Touch Domes Research Articles

Tenascin-C expressing touch dome keratinocytes exhibit characteristics of all epidermal lineages.

The touch dome (TD) keratinocytes are specialized epidermal cells that intimately associate with the light touch sensing Merkel cells (MCs). The TD keratinocytes function as a niche for the MCs and can induce de novo hair follicles upon stimulation; however, how the TD keratinocytes are maintained during homeostasis remains unclear. scRNA-seq identified a specific TD keratinocyte marker, Tenascin-C (TNC). Lineage tracing of Tnc-expressing TD keratinocytes revealed that these cells maintain themselves as an autonomous epidermal compartment and give rise to MCs upon injury. Molecular characterization uncovered that, while the transcriptional and chromatin landscape of the TD keratinocytes is remarkably similar to that of the interfollicular epidermal keratinocytes, it also shares certain molecular signatures with the hair follicle keratinocytes. Our study highlights that the TD keratinocytes in the adult skin have molecular characteristics of keratinocytes of diverse epidermal lineages.

329 In vitro co-culture models of merkel cell-neurite complex in touch domes

Touch dome, a specialized epidermal structure comprised from mechanosensory Merkel cell-neurite complex, has been known to change their numbers in the spontaneous hair cycle in mice. At the anagen phase, the number of Merkel cells are dramatically reduced with the loss of terminal neurites in touch dome afferents, indicating neuronal plasticity in healthy skin during hair-follicle regeneration. In this study, we focused on the role of Merkel cell-neurite complex in hair growth on the basis of the reports that the mechanical stimulation of human scalp by a massage improves hair thickness.

Effects on tactile transmission by serotonin transporter inhibitors at Merkel discs of mouse whisker hair follicles.

The Merkel disc is a main type of tactile end organs formed by Merkel cells and Aβ-afferent endings as first tactile sensory synapses. They are highly abundant in fingertips, touch domes, and whisker hair follicles of mammals and are essential for sensory tasks including social interaction, environmental exploration, and tactile discrimination. We have recently shown that Merkel discs use serotonin to transmit tactile signals from Merkel cells to Aβ-afferent endings to drive slowly adapting type 1 impulses on the Aβ-afferent nerves. This raises a question as whether the serotoninergic transmission at Merkel discs may be regulated by serotonin transporters and whether serotonin transporter inhibitors may affect the tactile transmission. Here, we made recordings from whisker afferent nerves of mouse whisker hair follicles and tested the effects of monoamine transporter inhibitors on slowly adapting type 1 impulses. We show that methamphetamine, a monoamine releasing facilitator and reuptake inhibitor, elicited spontaneous impulses as well as increased the numbers of slowly adapting type 1 impulses elicited by whisker hair deflections. S-duloxetine, a potent inhibitor of transporters of serotonin and norepinephrine, and fluoxetine, a selective inhibitor of serotonin transporters, both also increased the numbers of slowly adapting type 1 impulses. Prolonged treatment of whisker hair follicles with methamphetamine abolished most of slowly adapting type 1 impulses. Furthermore, the treatment of whisker hair follicles with methamphetamine resulted in serotonin release from whisker hair follicles. Taken together, our results suggest that serotonin transporters play a role in regulating tactile transmission at Merkel discs.

869 The specification of Merkel cell in the back skin and glabrous paw skin is controlled by FGFR2-meditated signaling

Merkel cells are mechanosensory cells responsible for light touch sensations. In murine hairy skin, Merkel cells are located in touch domes in back skin and in whisker pads. Merkel cells are also present in the glabrous paw skin. While it has been shown that Merkel cells are of epidermal lineage, their progenitor and the regulators that are critical for their specification have not been extensively study. Here, we show that during embryogenesis, SOX9 positive (+) cells located in hair placodes, which were previously known to give rise to hair follicle stem cells, also give rise to Merkel cells in the back skin. In contrast, in the glabrous paw skin, SOX9(+) cells do not give rise to Merkel cells suggesting that hairy and glabrous skins have different Merkel cell precursor populations. We performed RNA-seq transcriptional profiling of Merkel cells on the back and glabrous paw skin and showed that their transcriptomes are similar suggesting that their formation is controlled by similar regulators. Among these regulators is FGFR2-mediated signaling that is essential for Merkel cell formation in both the back skin and glabrous paw skin. Taken together, our results demonstrate that although the precursor cells of Merkel cells in the murine back skin and glabrous paw skin might differ, their formation is controlled by a common genetic program and that FGFR2 plays a critical role in this process.

Dissection of Merkel cell formation in hairy and glabrous skin reveals a common requirement for FGFR2‐mediated signalling

Merkel cells are mechanosensory cells involved in tactile discrimination. Merkel cells have been primarily studied in the murine back skin, where they are found in specialized structures called touch domes located around primary hair follicles. Yet, little is known about the morphogenesis of Merkel cells in areas of the skin devoid of hair, such as the glabrous paw skin. Here, we describe Merkel cell formation in the glabrous paw skin during embryogenesis. We first found in the glabrous paw skin that Merkel cells were specified at E15.5, 24hours later, compared to in the back skin. Additionally, by performing lineage-tracing experiments, we found that unlike in the back skin, SOX9(+) cells do not give rise to Merkel cells in the glabrous paw skin. Finally, we compared the transcriptomes of Merkel cells in the back and the glabrous paw skin and showed that they are similar. Genetic and transcriptome studies showed that the formation of Merkel cells in both regions was controlled by similar regulators. Among them was FGFR2, an upstream factor of MAPK signalling that was reported to have a critical function in Merkel cell formation in the back skin. Here, we showed that FGFR2 is also required for Merkel cell development in the glabrous paw skin. Taken together, our results demonstrate that Merkel cells in the murine back skin and glabrous paw skin are similar, and even though their formation is controlled by a common genetic programme, their precursor cells might differ.

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.

All in the Family – Touch Versus Olfaction in Moles

Here I review, compare, and contrast the neurobiology and behavior of the common, eastern mole (Scalopus aquaticus) and the star-nosed mole (Condylura cristata). These two species are part of the same family (Talpidae) and have similar body size and general morphology. But they differ in sensory specializations, complexity of neocortical organization, and behavior. The star-nosed mole has an elaborate mechanosensory organ-the star-consisting of 22 epidermal appendages (rays) covered with 25,000 touch domes called Eimer's organs. This densely innervated structure is represented in the neocortex in three different somatosensory maps, each visible in flattened neocortical sections as a series of 11 modules representing the 11 rays from the contralateral body. The 11th ray is greatly magnified in primary somatosensory cortex (S1). Behavioral studies show the star is moved in a saccadic manner and the 11th ray is a high-resolution tactile fovea, allowing star-nosed moles to forage on small prey with unprecedented speed and efficiency. In contrast, common mole noses lack Eimer's organs, their neocortex contains only two cortical maps of the nose, and they cannot localize small prey. Yet common moles have exceptional olfactory abilities, sniffing in stereo to rapidly localize discrete odor sources originating from larger prey. In addition, common moles are shown to track odorant trails laid down by moving prey. These results highlight the surprising abilities of species once thought to be simple, and the usefulness of diverse species in revealing general principles of brain organization and behavior. Anat Rec, 2019. © 2019 American Association for Anatomy.

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.

Sonic hedgehog signaling in epithelial tissue development.

The Sonic hedgehog (SHH) signaling pathway is essential for embryonic development and tissue regeneration. The dysfunction of SHH pathway is involved in a variety of diseases, including cancer, birth defects, and other diseases. Here we reviewed recent studies on main molecules involved in the SHH signaling pathway, specifically focused on their function in epithelial tissue and appendages development, including epidermis, touch dome, hair, sebaceous gland, mammary gland, tooth, nail, gastric epithelium, and intestinal epithelium. The advance in understanding the SHH signaling pathway will give us more clues to the mechanisms of tissue repair and regeneration, as well as the development of new treatment for diseases related to dysregulation of SHH signaling pathway.

Disinhibition of Touch-Evoked Itch in a Mouse Model of Psoriasis

Disinhibition of Touch-Evoked Itch in a Mouse Model of Psoriasis

FGF signalling controls the specification of hair placode-derived SOX9 positive progenitors to Merkel cells

Merkel cells are innervated mechanosensory cells responsible for light-touch sensations. In murine dorsal skin, Merkel cells are located in touch domes and found in the epidermis around primary hairs. While it has been shown that Merkel cells are skin epithelial cells, the progenitor cell population that gives rise to these cells is unknown. Here, we show that during embryogenesis, SOX9-positive (+) cells inside hair follicles, which were previously known to give rise to hair follicle stem cells (HFSCs) and cells of the hair follicle lineage, can also give rise to Merkel Cells. Interestingly, while SOX9 is critical for HFSC specification, it is dispensable for Merkel cell formation. Conversely, FGFR2 is required for Merkel cell formation but is dispensable for HFSCs. Together, our studies uncover SOX9(+) cells as precursors of Merkel cells and show the requirement for FGFR2-mediated epithelial signalling in Merkel cell specification.

Notch pathway signaling in the skin antagonizes Merkel cell development

Merkel cells are mechanosensitive skin cells derived from the epidermal lineage whose development requires expression of the basic helix-loop-helix transcription factor Atoh1. The genes and pathways involved in regulating Merkel cell development during embryogenesis are poorly understood. Notch pathway signaling antagonizes Atoh1 expression in many developing body regions, so we hypothesized that Notch signaling might inhibit Merkel cell development. We found that conditional, constitutive overexpression of the Notch intracellular domain (NICD) in mouse epidermis significantly decreased Merkel cell numbers in whisker follicles and touch domes of hairy skin. Conversely, conditional deletion of the obligate NICD binding partner RBPj in the epidermis significantly increased Merkel cell numbers in whisker follicles, led to the development of ectopic Merkel cells outside of touch domes in hairy skin epidermis, and altered the distribution of Merkel cells in touch domes. Deletion of the downstream Notch effector gene Hes1 also significantly increased Merkel cell numbers in whisker follicles. Together, these data demonstrate that Notch signaling regulates Merkel cell production and patterning.

422 The distribution and ultrastructure of the touch dome in human hairy skin

422 The distribution and ultrastructure of the touch dome in human hairy skin

Merkel cells are long-lived cells whose production is stimulated by skin injury

Mechanosensitive Merkel cells are thought to have finite lifespans, but controversy surrounds the frequency of their replacement and which precursor cells maintain the population. We found by embryonic EdU administration that Merkel cells undergo terminal cell division in late embryogenesis and survive long into adulthood. We also found that new Merkel cells are produced infrequently during normal skin homeostasis and that their numbers do not change during natural or induced hair cycles. In contrast, live imaging and EdU experiments showed that mild mechanical injury produced by skin shaving dramatically increases Merkel cell production. We confirmed with genetic cell ablation and fate-mapping experiments that new touch dome Merkel cells in adult mice arise from touch dome keratinocytes. Together, these independent lines of evidence show that Merkel cells in adult mice are long-lived, are replaced rarely during normal adult skin homeostasis, and that their production can be induced by repeated shaving. These results have profound implications for understanding sensory neurobiology and human diseases such as Merkel cell carcinoma.

A Cascade of Wnt, Eda, and Shh Signaling Is Essential for Touch Dome Merkel Cell Development.

The Sonic hedgehog (Shh) signaling pathway regulates developmental, homeostatic, and repair processes throughout the body. In the skin, touch domes develop in tandem with primary hair follicles and contain sensory Merkel cells. The developmental signaling requirements for touch dome specification are largely unknown. We found dermal Wnt signaling and subsequent epidermal Eda/Edar signaling promoted Merkel cell morphogenesis by inducing Shh expression in early follicles. Lineage-specific gene deletions revealed intraepithelial Shh signaling was necessary for Merkel cell specification. Additionally, a Shh signaling agonist was sufficient to rescue Merkel cell differentiation in Edar-deficient skin. Moreover, Merkel cells formed in Fgf20 mutant skin where primary hair formation was defective but Shh production was preserved. Although developmentally associated with hair follicles, fate mapping demonstrated Merkel cells primarily originated outside the hair follicle lineage. These findings suggest that touch dome development requires Wnt-dependent mesenchymal signals to establish reciprocal signaling within the developing ectoderm, including Eda signaling to primary hair placodes and ultimately Shh signaling from primary follicles to extrafollicular Merkel cell progenitors. Shh signaling often demonstrates pleiotropic effects within a structure over time. In postnatal skin, Shh is known to regulate the self-renewal, but not the differentiation, of touch dome stem cells. Our findings relate the varied effects of Shh in the touch dome to the ligand source, with locally produced Shh acting as a morphogen essential for lineage specification during development and neural Shh regulating postnatal touch dome stem cell maintenance.

Polycomb-Mediated Repression and Sonic Hedgehog Signaling Interact to Regulate Merkel Cell Specification during Skin Development.

An increasing amount of evidence indicates that developmental programs are tightly regulated by the complex interplay between signaling pathways, as well as transcriptional and epigenetic processes. Here, we have uncovered coordination between transcriptional and morphogen cues to specify Merkel cells, poorly understood skin cells that mediate light touch sensations. In murine dorsal skin, Merkel cells are part of touch domes, which are skin structures consisting of specialized keratinocytes, Merkel cells, and afferent neurons, and are located exclusively around primary hair follicles. We show that the developing primary hair follicle functions as a niche required for Merkel cell specification. We find that intraepidermal Sonic hedgehog (Shh) signaling, initiated by the production of Shh ligand in the developing hair follicles, is required for Merkel cell specification. The importance of Shh for Merkel cell formation is further reinforced by the fact that Shh overexpression in embryonic epidermal progenitors leads to ectopic Merkel cells. Interestingly, Shh signaling is common to primary, secondary, and tertiary hair follicles, raising the possibility that there are restrictive mechanisms that regulate Merkel cell specification exclusively around primary hair follicles. Indeed, we find that loss of Polycomb repressive complex 2 (PRC2) in the epidermis results in the formation of ectopic Merkel cells that are associated with all hair types. We show that PRC2 loss expands the field of epidermal cells competent to differentiate into Merkel cells through the upregulation of key Merkel-differentiation genes, which are known PRC2 targets. Importantly, PRC2-mediated repression of the Merkel cell differentiation program requires inductive Shh signaling to form mature Merkel cells. Our study exemplifies how the interplay between epigenetic and morphogen cues regulates the complex patterning and formation of the mammalian skin structures.

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 Cell Polyomavirus Small T Antigen Induces Cancer and Embryonic Merkel Cell Proliferation in a Transgenic Mouse Model.

Merkel cell polyomavirus (MCV) causes the majority of human Merkel cell carcinomas (MCC) and encodes a small T (sT) antigen that transforms immortalized rodent fibroblasts in vitro. To develop a mouse model for MCV sT-induced carcinogenesis, we generated transgenic mice with a flox-stop-flox MCV sT sequence homologously recombined at the ROSA locus (ROSA sT), allowing Cre-mediated, conditional MCV sT expression. Standard tamoxifen (TMX) administration to adult Ubc CreERT2; ROSA sT mice, in which Cre is ubiquitously expressed, resulted in MCV sT expression in multiple organs that was uniformly lethal within 5 days. Conversely, most adult Ubc CreERT2; ROSA sT mice survived low-dose tamoxifen administration but developed ear lobe dermal hyperkeratosis and hypergranulosis. Simultaneous MCV sT expression and conditional homozygous p53 deletion generated multi-focal, poorly-differentiated, highly anaplastic tumors in the spleens and livers of mice after 60 days of TMX treatment. Mouse embryonic fibroblasts from these mice induced to express MCV sT exhibited anchorage-independent cell growth. To examine Merkel cell pathology, MCV sT expression was also induced during mid-embryogenesis in Merkel cells of Atoh1 CreERT2/+; ROSA sT mice, which lead to significantly increased Merkel cell numbers in touch domes at late embryonic ages that normalized postnatally. Tamoxifen administration to adult Atoh1 CreERT2/+; ROSA sT and Atoh1 CreERT2/+; ROSA sT; p53f lox/flox mice had no effects on Merkel cell numbers and did not induce tumor formation. Taken together, these results show that MCV sT stimulates progenitor Merkel cell proliferation in embryonic mice and is a bona fide viral oncoprotein that induces full cancer cell transformation in the p53-null setting.

Neural Hedgehog signaling maintains stem cell renewal in the sensory touch dome epithelium

The touch dome is a highly patterned mechanosensory structure in the epidermis composed of specialized keratinocytes in juxtaposition with innervated Merkel cells. The touch dome epithelium is maintained by tissue-specific stem cells, but the signals that regulate the touch dome are not known. We identify touch dome stem cells that are unique among epidermal cells in their activated Hedgehog signaling and ability to maintain the touch dome as a distinct lineage compartment. Skin denervation reveals that renewal of touch dome stem cells requires a perineural microenvironment, and deleting Sonic hedgehog (Shh) in neurons or Smoothened in the epidermis demonstrates that Shh is an essential niche factor that maintains touch dome stem cells. Up-regulation of Hedgehog signaling results in neoplastic expansion of touch dome keratinocytes but no Merkel cell neoplasia. These findings demonstrate that nerve-derived Shh is a critical regulator of lineage-specific stem cells that maintain specialized sensory compartments in the epidermis.

Tumorigenesis: A hairy and nervous start.

Peterson et al. report that basal cell carcinomas primarily arise from stem cells within hair follicle and touch dome epithelia, and that cutaneous nerves promote tumorigenesis.