fundamental frequency component corresponding to the finger velocity divided by the spatial period of the stimulus. This regularity was found for a wide range of textural length scales and scanning velocities. For non-periodic textures, the spectrum of the vibration did not contain obvious features that would enable discrimination between the different stimuli. However, for both periodic and non-periodic stimuli, the intensity of the vibrations could be related to the microgeometry of the scanned surfaces.

Collagen XXVIII is a distinctive component of the peripheral nervous system nodes of ranvier and surrounds nonmyelinating glial cells

Growing evidence indicates that collagens perform crucial functions during the development and organization of the nervous system. Collagen XXVIII is a recently discovered collagen almost exclusively expressed in the peripheral nervous system (PNS). In this study, we show that this collagen is associated with nonmyelinated regions of the PNS. With the notable exception of type II terminal Schwann cell in the hairy skin, collagen XXVIII surrounds all nonmyelinating glial cells studied. This includes satellite glial cells of the dorsal root ganglia, terminal Schwann cells type I around mechanoceptors in the skin, terminal Schwann cells around proprioceptors in the muscle spindle or at the neuromuscular junction and olfactory ensheathing cells. Collagen XXVIII is also detected at nodes of Ranvier where the myelin sheath of myelinated fibers is interrupted and is thus a distinctive component of the PNS nodal gap. The correlation between the absence of myelin and the presence of collagen XXVIII is confirmed in a mouse model of Charcot-Marie-Tooth characterized by dysmyelinated nerve fibers, in which enhancement of collagen XXVIII labeling is observed.

Merkel Cells in Somatosensation

Merkel cells are rare epidermal cells whose function in the skin is still debated. These cells localize to highly touch-sensitive areas of vertebrate epithelia, including palatine ridges, touch domes and finger tips. In most cases, Merkel cells complex with somatosensory afferents to form slowly adapting touch receptors; it is unclear, however, whether mechanosensory transduction occurs in the Merkel cell, the somatosensory afferent or both. Classic anatomical results suggests that Merkel cells are sensory cells that transduce mechanical stimuli and then communicate with sensory afferents via neurotransmission. This model is supported by recent molecular, immunohistochemical and physiological studies of Merkel cells in vitro and in intact tissues. For example, Merkel cells express essential components of presynaptic machinery, including molecules required for release of the excitatory neurotransmitter glutamate. Moreover, Merkel cells in vitro and in vivo are activated by mechanical stimuli, including hypotonic-induced cell swelling. Although these findings support the hypothesis that Merkel cells are sensory receptor cells, a definitive demonstration that Merkel cells are necessary and sufficient to transduce touch awaits future studies.

Overexpression of neurotrophin-3 enhances the mechanical response properties of slowly adapting type 1 afferents and myelinated nociceptors

Constitutive overexpression of neurotrophin-3 (NT3) in murine skin results in an increased number of sensory neurons within the dorsal root ganglia, an increase of myelinated axons in cutaneous nerves, hyperinnervation of the skin, and an increased number of Merkel cells found in flank skin. Here we used a saphenous skin/nerve preparation to determine if these anatomical changes affect the functional response characteristics of cutaneous sensory neurons. Overexpression of NT3 significantly increased the responses of slowly adapting type 1 (SA1) low-threshold mechanoreceptors and Adelta high-threshold mechanoreceptors to suprathreshold mechanical stimulation. It also resulted in significantly faster conduction velocities of SA1 fibers. In contrast to earlier findings in flank skin, no differences were noted in the numbers of Merkel cells in the touch domes in hindlimb skin of NT3-overexpressing mice. In addition, the number of dermal Merkel cells, located around hair follicles on the dorsum of the foot, was reduced by 55%. The increase in mechanical sensitivity was found to correlate with significant increases in the expression of acid-sensing ion channels (ASIC) 1 and 3. Additional experiments using intracellular recordings and staining procedures confirmed that at least some cutaneous myelinated nociceptors and SA1 mechanoreceptors stained positively for both trkC and ASIC3. These results indicate that cutaneous NT3 overexpression alters the response properties of specific cutaneous sensory neurons, and that these changes may be due to the modulation of putative mechanosensitive ion channels.

Role of peripheral hyperpolarization-activated cyclic nucleotide-modulated channel pacemaker channels in acute and chronic pain models in the rat

Hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channels contribute to rhythmic spontaneous activity in the heart and CNS. Ectopic spontaneous neuronal activity has been implicated in the development and maintenance of acute and chronic hyperalgesia, allodynia and spontaneous pain. Previously, we documented that systemic administration of ZD7288, a specific blocker of pacemaker current (I(h)), decreased ectopic activity in dorsal root ganglion (DRG) and reversed tactile allodynia in spinal nerve ligated (SNL) rats [Chaplan SR, Guo HQ, Lee DH, Luo L, Liu C, Kuei C, Velumian AA, Butler MP, Brown SM, Dubin AE (2003) Neuronal hyperpolarization-activated pacemaker channels drive neuropathic pain. J Neurosci 23:1169-1178]. Spontaneous pain is the chief clinical manifestation of peripheral nerve injury; however, a role for I(h) in spontaneous pain has not been described. Here, in further rat studies, we report that systemic administration of ZD7288 reversed spontaneous pain induced by mild thermal injury (MTI) and tactile allodynia induced by SNL and MTI. In contrast, ZD7288 did not reduce thermal hyperalgesia. An important locus of action appears to be in the skin since intraplantar (local) administration of ZD7288 completely suppressed tactile allodynia arising from MTI and SNL and reduced spontaneous pain due to MTI. Immunohistochemical staining of plantar skin sections detected HCN1-HCN4 expression in mechanosensory structures (e.g., Meissner's corpuscles and Merkel cells). Collectively, these data suggest that expression and modulation of I(h) in the peripheral nervous system, including specialized sensory structures, may play a significant role in sensory processing and contribute to spontaneous pain and tactile allodynia.

Mechanosensory afferent input and neuronal firing properties in rodent spinal laminae III-V: re-examination of relationships with analysis of responses to static and time-varying stimuli

Relationships between neuronal firing pattern and mechanosensory input in the deep dorsal horn were investigated using whole-cell recordings from isolated hamster spinal cord with innervation from an attached skin patch. Neurons that fired repetitively to depolarizing current (tonic cells) responded to both moving and static stimulation of their cutaneous receptive fields, and discharged continuously for the duration of stimulus application. Neurons responding to depolarizing current with transient, rapidly adapting firing (phasic cells) were significantly more responsive to stimulus movement than to static skin contact. Phasic cells typically issued a brief discharge at the onset or termination of a stimulus; their responses during static skin contact were weaker than tonic cells. Tonic cells were activated during both ramp and steady-state skin indentations, whereas phasic cells responded with their strongest excitation to displacement velocities exceeding 8 microm/ms. Mechanosensory input to phasic cells originated primarily from low threshold receptors, whereas tonic cells demonstrated a mixture of inputs from both low and high threshold sources. A third class of neurons responded to depolarizing current with a pronounced firing delay and displayed a sensitivity to cutaneous stimuli that was similar to tonic cells except they showed a modest decrease in firing as skin indentation velocity increased. The results suggest a correlation between functional properties of mechanoreceptive afferent fibers and intrinsic discharge properties of laminae III-V neurons that may significantly influence integration of cutaneous mechanosensory information at the first spinal relay.

Airway mechanosensors

A mechanosensory unit is a functional unit that contains multiple receptors (or encoders) with different characteristics, including rapidly adapting receptors, slowly adapting receptors, and deflation-activated-receptors. Each is capable of sensing different aspects of lung mechanics. The sensory unit is both a transducer and a processor. Significant information integration occurs at the intra-encoder and inter-encoder levels. Within an encoder, the information is encoded as analog signals and integrated by amplitude modulation. Information from each single stretch-activated channel is processed through several levels of temporal and spatial summation, producing a generator potential that encodes averaged overall information within the encoder. This analog signal is transformed into a digital signal in the form of action potentials that are encoded as frequency (frequency modulation). These all-or-none propagated action potentials from different encoders interact through a competitive selection mechanism. Such inter-encoder interaction may occur at several levels, because of the fractal nature of the sensory unit. Inter-encoder interaction retains representative information but eliminates redundant information, resulting in the final output to the central nervous system, where multiple decoders specific for different variables decipher the encoded information for further processing.

Proneural and proneuroendocrine transcription factor expression in cutaneous mechanoreceptor (Merkel) cells and Merkel cell carcinoma

Merkel cells form part of the peripheral neuroendocrine system of the skin and act as mechanoreceptors in touch response. Merkel cell carcinoma (MCC) is a rare, aggressive disease with similarities to small cell lung cancer (SCLC), which is also of neuroendocrine origin. We previously identified a novel DNA binding protein complex specific for MCC suspension cell lines, termed Merkel nuclear factor (MNF) by its binding to the POU-IV family DNA binding consensus sequence. Here we report that MNF contains the POU-IV family member Brn-3c and that Brn-3c is expressed in normal Merkel cells. Additionally, Brn-3c protein reactivity is restricted to a subset of MCC biopsies and is not seen in biopsies revealing adherent, variant cell lines lacking neuroendocrine markers. Recently, proper development of murine Merkel cells was shown to require the proneural basic helix-loop-helix transcription factor, atonal family member, MATH1. We demonstrate a correlation between Brn-3c and HATH1 reactivity in MCC biopsies and cell lines with retention of neuroendocrine phenotype. In SCLC, the related basic helix-loop-helix transcription factor HASH1 is responsible for neuroendocrine phenotype, but HASH1 transcripts were not detected in MCC cell lines. We propose that HATH1 and Brn-3c may form a transcriptional hierarchy responsible for determining neuroendocrine phenotype in Merkel cells and that lack of Brn-3c and/or HATH1 in MCC may indicate a more aggressive disease requiring closer patient follow-up.

Hair cycle-dependent plasticity of skin and hair follicle innervation in normal murine skin

The innervation of normal, mature mammalian skin is widely thought to be constant. However, the extensive skin remodeling accompanying the transformation of hair follicles from resting stage through growth and regression back to resting (telogen-anagen-catagen-telogen) may also be associated with alteration of skin innervation. We, therefore, have investigated the innervation of the back skin of adolescent C57BL/6 mice at various stages of the depilation-induced hair cycle. By using antisera against neuronal (protein gene product 9.5 [PGP 9.5], neurofilament 150) and Schwann cell (S-100, myelin basic protein) markers, as well as against neural cell adhesion molecule (NCAM) and growth-associated protein-43 (GAP-43), we found a dramatic increase of single fibers within the dermis and subcutis during early anagen. This was paralleled by an increase in the number of anastomoses between the cutaneous nerve plexuses and by distinct changes in the nerve fiber supply of anagen vs. telogen hair follicles. The follicular isthmus, including the bulge, the seat of epithelial follicle stem cells, was found to be the most densely innervated skin area. Here, a defined subpopulation of nerve fibers increased in number during anagen and declined during catagen, accompanied by dynamic alterations in the expression of NCAM and GAP-43. Thus, our study provides evidence for a surprising degree of plasticity of murine skin innervation. Because hair cycle-associated tissue remodeling evidently is associated with tightly regulated sprouting and regression of nerve fibers, hair cycle-dependent alterations in murine skin and hair follicle innervation offer an intriguing model for studying the controlled rearrangement of neuronal networks in peripheral tissues under physiological conditions.