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World J Stem Cells. Jun 26, 2015; 7(5): 866-872
Published online Jun 26, 2015. doi: 10.4252/wjsc.v7.i5.866
Hair follicle stem cells: In vitro and in vivo neural differentiation
Nowruz Najafzadeh, Banafshe Esmaeilzade, Maryam Dastan Imcheh
Nowruz Najafzadeh, Maryam Dastan Imcheh, Research Laboratory for Embryology and Stem Cells, Department of Anatomical Sciences and Pathology, School of Medicine, Ardabil University of Medical Sciences, Ardabil 5618985991, Iran
Banafshe Esmaeilzade, Department of Anatomical Sciences, School of Medicine, Bushehr University of Medical Sciences, Bushehr 5756151819, Iran
Maryam Dastan Imcheh, Department of Biology, Faculty of Sciences, Urmia University, Urmia 5756151818, Iran
Author contributions: Najafzadeh N wrote the paper; Esmaeilzade B and Dastan Imcheh M reviewed and edited the manuscript.
Conflict-of-interest: The authors declare that there are no conflicts of interest.
Open-Access: This article is an open-access article which was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/
Correspondence to: Nowruz Najafzadeh, PhD, Research Laboratory for Embryology and Stem Cells, Department of Anatomical Sciences and Pathology, School of Medicine, Ardabil University of Medical Sciences, Ardabil 5618985991, Iran. n.najafzade@arums.ac.ir
Telephone: +98-453-3513776 Fax: +98-453-3513424
Received: November 28, 2014
Peer-review started: December 2, 2014
First decision: January 20, 2015
Revised: February 22, 2015
Accepted: April 1, 2015
Article in press: April 7, 2015
Published online: June 26, 2015

Abstract

Hair follicle stem cells (HFSCs) normally give rise to keratinocytes, sebocytes, and transient amplifying progenitor cells. Along with the capacity to proliferate rapidly, HFSCs provide the basis for establishing a putative source of stem cells for cell therapy. HFSCs are multipotent stem cells originating from the bulge area. The importance of these cells arises from two important characteristics, distinguishing them from all other adult stem cells. First, they are accessible and proliferate for long periods. Second, they are multipotent, possessing the ability to differentiate into mesodermal and ectodermal cell types. In addition to a developmental capacity in vitro, HFSCs display an ability to form differentiated cells in vivo. During the last two decades, numerous studies have led to the development of an appropriate culture condition for producing various cell lineages from HFSCs. Therefore, these stem cells are considered as a novel source for cell therapy of a broad spectrum of neurodegenerative disorders. This review presents the current status of human, rat, and mouse HFSCs from both the cellular and molecular biology and cell therapy perspectives. The first section of this review highlights the importance of HFSCs and in vitro differentiation, while the final section emphasizes the significance of cell differentiation in vivo.

Key Words: Hair follicle, Stem cells, Bulge area, Neuron, Differentiation

Core tip: Hair follicle stem cells (HFSCs) can proliferate in vitro and retain the label for a long time. Various types of stem cells, including epidermal-neural crest stem cells, nestin-positive, keratin 15-negative cells, and CD34-positive cells have been demonstrated in hair follicles. HFSCs normally give rise to keratinocytes, sebocytes, and transient amplifying cells in vivo. In addition, these cells differentiate into ectodermal lineages including oligodendrocytes, astrocytes, and neurons. Neural cells derived from HFSCs can replace lost cells in neurodegenerative diseases. Their easy accessibility along with their potential for neural differentiation makes HFSCs an ideal stem cell source for treatment of neurodegenerative disorders.


Citation: Najafzadeh N, Esmaeilzade B, Dastan Imcheh M. Hair follicle stem cells: In vitro and in vivo neural differentiation. World J Stem Cells 2015; 7(5): 866-872
INTRODUCTION

Skin stem cells reside in the stratum basale of the epidermis, sebaceous glands, and bulge area of the hair follicles. A promising start to derive hair follicle stem cells (HFSCs) was made in the early 1990s, when it was found that label-retaining cells were located in the upper portion of the hair follicle (bulge area). Subsequently, it was clarified that during the early anagen phase, the bulge cells grew downward in response to the stimulating factors of the dermal papilla cells and formed half of a hair follicle, which was degenerated during the catagen phase. Studies on hair follicles suggest that mouse HFSCs occupy a relatively fixed location in the hair follicle called the bulge area, which is important for hair follicle cycling[1]. HFSCs continuously supply new cells to the bulb during the anagen phase[2], and have the ability to differentiate into most of the ectodermal lineages[3]. These characteristics make them a promising cell source for grafting in various skin and nervous system diseases[4].

Kobayashi et al[5] has shown that the bulge area of rat whisker follicles contains colony-forming stem cells that can be expanded continuously on the feeder cell layer and form 95% of the total colonies. Moreover, the bulge cells form larger colonies than the non-bulge keratinocytes[6]. HFSCs have been seen to be able to differentiate either in vitro or in vivo upon grafting into a spinal cord injury (SCI)[7,8] and Alzheimer disease (AD)[9] animal models. The isolation of human HFSCs has generated tremendous interest, because the information gained from the study of human HFSCs, particularly those regarding cell differentiation, is of particular importance[10].

The neural potential of HFSCs was first reported by Sieber-Blum et al[11] and Amoh et al[12] who demonstrated that the population of HFSCs could be converted into Schwann cells after pretreatment with neuregulin-1 or upon transplantation into a severed sciatic nerve and SCI. Since then, many studies used inducers for neural differentiation of HFSCs, including neurotrophin-3 (NT-3)[13], a glial cell line-derived neurotrophic factor (GDNF), a brain-derived neurotrophic factor (BDNF)[14], a serum-free medium, all trans retinoic acid (RA), and other chemical neural inducers[15].

In this review, we have summarized the progress achieved in both the proliferation and neural lineage differentiation of HFSCs. Research on the application of stem cells for the treatment of neurodegenerative diseases such as peripheral nerve lesions, SCI, and AD has created a growing interest in the field of biology. Ultimately, a good knowledge of the wide variety of cell differentiation will provide the therapeutic application of HFSCs in degenerative nervous system diseases and skin pathologies.

MAINTAINING HFSCS

HFSCs were initially identified in the bulge area of rat hair follicles[5]. In 1990, Cotsarelis et al[1] suggested that follicular stem cells retain the label and are slow cycling. Similarly, Morris et al[16] demonstrated that bulge cells are quiescent and maintain a label for a long time following induction of anagen. The label-retaining cells divide asymmetrically into both identical copies of themselves and transient amplifying cells[1,17]. The initial HFSCs culture is established and maintained by a co-culture, with a feeder layer of 3T3-J2 fibroblasts[5]. Subsequent studies have revealed that the epidermal growth factor (EGF) and a basic fibroblast growth factor (b-FGF) are the feeder molecules that play a significant role in the maintenance and proliferation of these cells[18,19]. HFSCs can also be grown in a medium supplemented with EGF, cholera toxin, insulin, and hydrocortisone[13]. Sieber-Blum et al[11] suggests that epidermal neural crest stem cells (Epi-NCSCs) are pluripotent stem cells residing in the bulge area and can also be expanded continuously in the presence of an alpha-modified MEM medium, 5% chick embryo extract, and 10% of fetal calf serum. With these new culture conditions, it is now possible to grow HFSCs with defined factors in the absence of a feeder cell layer.

CELL SURFACE MARKERS

HFSCs can be routinely isolated from hair follicles and expanded in vitro to cell populations that are similar to the adult stem cells in respect to morphology and cell surface markers. In fact, numerous subsets of stem cells with varying differentiation potentials have been demonstrated in hair follicles, such as, Epi-NCSCs[9,11], nestin-positive, and keratin 15-negative cells[20], CD34-positive cells[15,21], and CD200 -positive cells[22].

Mouse bulges cells express specific markers including CD34 and K19, as well as CD200 and K15[23]. Expression of CD200 protects the bulge area from inflammation and hair loss in alopecia areata[24] and these cell surface protein is the best positive marker for the human bulge stem cells[22]. A central problem is the lack of a specific and reliable marker for the identification of HFSCs. Among the different markers, CD34 is possibly the most promising marker for HFSCs[25]. Previously, bulge cells were isolated from mouse, rat, and human cells by manual dissection, but the purity of the isolated cells was unclear. Several years ago, Ohyama et al[22] used laser capture microdissection for precise bulge cell isolation. Now different isolation methods for bulge stem cells are accessible using CD34 as a hair follicle stem cell surface marker, by fluorescence-activated cell sorting (FACS)[26] and magnetic activated cell sorting (MACS)[8,27]. Therefore, the precise isolation of HFSCs having the ability to differentiate into many different cell types has generated a promising field for therapeutic application.

Some bulge stem cells are K15-positive and nestin-negative and differentiate into keratinocytes[28]. More recently, Snippert et al[29], reported Lgr6 expression in HFSCs, which differs from Lgr5/CD34+ HFSCs. Lgr6+ HFSCs particularly express Sca-1, α6-integrin, β1-integrin, sox9, and Lhx2, and contribute to wound repair. Sieber-Blum et al[7], showed that the other type of HFSCs express neural crest genes as well as neural stem cell markers, indicating a neural crest origin and neural differentiation potential. The other types of HFSCs in mouse hair follicles are called nestin-positive stem cells. These cells express some stem cell markers such as CD34, and can differentiate into non-follicular cell types, such as, neural cells[30], which are maintained in the presence of FGF2 and a low concentration of B27[20]. Nestin-positive cells exhibit a colony forming potential in vitro and are small oval or round shaped, with a dendrite-like structure. At the anagen stage, nestin-expressing cells form the lower part of the hair follicle[30].

MULTIPOTENCY OR PLURIPOTENCY

This is an open question of whether bulge stem cells isolated from the hair follicle are pluripotent or multipotent cells and whether truly pluripotent cells can be isolated from the hair follicle by expansion, in vitro. HFSCs are considered to be multipotent cells because they differentiate into ectodermal derivatives such as neural cells, astrocytes, oligodendrocytes, and Schwann cells[14], as well as mesodermal lineages, including chondrocytes[11], myocytes[31], adipocytes, and osteocytes[17]. However, the expression of differentiation markers is often insufficient to conclude whether a cell has converted to a new state of differentiation[32]. Bulge cells can proliferate and differentiate into the bulb keratinocytes at the onset of the anagen phase[2,33]. In addition it is shown that these stem cells can also emigrate and continuously provide progenitors, regenerating the epidermis and sebaceous gland[34-36].

EP-NCSCs derived from the mouse vibrissae hair follicle express neural stem cell markers, such as Nanog and Oct4[37]. The neural crest transcription factors such as nestin, Slug, Snail, Twist, Sox9, Sox10, Wnt-1 and BMP4 play essential roles in maintaining the undifferentiated state of bulge cells[11,31]. The role of these transcription factors in neural crest stem cell renewal has been recently reviewed[38,39] and we are not going to discuss further. Together, HFSCs has an extensive developmental potential, however, it is necessary to evaluate the multipotency of HFSCs by differencing, in vivo, after cell grafting. For this purpose, HFSCs are transplanted into the skin[40], SCI[8], peripheral nerve injury and AD[9] models. The results suggest that HFSCs have extensive developmental potential. Further studies are required to determine whether the population of HFSCs can give rise to endodermal derivatives. Finally, the pluripotency of all subpopulations of HFSCs remains an unproven concept, but it seems that only Epi-NCSCs express two pluripotency factors, such as, Oct-4 and nanog[11].

IN VITRO NEURAL DIFFERENTIATION OF HFSCS

HFSCs attract attention not only in dermatology, but also in the treatment of nervous system diseases. It has been shown that HFSCs retain the ability to generate differentiated progeny in vitro. Many differentiation approaches have been optimized using growth factors and other inducing factors. These protocols can also successfully generate certain lineages. HFSCs can be induced to produce an enriched population of glial and neuronal lineages following incubation in exogenous growth factors[14]. On account of the diversity of HFSCs, differentiation and the proliferative potential of HFSCs is strongly dependent on the exogenous growth factors. Neuroregulin-1 has been found to be required for differentiation of HFSCs to Schwann cell and β-III tubulin positive neural cells. The Epi-NCSCs can also differentiate into smooth muscle cells in response to bone morphogenetic protein-2 (BMP-2)[11].

Of late, Ghoroghi et al[13], isolated rat HFSCs and cultured them on a poly-l-lactic acid (PLLA) scaffold to provide a suitable microenvironment for neural differentiation, and then used NT-3 to induce neural differentiation in the HFSCs.

New protocols have been developed to generate neural cells from HFSCs. Of late, we have isolated CD34+ cells from the mouse hair follicle, using MACS, and then we applied RA, serum-free medium, and chemical treatments such as β-mercaptoethanol (BME), butylated hydroxyanisole (BHA), and dimethyl sulfoxide (DMSO), for the neural differentiation in these cells. We have found that the serum-free condition and 1 μmol/L RA appropriately triggered neurogenesis in HFSCs[15]. In addition, it seems that prolonged serum deprivation induces formation of the ectodermal derivatives[41]. However, chemical treatments with DMSO, BME, BHA, and potassium chloride (KCL) induced rapid changes in cell morphology, which led to the CD34+ cells dying within seven days of treatment[15]. It will be important to determine whether the neural cells derived from HFSCs possess the functional characteristics of neurons.

Therapeutic application of HFSCs

HFSCs have advantages and disadvantages regarding the potential use of cell-based regenerative therapies. The main potential advantage of the HFSCs is that they are easily accessible adult stem cells, which have ectodermal differentiation potential[15]. Moreover, HFSCs are attractive tools for future burn and neurodegenerative disease treatments.

HFSCs are similar to embryonic stem cells and have a high proliferative potential in vitro. Similar to other types of adult stem cells, they do not form tumors. Without a doubt, autologous transplantation and avoiding immune rejection has been demonstrated in HFSC-treated animal studies[7,12,42].

HFSCs have the properties to differentiate into various neural cell types including neurons, astrocytes, oligodendrocytes, and Schwann cells. Among them some specific cell types can be produced in high purity, such as, Schwann cells, motor neurons, and oligodendrocytes, which are the main cells that degenerate in SCI, peripheral nerve lesions, and AD. One of the goals of the researchers in this field is to generate neural cells from stem cells, with the aim of replacing lost tissue in degenerative disorders. Therefore, we have used HFSCs as an alternative cell source for cell therapy in nervous system diseases. AD is one of the most common neurodegenerative disorders affecting about four million people throughout the world each year. AD brains have been characterized by amyloid β peptide plaque and neurofibrillary tangle formation, which leads to axonal transport defects and synaptic loss responsible for cholinergic neuron degeneration[43].

HFSCs have the potential to constitute an anatomical nervous system structure. Thus, in the case of an AD model, these cells are capable of replacing the degenerated neurons. In a recent study, researchers transplanted a subtype of HFSCs (Epi-NCSCs) to differentiate into cholinergic neurons in the AD rat model. These stem cell-derived cholinergic neurons improved learning and memory deficits in the AD model. In spite of rare studies on the therapeutic effects of Epi-NCSCs in an AD model[9], it will be possible to modify HFSCs to deliver neurotropic factors that modify the course of the disease, in the future.

It is well known that SCI induces paralysis in about 2.5 million people, affecting the patients and society. Unfortunately, no effective treatment has been discovered for SCI to date[44]. However, some studies are under way to promote regeneration and neural repair in patients with SCI[45]. Cell therapy plays a major role in axonal regeneration and neuronal replacement in AD[9], SCI, and other nervous system diseases. Potential strategies to repair neurodegenerative diseases include neuronal and glial replacement, axonal regeneration and remyelination, and increased production of neurotrophins by grafted cells[46,47]. Various cell types have been assessed to repair central and peripheral nervous system disorders, such as, fetal tissue, olfactory ensheathing glia, Schwann cells, skin derived precursors, mesenchymal stem cells, and HFSCs[48]. To this end, in this part of the review, we have focused on the transplantation of HFSCs in SCI.

Preclinical studies have shown a novel type of HFSCs known as Epi-NCSCs, which survive and differentiate into βIII-tubulin, glutamic acid decarboxylase (GAD67), RIP positive, and myelin basic protein-positive neural cells, but not Schwann cells, when transplanted into the contusion model of the SCI; the cells can be even integrated with host neurites in the contused spinal cord. However, the transplanted cells do not form tumors[7]. In another study, extensive Schwann cell differentiation of nestin-positive HFSCs has been reported after transplantation into a severed spinal cord. They have found that the differentiated cells facilitated the repair of SCI and promoted hind limb function recovery[49]. Likewise, when rat HFSCs are transplanted into a compression model of SCI, they generate oligodendrocytes and neuron-like cells expressing RIP and β-III tubulin, respectively. Walking scale, limb coordination, and plantar stepping improved following HFSC transplantation[8]. Consistent with this study, Hu et al[50], also demonstrated that unilateral bulge-derived Epi-NCSC transplantation into a contused spinal cord promoted a 24% recovery in sensory connectivity and touch perception. Moreover, some grafted Epi-NCSCs differentiated into functional motor neurons. In vivo studies using Epi-NCSCs isolated from the mouse hair follicle showed similar results[7]. Briefly, these studies showed the safety of HFSC transplantation and enhanced recovery in animal models of SCI.

Damage to the peripheral nerve interrupts the axonal pathways and causes partial or total loss of motor, sensory, and autonomic functions. An important aim for the ongoing research is the development of therapeutic strategies that enhance axonal regeneration and replace lost neural cells, to bridge the gap of the lesion[51].

HFSCs possess a self-renewal ability and neural differentiation potential. Amoh et al[12,52] transplanted HFSCs in the severed sciatic nerves of mice. Eight weeks after grafting, many spindle cells grew in the severed sciatic nerve rather than in the control, differentiated extensively into Schwann cells, and promoted motor function.

In another study, extensive neuronal differentiation of the nestin-expressing cells was reported after transplantation of RA-pretreated cells into the transected distal sciatic nerve. In this study, the nestin-positive bulge cells grew in the distal sciatic nerve stump and caused locomotor recovery owing to the presence of many Tuj1, Is11/2, and EN1-positive cells and nerve fibers. Muscle atrophy was also reduced after grafting, and re-innervation was promoted[53]. HFSCs transplanted to the subcutis and severed sciatic nerve also differentiated into blood vessels[40] and Schwann cells[12], respectively. It seemed that the differentiation of HFSCs supported neural regeneration and motor function of the lower extremity muscles, such as, the gastrocnemius[12].

There are still many problems that need to be resolved before HFSCs can be widely used clinically. The main problem is that approximately only a few neural cells derive from these stem cells in vivo and further studies are required to determine whether the differentiated neurons are functionally integrated into the AD brain, SCI, and peripheral nerve lesion tissues. Some other forms of cell differentiation, such as keratinocytes, myocytes, and melanocytes are the other disadvantages of HFSC transplantation in neurodegenerative disorders. The successful directed differentiation of HFSCs into specific neurons with the help of different neural induction methods may make neural transplantation widely available for neurodegenerative disorders at some point in the future.

Collectively, these results indicate that HFSCs can promote the recovery of peripheral nerve injury, SCI, and AD. Thus, these cells have the desirable properties of neural replacement and remyelination.

CONCLUSION

In summary, we have explained that the hair follicle contains various multipotent stem cells. The ability of HFSCs to proliferate in vitro and the directed differentiation toward ectodermal and mesodermal lineage cell types render them potential for clinical application. These include the modeling of SCI, sciatic nerve injury, and AD, studies of cell therapy on progression, and amelioration of symptoms.

At present, HFSC transplantation provides the best chance of cure for many diseases. Future studies seek to characterize the signaling genes that regulate stem cell behavior.

ACKNOWLEDGMENTS

We gratefully acknowledge to M Sagha, M Arzanlou and M Amani for their precious contribution in terms of language editing.

Footnotes

P- Reviewer: Li ZJ, Liu L S- Editor: Ma YJ L- Editor: A E- Editor: Wang CH

References
1.  Cotsarelis G, Sun TT, Lavker RM. Label-retaining cells reside in the bulge area of pilosebaceous unit: implications for follicular stem cells, hair cycle, and skin carcinogenesis. Cell. 1990;61:1329-1337.  [PubMed]  [DOI]
2.  Wilson C, Cotsarelis G, Wei ZG, Fryer E, Margolis-Fryer J, Ostead M, Tokarek R, Sun TT, Lavker RM. Cells within the bulge region of mouse hair follicle transiently proliferate during early anagen: heterogeneity and functional differences of various hair cycles. Differentiation. 1994;55:127-136.  [PubMed]  [DOI]
3.  Blanpain C, Lowry WE, Geoghegan A, Polak L, Fuchs E. Self-renewal, multipotency, and the existence of two cell populations within an epithelial stem cell niche. Cell. 2004;118:635-648.  [PubMed]  [DOI]
4.  Kanno H. Regenerative therapy for neuronal diseases with transplantation of somatic stem cells. World J Stem Cells. 2013;5:163-171.  [PubMed]  [DOI]
5.  Kobayashi K, Rochat A, Barrandon Y. Segregation of keratinocyte colony-forming cells in the bulge of the rat vibrissa. Proc Natl Acad Sci USA. 1993;90:7391-7395.  [PubMed]  [DOI]
6.  Morris RJ, Liu Y, Marles L, Yang Z, Trempus C, Li S, Lin JS, Sawicki JA, Cotsarelis G. Capturing and profiling adult hair follicle stem cells. Nat Biotechnol. 2004;22:411-417.  [PubMed]  [DOI]
7.  Sieber-Blum M, Schnell L, Grim M, Hu YF, Schneider R, Schwab ME. Characterization of epidermal neural crest stem cell (EPI-NCSC) grafts in the lesioned spinal cord. Mol Cell Neurosci. 2006;32:67-81.  [PubMed]  [DOI]
8.  Najafzadeh N, Nobakht M, Pourheydar B, Golmohammadi MG. Rat hair follicle stem cells differentiate and promote recovery following spinal cord injury. Neural Regen Res. 2013;8:3365-3372.  [PubMed]  [DOI]
9.  Esmaeilzade B, Nobakht M, Joghataei MT, Rahbar Roshandel N, Rasouli H, Samadi Kuchaksaraei A, Hosseini SM, Najafzade N, Asalgoo S, Hejazian LB. Delivery of epidermal neural crest stem cells (EPI-NCSC) to hippocamp in Alzheimer’s disease rat model. Iran Biomed J. 2012;16:1-9.  [PubMed]  [DOI]
10.  Rochat A, Kobayashi K, Barrandon Y. Location of stem cells of human hair follicles by clonal analysis. Cell. 1994;76:1063-1073.  [PubMed]  [DOI]
11.  Sieber-Blum M, Grim M, Hu YF, Szeder V. Pluripotent neural crest stem cells in the adult hair follicle. Dev Dyn. 2004;231:258-269.  [PubMed]  [DOI]
12.  Amoh Y, Li L, Campillo R, Kawahara K, Katsuoka K, Penman S, Hoffman RM. Implanted hair follicle stem cells form Schwann cells that support repair of severed peripheral nerves. Proc Natl Acad Sci USA. 2005;102:17734-17738.  [PubMed]  [DOI]
13.  Ghoroghi FM, Hejazian LB, Esmaielzade B, Dodel M, Roudbari M, Nobakht M. Evaluation of the Effect of NT-3 and Biodegradable Poly-L-lactic Acid Nanofiber Scaffolds on Differentiation of Rat Hair Follicle Stem Cells into Neural Cells In Vitro. J Mol Neurosci. 2013;Epub ahead of print.  [PubMed]  [DOI]
14.  El Seady R, Huisman MA, Löwik CW, Frijns JH. Uncomplicated differentiation of stem cells into bipolar neurons and myelinating glia. Biochem Biophys Res Commun. 2008;376:358-362.  [PubMed]  [DOI]
15.  Najafzadeh N, Sagha M, Heydari Tajaddod S, Golmohammadi MG, Massahi Oskoui N, Deldadeh Moghaddam M. In vitro neural differentiation of CD34 (+) stem cell populations in hair follicles by three different neural induction protocols. In Vitro Cell Dev Biol Anim. 2015;51:192-203.  [PubMed]  [DOI]
16.  Morris RJ, Potten CS. Highly persistent label-retaining cells in the hair follicles of mice and their fate following induction of anagen. J Invest Dermatol. 1999;112:470-475.  [PubMed]  [DOI]
17.  Yu H, Kumar SM, Kossenkov AV, Showe L, Xu X. Stem cells with neural crest characteristics derived from the bulge region of cultured human hair follicles. J Invest Dermatol. 2010;130:1227-1236.  [PubMed]  [DOI]
18.  Yang JS, Lavker RM, Sun TT. Upper human hair follicle contains a subpopulation of keratinocytes with superior in vitro proliferative potential. J Invest Dermatol. 1993;101:652-659.  [PubMed]  [DOI]
19.  Mignone JL, Roig-Lopez JL, Fedtsova N, Schones DE, Manganas LN, Maletic-Savatic M, Keyes WM, Mills AA, Gleiberman A, Zhang MQ. Neural potential of a stem cell population in the hair follicle. Cell Cycle. 2007;6:2161-2170.  [PubMed]  [DOI]
20.  Amoh Y, Li L, Katsuoka K, Penman S, Hoffman RM. Multipotent nestin-positive, keratin-negative hair-follicle bulge stem cells can form neurons. Proc Natl Acad Sci USA. 2005;102:5530-5534.  [PubMed]  [DOI]
21.  Reali C, Scintu F, Pillai R, Cabras S, Argiolu F, Ristaldi MS, Sanna MA, Badiali M, Sogos V. Differentiation of human adult CD34+ stem cells into cells with a neural phenotype: role of astrocytes. Exp Neurol. 2006;197:399-406.  [PubMed]  [DOI]
22.  Ohyama M, Terunuma A, Tock CL, Radonovich MF, Pise-Masison CA, Hopping SB, Brady JN, Udey MC, Vogel JC. Characterization and isolation of stem cell-enriched human hair follicle bulge cells. J Clin Invest. 2006;116:249-260.  [PubMed]  [DOI]
23.  Cotsarelis G. Gene expression profiling gets to the root of human hair follicle stem cells. J Clin Invest. 2006;116:19-22.  [PubMed]  [DOI]
24.  Rosenblum MD, Olasz EB, Yancey KB, Woodliff JE, Lazarova Z, Gerber KA, Truitt RL. Expression of CD200 on epithelial cells of the murine hair follicle: a role in tissue-specific immune tolerance? J Invest Dermatol. 2004;123:880-887.  [PubMed]  [DOI]
25.  Cotsarelis G. Epithelial stem cells: a folliculocentric view. J Invest Dermatol. 2006;126:1459-1468.  [PubMed]  [DOI]
26.  Trempus CS, Morris RJ, Ehinger M, Elmore A, Bortner CD, Ito M, Cotsarelis G, Nijhof JG, Peckham J, Flagler N. CD34 expression by hair follicle stem cells is required for skin tumor development in mice. Cancer Res. 2007;67:4173-4181.  [PubMed]  [DOI]
27.  Huang E, Lian X, Chen W, Yang T, Yang L. Characterization of rat hair follicle stem cells selected by vario magnetic activated cell sorting system. Acta Histochem Cytochem. 2009;42:129-136.  [PubMed]  [DOI]
28.  Amoh Y, Kanoh M, Niiyama S, Kawahara K, Sato Y, Katsuoka K, Hoffman RM. Human and mouse hair follicles contain both multipotent and monopotent stem cells. Cell Cycle. 2009;8:176-177.  [PubMed]  [DOI]
29.  Snippert HJ, Haegebarth A, Kasper M, Jaks V, van Es JH, Barker N, van de Wetering M, van den Born M, Begthel H, Vries RG. Lgr6 marks stem cells in the hair follicle that generate all cell lineages of the skin. Science. 2010;327:1385-1389.  [PubMed]  [DOI]
30.  Li L, Mignone J, Yang M, Matic M, Penman S, Enikolopov G, Hoffman RM. Nestin expression in hair follicle sheath progenitor cells. Proc Natl Acad Sci USA. 2003;100:9958-9961.  [PubMed]  [DOI]
31.  Yu H, Fang D, Kumar SM, Li L, Nguyen TK, Acs G, Herlyn M, Xu X. Isolation of a novel population of multipotent adult stem cells from human hair follicles. Am J Pathol. 2006;168:1879-1888.  [PubMed]  [DOI]
32.  Jaenisch R, Young R. Stem cells, the molecular circuitry of pluripotency and nuclear reprogramming. Cell. 2008;132:567-582.  [PubMed]  [DOI]
33.  Ito M, Kizawa K, Hamada K, Cotsarelis G. Hair follicle stem cells in the lower bulge form the secondary germ, a biochemically distinct but functionally equivalent progenitor cell population, at the termination of catagen. Differentiation. 2004;72:548-557.  [PubMed]  [DOI]
34.  Taylor G, Lehrer MS, Jensen PJ, Sun TT, Lavker RM. Involvement of follicular stem cells in forming not only the follicle but also the epidermis. Cell. 2000;102:451-461.  [PubMed]  [DOI]
35.  Oshima H, Rochat A, Kedzia C, Kobayashi K, Barrandon Y. Morphogenesis and renewal of hair follicles from adult multipotent stem cells. Cell. 2001;104:233-245.  [PubMed]  [DOI]
36.  Ito M, Liu Y, Yang Z, Nguyen J, Liang F, Morris RJ, Cotsarelis G. Stem cells in the hair follicle bulge contribute to wound repair but not to homeostasis of the epidermis. Nat Med. 2005;11:1351-1354.  [PubMed]  [DOI]
37.  Sieber-Blum M, Hu Y. Epidermal neural crest stem cells (EPI-NCSC) and pluripotency. Stem Cell Rev. 2008;4:256-260.  [PubMed]  [DOI]
38.  Delfino-Machín M, Chipperfield TR, Rodrigues FS, Kelsh RN. The proliferating field of neural crest stem cells. Dev Dyn. 2007;236:3242-3254.  [PubMed]  [DOI]
39.  Dupin E, Sommer L. Neural crest progenitors and stem cells: from early development to adulthood. Dev Biol. 2012;366:83-95.  [PubMed]  [DOI]
40.  Amoh Y, Li L, Yang M, Moossa AR, Katsuoka K, Penman S, Hoffman RM. Nascent blood vessels in the skin arise from nestin-expressing hair-follicle cells. Proc Natl Acad Sci USA. 2004;101:13291-13295.  [PubMed]  [DOI]
41.  Tropepe V, Hitoshi S, Sirard C, Mak TW, Rossant J, van der Kooy D. Direct neural fate specification from embryonic stem cells: a primitive mammalian neural stem cell stage acquired through a default mechanism. Neuron. 2001;30:65-78.  [PubMed]  [DOI]
42.  Paus R, Nickoloff BJ, Ito T. A ‘hairy’ privilege. Trends Immunol. 2005;26:32-40.  [PubMed]  [DOI]
43.  Blennow K, de Leon MJ, Zetterberg H. Alzheimer’s disease. Lancet. 2006;368:387-403.  [PubMed]  [DOI]
44.  Thuret S, Moon LD, Gage FH. Therapeutic interventions after spinal cord injury. Nat Rev Neurosci. 2006;7:628-643.  [PubMed]  [DOI]
45.  Boulenguez P, Vinay L. Strategies to restore motor functions after spinal cord injury. Curr Opin Neurobiol. 2009;19:587-600.  [PubMed]  [DOI]
46.  Mothe AJ, Tator CH. Advances in stem cell therapy for spinal cord injury. J Clin Invest. 2012;122:3824-3834.  [PubMed]  [DOI]
47.  Lindvall O, Kokaia Z. Stem cells for the treatment of neurological disorders. Nature. 2006;441:1094-1096.  [PubMed]  [DOI]
48.  Fehlings MG, Vawda R. Cellular treatments for spinal cord injury: the time is right for clinical trials. Neurotherapeutics. 2011;8:704-720.  [PubMed]  [DOI]
49.  Amoh Y, Li L, Katsuoka K, Hoffman RM. Multipotent hair follicle stem cells promote repair of spinal cord injury and recovery of walking function. Cell Cycle. 2008;7:1865-1869.  [PubMed]  [DOI]
50.  Hu YF, Zhang ZJ, Sieber-Blum M. An epidermal neural crest stem cell (EPI-NCSC) molecular signature. Stem Cells. 2006;24:2692-2702.  [PubMed]  [DOI]
51.  Navarro X, Vivó M, Valero-Cabré A. Neural plasticity after peripheral nerve injury and regeneration. Prog Neurobiol. 2007;82:163-201.  [PubMed]  [DOI]
52.  Amoh Y, Kanoh M, Niiyama S, Hamada Y, Kawahara K, Sato Y, Hoffman RM, Katsuoka K. Human hair follicle pluripotent stem (hfPS) cells promote regeneration of peripheral-nerve injury: an advantageous alternative to ES and iPS cells. J Cell Biochem. 2009;107:1016-1020.  [PubMed]  [DOI]
53.  Liu F, Zhang C, Hoffman RM. Nestin-expressing stem cells from the hair follicle can differentiate into motor neurons and reduce muscle atrophy after transplantation to injured nerves. Tissue Eng Part A. 2014;20:656-662.  [PubMed]  [DOI]