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A major role of the skin is to serve as a barrier toward the environment. The skin's permeability barrier consists of a lipid structure positioned in the stratum corneum. Recent progress in high-resolution cryo-electron microscopy (cryo-EM) has allowed for elucidation of the architecture of the skin's barrier and its stepwise formation process representing the final stage of epidermal differentiation. In this review, we present an overview of the skin's barrier structure and its formation process, as evidenced by cryo-EM., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

Skin expansion during development is predominantly driven by growth of basal epithelial cell (BEC)-derived clonal populations, which often display varied sizes and shapes. However, little is known about the causes of clonal heterogeneity and the maximum size to which a single clone can grow. Here, we created a zebrafish model, basebow, for capturing clonal growth behavior in the BEC population on a whole-body, centimeter scale. By tracking 222 BECs over the course of a 28-fold expansion of body surface area, we determined that most BECs survive and grow clonal populations with an average size of 0.013 mm2. An extensive survey of 742 sparsely labeled BECs further revealed that giant dominant clones occasionally arise on specific body regions, covering up to 0.6% of the surface area. Additionally, a growth-induced extracellular matrix component, Lamb1a, mediates clonal growth in a cell-autonomous manner. Altogether, our findings demonstrate how clonal heterogeneity and clonal dominance may emerge to enable post-embryonic growth of a vertebrate organ, highlighting key cellular mechanisms that may only become evident when visualizing single cell behavior at the whole-animal level., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2021. Published by The Company of Biologists Ltd.)

The collective polarization of cellular structures and behaviors across a tissue plane is a near universal feature of epithelia known as planar cell polarity (PCP). This property is controlled by the core PCP pathway, which consists of highly conserved membrane-associated protein complexes that localize asymmetrically at cell junctions. Here, we introduce three new mouse models for investigating the localization and dynamics of transmembrane PCP proteins: Celsr1, Fz6 and Vangl2. Using the skin epidermis as a model, we characterize and verify the expression, localization and function of endogenously tagged Celsr1-3xGFP, Fz6-3xGFP and tdTomato-Vangl2 fusion proteins. Live imaging of Fz6-3xGFP in basal epidermal progenitors reveals that the polarity of the tissue is not fixed through time. Rather, asymmetry dynamically shifts during cell rearrangements and divisions, while global, average polarity of the tissue is preserved. We show using super-resolution STED imaging that Fz6-3xGFP and tdTomato-Vangl2 can be resolved, enabling us to observe their complex localization along junctions. We further explore PCP fusion protein localization in the trachea and neural tube, and discover new patterns of PCP expression and localization throughout the mouse embryo., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2021. Published by The Company of Biologists Ltd.)

How dermis maintains tissue homeostasis in cyclic growth and wounding is a fundamental unsolved question. Here, we study how dermal components of feather follicles undergo physiological (molting) and plucking injury-induced regeneration in chickens. Proliferation analyses reveal quiescent, transient-amplifying (TA) and long-term label-retaining dermal cell (LRDC) states. During the growth phase, LRDCs are activated to make new dermal components with distinct cellular flows. Dermal TA cells, enriched in the proximal follicle, generate both peripheral pulp, which extends distally to expand the epithelial-mesenchymal interactive interface for barb patterning, and central pulp, which provides nutrition. Entering the resting phase, LRDCs, accompanying collar bulge epidermal label-retaining cells, descend to the apical dermal papilla. In the next cycle, these apical dermal papilla LRDCs are re-activated to become new pulp progenitor TA cells. In the growth phase, lower dermal sheath can generate dermal papilla and pulp. Transcriptome analyses identify marker genes and highlight molecular signaling associated with dermal specification. We compare the cyclic topological changes with those of the hair follicle, a convergently evolved follicle configuration. This work presents a model for analyzing homeostasis and tissue remodeling of mesenchymal progenitors., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2021. Published by The Company of Biologists Ltd.)

Teleost fish fins, like all vertebrate limbs, comprise a series of bones laid out in characteristic pattern. Each fin's distal bony rays typically branch to elaborate skeletal networks providing form and function. Zebrafish caudal fin regeneration studies suggest basal epidermal-expressed Sonic hedgehog (Shh) promotes ray branching by partitioning pools of adjacent pre-osteoblasts. This Shh role is distinct from its well-studied Zone of Polarizing Activity role establishing paired limb positional information. Therefore, we investigated if and how Shh signaling similarly functions during developmental ray branching of both paired and unpaired fins while resolving cellular dynamics of branching by live imaging. We found shha is expressed uniquely by basal epidermal cells overlying pre-osteoblast pools at the distal aspect of outgrowing juvenile fins. Lateral splitting of each shha-expressing epidermal domain followed by the pre-osteoblast pools precedes overt ray branching. We use ptch2:Kaede fish and Kaede photoconversion to identify short stretches of shha+basal epidermis and juxtaposed pre-osteoblasts as the Shh/Smoothened (Smo) active zone. Basal epidermal distal collective movements continuously replenish each shha+domain with individual cells transiently expressing and responding to Shh. In contrast, pre-osteoblasts maintain Shh/Smo activity until differentiating. The Smo inhibitor BMS-833923 prevents branching in all fins, paired and unpaired, with surprisingly minimal effects on caudal fin initial skeletal patterning, ray outgrowth or bone differentiation. Staggered BMS-833923 addition indicates Shh/Smo signaling acts throughout the branching process. We use live cell tracking to find Shh/Smo restrains the distal movement of basal epidermal cells by apparent 'tethering' to pre-osteoblasts. We propose short-range Shh/Smo signaling promotes these heterotypic associations to couple instructive basal epidermal collective movements to pre-osteoblast repositioning as a unique mode of branching morphogenesis., Competing Interests: Declaration of competing interest None., (Copyright © 2021 Elsevier Inc. All rights reserved.)

A fundamental question in medical genetics is how the genetic background modifies the phenotypic outcome of mutations. We address this question by focusing on the seam cells, which display stem cell properties in the epidermis of Caenorhabditis elegans. We demonstrate that a putative null mutation in the GATA transcription factor egl-18, which is involved in seam cell fate maintenance, is more tolerated in the CB4856 isolate from Hawaii than the lab reference strain N2 from Bristol. We identify multiple quantitative trait loci (QTLs) underlying the difference in phenotype expressivity between the two isolates. These QTLs reveal cryptic genetic variation that reinforces seam cell fate through potentiating Wnt signalling. Within one QTL region, a single amino acid deletion in the heat shock protein HSP-110 in CB4856 is sufficient to modify Wnt signalling and seam cell development, highlighting that natural variation in conserved heat shock proteins can shape phenotype expressivity.

Healthy tissues harbour a surprisingly high number of cells that carry well-known cancer-causing mutations without impacting their physiological function. In recent years, strong evidence accumulated that the immediate environment of mutant cells profoundly impact their prospect of malignant progression. In this review, focusing on the skin, we investigate potential key mechanisms that ensure tissue homeostasis despite the presence of mutant cells, as well as critical factors that may nudge the balance from homeostasis to tumour formation. Functional in vivo studies and single-cell transcriptome analyses have revealed a tremendous cellular heterogeneity and plasticity within epidermal (stem) cells and their respective niches, revealing for example wild-type epithelial cells, fibroblasts or immune-cell subsets as critical in preventing cancer formation and malignant progression. It's the same cells, however, that can drive carcinogenesis. Therefore, understanding the abundance and molecular variation of cell types in health and disease, and how they interact and modulate the local signalling environment will thus be key for new therapeutic avenues in our battle against cancer., (© 2020 The Association for the Publication of the Journal of Internal Medicine.)

Dynamic chemical modifications of RNA represent novel and fundamental mechanisms that regulate stemness and tissue homeostasis. Rejuvenation and wound repair of mammalian skin are sustained by epidermal progenitor cells, which are localized within the basal layer of the skin epidermis. N 6 -methyladenosine (m 6 A) is one of the most abundant modifications found in eukaryotic mRNA and lncRNA (long noncoding RNA). In this report, we survey changes of m 6 A RNA methylomes upon epidermal differentiation and identify Pvt1, a lncRNA whose m 6 A modification is critically involved in sustaining stemness of epidermal progenitor cells. With genome-editing and a mouse genetics approach, we show that ablation of m 6 A methyltransferase or Pvt1 impairs the self-renewal and wound healing capability of skin. Mechanistically, methylation of Pvt1 transcripts enhances its interaction with MYC and stabilizes the MYC protein in epidermal progenitor cells. Our study presents a global view of epitranscriptomic dynamics that occur during epidermal differentiation and identifies the m 6 A modification of Pvt1 as a key signaling event involved in skin tissue homeostasis and wound repair., (© 2021 The Authors.)

How global patterns emerge from individual cell behaviors is poorly understood. In the Xenopus embryonic epidermis, multiciliated cells (MCCs) are born in a random pattern within an inner mesenchymal layer and subsequently intercalate at regular intervals into an outer epithelial layer. Using video microscopy and mathematical modeling, we found that regular pattern emergence involves mutual repulsion among motile immature MCCs and affinity toward outer-layer intercellular junctions. Consistently, Arp2/3-mediated actin remodeling is required for MCC patterning. Mechanistically, we show that the Kit tyrosine kinase receptor, expressed in MCCs, and its ligand Scf, expressed in outer-layer cells, are both required for regular MCC distribution. Membrane-associated Scf behaves as a potent adhesive cue for MCCs, while its soluble form promotes their mutual repulsion. Finally, Kit expression is sufficient to confer order to a disordered heterologous cell population. This work reveals how a single signaling system can implement self-organized large-scale patterning., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Nails are highly keratinized skin appendages that exhibit continuous growth under physiological conditions and full regeneration upon removal. These mini-organs are maintained by two autonomous populations of skin stem cells. The fast-cycling, highly proliferative stem cells of the nail matrix (nail stem cells (NSCs)) predominantly replenish the nail plate. Furthermore, the slow-cycling population of the nail proximal fold (nail proximal fold stem cells (NPFSCs)) displays bifunctional properties by contributing to the peri-nail epidermis under the normal homeostasis and the nail structure upon injury. Here, we discuss nail mini-organ stem cells' location and their role in skin and nail homeostasis and regeneration, emphasizing their importance to orchestrate the whole digit tip regeneration. Such endogenous regeneration capabilities are observed in rodents and primates. However, they are limited to the region adjacent to the nail's proximal area, indicating the crucial role of nail mini-organ stem cells in digit restoration. Further, we explore the molecular characteristics of nail mini-organ stem cells and the critical role of the bone morphogenetic protein (BMP) and Wnt signaling pathways in homeostatic nail growth and digit restoration. Finally, we investigate the latest accomplishments in stimulating regenerative responses in regeneration-incompetent injuries. These pioneer results might open up new opportunities to overcome amputated mammalian digits and limbs' regenerative failures in the future.

In order to cope with external stressors such as changes in humidity and temperature or irritating substances, the epidermis as the outermost skin layer forms a continuously renewing and ideally intact protective barrier. Under certain circumstances, this barrier can be impaired and epidermal cells have to counteract cell swelling or shrinkage induced by osmotic stress via regulatory volume decrease (RVD) or increase (RVI). Here, we will review the current knowledge regarding the molecular machinery underlying RVD and RVI in the epidermis. Furthermore, we will discuss the current understanding how cell volume changes and its regulators are associated with epidermal renewal and barrier formation., Competing Interests: CB and TF are inventors of patent application WO 2019/158696., (© Copyright by the Author(s). Published by Cell Physiol Biochem Press.)

Skin epithelium can accumulate a high burden of oncogenic mutations without morphological or functional consequences. To investigate the mechanism of oncogenic tolerance, we induced Hras G12V in single murine epidermal cells and followed them long term. We observed that Hras G12V promotes an early and transient clonal expansion driven by increased progenitor renewal that is replaced with an increase in progenitor differentiation leading to reduced growth. We attribute this dynamic effect to emergence of two populations within oncogenic clones: renewing progenitors along the edge and differentiating ones within the central core. As clone expansion is accompanied by progressive enlargement of the core and diminishment of the edge compartment, the intraclonal competition between the two populations results in stabilized oncogenic growth. To identify the molecular mechanism of Hras G12V -driven differentiation, we screened known Ras-effector in vivo and identified Rassf5 as a novel regulator of progenitor fate choice that is necessary and sufficient for oncogene-specific differentiation., Competing Interests: MS, ZY, SB No competing interests declared, (© 2021, Sandoval et al.)

The tuatara (Sphenodon punctatus) is the phylogenetically closest relative of squamates (including lizards and snakes) from which it diverged around 250 million years ago. Together, they constitute the clade Lepidosauria. Fully terrestrial vertebrates (amniotes) form their skin barrier to the environment under the control of a gene cluster, termed the epidermal differentiation complex (EDC). Here we identified EDC genes in the genome of the tuatara and compared them to those of other amniotes. The organization of the EDC and proteins encoded by EDC genes are most similar in the tuatara and squamates. A subcluster of lepidosaurian EDC genes encodes corneous beta-proteins (CBPs) of which three different types are conserved in the tuatara. Small proline-rich proteins have undergone independent expansions in the tuatara and some, but not all subgroups of squamates. Two genes encoding S100 filaggrin-type proteins (SFTPs) are expressed during embryonic skin development of the tuatara whereas SFTP numbers vary between 1 and 3 in squamates. Our comparative analysis of the EDC in the tuatara genome suggests that many molecular features of the skin that were previously identified in squamates have evolved prior to their divergence from the lineage leading to the tuatara.

Cold atmospheric plasma (CAP) is a group of various chemical active species, such as ozone and nitric oxide, generated by working gas. CAP was demonstrated to have an effect on tissue regeneration and wound healing. We conducted this study to evaluate the efficacy and safety of CAP as a novel therapy for diabetic wounds in vitro and in vivo. The plasma consists of ionised helium gas that is produced by a high-voltage and high-frequency power supply. Eight-week-old male db/db mice and C57BL mice were treated with helium gas (control group), 90s' CAP (low-dose group), and 180s' CAP (high-dose group). Mice were treated and observed for 2 weeks. Skin samples from around the wound and blood samples were collected. Our in vitro analysis included scratch wound-healing assays by using human HaCaT immortalised human epidermal cells. After 14 days of treatment, CAP could obviously promote diabetic wound healing. Wound closure rates were significantly higher in the low-dose group and high-dose groups compared with the control group. Meanwhile, compared with the control group, the protein expression of IL-6, tumour necrosis factor-α, inducible nitric oxide synthase, and superoxide dismutase in two CAP groups significantly decreased, while the protein expression of vascular endothelial growth factor and transforming growth factor-β in two CAP groups significantly increased (all P < .05); these data show good agreement with the change in mRNA level (all P < .05). In vitro, scratch wound-healing assays showed that plasma treatment could effectively ensure healing within 3 minutes of exposure (all P < .05). In addition, no difference was found in histological observations of normal skin and the level of serum alanine transaminase, aspartate aminotransferase, blood urea nitrogen, creatinine, and white blood cells among the CAP groups and control group. CAP treatment for 3 minutes every day improves wound healing in diabetic mice by suppressing inflammation, reducing oxidative stress, and enhancing angiogenesis, involving several proteins signalling, and it is safe for the liver and kidney., (© 2020 Medicalhelplines.com Inc and John Wiley & Sons Ltd.)

During prolonged nutrient restriction, developing animals redistribute vital nutrients to favor brain growth at the expense of other organs. In Drosophila , such brain sparing relies on a glia-derived growth factor to sustain proliferation of neural stem cells. However, whether other aspects of neural development are also spared under nutrient restriction is unknown. Here we show that dynamically growing somatosensory neurons in the Drosophila peripheral nervous system exhibit organ sparing at the level of arbor growth: Under nutrient stress, sensory dendrites preferentially grow as compared to neighboring non-neural tissues, resulting in dendrite overgrowth. These neurons express lower levels of the stress sensor FoxO than neighboring epidermal cells, and hence exhibit no marked induction of autophagy and a milder suppression of Tor signaling under nutrient stress. Preferential dendrite growth allows for heightened animal responses to sensory stimuli, indicative of a potential survival advantage under environmental challenges., Competing Interests: AP, YX, CZ, JL, KL, DL, CH No competing interests declared, (© 2020, Poe et al.)

Desmosomes are cell-cell adhesions necessary for the maintenance of tissue integrity in the skin and heart. While the core components of desmosomes have been identified, peripheral components that modulate canonical or noncanonical desmosome functions still remain largely unexplored. Here we used targeted proximity labeling approaches to further elaborate the desmosome proteome in epidermal keratinocytes. Quantitative mass spectrometry analysis identified all core desmosomal proteins while uncovering a diverse array of new constituents with broad molecular functions. By individually targeting the inner and outer dense plaques, we defined proteins enriched within these subcompartments. We validated a number of these novel desmosome-associated proteins and find that many are membrane proximal proteins that show a dependence on functional desmosomes for their cortical localization. We further explored the mechanism of localization and function of two novel desmosome-associated adaptor proteins enriched in the desmosome proteome, Crk and Crk-like (CrkL). These proteins interacted with Dsg1 and rely on Dsg1 and desmoplakin for robust cortical localization. Epidermal deletion of both Crk and CrkL resulted in perinatal lethality with defects in desmosome morphology and keratin organization, thus demonstrating the utility of this dataset in identifying novel proteins required for desmosome-dependent epidermal integrity.

Synaptic positions underlie precise circuit connectivity. Synaptic positions can be established during embryogenesis and sustained during growth. The mechanisms that sustain synaptic specificity during allometric growth are largely unknown. We performed forward genetic screens in C. elegans for regulators of this process and identified mig-17 , a conserved ADAMTS metalloprotease. Proteomic mass spectrometry, cell biological and genetic studies demonstrate that MIG-17 is secreted from cells like muscles to regulate basement membrane proteins. In the nematode brain, the basement membrane does not directly contact synapses. Instead, muscle-derived basement membrane coats one side of the glia, while glia contact synapses on their other side. MIG-17 modifies the muscle-derived basement membrane to modulate epidermal-glial crosstalk and sustain glia location and morphology during growth. Glia position in turn sustains the synaptic pattern established during embryogenesis. Our findings uncover a muscle-epidermis-glia signaling axis that sustains synaptic specificity during the organism's allometric growth., Competing Interests: JF, TJ, KW, JH, MW, LM, XD, YS, XZ, ZS, DC No competing interests declared, (© 2020, Fan et al.)

Aging manifests with architectural alteration and functional decline of multiple organs throughout an organism. In mammals, aged skin is accompanied by a marked reduction in hair cycling and appearance of bald patches, leading researchers to propose that hair follicle stem cells (HFSCs) are either lost, differentiate, or change to an epidermal fate during aging. Here, we employed single-cell RNA-sequencing to interrogate aging-related changes in the HFSCs. Surprisingly, although numbers declined, aging HFSCs were present, maintained their identity, and showed no overt signs of shifting to an epidermal fate. However, they did exhibit prevalent transcriptional changes particularly in extracellular matrix genes, and this was accompanied by profound structural perturbations in the aging SC niche. Moreover, marked age-related changes occurred in many nonepithelial cell types, including resident immune cells, sensory neurons, and arrector pili muscles. Each of these SC niche components has been shown to influence HF regeneration. When we performed skin injuries that are known to mobilize young HFSCs to exit their niche and regenerate HFs, we discovered that aged skin is defective at doing so. Interestingly, however, in transplantation assays in vivo, aged HFSCs regenerated HFs when supported with young dermis, while young HFSCs failed to regenerate HFs when combined with aged dermis. Together, our findings highlight the importance of SC:niche interactions and favor a model where youthfulness of the niche microenvironment plays a dominant role in dictating the properties of its SCs and tissue health and fitness., Competing Interests: The authors declare no competing interest.

Cell size homeostasis is often achieved by coupling cell-cycle progression to cell growth. Growth has been shown to drive cell-cycle progression in bacteria and yeast through "sizers," wherein cells of varying birth size divide at similar final sizes [1-3], and "adders," wherein cells increase in size a fixed amount per cell cycle [4-6]. Intermediate control phenomena are also observed, and even the same organism can exhibit different control phenomena depending on growth conditions [2, 7, 8]. Although studying unicellular organisms in laboratory conditions may give insight into their growth control in the wild, this is less apparent for studies of mammalian cells growing outside the organism. Sizers, adders, and intermediate phenomena have been observed in vitro [9-12], but it is unclear how this relates to mammalian cell proliferation in vivo. To address this question, we analyzed time-lapse images of the mouse epidermis taken over 1 week during normal tissue turnover [13]. We quantified the 3D volume growth and cell-cycle progression of single cells within the mouse skin. In dividing epidermal stem cells, we found that cell growth is coupled to division through a sizer operating largely in the G1 phase of the cell cycle. Thus, although the majority of tissue culture studies have identified adders, our analysis demonstrates that sizers are important in vivo and highlights the need to determine their underlying molecular origin., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

In this Primer, Moreci and Lechler follow the lifetime of an epidermal cell from its birth to its ultimate death, and detail how this journey is necessary for epidermal function., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The majority of pressure ulcers (PUs) including deep tissue injuries (DTIs) are preventable, and even reversible if detected in their early phase. One of the greatest barriers in PU prevention is that clinicians traditionally depended on subjective and qualitative techniques, particularly routine visual skin assessments that would only document existing, macroscopic PUs/DTIs, rather than preventing them or detecting them at their microscopic phase. At the early phase of cell damage, when a forming PU is still microscopic, there is a local increase in extracellular fluid contents within affected tissues, which is called sub-epidermal moisture (SEM). This new understanding has led to an emerging technology, a SEM Scanner (BBI LLC, Bruin Biometrics) that has been designed to effectively examine the health status of tissues, by measuring local changes in the biophysical SEM marker. In the present work, the SEM Scanner was tested under controlled laboratory conditions to experimentally determine its sensitivity and precision in identifying small (1 mL) water content changes in phantoms of the human heel and skull/face, which simulated common PU development scenarios. In both phantom configurations, the locally increased water contents resulted in consistent, statistically significant elevated SEM readings, which confirms that the SEM Scanner is able to detect fluid content changes that are as small as 1 mL. In agreement with a simplified theoretical (mathematical) SEM model, which was also developed here, changes in water contents had a consistent trend of effect on SEM delta values, which increased with each 1 mL increment in intra-tissue-substitute water contents., (© 2019 Medicalhelplines.com Inc and John Wiley & Sons Ltd.)

How tissues from different developmental origins interact to achieve coordinated morphogenesis at the level of a whole organism is a fundamental question in developmental biology. While biochemical signaling pathways controlling morphogenesis have been extensively studied [1-3], morphogenesis of epithelial tissues can also be directed by mechanotransduction pathways physically linking two tissues [4-8]. C. elegans embryonic elongation requires the coordination of three tissues: muscles, the dorsal and ventral epidermis, and the lateral epidermis. Elongation starts by cell-shape changes driven by actomyosin contractions in the lateral epidermis [9, 10]. At mid-elongation, muscles become connected to the apical surface of the dorsal and ventral epidermis by molecular tendons formed by muscle integrins, extracellular matrix, and C. elegans hemidesmosomes (CeHDs). The mechanical signal generated by the onset of muscle contractions in the antero-posterior axis from mid-elongation is translated into a biochemical pathway controlling the maturation of CeHDs in the dorsal and ventral epidermis [11]. Consistently, mutations affecting muscle contractions or molecular tendons lead to a mid-elongation arrest [12]. Here, we found that the mechanical force generated by muscle contractions and relayed by molecular tendons is transmitted by adherens junctions to lateral epidermal cells, where it establishes a newly identified bipolar planar polarity of the apical PAR module. The planar polarized PAR module is then required for actin planar organization, thus contributing to the determination of the orientation of cell-shape changes and the elongation axis of the whole embryo. This mechanotransduction pathway is therefore essential to coordinate the morphogenesis of three embryonic tissues., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Interactions between epithelial cells and neurons influence a range of sensory modalities including taste, touch, and smell. Vertebrate and invertebrate epidermal cells ensheath peripheral arbors of somatosensory neurons, including nociceptors, yet the developmental origins and functional roles of this ensheathment are largely unknown. Here, we describe an evolutionarily conserved morphogenetic mechanism for epidermal ensheathment of somatosensory neurites. We found that somatosensory neurons in Drosophila and zebrafish induce formation of epidermal sheaths, which wrap neurites of different types of neurons to different extents. Neurites induce formation of plasma membrane phosphatidylinositol 4,5-bisphosphate microdomains at nascent sheaths, followed by a filamentous actin network, and recruitment of junctional proteins that likely form autotypic junctions to seal sheaths. Finally, blocking epidermal sheath formation destabilized dendrite branches and reduced nociceptive sensitivity in Drosophila . Epidermal somatosensory neurite ensheathment is thus a deeply conserved cellular process that contributes to the morphogenesis and function of nociceptive sensory neurons., Competing Interests: NJ, JR, JC, MR, KL, MC, EP, HK, JV, AS, JP No competing interests declared, (© 2019, Jiang et al.)

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., Competing Interests: BJ, NF, CL, EL No competing interests declared, EF Reviewing editor, eLife, (© 2019, Jenkins et al.)

Local transplantation of stem cells has therapeutic effects on skin damage but cannot provide satisfactory wound healing. Studies on the mechanisms underlying the therapeutic effects of stem cells on skin wound healing will be needed. Hence, in the present study, we explored the role of Caveolin-1 in epidermal stem cells (EpiSCs) in the modulation of wound healing. We first isolated EpiSCs from mouse skin tissues and established stable EpiSCs with overexpression of Caveolin-1 using a lentiviral construct. We then evaluated the epidermal growth factor (EGF)-induced cell proliferation ability using cell counting Kit-8 (CCK-8) assay and assessed EpiSC pluripotency by examining Nanog mRNA levels in EpiSCs. Furthermore, we treated mice with skin burn injury using EpiSCs with overexpression of Caveolin-1. Histological examinations were conducted to evaluate re-epithelialization, wound scores, cell proliferation and capillary density in wounds. We found that overexpression of Caveolin-1 in EpiSCs promoted EGF-induced cell proliferation ability and increased wound closure in a mouse model of skin burn injury. Histological evaluation demonstrated that overexpression of Caveolin-1 in EpiSCs promoted re-epithelialization in wounds, enhanced cellularity, and increased vasculature, as well as increased wound scores. Taken together, our results suggested that Caveolin-1 expression in the EpiSCs play a critical role in the regulation of EpiSC proliferation ability and alteration of EpiSC proliferation ability may be an effective approach in promoting EpiSC-based therapy in skin wound healing., (Copyright © 2018 Elsevier Inc. All rights reserved.)

During the process of skin regeneration following a skin injury, de novo hair follicle regeneration is initiated after wounding; however, these regenerated hairs are mostly unpigmented. The activation of epidermal melanocyte stem cells and their differentiation into regenerating hair follicles have been shown to be necessary for the pigmented hair regeneration after wounding. To determine the role of flavonoids in the regeneration of pigmented hairs, we applied the candidate flavonoids to the regenerating hair follicles after wounding and identified the flavonoid species that maximally induced pigmented hair regeneration. Flavonoids with two OH groups in the B-ring, such as sterubin, luteolin, and hydroxygenkwanin, showed promising effects in regenerating black pigmented hairs, while those with one OH group in the B-ring showed no significant change. Thus, flavonoids with two OH groups in their B-ring could be studied further as potential wound healing agents with the ability to regenerate pigmented hair.

Polycomb-repressive complex 1 (PRC1) and PRC2 are critical chromatin regulators of gene expression and tissue development. Here, we show that despite extensive genomic cobinding, PRC1 is essential for epidermal integrity, whereas PRC2 is dispensable. Loss of PRC1 resulted in blistering skin, reminiscent of human skin fragility syndromes. Conversely, PRC1 does not restrict epidermal stratification during skin morphogenesis, whereas PRC2 does. Molecular dissection demonstrated that PRC1 functions with PRC2 to silence/dampen expression of adhesion genes. In contrast, PRC1 promotes expression of critical epidermal adhesion genes independently of PRC2-mediated H3K27me3. Together, we demonstrate a functional link between epigenetic regulation and skin diseases., (© 2019 Cohen et al.; Published by Cold Spring Harbor Laboratory Press.)

Detection of subcutaneous tissue damage before it is visible can trigger early intervention and decrease hospital-acquired pressure ulcer (HAPU) rates. The objective of this two-phase study was to evaluate the clinical utility of the Sub-Epidermal Moisture (SEM) Scanner (Bruin Biometrics (BBI), LLC), a hand-held device that assesses increases in interstitial fluid or subepidermal moisture, indicating early tissue damage. Phase 1: Patients were provided standard-of-care risk assessment and interventions and were scanned with the SEM Scanner, but the resulting SEM scores were not used to determine interventions. This gave a baseline pressure ulcer incidence rate. Phase 2: This phase is the same as Phase 1 except the resulting SEM scores were used in conjunction with risk assessment scores to determine appropriate interventions and care planning. In Phase 1, 12 of the 89 subjects or 13.5% developed visible pressure ulcers-4 Stage I's, 6 Stage II's, 1 Stage III, and 1 deep tissue injury. In Phase 2, 2 of the 195 subjects or 1.0% developed visible pressure ulcers-1 Stage I and 1 Stage II. Patients in Phase 2 were more incontinent, less mobile, and had longer lengths of stay than those in Phase 1. Use of the Scanner resulted in a 93% decrease in HAPU. No deep injuries developed in Phase 2., (© 2018 Medicalhelplines.com Inc and John Wiley & Sons Ltd.)

Aryl hydrocarbon receptor (AHR) is a chemical sensor that is expressed abundantly in epidermal keratinocytes. Oxidative AHR ligands induce the production of reactive oxygen species. However, antioxidant AHR ligands inhibit reactive oxygen species generation via activation of nuclear factor-erythroid 2-related factor-2, which is a master switch for antioxidative signalling. In addition, AHR signalling accelerates epidermal terminal differentiation, but excessive acceleration by oxidative ligands, such as dioxins, may induce chloracne and inflammation. However, antioxidative phytochemical ligands induce the beneficial acceleration of epidermal differentiation that repairs skin barrier disruption. The upregulated expression of differentiation molecules, such as filaggrin, is mediated via the AHR-OVOL1 axis. This AHR-OVOL1 system is capable of counteracting skin barrier dysfunction in T-helper type 2-shifted inflammation. This article reviews the dynamic and multifaceted role of AHR in epidermal biology and discusses the potential use of antioxidative phytochemical ligands for AHR in inflammatory skin diseases, such as atopic dermatitis.

Epidermal stem cells residing in the skin play an essential role in epidermal regeneration. When skin is injured, the stem cells are first activated to proliferate, and subsequently the progeny migrate and differentiate to regenerate the epidermis. Here, we demonstrate that the vitamin D receptor (VDR) is essential for these processes to occur. The requirement for VDR on epidermal stem cell function was revealed in conditional VDR knockout mice, in which VDR was deleted from stem cells and progeny, and mice were maintained on a low calcium diet. First, self-renewal and niche formation of epidermal stem cells were impaired. Wound-induced activation of epidermal stem cells was blunted associated with a reduction of β-catenin signaling. Second, wound induced migration of stem cells and progeny was impaired as shown by lineage tracing and delayed migration of VDR silenced cells. Epidermal differentiation of progeny was impaired at the wounding site associated with reduced E-cadherin expression. Deletion of VDR also changed stem cell fate blunting hair development, increasing sebaceous glands, and altering expression and location of epidermal markers. These results suggest that VDR is required for self-renewal, migration, and differentiation of epidermal stem cells and progeny during cutaneous wound healing., (Copyright © 2018 The Authors. Published by Elsevier Inc. All rights reserved.)

The sense of touch is fundamental as it provides vital, moment-to-moment information about the nature of our physical environment. Primary sensory neurons provide the basis for this sensation in the periphery; however, recent work demonstrates that touch transduction mechanisms also occur upstream of the sensory neurons via non-neuronal cells such as Merkel cells and keratinocytes. Within the spinal cord, deep dorsal horn circuits transmit innocuous touch centrally and also transform touch into pain in the setting of injury. Here non-neuronal cells play a key role in the induction and maintenance of persistent mechanical pain. This review highlights recent advances in our understanding of mechanosensation, including a growing appreciation for the role of non-neuronal cells in both touch and pain., (Copyright © 2018. Published by Elsevier Inc.)

Soil salinity is destroying arable land and is considered to be one of the major threats to global food security in the 21st century. Therefore, the ability of naturally salt-tolerant halophyte plants to sequester large quantities of salt in external structures, such as epidermal bladder cells (EBCs), is of great interest. Using Chenopodium quinoa, a pseudo-cereal halophyte of great economic potential, we have shown previously that, upon removal of salt bladders, quinoa becomes salt sensitive. In this work, we analyzed the molecular mechanism underlying the unique salt dumping capabilities of bladder cells in quinoa. The transporters differentially expressed in the EBC transcriptome and functional electrophysiological testing of key EBC transporters in Xenopus oocytes revealed that loading of Na + and Cl - into EBCs is mediated by a set of tailored plasma and vacuole membrane-based sodium-selective channel and chloride-permeable transporter., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

How embryonic progenitors coordinate cell fate specification and establish transcriptional and signaling competence is a fundamental question in developmental biology. Here, we show that transcription factor ΔNp63 profoundly changes the transcriptome and remodels thousands of open chromatin regions of Krt8+ progenitors during epidermal fate specification. ATAC-seq and single-cell RNA-seq reveal that ΔNp63-dependent programs govern epidermal lineage formation, and ΔNp63-independent programs, mediated by AP2 and AP1 transcription factors, promote epidermal differentiation and epithelial-to-mesenchymal transition. ΔNp63 promotes Wnt signaling by directly upregulating Wnt ligands, Frizzled receptors, and transcription factors. Deletion of β-catenin in Krt8+ progenitors delays their maturation into Krt5+ progenitors. The lack of epidermal Wnt production in the absence of ΔNp63 also incapacitates Wnt activation in the underlying dermal cells. These findings reveal the remarkable changes of the transcriptome, open chromatin, and signaling pathways at the onset of skin development and uncover the molecular cascade for epidermal lineage formation., (Copyright © 2018 Elsevier Inc. All rights reserved.)

The transcription factor FOXJ1 is essential for the formation of motile cilia throughout the animal kingdom. Target genes therefore likely constitute an important part of the motile cilia program. Here, we report on the analysis of one of these targets, Fam183b, in Xenopus and mice. Fam183b encodes a protein with unknown function which is conserved from the green algae Chlamydomonas to humans. Fam183b is expressed in tissues harbouring motile cilia in both mouse and frog embryos. FAM183b protein localises to basal bodies of cilia in mIMCD3 cells and of multiciliated cells of the frog larval epidermis. In addition, FAM183b interacts with NUP93, which also localises to basal bodies. During frog embryogenesis, Fam183b was dispensable for laterality specification and brain development, but required for ciliogenesis and motility of epidermal multiciliated cells and nephrostomes, i.e. the embryonic kidney. Surprisingly, mice homozygous for a null allele did not display any defects indicative of disrupted motile ciliary function. The lack of a cilia phenotype in mouse and the limited requirements in frog contrast with high sequence conservation and the correlation of gene expression with the presence of motile cilia. This finding may be explained through compensatory mechanisms at sites where no defects were observed in our FAM183b-loss-of-function studies.

FGF signaling is a potent inducer of lacrimal gland development in the eye, capable of transforming the corneal epithelium into glandular tissues. Here, we show that genetic ablation of the Pea3 family of transcription factors not only disrupted the ductal elongation and branching of the lacrimal gland, but also biased the lacrimal gland epithelium toward an epidermal cell fate. Analysis of high-throughput gene expression and chromatin immunoprecipitation data revealed that the Pea3 genes directly control both the positive and negative feedback loops of FGF signaling. Importantly, Pea3 genes are also required to suppress aberrant Notch signaling which, if gone unchecked, can compromise lacrimal gland development by preventing the expression of both Sox and Six family genes. These results demonstrate that Pea3 genes are key FGF early response transcriptional factors, programing the genetic landscape for cell fate determination., Competing Interests: Mukesh Bansal is employed by PsychoGenics Inc., which did not have any role in the conception, analysis or writing of the manuscript.

Large excisional wounds in mice prominently regenerate new hair follicles (HFs) and fat, yet humans are deficient for this regenerative behavior. Currently, wound-induced regeneration remains a clinically desirable, but only partially understood phenomenon. We show that large excisional wounds in rats across seven strains fail to regenerate new HFs. We compared wound transcriptomes between mice and rats at the time of scab detachment, which coincides with the onset of HF regeneration in mice. In both species, wound dermis and epidermis share core dermal and epidermal transcriptional programs, respectively, yet prominent interspecies differences exist. Compared with mice, rat epidermis expresses distinct transcriptional and epigenetic factors, markers of epidermal repair, hyperplasia, and inflammation, and lower levels of WNT signaling effectors and regulators. When recombined on the surface of excisional wounds with vibrissa dermal papillae, partial-thickness skin grafts containing distal pelage HF segments, but not interfollicular epidermis, readily regenerated new vibrissa-like HFs. Together, our findings establish rats as a nonregenerating rodent model for excisional wound healing and suggest that low epidermal competence and associated transcriptional profile may contribute to its regenerative deficiency. Future comparison between rat and mouse may lend further insight into the mechanism of wounding-induced regeneration and causes for its deficit., (Copyright © 2018 The Authors. Published by Elsevier Inc. All rights reserved.)

The basal stem/progenitor cell maintains homeostasis of the epidermis. Progressive disturbance of this homeostasis has been implicated as a possible cause in the pathogenesis of epithelial disease, such as middle ear cholesteatoma. In many cases of stem/progenitor cell regulation, the importance of extracellular signals provided by the surrounding cells is well-recognized. Keratinocyte growth factor (KGF) is a mesenchymal-cell-derived paracrine growth factor that specifically participates in skin homeostasis; however, the overexpression of KGF induces middle ear cholesteatoma. In this study, two kinds of thymidine analogs were transferred at different time points and we investigated the effects of overexpressed KGF on the cell kinetics of stem/progenitor cells in vivo. As a result, BrdU(+)EdU(+) cells (stem/progenitor cells) were detected in the thickened epithelium of KGF-transfected specimens. The use of a high-resolution microscope enabled us to analyze the phosphorylated level of p63 in individual nuclei, and the results clearly demonstrated that BrdU(+)EdU(+) cells are regarded as progenitor cells. In the overexpression of KGF, the stimulation of progenitor cell proliferation was inhibited by SU5402, an inhibitor for tyrosine kinase of KGFR. These findings indicate that KGF overexpression may increase stem/progenitor cell proliferation and block terminal differentiation, resulting in epithelial hyperplasia, which is typical in middle ear cholesteatoma.

The shape and function of plant cells are often highly interdependent. The puzzle-shaped cells that appear in the epidermis of many plants are a striking example of a complex cell shape, however their functional benefit has remained elusive. We propose that these intricate forms provide an effective strategy to reduce mechanical stress in the cell wall of the epidermis. When tissue-level growth is isotropic, we hypothesize that lobes emerge at the cellular level to prevent formation of large isodiametric cells that would bulge under the stress produced by turgor pressure. Data from various plant organs and species support the relationship between lobes and growth isotropy, which we test with mutants where growth direction is perturbed. Using simulation models we show that a mechanism actively regulating cellular stress plausibly reproduces the development of epidermal cell shape. Together, our results suggest that mechanical stress is a key driver of cell-shape morphogenesis., Competing Interests: AS, AR, AR, MD, LH, HH, SV, GM, CL, AH, OH, AR, MT, PP, RS No competing interests declared, (© 2018, Sapala et al.)