The cell biology of human hair follicle pigmentation

Sequential cyclic changes of hair roots revealed by dermoscopy demonstrate a progressive mechanism of diffuse alopecia areata over time

BACKGROUND

Diffuse alopecia areata (DAA) often leads to a complete hair shedding within a few months.

OBJECTIVE

To explore features and mechanisms underlying DAA.

MATERIALS AND METHODS

Scalp and hair root dermoscopy were conducted on 23 DAA patients throughout the disease process, 20 patchy Alopecia areata patients, 23 acute telogen effluvium (ATE) patients and 10 normal controls. Histopathology was also evaluated.

RESULTS

We found almost all hair roots were anagen in early stage DAA in 18 patients (18/23, 78.3%) within the first 4-8 weeks after hair loss onset. Anagen effluvium (~4 weeks) was followed by catagen (~4 weeks) and then telogen/exogen (~8 weeks) effluvium with overlap. Hair root and proximal hair shaft depigmentation was more prominent in later DAA disease stages. Black dots, exclamation mark hairs and inconsistent thickness of hair shafts were found more often in early than later DAA (Ps < 0.01). Early DAA histopathology revealed more prominent inflammation and hair follicle regression than that observed in the later stages. Patchy alopecia areata patients showed mixed anagen, catagen and telogen hair roots while ATE patients showed increased exogen and mildly decreased hair root pigmentation.

CONCLUSION

Sequential cyclic staging of shed hairs in DAA indicates the insult may be hair-cycle specific. We suggest that DAA is initially an anagen effluvium disease involving an intense inflammatory insult, later progressing to a brief catagen effluvium, and then to telogen effluvium with premature exogen, in later stages of DAA.

Efficacy of an agonist of α-MSH, the palmitoyl tetrapeptide-20, in hair pigmentation

OBJECTIVE

Hair greying (i.e., canities) is a component of chronological ageing and occurs regardless of gender or ethnicity. Canities is directly linked to the loss of melanin and increase in oxidative stress in the hair follicle and shaft. To promote hair pigmentation and reduce the hair greying process, an agonist of α-melanocyte-stimulating hormone (α-MSH), a biomimetic peptide (palmitoyl tetrapeptide-20; PTP20) was developed. The aim of this study was to describe the effects of the designed peptide on hair greying.

METHODS

Effect of the PTP20 on the enzymatic activity of catalase and the production of H₂ O₂ by Human Follicle Dermal Papilla Cells (HFDPC) was evaluated. Influence of PTP20 on the expression of melanocortin receptor-1 (MC1-R) and the production of melanin were investigated. Enzymatic activity of sirtuin 1 (SIRT1) after treatment with PTP20 was also determined. Ex vivo studies using human micro-dissected hairs allowed to visualize the effect of PTP20 on the expression in hair follicle of catalase, TRP-1, TRP-2, Melan-A, ASIP, and MC1-R. These investigations were completed by a clinical study on 15 human male volunteers suffering from premature canities.

RESULTS

The in vitro and ex vivo studies revealed the capacity of the examined PTP20 peptide to enhance the expression of catalase and to decrease (30%) the intracellular level of H₂ O₂ . Moreover, PTP20 was shown to activate in vitro and ex vivo the melanogenesis process. In fact, an increase in the production of melanin was shown to be correlated with elevated expression of MC1-R, TRP-1, and Melan-A, and with the reduction in ASIP expression. A modulation on TRP-2 was also observed. The pivotal role of MC1-R was confirmed on protein expression analysed on volunteer's plucked hairs after 3 months of the daily application of lotion containing 10 ppm of PTP20 peptide.

CONCLUSION

The current findings demonstrate the ability of the biomimetic PTP20 peptide to preserve the function of follicular melanocytes. The present results suggest potential cosmetic application of this newly designed agonist of α-MSH to promote hair pigmentation and thus, reduce the hair greying process.

An explanation for the mysterious distribution of melanin in human skin: a rare example of asymmetric (melanin) organelle distribution during mitosis of basal layer progenitor keratinocytes

BACKGROUND

Melanin is synthesized by melanocytes in the basal layer of the epidermis. When transferred to surrounding keratinocytes melanin is the key ultraviolet radiation-protective biopolymer responsible for skin pigmentation. Most melanin is observable in the proliferative basal layer of the epidermis and only sparsely distributed in the stratifying/differentiating epidermis. The latter has been explained as 'melanin degradation' in suprabasal layers.

OBJECTIVES

To re-evaluate the currently accepted basis for melanin distribution in human epidermis and to discover whether this pattern is altered after a regenerative stimulus.

METHODS

Normal epidermis of adult human skin, at rest and after tape-stripping, was analysed by a range of (immuno)histochemical and high-resolution microscopy techniques. In vitro models of melanin granule uptake by human keratinocytes were attempted.

RESULTS

We propose a different fate for melanin in the human epidermis. Our evidence indicates that the bulk of melanin is inherited only by the nondifferentiating daughter cell postmitosis in progenitor keratinocytes via asymmetric organelle inheritance. Moreover, this preferred pattern of melanin distribution can switch to a symmetric or equal daughter cell inheritance mode under conditions of stress, including regeneration.

CONCLUSIONS

In this preliminary report, we provide a plausible and histologically supported explanation for how human skin pigmentation is efficiently organized in the epidermis. Steady-state epidermis pigmentation may involve much less redox-sensitive melanogenesis than previously thought, and at least some premade melanin may be available for reuse. The epidermal melanin unit may be an excellent example with which to study organelle distribution via asymmetric or symmetric inheritance in response to microenvironment and tissue demands.

Ultraviolet radiation induces Melan-A-expressing cells in interfollicular epidermis in wild-type mice

Adult wild-type mice are not supposed to be proper models for ultraviolet radiation (UVR)-induced melanoma since melanocytes are confined to hair follicles and cannot be sufficiently reached by UVR. On the other hand, in mutated mouse models used for melanoma research limitations, including an altered immune system and selection of affected pathways, lead to tumors phenotypically quite different from naturally occurring melanomas. We compared the distribution of epidermal melanocytes in UVR and not-UVR-exposed wild-type C57BL/6 mice. Starting at the age of 8 weeks, mice were exposed to physiologic doses of UVR three times weekly over 16 weeks. Back skin biopsies were taken 4, 8, 12 and 16 weeks after initiation of exposure, and stained for Melan-A, representing a highly selective marker for melanocytes. Surprisingly, after exposure to UVR, Melan-A positive cells were detected also in the interfollicular epidermis of C57BL/6 mice. We conclude that UVR is capable of inducing interfollicular epidermal melanocytes in wild-type mice.

IDH2 deficiency accelerates skin pigmentation in mice via enhancing melanogenesis

Melanogenesis is a complex biosynthetic pathway regulated by multiple agents, which are involved in the production, transport, and release of melanin. Melanin has diverse roles, including determination of visible skin color and photoprotection. Studies indicate that melanin synthesis is tightly linked to the interaction between melanocytes and keratinocytes. α-melanocyte-stimulating hormone (α-MSH) is known as a trigger that enhances melanin biosynthesis in melanocytes through paracrine effects. Accumulated reactive oxygen species (ROS) in skin affects both keratinocytes and melanocytes by causing DNA damage, which eventually leads to the stimulation of α-MSH production. Mitochondria are one of the main sources of ROS in the skin and play a central role in modulating redox-dependent cellular processes such as metabolism and apoptosis. Therefore, mitochondrial dysfunction may serve as a key for the pathogenesis of skin melanogenesis. Mitochondrial NADP⁺-dependent isocitrate dehydrogenase (IDH2) is a key enzyme that regulates mitochondrial redox balance and reduces oxidative stress-induced cell injury through the generation of NADPH. Downregulation of IDH2 expression resulted in an increase in oxidative DNA damage in mice skin through ROS-dependent ATM-mediated p53 signaling. IDH2 deficiency also promoted pigmentation on the dorsal skin of mice, as evident from the elevated levels of melanin synthesis markers. Furthermore, pretreatment with mitochondria-targeted antioxidant mito-TEMPO alleviated oxidative DNA damage and melanogenesis induced by IDH2 deficiency both in vitro and in vivo. Together, our findings highlight the role of IDH2 in skin melanogenesis in association with mitochondrial ROS and suggest unique therapeutic strategies for the prevention of skin pigmentation.

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Melanogenesis is associated with the production of ROS.

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IDH2 is an essential enzyme in the mitochondrial antioxidant system.

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Downregulation of IDH2 induces ROS-dependent ATM-mediated p53 signaling.

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IDH2 deficiency promotes skin pigmentation.

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mito-TEMPO alleviates melanogenesis caused by IDH2 deficiency.

E-cadherin mediates ultraviolet radiation- and calcium-induced melanin transfer in human skin cells

Skin pigmentation is directed by epidermal melanin units, characterized by long-lived and dendritic epidermal melanocytes (MC) that interact with viable keratinocytes (KC) to contribute melanin to the epidermis. Previously, we reported that MC:KC contact is required for melanosome transfer that can be enhanced by filopodi, and by UVR/UVA irradiation, which can upregulate melanosome transfer via Myosin X-mediated control of MC filopodia. Both MC and KC express Ca²⁺ -dependent E-cadherins. These homophilic adhesion contacts induce transient increases in intra-KC Ca²⁺ , while ultraviolet radiation (UVR) raises intra-MC Ca²⁺ via calcium-selective ORAI1 ion channels; both are associated with regulating melanogenesis. However, how Ca²⁺ triggers melanin transfer remains unclear. Here we evaluated the role of E-cadherin in UVR-mediated melanin transfer in human skin cells. MC and KC in human epidermis variably express filopodia-associated E-cadherin, Cdc42, VASP and β-catenin, all of which were upregulated by UVR in human MC in vitro. Knockdown of E-cadherin revealed that this cadherin is essential for UVR-induced MC filopodia formation and melanin transfer. Moreover, Ca²⁺ induced a dose-dependent increase in filopodia formation and melanin transfer, as well as increased β-catenin, Cdc42, Myosin X and E-cadherin expression in these skin cells. Together, these data suggest that filopodial proteins and E-cadherin, which are upregulated by intracellular (UVR-stimulated) and extracellular Ca²⁺ availability, are required for filopodia formation and melanin transfer. This may open new avenues to explore how Ca²⁺ signalling influences human pigmentation.

Electrophoto-Biomodulation in Aesthetic Treatment of Postburn Hypopigmentation: Clinical Response in Relation to Histopathological Changes

BACKGROUND

Hypopigmentation is a troublesome often-permanent sequelae after burn injury, particularly in dark races. A number of methods have been described to treat this phenomenon. In this work, we are going to study the effect of E light (intensive pulsed light+radiofrequency+cooling) in repigmentation of partial thickness and full thickness burn wounds in adult patients and light microscopic changes of biopsy material at various stages of repigmentation.

PATIENTS AND METHODS

In this study, 24 patients with postburn hypopigmentation were selected and E light was used, using different filters, according to the skin color. The intensive pulsed light fluence varied between 38 and 42 J. Radiofrequency fluencies varied between 6 and 10 J, and the spot diameter was 8 × 32 mm. Pulse durations of 2 to 7 milliseconds and pulse delays of 15 to 30 milliseconds were used on all patients.The patients were evaluated by comparing pretreatment and posttreatment photos. Skin biopsies were taken from depigmented area before and after interventions. Melanocytes were immunostained using Hwenty-four homatropine methylbromide 45 and were counted in ×200 magnification fields. Melanin can be demonstrated after staining with hematoxylin and eosin.

RESULTS

Seventeen cases were evaluated as excellent, 4 cases were evaluated as good, and 3 cases were evaluated as fair, but all patients stressed that their wounds have been improved.Light microscopy at 6 weeks posttreatment also confirmed the increased melanocyte number per field in all cases.

CONCLUSIONS

E light induced significant overall clinical improvement in postburn hypopigmentation, particularly when E light was applied early after burn.

Epilation induces hair and skin pigmentation through an EDN3/EDNRB-dependent regenerative response of melanocyte stem cells

In response to various types of injury, melanocyte stem cells (McSCs) located in the bulge of hair follicles can regenerate mature melanocytes for hair and skin pigmentation. How McSCs respond to injury, however, remains largely unknown. Here we show that after epilation of mice, McSCs regenerate follicular and epidermal melanocytes, resulting in skin and hair hyperpigmentation. We further show that epilation leads to endogenous EDN3 upregulation in the dermal papilla, the secondary hair germ cells, and the epidermis. Genetic and pharmacological disruption of the EDN3 receptor EDNRB in vivo significantly blocks the effect of epilation on follicular and epidermal melanocyte regeneration as well as skin and hair hyperpigmentation. Taken together, these results indicate that epilation induces McSCs activation through EDN3/EDNRB signaling and in turn leads to skin and hair hyperpigmentation. The findings suggest that EDN/EDNRB signaling may serve as a potential therapeutic target to promote repigmentation in hypopigmentation disorders.

Selective Coloration of Melanin Nanospheres through Resonant Mie Scattering

Black melanin inks are prepared to selectively exhibit colors under strong light, inspired by human hair. High absorbance of melanin suppresses multiple scattering, causing resonant Mie scattering predominant. Various colors can be developed as the resonant wavelength dictated by nanosphere diameter. Therefore, the melanin inks can be used to encrypt and selectively disclose multicolor patterns for anticounterfeiting applications.

Polygonum multiflorum Radix extract protects human foreskin melanocytes from oxidative stress in vitro and potentiates hair follicle pigmentation ex vivo

OBJECTIVE

To examine the ability of an extract from traditional Chinese medicine, Polygonum multiflorum Radix, to protect melanocyte viability from oxidative stress, a key mechanism in the initiation and progression of hair greying.

METHODS

To assess the antioxidant capacity of Polygonum multiflorum Radix extract, primary human foreskin melanocytes were treated with a commercially available Polygonum multiflorum Radix extract added to culture medium and exposed to hydrogen peroxide (H₂ O₂ ), using intracellular reactive oxygen species concentrations and glutathione/protein ratios as endpoints. To improve solubility for cosmetic uses, a new Polygonum multiflorum Radix extract was derived. As hair greying is the consequence of melanocyte disappearance in an oxidative stress environment, we checked whether the antioxidant capacity of the new Polygonum multiflorum Radix extract could preserve melanocyte viability in response to H₂ O₂ -induced oxidative stress, and preserve pigmentation within ex vivo human hair follicles.

RESULTS

In vitro treatment of primary human foreskin melanocytes with traditional available Polygonum multiflorum Radix extract resulted in decreased intracellular ROS accumulation in response to H₂ O₂ exposure with a concomitant preservation of glutathione-to-protein ratio, consistent with a protective response against H₂ O₂ exposure and demonstrating the promise of this extract for protecting melanocytes against oxidative stress. Melanocytes treated with the improved Polygonum multiflorum Radix extract exhibited attenuated H₂ O₂ -induced cell death, demonstrating a clear cytoprotective effect. Treatment of ex vivo human hair follicles with the improved Polygonum multiflorum Radix extract resulted in a higher level of melanin compared to vehicle-treated controls, demonstrating an ex vivo protective effect on hair pigmentation.

CONCLUSION

Polygonum multiflorum Radix extract protects in vitro primary human foreskin melanocytes from the deleterious effects of H₂ O₂ exposure and improves pigmentation within ex vivo human hair follicles, demonstrating the utility of Polygonum multiflorum Radix extract as a potential active ingredient for the protection of melanocytes against premature death. This data provides in vitro mechanistic evidence consistent with existing in vivo studies for the use of Polygonum multiflorum Radix extract as a strategy for the prevention of oxidative stress-induced hair greying, in line with traditional Polygonum multiflorum Radix uses.

Hair and stress: A pilot study of hair and cytokine balance alteration in healthy young women under major exam stress

Mouse models show that experimental stress mimicking prolonged life-stress exposure enhances neurogenic inflammation, induces adaptive immunity cytokine-imbalance characterized by a shift to Type 1 T-helper cell cytokines and increases apoptosis of epithelial cells. This affects hair growth in otherwise healthy animals. In this study, we investigate whether a prolonged naturalistic life-stress exposure affects cytokine balance and hair parameters in healthy humans. 33 (18 exam, 15 comparison) female medical students with comparable sociobiological status were analyzed during a stressful final examination period, at three points in time (T) 12 weeks apart. T1 was before start of the learning period, T2 between the three-day written exam and an oral examination, and T3 after a 12 week rest and recovery from the stress of the examination period. Assessments included: self-reported distress and coping strategies (Perceived Stress Questionnaire [PSQ], Trier Inventory for the Assessment of Chronic Stress [TICS]), COPE), cytokines in supernatants of stimulated peripheral blood mononucleocytes (PBMCs), and trichogram (hair cycle and pigmentation analysis). Comparison between students participating in the final medical exam at T2 and non-exam students, revealed significantly higher stress perception in exam students. Time-wise comparison revealed that stress level, TH1/TH2 cytokine balance and hair parameters changed significantly from T1 to T2 in the exam group, but not the control. However, no group differences were found for cytokine balance or hair parameters at T2. The study concludes that in humans, naturalistic stress, as perceived during participation in a major medical exam, has the potential to shift the immune response to TH1 and transiently hamper hair growth, but these changes stay within a physiological range. Findings are instructive for patients suffering from hair loss in times of high stress. Replication in larger and more diverse sample populations is required, to assess suitability of trichogram analysis as biological outcome for stress studies.

Paracrine Secreted Frizzled-Related Protein 4 Inhibits Melanocytes Differentiation in Hair Follicle

Wnt signaling plays crucial role in regulating melanocyte stem cells/melanocyte differentiation in the hair follicle. However, how the Wnt signaling is balanced to be overactivated to control follicular melanocytes behavior remains unknown. Here, by using immunofluorescence staining, we showed that secreted frizzled-related protein 4 (sFRP4) is preferentially expressed in the skin epidermal cells rather than in melanocytes. By overexpression of sFRP4 in skin cells in vivo and in vitro, we found that sFRP4 attenuates activation of Wnt signaling, resulting in decrease of melanocytes differentiation in the regenerating hair follicle. Our findings unveiled a new regulator that involves modulating melanocytes differentiation through a paracrine mechanism in hair follicle, supplying a hope for potential therapeutic application to treat skin pigmentation disorders.

The lactoferricin B-derived peptide, LfB17-34, induces melanogenesis in B16F10 cells

Lactoferricin B (LfcinB), a peptide of bovine lactoferrin (LfB), exhibits multiple biological functions, including antimicrobial, antiviral, antioxidant and immuno-modulatory activities. However, the role of LfcinB-related peptides in melanogenesis remains unclear. In this study, a set of five LfcinB-related peptides was examined. We found that LfB17-34, an 18-mer LfcinB-derived peptide, increased melanogenesis in B16F10 melanoma cells without significantly affecting cell viability. LfB17-34 increased in vitro tyrosinase activity and melanin content in a dose-dependent manner. The results of RT-qPCR and western blot analyses showed that LfB17-34 increased the mRNA and protein expression of tyrosinase and tyrosinase-related protein 1 (Trp1). Moreover, LfB17-34 inhibited the phosphorylation of MAPK/Erk, but not p38 and Akt, and constitutively active MEK was able to reverse the LfB17-34-enhanced pigmentation, melanin content, and tyrosinase activity, suggesting a role of Erk signaling in the process of LfB17-34-mediated pigmentation. Taken together, these results suggest that LfB17-34 induces melanogenesis in B16F10 cells primarily through increased tyrosinase expression and activity and that LfB17-34 could be further developed for the treatment of hypopigmentation disorders.

Comparative Transcriptome Analysis of Raccoon Dog Skin to Determine Melanin Content in Hair and Melanin Distribution in Skin

The raccoon dog (Nyctereutes procyonoides) is an important canid fur-bearing animal species worldwide. Chinese raccoon dogs that present a white mutation, especially those with a white coat. Exploring melanin biosynthesis in the hair and skin of raccoon dogs is important for understanding the survival and evolutionary mechanisms of them. In this study, we measured the content of melanin in the hair of two types of raccoon dog and generated stained slices of skin tissue. The results indicated that melanin biosynthesis occurs in the wild-type (W) and white-type (B) raccoon dog skin, although less melanin is produced in B skin. We then sequenced the skin transcriptomes of W and B, compared the similarities and differences in expressed genes. A comparison of the gene expression showed 60 up-regulated genes and 127 down-regulated genes in B skin. We analyzed the unigenes and pathways related to the melanogenesis pathway and found that TYR, TYRP1, MC1R, SLC24a5, SLC45a2 and OCA2 were significantly down-regulated in B skin and these results were verified via qRT-PCR. We surmised that the phenotypic characteristics of the white mutation might be caused by the reduced expression of these genes and this finding provides new insights for future experiments in raccoon dogs.

The expression of KRT2 and its effect on melanogenesis in alpaca skins

In order to investigate the effects of the keratin 2 (KRT2) on alpaca melanocyte in vivo and vitro, the immunohistochemistry (IHC), quantitative real-time PCR (qPCR), Western blot, and alpaca melanocytes transfection methods were used. The results showed that mRNA and protein expression of KRT2 was highly expressed in brown skin in comparison with that in white skin. Moreover, we found that KRT2 was expressed in alpaca melanocytes in vitro by immunocytochemistry. After transfection with KRT2 in alpaca melanocytes, the relative mRNA and protein expression of KRT2, microphthalmia-associtated transcription factor (MITF), tyrosinase (TYR) and tyrosinase-related protein 1 (TYRP1) in alpaca skin melanocytes was increased with significant differences; a further result was the increase of melanin production. The results suggested that KRT2 functions in alpaca hair color formation, which offered an essential theoretical basis for further exploration of the role of melanogenesis.

Follicular vitiligo: A report of 8 cases

BACKGROUND

Follicular vitiligo, a recently proposed new subtype of vitiligo, has primary involvement of the hair follicle melanocytic reservoir.

OBJECTIVE

We sought to characterize follicular vitiligo through a case series of 8 patients.

METHODS

Patients with features of follicular vitiligo who were seen at the vitiligo clinic in the National Center for Rare Skin Disorders in Bordeaux, France, were recruited. A retrospective review of case records and clinical photographs was carried out.

RESULTS

There were 8 male patients with a mean age of 48 years. All patients reported significant whitening of their body and, in some, scalp hairs before cutaneous depigmentation. Examination revealed classic generalized depigmented lesions of vitiligo and an impressive presence of leukotrichia, not only in the vitiliginous areas, but also in areas with clinically normal-appearing skin. Punch biopsy specimen of the leukotrichia and vitiligo lesions demonstrated loss of melanocytes and precursors in the basal epidermis and hair follicle.

LIMITATIONS

This was a cross-sectional study based on a single-center experience.

CONCLUSION

Follicular vitiligo is a distinct entity within the spectrum of vitiligo. This entity may serve as the missing link between alopecia areata and vitiligo, with probable physiopathological similarities between these conditions.

Biology of the eyelash hair follicle: an enigma in plain sight

Because of their crucial impact on our perception of beauty, eyelashes constitute a prime target for the cosmetic industry. However, when compared with other hair shafts and the mini-organs that produce them [eyelash hair follicles (ELHFs)], knowledge on the biology underlying growth and pigmentation of eyelashes is still rudimentary. This is due in part to the extremely restricted availability of human ELHFs for experimental study, underappreciation of their important sensory and protective functions and insufficient interest in understanding why they are distinct from scalp hair follicles (HFs) (e.g. ELHFs produce shorter hair shafts, do not possess an arrector pili muscle, have a shorter hair cycle and undergo greying significantly later than scalp HFs). Here we synthesize the limited current knowledge on the biology of ELHFs, in humans and other species, their role in health and disease, the known similarities with and differences from other HF populations, and their intrinsic interethnic variations. We define major open questions in the biology of these intriguing mini-organs and conclude by proposing future research directions. These include dissecting the molecular and cellular mechanisms that underlie trichomegaly and the development of in vitro models in order to interrogate the distinct molecular controls of ELHF growth, cycling and pigmentation and to probe novel strategies for the therapeutic and cosmetic manipulation of ELHFs beyond prostaglandin receptor stimulation.

A Guide to Studying Human Hair Follicle Cycling In Vivo

Hair follicles (HFs) undergo lifelong cyclical transformations, progressing through stages of rapid growth (anagen), regression (catagen), and relative "quiescence" (telogen). Given that HF cycling abnormalities underlie many human hair growth disorders, the accurate classification of individual cycle stages within skin biopsies is clinically important and essential for hair research. For preclinical human hair research purposes, human scalp skin can be xenografted onto immunocompromised mice to study human HF cycling and manipulate long-lasting anagen in vivo. Although available for mice, a comprehensive guide on how to recognize different human hair cycle stages in vivo is lacking. In this article, we present such a guide, which uses objective, well-defined, and reproducible criteria, and integrates simple morphological indicators with advanced, (immuno)-histochemical markers. This guide also characterizes human HF cycling in xenografts and highlights the utility of this model for in vivo hair research. Detailed schematic drawings and representative micrographs provide examples of how best to identify human HF stages, even in suboptimally sectioned tissue, and practical recommendations are given for designing human-on-mouse hair cycle experiments. Thus, this guide seeks to offer a benchmark for human hair cycle stage classification, for both hair research experts and newcomers to the field.

A primer on pigmentation

There is at least a temporary loss of skin pigmentation with all but first-degree burns. Commonly, pigment changes persist for months, and sometimes, permanent changes in skin color add to the ultimate change in appearance that commonly affects burn patients. There are many different treatment modalities for the treatment of pigment changes, but most of them have little scientific basis and often lead to disappointing results. The purpose of this review is to discuss the molecular and cellular mechanisms of skin pigmentation, mechanisms of repigmentation after burns, treatment options for dealing with pigmentation changes, and advice for dealing with the sun after burn injury.

Expression and tissue distribution of hepatocyte growth factor (HGF) and its receptor (c-Met) in alpacas (Vicugna pacos) skins associated with white and brown coat colors

Hepatocyte growth factor (HGF)/c-Met signaling has been considered as a key pathway in both melanocyte development and melanogenesis. To understand better the expression patterns and tissue distribution characterization of HGF and its receptor c-Met in skin of white versus brown alpaca (Vicugna pacos), we detected the tissue distribution of HGF and c-Met using immunohistochemistry and analyzed the expression patterns by using Western blot and quantitative real time PCR (qPCR). Immunohistochemistry analysis demonstrated that HGF staining robustly increased in the dermal papilla and mesenchymal cells of white alpaca skin compared with that of brown. However, c-Met staining showed strongly positive result, particularly inhair matrix and root sheath in brown alpaca skin. Western blot and qPCR results suggested that HGF and c-Met were expressed at significantly high levels in white and brown alpaca skins, respectively, and protein and transcripts possessed the same expression pattern in white and brown alpaca skins. The results suggested that HGF/c-Met signaling functions in alpaca coat color formation offer essential theoretical basis for further exploration of the role of HGF/c-Met signaling in pigment formation.

Angiotensin II stimulates melanogenesis via the protein kinase C pathway

Melanogenesis is a physiological process that results in the synthesis of melanin pigments, which serve a crucial function in hyperpigmentation. The aim of the present study was to determine the effects of angiotensin II (Ang II) on melanogenesis and to elucidate the molecular events of Ang II-induced melanogenesis. Experiments were performed on human melanocytes to elucidate the pigmenting effect of Ang II and the underlying mechanisms. The elements involved in melanogenesis, including melanin content, tyrosinase (TYR) activity, and microphthalmia-associated transcription factor (MITF) and TYR expression at the mRNA and protein levels were evaluated. Melanin content and TYR activity increased in response to Ang II treatment in a concentration-dependent manner. MITF and TYR mRNA and protein expression levels were increased significantly in response to Ang II in a concentration-dependent manner. The Ang II-induced increase in melanin synthesis was reduced significantly in response to co-treatment with Ro-32-0432, a protein kinase C (PKC) inhibitor, whereas co-treatment with H-89, a PKA inhibitor, did not attenuate the Ang II-induced increase in melanin levels. These results suggest that PKC is required for Ang II-induced pigmentation in human melanocytes and that the mechanism involves the PKC pathway and MITF upregulation.

Tryptophan hydroxylase Is Required for Eye Melanogenesis in the Planarian Schmidtea mediterranea

Melanins are ubiquitous and biologically important pigments, yet the molecular mechanisms that regulate their synthesis and biochemical composition are not fully understood. Here we present a study that supports a role for serotonin in melanin synthesis in the planarian Schmidtea mediterranea. We characterize the tryptophan hydroxylase (tph) gene, which encodes the rate-limiting enzyme in serotonin synthesis, and demonstrate by RNA interference that tph is essential for melanin production in the pigment cups of the planarian photoreceptors. We exploit this phenotype to investigate the biological function of pigment cups using a quantitative light-avoidance behavioral assay. Planarians lacking eye pigment remain phototactic, indicating that eye pigmentation is not essential for light avoidance in S. mediterranea, though it improves the efficiency of the photophobic response. Finally, we show that the eye pigmentation defect observed in tph knockdown animals can be rescued by injection of either the product of TPH, 5-hydroxytryptophan (5-HTP), or serotonin. Together, these results highlight a role for serotonin in melanogenesis, perhaps as a regulatory signal or as a pigment substrate. To our knowledge, this is the first example of this relationship to be reported outside of mammalian systems.

The peripheral clock regulates human pigmentation

Although the regulation of pigmentation is well characterized, it remains unclear whether cell-autonomous controls regulate the cyclic on-off switching of pigmentation in the hair follicle (HF). As human HFs and epidermal melanocytes express clock genes and proteins, and given that core clock genes (PER1, BMAL1) modulate human HF cycling, we investigated whether peripheral clock activity influences human HF pigmentation. We found that silencing BMAL1 or PER1 in human HFs increased HF melanin content. Furthermore, tyrosinase expression and activity, as well as TYRP1 and TYRP2 mRNA levels, gp100 protein expression, melanocyte dendricity, and the number gp100+ HF melanocytes, were all significantly increased in BMAL1 and/or PER1-silenced HFs. BMAL1 or PER1 silencing also increased epidermal melanin content, gp100 protein expression, and tyrosinase activity in human skin. These effects reflect direct modulation of melanocytes, as BMAL1 and/or PER1 silencing in isolated melanocytes increased tyrosinase activity and TYRP1/2 expression. Mechanistically, BMAL1 knockdown reduces PER1 transcription, and PER1 silencing induces phosphorylation of the master regulator of melanogenesis, microphthalmia-associated transcription factor, thus stimulating human melanogenesis and melanocyte activity in situ and in vitro. Therefore, the molecular clock operates as a cell-autonomous modulator of human pigmentation and may be targeted for future therapeutic strategies.

The role of P-cadherin in skin biology and skin pathology: lessons from the hair follicle

Adherens junctions (AJs) are one of the major intercellular junctions in various epithelia including the epidermis and the follicular epithelium. AJs connect the cell surface to the actin cytoskeleton and comprise classic transmembrane cadherins, such as P-cadherin, armadillo family proteins, and actin microfilaments. Loss-of-function mutations in CDH3, which encodes P-cadherin, result in two allelic autosomal recessive disorders: hypotrichosis with juvenile macular dystrophy (HJMD) and ectodermal dysplasia, ectrodactyly, and macular dystrophy (EEM) syndromes. Both syndromes feature sparse hair heralding progressive macular dystrophy. EEM syndrome is characterized in addition by ectodermal and limb defects. Recent studies have demonstrated that, together with its involvement in cell-cell adhesion, P-cadherin plays a crucial role in regulating cell signaling, malignant transformation, and other major intercellular processes. Here, we review the roles of P-cadherin in skin and hair biology, with emphasize on human hair growth, cycling and pigmentation.

Ectopic differentiation of melanocyte stem cells is influenced by genetic background

Hair graying in mouse is attributed to the loss of melanocyte stem cell function and the progressive depletion of the follicular melanocyte population. Single-gene, hair graying mouse models have pointed to a number of critical pathways involved in melanocyte stem cell biology; however, the broad range of phenotypic variation observed in human hair graying suggests that additional genetic variants involved in this process may yet be discovered. Using a sensitized approach, we ask here whether natural genetic variation influences a predominant cellular mechanism of hair graying in mouse, melanocyte stem cell differentiation. We developed an innovative method to quantify melanocyte stem cell differentiation by measuring ectopically pigmented melanocyte stem cells in response to the melanocyte-specific transgene Tg(Dct-Sox10). We make the novel observation that the production of ectopically pigmented melanocyte stem cells varies considerably across strains. The success of sensitizing for melanocyte stem cell differentiation by Tg(Dct-Sox10) sets the stage for future investigations into the genetic basis of strain-specific contributions to melanocyte stem cell biology.

The melanocortin 1 receptor (Mc1r) variants do not account for the co-occurrence of Parkinson's disease and malignant melanoma

Parkinson's disease (PD) is characterized by loss of melanin-positive dopaminergic neurons in the substantia nigra. Malignant melanoma (MM), a melanocyte-derived neoplasm, occurs with higher than expected frequency among PD patients. Red-haired individuals exhibit a threefold risk for developing MM than dark-haired people; PD risk also increases with lighter hair color. One plausible explanation for the associations between MM, hair color, and PD is the melanocortin-1 receptor (MC1R) gene that plays a key role in hair and skin pigmentation as well as in MM predisposition. We hypothesized that specific MC1R variants may predispose to both MM and PD. Genotyping of the MC1R gene was performed for 16 PD patients with MM (PD+ MM+) and for three sets of age, sex, and ethnically matched controls, including 36 patients with PD (PD+ MM-), 37 with MM (PD- MM+) and 37 with neither diagnosis (PD- MM-). No association was found between MC1R variants and the co-occurrence of PD and MM. The risk for MM was higher in carriers of two MC1R variants versus with no MC1R variant (odds ratio (OR)=5.0, 95% confidence interval (CI) 1.7-14.4, p=0.003). The risk for PD in carriers of two MC1R variants was markedly lower (OR=0.213, 95% CI 0.063-0.725) compared with individuals with no MC1R variant (p=0.013). In this study, MC1R variants were not associated with both MM and PD. Further studies in larger cohorts are necessary to confirm these preliminary results.

Melanocytes and their diseases

Human melanocytes are distributed not only in the epidermis and in hair follicles but also in mucosa, cochlea (ear), iris (eye), and mesencephalon (brain) among other tissues. Melanocytes, which are derived from the neural crest, are unique in that they produce eu-/pheo-melanin pigments in unique membrane-bound organelles termed melanosomes, which can be divided into four stages depending on their degree of maturation. Pigmentation production is determined by three distinct elements: enzymes involved in melanin synthesis, proteins required for melanosome structure, and proteins required for their trafficking and distribution. Many genes are involved in regulating pigmentation at various levels, and mutations in many of them cause pigmentary disorders, which can be classified into three types: hyperpigmentation (including melasma), hypopigmentation (including oculocutaneous albinism [OCA]), and mixed hyper-/hypopigmentation (including dyschromatosis symmetrica hereditaria). We briefly review vitiligo as a representative of an acquired hypopigmentation disorder.

Premature graying as a consequence of compromised antioxidant activity in hair bulb melanocytes and their precursors

Intricate coordinated mechanisms that govern the synchrony of hair growth and melanin synthesis remain largely unclear. These two events can be uncoupled in prematurely gray hair, probably due to oxidative insults that lead to the death of oxidative stress-sensitive melanocytes. In this study, we examined the gene expression profiles of middle (bulge) and lower (hair bulb) segments that had been micro-dissected from unpigmented and from normally pigmented hair follicles from the same donors using quantitative real-time RT-PCR (qPCR) arrays. We found a significant down-regulation of melanogenesis-related genes (TYR, TYRP1, MITF, PAX3, POMC) in unpigmented hair bulbs and of marker genes typical for melanocyte precursor cells (PAX3, SOX10, DCT) in unpigmented mid-segments compared with their pigmented analogues. qPCR, western blotting and spin trapping assays revealed that catalase protein expression and hydroxyl radical scavenging activities are strongly repressed in unpigmented hair follicles. These data provide the first clear evidence that compromised antioxidant activity in gray hair follicles simultaneously affects mature hair bulb melanocytes and their immature precursor cells in the bulge region.

Key role of CRF in the skin stress response system

The discovery of corticotropin-releasing factor (CRF) or CRH defining the upper regulatory arm of the hypothalamic-pituitary-adrenal (HPA) axis, along with the identification of the corresponding receptors (CRFRs 1 and 2), represents a milestone in our understanding of central mechanisms regulating body and local homeostasis. We focused on the CRF-led signaling systems in the skin and offer a model for regulation of peripheral homeostasis based on the interaction of CRF and the structurally related urocortins with corresponding receptors and the resulting direct or indirect phenotypic effects that include regulation of epidermal barrier function, skin immune, pigmentary, adnexal, and dermal functions necessary to maintain local and systemic homeostasis. The regulatory modes of action include the classical CRF-led cutaneous equivalent of the central HPA axis, the expression and function of CRF and related peptides, and the stimulation of pro-opiomelanocortin peptides or cytokines. The key regulatory role is assigned to the CRFR-1α receptor, with other isoforms having modulatory effects. CRF can be released from sensory nerves and immune cells in response to emotional and environmental stressors. The expression sequence of peptides includes urocortin/CRF→pro-opiomelanocortin→ACTH, MSH, and β-endorphin. Expression of these peptides and of CRFR-1α is environmentally regulated, and their dysfunction can lead to skin and systemic diseases. Environmentally stressed skin can activate both the central and local HPA axis through either sensory nerves or humoral factors to turn on homeostatic responses counteracting cutaneous and systemic environmental damage. CRF and CRFR-1 may constitute novel targets through the use of specific agonists or antagonists, especially for therapy of skin diseases that worsen with stress, such as atopic dermatitis and psoriasis.

Extracted hair follicle outer root sheath cell suspension for pigment cell restoration in vitiligo

Vitiligo surgery has come up a long way from punch skin grafts to epidermal cell suspension and latest to the extracted hair follicle outer root sheath cell suspension (EHF-ORS-CS) transplantation. The progressive development from one technique to the other is always in a quest for the best. In the latest development- EHF-ORS-CS, which is an enriched source of follicular inactive melanocyte (melanocyte stem cells), seems to be a good addition to the prevailing cell-based therapies for vitiligo; however, need to be explored further in larger, and preferably randomized blinded studies. This review discusses the principle, technical details, and stem cell composition of hair follicular outer root sheath cell suspension.

What causes alopecia areata?

The pathobiology of alopecia areata (AA), one of the most frequent autoimmune diseases and a major unsolved clinical problem, has intrigued dermatologists, hair biologists and immunologists for decades. Simultaneously, both affected patients and the physicians who take care of them are increasingly frustrated that there is still no fully satisfactory treatment. Much of this frustration results from the fact that the pathobiology of AA remains unclear, and no single AA pathogenesis concept can claim to be universally accepted. In fact, some investigators still harbour doubts whether this even is an autoimmune disease, and the relative importance of CD8(+) T cells, CD4(+) T cells and NKGD2(+) NK or NKT cells and the exact role of genetic factors in AA pathogenesis remain bones of contention. Also, is AA one disease, a spectrum of distinct disease entities or only a response pattern of normal hair follicles to immunologically mediated damage? During the past decade, substantial progress has been made in basic AA-related research, in the development of new models for translationally relevant AA research and in the identification of new therapeutic agents and targets for future AA management. This calls for a re-evaluation and public debate of currently prevalent AA pathobiology concepts. The present Controversies feature takes on this challenge, hoping to attract more skin biologists, immunologists and professional autoimmunity experts to this biologically fascinating and clinically important model disease.

Culture of amelanotic melanocytes derived from human fetal hair follicles

Human melanocyte stem cells (MSCs) or melanoblasts are not well-investigated owing to the devoid of suitable culture system. Establishing cell lines of MSCs and/or their progenies from human hair follicles will provide a better opportunity to satisfy clinical needs and to enable a deeper understanding of hair-related diseases. In the present study, we cultured melanocytes derived from human fetal hair follicles, perform immunocytochemistry and Fontana Masson staining on them, and employed atomic force microscopy (AFM) and scanning electron microscopy to observe their subtle morphologies. The results show that the cultured melanocytes have a bipolar or tripolar appearance, which obviously differ from cultured epidermal melanocytes. Compared to cells derived from adult human hair follicles, these cells display a high proliferative capability and exhibit a clonal growth behavior. At the second passage, all these cells were positive for immunocytochemical staining with the NKI/beteb monoclonal antibody and Fontana Masson staining. Under AFM, the cells exhibited rounded, oval, triangular, or quadrangular perikarya, from which two or three dendrites arose. The dendritic arbor was not homogeneous but appeared as spindle-shaped dendritic swellings, knob-like processes, without any filopodia arising from the dendrites or the cell body. Without using a feeder layer, we successfully obtained the clonal growth of melanocytes from human fetal HFs, suggesting that the medium was suitable for the growth of MSCs and their progenies.

Tolerance induction by hair-specific keratins in murine alopecia areata

AA is a presumptive autoimmune disease, severely damaging the hair follicle. Hair- and nail-specific keratins are discussed as potential candidates, which we controlled in C3H/HeJ mice that develop AA spontaneously or after skin transplantation. From nine keratins, K71 and K31 peptides supported T cell activation when presented by DCs to syngeneic naive T cells, and young C3H/HeJ mice receiving s.c. injections of peptide-loaded DC developed AA. The frequency of K71- and K31-specific CD4(+) and CD8(+) T cells increased four- to fivefold by vaccination, which corresponds with the frequency seen in skin transplantation-induced AA mice. Also, accessory molecule expression, the cytokine profile with a dominance of IFN-γ-expressing T cells, the proliferative response against AA lysate or peptide-loaded DCs, as well as peptide-specific cytotoxic T cells were similar in keratin peptide- and skin transplantation-induced AA. Instead, vaccination with soluble K71 or K31 peptides significantly retarded AA induction and prevented progression. Soluble peptide vaccination did not provoke immunosuppression but induced long-lasting T cell anergy with unresponsiveness to DC-presented K71 and K31 peptides. Thus, keratins K71 and K31 contribute to AA induction, and peptide application in a nonimmunogenic form serves as an efficient therapeutic.

Repigmentation of cutaneous scars depends on original wound type

Cutaneous scarring is currently an inevitable outcome following skin injury. Abnormal pigmentation within scars makes them more noticeable, causing distress for patients, particularly as there is no reliable and effective treatment available to date. The Duroc pig, known to scar badly, was used to investigate repigmentation of scars resulting from three different wound types: incisional, partial thickness excisional and full thickness excisional. Wounds were created on the backs of Duroc pigs and the resulting scars harvested at days 35, 56, 70 and 90 days post-injury. Scars were processed for histology and immunohistochemistry, quantitatively analysed using image analysis software and subjected to statistical analysis. Photographs of the macroscopic appearance of scars were scored for pigmentation using a visual analogue scale. Results demonstrated temporal and spatial differences in melanocyte repopulation and function within scars from different wound types. The microscopic pigment deposition did not correlate with macroscopic appearances in mature scars. Pigmentation of scars is dependent on the width and depth of wounds. This study has provided important information on which we can base future studies to investigate factors controlling the repigmentation of scars.

Skin melanocytes: biology and development

In the human skin, melanocytes are present in the epidermis and hair follicles. The basic features of these cells are the ability to melanin production and the origin from neural crest cells. This last element is important because there are other cells able to produce melanin but of different embryonic origin (pigmented epithelium of retina, some neurons, adipocytes). The life cycle of melanocyte consists of several steps including differentiation of melanocyte lineage/s from neural crest, migration and proliferation of melanoblasts, differentiation of melanoblasts into melanocytes, proliferation and maturation of melanocytes at the target places (activity of melanogenic enzymes, melanosome formation and transport to keratinocytes) and eventual cell death (hair melanocytes). Melanocytes of the epidermis and hair are cells sharing some common features but in general they form biologically different populations living in unique niches of the skin.

Topobiology of human pigmentation: P-cadherin selectively stimulates hair follicle melanogenesis

P-cadherin serves as a major topobiological cue in mammalian epithelium. In human hair follicles (HFs), it is prominently expressed in the inner hair matrix that harbors the HF pigmentary unit. However, the role of P-cadherin in normal human pigmentation remains unknown. As patients with mutations in the gene that encodes P-cadherin show hypotrichosis and fair hair, we explored the hypothesis that P-cadherin may control HF pigmentation. When P-cadherin was silenced in melanogenically active organ-cultured human scalp HFs, this significantly reduced HF melanogenesis and tyrosinase activity as well as gene and/or protein expression of gp100, stem cell factor, c-Kit, and microphthalmia-associated transcription factor (MITF), both in situ and in isolated human HF melanocytes. Instead, epidermal pigmentation was unaffected by P-cadherin knockdown in organ-cultured human skin. In hair matrix keratinocytes, P-cadherin silencing reduced plasma membrane β-catenin, whereas glycogen synthase kinase 3 beta (GSK3β) and phospho-β-catenin expression were significantly upregulated. This suggests that P-cadherin-GSK3β/Wnt signaling is required for maintaining the expression of MITF to sustain intrafollicular melanogenesis. Thus, P-cadherin-mediated signaling is a melanocyte subtype-specific topobiological regulator of normal human pigmentation, possibly via GSK3β-mediated canonical Wnt signaling.

Profiling mRNA of the graying human hair follicle constitutes a promising state-of-the-art tool to assess its aging: an exemplary report

Determining hitherto uninvestigated and safe targets to halt the aging process is important in our aging society. Graying is a hallmark of the aging process and may be used to identify aging tissue for comparative analysis. Here we analyzed differential gene expressions between pigmented, gray, and white human scalp skin hair follicles (HFs) from identical donors. Forming intersections between five donors identified 194/192 downregulated and 186/177 upregulated genes in gray/white HFs. These included melanogenesis (tyrosinase; tyrosinase-related protein 1)- and melanosome structure (Melan-A; Pmel17)-associated genes and regulation of melanocyte relevant tyrosine kinases. Alongside these expected changes, regulated genes included nonmelanocyte-related genes associated with aging as well as nonaging-related genes associated with melanocytes. Intriguingly, among them, genes associated with energy metabolism (i.e., glutaminase) and axon guidance (plexin C1) were altered. These results were reflected by pathway analysis and exemplarily confirmed by PCR and immunohistochemical studies. Supplementing cultured HFs with glutamine or plexin C1 revealed biological relevance and pharmacointerventional potential of these microarray results in altering the HF aging process. Together, we present intriguing data obtained from intra-individual sample comparison that suggest the graying HF to be a valid aging model and a promising target for testing therapeutic interventions.

Abnormal pigmentation within cutaneous scars: A complication of wound healing

Abnormally pigmented scars are an undesirable consequence of cutaneous wound healing and are a complication every single individual worldwide is at risk of. They present a challenge for clinicians, as there are currently no definitive treatment options available, and render scars much more noticeable making them highly distressing for patients. Despite extensive research into both wound healing and the pigment cell, there remains a scarcity of knowledge surrounding the repigmentation of cutaneous scars. Pigment production is complex and under the control of many extrinsic and intrinsic factors and patterns of scar repigmentation are unpredictable. This article gives an overview of human skin pigmentation, repigmentation following wounding and current treatment options.