Melanocyte Stem Cells as Potential Therapeutics in Skin Disorders

Abstract

Introduction

Melanocytes produce pigment granules that color both skin and hair. In the hair follicles melanocytes are derived from stem cells (MelSC) that are present in hair bulges or sub-bulge regions and function as melanocyte reservoirs. Quiescence, maintenance, activation, and proliferation of MelSC are controlled by specific activities in the microenvironment that can influence the differentiation and regeneration of melanocytes. Therefore, understanding MelSC and their niche may lead to use of MelSC in new treatments for various pigmentation disorders.

Areas covered

We describe here pathophysiological mechanisms by which melanocyte defects lead to skin pigmentation disorders such as vitiligo and hair graying. The development, migration, and proliferation of melanocytes and factors involved in the survival, maintenance, and regeneration of MelSC are reviewed with regard to the biological roles and potential therapeutic applications in skin pigmentation diseases.

Expert Opinion

MelSC biology and niche factors have been studied mainly in murine experimental models. Human MelSC markers or methods to isolate them are much less well understood. Identification, isolation and culturing of human MelSC would represent a major step toward new biological therapeutic options for patients with recalcitrant pigmentary disorders or hair graying. By modulating the niche factors for MelSC it may one day be possible to control skin pigmentary disorders and prevent or reverse hair graying.

1. INTRODUCTION

Melanocytes that are located in the epidermis and hair follicles of the skin play a major role in pigmentation of the skin or hair. Pigment producing cells are also distributed in the eyes, ears, brain, heart, lung, and bone _[1–3]. The functions of melanocytes in these other locations are not known in detail, although a role in scavenging reactive oxygen species has been reported _[4,5]. In the skin, melanin pigment is taken up by skin keratinocytes and organized into a shield around the nucleus where it is thought to protect genomic DNA from the harmful effects of ultraviolet light. Pigment produced by the melanocytes in hair follicles is incorporated into the growing hair and therefore determines the coat color in mammals. : The maintenance of the melanocyte is dependent on a population of melanocyte stem cells (MelSCs), a quiescent population that is present in the bulge region of the hair follicle and acts as a melanocyte reservoir. After migration into epidermis, MelSCs give rise to differentiated, pigment-producing melanocyte. Also, as many local and systemic factors are thought to participate in the pathogenesis of skin diseases such as vitiligo and hair growth disorders, it is important to understand the environmental effects on melanocytes including serotoninergic/melatoninergic system in the skin response to stresses as well as cytochrome-dependent and proopiomelanocortin (POMC) systems _[6–10].

Skin and hair melanocytes are derived from neural crest cells early in development _[11–14]. The cranial and trunk-located neural crest stem cells differentiate during migration. Melanoblasts are precursor cells with properties similar to Schwann cell precursors and they share various signaling pathways with neurons _[15–18]. After migration into the epidermis, melanocyte precursor cells are positioned in the lower permanent portion (LPP) during formation of the hair placode. The melanocytic lineage population in this region is thought to include melanocyte stem cells (MelSCs), previously known as amelanotic melanocytes in human follicles. These melanocyte precursor cells can remain and persist in the dermis of the skin, and have been suggested to have overlapping characteristics with cells of the nervous system _[19]. For example, cultured melanoblasts can be differentiated towards neurons, glial cells, or smooth muscle cells _[20–22]. This indicates that neural crest stem cell-derived melanoblasts in the skin retain reprogramming ability and show multipotency related to signals from the microenvironment. Also, melanocytes are sensory and regulatory cells for the maintenance of the cutaneous homeostasis and have been defined as neuroendocrine cells that could efficiently regulate local and systemic homeostasis _[7,14,23].

During the hair cycle, MelSCs differentiate into terminally differentiated melanocytes that produce mature pigment containing melanosomes, and that are incorporated into the growing hair shaft (anagen coupled melanogenesis) resulting in pigmented hair _[24–30]. In catagen, melanin formation is switched off and is absent throughout telogen _[26]. A key factor during melanocyte differentiation is microphthalmia associated transcription factor (MITF) _[31–34]. MITF controls the expression of key pigment synthetic genes including TRP-1, DCT, and tyrosinase _[32]. Various extracelluar signaling pathways converge on MITF to control both migration and survival of melanoblasts _[35–40].

2. MELANOCYTE STEM CELLS IN THE HAIR BULGE

Differentiated melanocytes in the hair bulb and melanocyte precursor cells (transient amplifying (TA) cells) in the outer root sheath originate from MelSCs in the hair follicle bulge region (Figure 1). In mice, MelSCs can be identified by use of dopachrometautomerase (Dct) promoter. Specifically in Dct promoter-LacZ reporter engineered mice, non-pigmented small oval-shaped cells with scant cytoplasm that share similarity with amelanotic melanocytes localized in the lower permanent portion of the hair follicle stain LacZ-positive and therefore are identified as MelSCs _[41]. In contrast to bulb melanocytes, these MelSCs exhibit low expression of pigmentation-related genes _[42–45]. Osawa et al. isolated murine MelSCs and analyzed their gene expression patterns, finding that multiple key genes such as Dct and Pax3, potential candidate MelSC markers, were also expressed in transient amplifying (TA) cells. Other important melanocytic genes such as Kit, Si, Tyr, Tyrp1, Mki67, Lef1, Sox10, and Mitf were expressed at higher levels only in TA cells, not in MelSC _[42].

Figure 1.

Possible related pathways in MelSC survival, maintenance, and differentiation. MITF, Bcl-2, B-Raf, C-Raf, TGF-b, Notch pathways are involved in MelSC survival and maintenance. PAX-3 and Wnt pathways are related to MelSC differentiation.

MelSCs remain in a quiescent state during the telogen phase of the hair cycle without transcription of pigmentation-related genes _{[29,42,44,45]}. However, once activated in the anagen phase of the hair cycle, pigmentation-related genes are upregulated and the cells proliferate with dendrite formation _[41]. After the mid-anagen phase, the pigmentation-related genes are downregulated and MelSCs become inactivated again _[46–48]. Repeated hair cycles result in melanocyte differentiation from the MelSCs, and eventually differentiated melanocytes undergo apoptosis during the catagen stage of the hair cycle _[49,50], although MelSCs still remain in the LPP of the hair follicle. The terminal differentiation of melanocytes within the hair follicle is coupled with hair cycle progression _[51]. During the catagen and telogen stages, MelSCs reside in the LPP region of the hair, indicating that MelSC activation is related to the niche factors during the hair follicle cycle _[24,41,52]. In humans, a selective MelSC marker has not been elucidated, in part because of the limited availability of genetic studies of these human amelanotic melanocytes _[53]. Human amelanotic melanocytes are considered equivalent to MelSCs in mice because they show similar morphology and location in the LPP _[46,53–55].

3. MELANOCYTE STEM CELL NICHE

The specific niche environment that surrounds MelSC plays a major role in regulating quiescence, differentiation, proliferation, and survival of MelSC _{[41,43,46,56]}. For example, certain environmental conditions can maintain quiescent characteristics of MelSCs by downregulating basal transcription and translational levels of some housekeeping genes _[42,57].

Although many factors, including genes, transcription factors, and signaling pathways are implicated in embryonic development, differentiation, and proliferation of the melanoblast and melanocyte (e.g. c-Kit, SCF, Ednrb, Wnt, Mitf, Pax-3, Sox-10, and c-myc), the niche factors that directly regulate the MelSCs are incompletely understood _[58–61]. Some niche factors have been proposed to affect MelSC maintenance and quiescence in mice, and have been summarized in Table 1. For example, MITF, known as the master regulator of melanocyte development, differentiation, and pigmentation, also plays an important role in MelSC maintenance _[46,59,62]. Bcl2, a survival gene which is a downstream target of MITF, plays a central role in MelSCs _[46,63]. Bcl2 protects against apoptosis of melanocytes and promotes the survival of MelSCs, thus mutation of Bcl2 causes melanocyte loss _[63–65]. More specifically, it was observed that germ line BCL2 knockout results in complete loss of melanocyte stem cells at post-natal day 8, but this death did not occur for bulb melanocytes. As a consequence, fur remained fully pigmented through the initial hair follicle cycle, but became white starting at the second hair follicle cycle. Additional analysis revealed that TGF-β signaling is related to MelSC quiescence or MelSC depletion (in the absence of Bcl2) _{[41,46,63–65]}.

Table 1.

Niche factors that affect melanocyte stem cell maintenance and quiescence in mice

Protein	Function	Mouse model	Result	Reference
Bcl-2	MelSC maintenance	Bcl-2 null mice	Pigmentation loss	Veis et al. [111] Kamada et al.[112] Yamamura et al. [64] Mak et al. [63]
Mitf	MelSC survival MelSC maintenance	Bcl-2 deficient mice	Pigmentation loss LPP colonization	Nishimura et al. [46]
C-Kit	Bcl activation Not required for MelSC	c-Kit KO	No hair color change	McGill et al. [62]
BRAF, CRAF	MelSC maintenance	Raf KO mouse	Graying hair	Valluet et al [68]
TGF-β	MelSC maintenance	TGFbRII-deficient mice	Loss of MSC	Nishimura et al. [47]
Pax3	MelSC differentiation MelSC development		Mitf activation Coexpression with Mitf and Sox 10	Lang et al. [72]
Wnt	MelSC differentiation MelSC maintenance		Control Pax3, Mitf function	Kubic et al. [71] Lang et al. [72] Moriyama et al. [74]
Notch	MelSC survival MelSC maintenance	RBP-J KO mice	Premature hair graying	Osawa et al. [76] Moriyama et al. [74] Schouwey et al. [75]

B-Raf and C-Raf protein kinases are important effectors of the MAPK pathway downstream of RAS _[66,67]. Recently, a double-knockout of B-raf and C-raf in mice showed marked abnormality in coat color although single mutation of B-raf or C-raf did not show this phenotypic change, demonstrating a key function for these kinases (and likely for the MAPK pathway) in the self-maintenance of MelSCs _[68].

MelSCs reside in the hair follicle bulge area where epidermal stem cells are located _[48]. It is likely that factors related to epidermal stem cells may also affect MelSCs in the niche because of the proximity of these stem cell populations within the bulge. Wnt ligands are responsible for the activation of MelSCs to proliferate into melanocyte precursor cells whereas transforming growth factor-beta (TGF-β) is vital for the quiescence maintenance of MelSCs, a vital aspect of stemness _[47,69,70]. Wnt signaling is upstream of Mitf and Pax3, which are also related to MelSC maintenance _[71]. These Wnt pathway targets-- especially Pax3, Sox10, and Mitf--are likely to regulate MelSC maintenance. In particular Pax3 prevents terminal differentiation of MelSCs into melanocytes, a process which is antagonized by β-catenin _[21,71,72]. Activation of the Wnt signaling pathway results in MelSC differentiation into melanocytes, whereas inhibition maintains the MelSC phenotype. The niche expresses Wnt inhibitors such as DKK3, Sfrp1, and Dab2 _[71,73]. Also, MelSCs themselves express DKK5, Sfrp1, Dab2, or Wif1 _[42,44,71] which may help to maintain MelSC progeny in the niche.

Notch signaling is also involved in MelSC survival and maintenance _[74–76]. Genetic ablation of notch signaling resulted in premature hair graying in mice _[74,75]. Moriyama et al. demonstrated that notch signaling acts through the Hes1 downstream transcription factor. This finding suggests that, as with melanoblast development and differentiation, interactions and collaborations between the melanocyte lineage cells and hair follicle stem cells (HFSCs) plays an important role in the regulation of MelSC maintenance _[77]. It has also been shown that Col17a1-mediated HFSC depletion results in MelSC defects _[47]. Tanimura et al. _[70] demonstrated that MelSC maintenance was rescued via TGF-β in Col17a1-knockout mice expressing COL17A1 under control of the Keratin promoter, which targets epidermal keratinocytes.

Chang et al. _[78] also showed that the transcription factor NFIB that controls endothelin 2 (Edn2) expression, plays the role of a coordinator of HFSC-MelSC behaviors within the niche. The uncoupling of stem cell synchrony by HFSC-specific conditional targeting of Nfib occurs by promoting MelSCs to produce premature melanin pigmentation and results in precocious MelSC differentiation and HFSC apoptosis. This finding suggests the importance of cooperation between stem cells within the niche in skin injury, stress, and disease states, including skin cancer development involving the NFIB pathway.

Because of the convenience of genetic manipulation and identification of phenotypic changes of coat color, experimental results regarding MelSC maintenance are almost all derived from mouse models. The exact mechanisms controlling human MelSC biology are poorly understood and may have differences from what we have learned in mouse models. Evidence from the clinical the setting can provide precious clues to melanocyte stem cell biology and potential therapeutic applications for the future.

4. MELANOCYTE STEM CELLS IN HUMAN SKIN DISORDERS

4.1 VITILIGO

Vitiligo is a condition that causes skin depigmentation and occurs in 0.5–1% of the population _[79,80]. Autoimmune, genetic, viral, or oxidative stresses have been proposed as the pathogenic mechanism(s) of melanocyte loss although the most common subtypes are likely to be autoimmune-based _[81]. There is still debate over the complete versus partial absence of melanocytes in the vitiligo lesions, but it is generally accepted that melanocyte number is reduced and some patients show complete melanocyte loss in severe cases.

After therapy for vitiligo, such as immune-suppressive modalities, repigmentation frequently begins in the peri-follicular area. This likely arises from the reservoir of MelSCs in the hair follicle bulge _[82]. Previously described amelanotic melanocytes from the outer root sheath are thought to be a reservoir for this migration _[41,83]. Although it is difficult to identify the presence of melanoblasts or MelSCs in the clinical setting, non-pigmented melanocytes have been identified microscopically in chronic recalcitrant vitiligo _[84], which suggests that MelSCs can remain in the niche and potentially provide a chance of repigmentation. It is also possible that the undifferentiated state of melanocyte stem cells prevents autoimmune recognition if such recognition would have required expression of melanocytic differentiation factors/antigens. Seleit et al. also showed that 54% and 63% of melanocyte precursor cells/MelSCs remain at the interfollicular and follicular areas of vitiligo lesional skin respectively _[85]. Another clinical study demonstrated that 65.5% of 352 vitiligo patches showed a perifollicular repigmentation pattern on systemic PUVA (psoralin UVA) treatment _[86]. Another vitiligo treatment option, narrow band UVB (NB-UVB (311-nm)), is a relatively effective therapy and has been substituted for conventional PUVA therapy _[87]. NB-UVB treatment induces Sox10, Kit, and MC1R and enhances differentiation of melanocytes, possibly from the MelSCs _[88]. The mechanism of repigmentation after UV has been also studied. UVB irradiation induces wnt7a activation, which triggers MelSC differentiation through the activation of β-catenin and migration of melanocyte precursor cells to the epidermis with Kit induction _[89]. The distribution of MelSCs and melanocytes in vitiliginous lesions is schematically illustrated in Figure 1.

Several areas are known to resist repigmentation during vitiligo treatment, such as the hands and feet _[79] or leucotrichia-associated lesions _[90]. The most relevant reasons for recalcitrance to photochemotherapy in acral vitiligo lesions seem to be inherent lower melanocyte density, lower melanocyte stem cell reservoirs, and lower baseline levels of epidermal stem cell requiring factors _[91].

Becasue MelSCs reside in the outer root sheath of hair follicles, suspensions of outer root sheath cells have functioned as a source of MelSC when transplanted into patients with vitiligo or leukoderma _[92,93]. Vanscheidt and Hunziker _[92] have used single-cell suspensions of plucked hair follicles in the treatment of vitiligo with good results. Mohanty et al. _[93] reported that application of non-cultured autologous outer root sheath cell suspensions resulted in repigmentation in 65.7% of vitiligo patches. Although a larger long-term study is essential for validation of the efficacy, these studies demonstrated a therapeutic potential of using MelSCs in pigmentary disorders. This approach is a more focused strategy than the use of epidermal suction blister transplants for treatment of vitiligo, which are used quite widely throughout the world. Considerable research is still required in order to refine methods for use of melanocyte stem cells in vitiligo treatment. These will include methods for isolation, culture, supplementation, protection, and reactivation to repopulate epidermal melanocytes within hypopigmented lesions.

Epidermal melanocytes originate from follicular MelSCs after skin injury by a mechanism dependent on the melanocortin 1 receptor (MC1R) pathway. Thus, it is plausible that modulating the MC1R pathway might contribute to improvement to pigmentary disorders such as vitiligo, melasma, or postinflammatory hyper/hypopigmentation _[94]. Furthermore, other clinical and cosmetic implications of MelSCs have been postulated in a review by Stanley _[95]. Considering stem cell anatomy and biology, permanent hair removal by electrolysis or laser treatment should be focused on eliminating the hair bulge, even though there is some difficulty in targeting selectively the bulge areas. Because MelSC are unpigmented, they present challenges as targets of selective photothermolysis.

4.2 GRAYING HAIR

To study the mechanism by which MelSCs differentiation into hair follicle melanocytes, observational experiments of the coat color phenotype in genetically manipulated or mutant mice have been used. MelSCs reside in the lower permanent portion (LPP) of the follicle, also called the bulge region due to the insertion of the arector pili muscle at that location. Nishimura et al. _[46] identified a mechanism for age-related hair graying in mice as MelSC depletion in the bulge and sub-bulge area. Loss of the same melanocyte population was also observed in an age-correlated fashion in human hair follicles (Figure 2). MelSC depletion was accompanied by ectopic pigmentation of bulge melanocytes -- a phenomenon which is predicted to be inconsistent with maintenance of the non-differentiated state required for maintenance of “stemness”. Indeed the presence of pigmented bulge melanocytes inversely correlated with melanocyte stem cell loss, and was also associated with apoptosis of these cells, suggesting that this ectopic pigmentation event represents a mode of melanocyte stem cell attrition during aging. Importantly, Nishimura and colleagues went on to demonstrate that MelSC depletion and subsequent graying hair is induced by genotoxic stresses (e.g., ionizing radiation, H2O2 treatment, DNA damaging drugs, or DNA repair deficiencies) in mouse models, and these stresses also induce premature ectopic pigmentation in the bulge–sub-bulge area where the MelSCs are located _[96] (Figure 2).

Figure 2.

MelSC in vitiligo. In the vitiligious lesion, the number of functional melanocytes and MelSC is decreased compared with the adjacent non-vitiligious lesion. MelSC or melanocyte precursor cells can remain in the hair bulge and provide the chance of repigmentation. During the process of repigmentation after the treatment of vitiligo, repigmentation frequently starts in the perifollicular area from the hair bulge MelSC. Sox10, Kit, MC1R, and wnt7a are related to MelSC activation, migration, and differentiation.

Although the mechanism of hair graying is generally accepted to be incomplete MelSC maintenance and MelSC depletion, the ability to utilize MelSC for expansion and/or transplantation as a treatment for graying hair or leukotrichia remains a significant technical challenge. The bulge area of hair follicles shows a decrease in MelSCs and ectopic differentiation in the bulge–sub-bulge area upon aging _[46]—findings which are seen in different mammalian species, suggesting that MelSC maintenance is incomplete with aging and results in stem cell depletion as well as hair graying. Two broad strategies which may conceptually improve the hair graying phenotype—but which carry technical challenges at this time—are to 1) identify a means of expanding and transplanting the melanocyte stem cells, or 2) identify a means of preventing the process by which melanocyte stem cells become ectopically pigmented and lose their stemness capabilities (Figure 2).

4.3 WOUND HEALING

Stem cells are activated for differentiation during tissue regeneration to provide functional mature cells _[97]. Apart from the conventional function of melanocytes in pigmentation, a role for melanocytes in epithelial regeneration has recently been proposed. Chou et al. _[94] performed an experiment that demonstrated migration and proliferation of MelSCs out of the niche after skin wounding or UVB irradiation (Figure 3). They further showed that this phenomenon is associated with the MC1R–ACTH-α-MSH signaling pathway and that MelSC migration preceded melanocyte proliferation. These findings imply that MelSCs play a role in wound healing after skin injury rather than simply maintaining quiescence, and suggest that follicular MelSCs may indeed be a source of epidermal melanocytes.

Figure 3.

MelSC in graying hair. Factors such as aging and genotoxic stress can induce hair graying through MelSC loss or MelSC differentiation into melanocytes. Repeated MelSC loss induces MelSC depletion and leads to hair graying. For hair pigmentation, not only MelSC maintenance but also melanogenesis of hair matrix melanocytes and melanin transfer to the hair shaft should occur. MelSC maintenance and activation are regulated by niche factors including Bcl-2, MITF, B-raf, C-raf, TGF-β, PAX3, Wnt, and Notch.

5. EXPERT OPINION: POTENTIAL CLINICAL APPLICATIONS OF MELANOCYTE STEM CELLS

Development of MelSCs as a biologic therapeutic method will require a better understanding of MelSC characteristics and identifying markers. So far, studies on MelSCs have been performed in murine models rather than in humans. Modeling of the interactions of stem cells and niche components observed using in vivo systems by isolation and culture of MelSCs has been attempted. Using keratinocyte XB2 cells as feeder cells, stem cell factor, and basic fibroblast growth factor (bFGF), Nishikawa-Torikai et al. _[98] tried to culture MelSCs in Dct(tm1(Cre)Bee)/CAG-CAT-GFP mice and demonstrated replication/proliferation and differentiation of the MelSCs although there was no evidence of dormant MelSCs. Furthermore, MelSCs could be isolated by a fluorescent activated cell sorter using the markers of c-Kit^low, side scatter^low, suggesting an undifferentiated state of MelSCs in mice. These cells could differentiate into melanocytes and application of this technology to human MelSCs will allow further investigation of human MelSC isolation.

For repigmentation in vitiligo, various medical therapies may be employed including corticosteroids, immunomodulators, phototherapy, 308-nm Excimer laser, or other adjuvant therapies, whereas for stable hypopigmentary lesions, surgical therapies using cells or grafts may be utilized _[99]. Although isolated or cultured MelSCs have not yet been used in the clinic, mature melanocytes have been transplanted into vitiligo lesions _[100,101]. To date, many studies have used cultured/non-cultured melanocytes with/without keratinocytes or epidermal grafts containing melanocytes as treatment for depigmented lesions. Compared with epidermal grafting, transplantation of a non-cultured epidermal suspension showed better results, suggesting that keratinocytes and niche factors are crucial for repigmentation _[102].

In addition, non-cultured epidermal suspension and outer root sheath (ORS) cells from extracted hairs showed statistically similar clinical repigmentation results in patients with stable vitiligo, indicating that epidermal melanocytes and follicular MelSCs may have similar effects _[103]. However these modalities are mixtures of various cells, thus we cannot define the effect of MelSCs or melanocytes only. Although transplantation of cultured melanocyte suspensions showed excellent results in stable localized vitiligo with 84–94% repigmentation _{[101,104,105]}, in theory MelSCs may have the greater therapeutic potential of a more long-term benefit. The superiority of transplantation of MelSCs with renewal factors or terminally differentiated melanocytes should be determined after MelSCs can be isolated.

In addition, there has been an attempt to culture ORS cells from the hair follicles to regenerate melanocytes because the bulge area of ORS contains MelSCs as well as other stem cells. As ORS cells include pluripotent neural crest stem cell (NCSC)-like stem cells and quiescent stem cells that have potential to be differentiated into various cell types, ORS cells are a valuable resource in regenerative medicine _[106]. Dieckmann et al. demonstrated methods to isolate melanocytes from ORS cells in human anagen hair and propagated them in vivo; moreover, the yields of the melanocytes could be improved by a sequential method _[107,108]. Although this method is not yet used in the clinical setting, the source of the melanocytes could be the MelSCs in ORS and this represents a promising method for biologic therapeutics in the near future. Moreover, with narrow band-UVB (NB-UVB) treatment, hair follicle NCSCs were directly affected and differentiated into a melanocyte lineage that produced pigmentation in vitro _[88]. Such studies suggest that these MelSC reservoirs are important for repigmentation of the skin. Thus, NB-UVB treatment together with biologic therapeutics using MelSCs is expected to have efficacy for pigment production in hypopigmentary disorders. The use of induced pluripotent stem (iPS) cells to generate melanocytes has also have been explored in several experiments, but has not yet been proved in vivo _[109].

MelSCs are also responsible for hair graying; however, effective therapeutic methods to prevent or reverse hair graying have not been reported. Considering the pathogenesis of the graying hair, prevention of MelSC depletion, enhancement of MelSC renewal or maintenance, and control of the MelSC niche should be therapeutic directions for hair graying. For hyperpigmentary skin disorders, few studies involving MelSCs have been reported so far. Although congenital melanocytic nevi were revealed to be derived from skin stem cells that have melanocytic differentiation in immunohistochemical studies, further studies of MelSC biology in recalcitrant pigmentary disorders such as melasma and melanocytic nevi should be performed _[110].

In summary, methods for identification, isolation, and culturing of murine MelSCs have been developed and provide the biomolecular basis for research on the characterization and function of MelSCs. These findings will facilitate updated research in humans and further clinical applications of MelSCs can be expected. With the identification of specific markers for human MelSCs and in vivo tracing, we may be enable to elucidate the biology of MelSCs and melanocyte lineages. Furthermore, such efforts will provide novel therapeutics using MelSCs for various pigmentary disorders and hair graying, as well as a valuable model pertinent to other stem cell research.

Figure 4.

MelSC in wound healing. Wounded skin lacks melanocytes on the basal layer of the epidermis. As wound healing progresses, activated MelSC migrate to the epidermis from the hair bulge area rather than proliferating. The MelSC then proliferate into melanocytes to be the source of epidermal melanocytes and maintain the cutaneous epithelium biology. This migration and proliferation of MelSC after skin wounding is dependent on the MC1R–ACTH-α-MSH signaling pathway.

Abbreviations

MelSC

melanocyte stem cell

LPP

lower permanent portion

MITF

microphthalmia associated transcription factor

SCF

stem cell factor

TGF-β

transforming growth factor-beta

NB-UVB

narrow band UVB

ORS

outer root sheath.