Epidermal stem cells arise from the hair follicle after wounding

Accelerated closure of skin wounds in mice deficient in the homeobox gene Msx2

Differences in cellular competence offer an explanation for the differences in the healing capacity of tissues of various ages and conditions. The homeobox family of genes plays key roles in governing cellular competence. Of these, we hypothesize that Msx2 is a strong candidate regulator of competence in skin wound healing because it is expressed in the skin during fetal development in the stage of scarless healing, affects postnatal digit regeneration, and is reexpressed transiently during postnatal skin wound repair. To address whether Msx2 affects cellular competence in injury repair, 3 mm full-thickness excisional wounds were created on the back of C.Cg-Msx2(tm1Rilm)/Mmcd (Msx2 null) mice and the healing pattern was compared with that of the wild type mice. The results show that Msx2 null mice exhibited faster wound closure with accelerated reepithelialization plus earlier appearance of keratin markers for differentiation and an increased level of smooth muscle actin and tenascin in the granulation tissue. In vitro, keratinocytes of Msx2 null mice exhibit increased cell migration and the fibroblasts show stronger collagen gel contraction. Thus, our results suggest that Msx2 regulates the cellular competence of keratinocytes and fibroblasts in skin injury repair.

Hedgehog signaling maintains hair follicle stem cell phenotype in young and aged human skin

Skin hair follicles (HF) contain bulge stem cells (SC) that regenerate HFs during hair cycles, and repair skin epithelia following injury. As natural aging is associated with decreased skin repair capacity in humans, we have investigated the impact of age on human scalp HF bulge cell number and function. Here, we isolated human bulge cells, characterized as CD200+/KRT15+/KRT19+ cells of the HF, by dissection-combined CD200 selection in young and aged human skin. Targeted transcriptional profiling indicates that KRT15, KRT19, Dkk3, Dkk4, Tcf3, S100A4, Gas1, EGFR and CTGF/CCN2 are also preferentially expressed by human bulge cells, compared to differentiated HF keratinocytes (KC). Our results demonstrate that aging does not alter expression or localization of these HF SC markers. In addition, we could not detect significant differences in HF density or bulge cell number between young and aged human scalp skin. Interestingly, hedgehog (Hh) signaling is activated in human bulge cells in vivo, and down-regulated in differentiated HF KCs, both in young and aged skin. In addition, activation of Hh signaling by lentivirus-mediated overexpression of transcription factor Gli1 induces transcription of HF SC markers KRT15, KRT19, and Gas1, in cultured KCs. Together with previously reported knock-out mouse results, these data suggest a role for Hh signaling in maintaining bulge cell phenotype in young and aged human skin.

De novo follicular regeneration of the skin by wingless int 3 and bone morphogenetic protein 2 genes introduced into dermal fibroblasts and fibroblast growth factor-2 protein

In this study, we regenerated skin and its appendages by transplanting cultured normal dermal fibroblasts, into which morphogen genes had been introduced. We cultured normal dermal fibroblasts obtained from Fisher 344 rats on the surface of hydroxyapatite beads, and then adsorbed them onto the surface of a collagen sponge, which was transplanted into a full-thickness skin defect prepared on the backs of rats. Before transplantation, genes were introduced into the dermal fibroblasts via adenovirus vector (ad)-bone morphogenetic protein 2 and ad-wingless int 3 genes in addition to fibroblast growth factor-2 protein. By Week 4, the appearance of follicle germs or primitive hair germs was observed only in the ad-bone morphogenetic protein 2+ad-wingless int 3 combined with the fibroblast growth factor-2 protein group. By Week 16, in that same group, hair follicles having mature pilosebaceous systems with equally spaced localization had formed in the ulcer wound.

Mice lacking beta6 integrin in skin show accelerated wound repair in dexamethasone impaired wound healing model

Integrin alphavbeta6 is an epithelial-specific receptor that is absent from the healthy epidermis but synthesized de novo during wound repair. However, its function in wound repair is unknown. Integrin-mediated transforming growth factor-beta1 (TGF-beta1) activation is the main activation mechanism of this key cytokine in vivo. Impaired wound healing caused by glucocorticoids is a major clinical problem and is associated with a disturbed balance of TGF-beta1 activity. Therefore, alphavbeta6 integrin-mediated regulation of TGF-beta1 activity may be involved in this process. To determine the function of alphavbeta6 integrin in glucocorticoid-induced impaired wound healing, both beta6 integrin-deficient (beta6-/-) and wild-type mice were exposed to dexamethasone treatment. Multiple wound parameters, keratinocyte proliferation, inflammation, and TGF-beta1 activation were assessed. Wound healing was significantly accelerated in the dexamethasone-treated beta6-/- mice compared with the corresponding wild-type mice. The dexamethasone-treated beta6-/- mice showed enhanced keratinocyte proliferation in both wound epithelium and hair follicles while the production of proinflammatory cytokines and TGF-beta1 activation were reduced. Accelerated wound repair in the dexamethasone-treated beta6-/- mice might be associated with the reduced antiproliferative and proinflammatory effects of TGF-beta1. Inhibition of alphavbeta6 integrin may provide a future target for treatment of impaired wound healing.

Finding one's niche in the skin

Skin epithelia must rejuvenate constantly during normal homeostasis and repair damage after wounding. Fulfilling these roles necessitates reservoirs of stem cells that persist for life. This review focuses on the elusive stem cell niche of the epidermis, long thought to reside within the basal layer that is sandwiched between the basement membrane and the suprabasal interface.

Recent advances on skin-resident stem/progenitor cell functions in skin regeneration, aging and cancers and novel anti-aging and cancer therapies

Recent advances in skin-resident adult stem/progenitor cell research have revealed that these immature and regenerative cells with a high longevity provide critical functions in maintaining skin homeostasis and repair after severe injuries along the lifespan of individuals. The establishment of the functional properties of distinct adult stem/progenitor cells found in skin epidermis and hair follicles and extrinsic signals from their niches, which are deregulated during their aging and malignant transformation, has significantly improved our understanding on the etiopathogenesis of diverse human skin disorders and cancers. Particularly, enhanced ultraviolet radiation exposure, inflammation and oxidative stress and telomere attrition during chronological aging may induce severe DNA damages and genomic instability in the skin-resident stem/progenitor cells and their progenies. These molecular events may result in the alterations in key signalling components controlling their self-renewal and/or regenerative capacities as well as the activation of tumour suppressor gene products that trigger their growth arrest and senescence or apoptotic death. The progressive decline in the regenerative functions and/or number of skin-resident adult stem/progenitor cells may cause diverse skin diseases with advancing age. Moreover, the photoaging, telomerase re-activation and occurrence of different oncogenic events in skin-resident adult stem/progenitor cells may also culminate in their malignant transformation into cancer stem/progenitor cells and skin cancer initiation and progression. Therefore, the anti-inflammatory and anti-oxidant treatments and stem cell-replacement and gene therapies as well as the molecular targeting of their malignant counterpart, skin cancer-initiating cells offer great promise to treat diverse skin disorders and cancers.

Exploring the role of stem cells in cutaneous wound healing

The skin offers a perfect model system for studying the wound healing cascade, which involves a finely tuned interplay between several cell types, pathways and processes. The dysregulation of these factors may lead to wound healing disorders resulting in chronic wounds, as well as abnormal scars such as hypertrophic and keloid scars. As the contribution of stem cells towards tissue regeneration and wound healing is increasingly appreciated, a rising number of stem cell therapies for cutaneous wounds are currently under development, encouraged by emerging preliminary findings in both animal models and human studies. However, we still lack an in-depth understanding of the underlying mechanisms through which stem cells contribute to cutaneous wound healing. The aim of this review is, therefore, to present a critical synthesis of our current understanding of the role of stem cells in normal cutaneous wound healing. In addition to summarizing wound healing principles and related key molecular and cellular players, we discuss the potential participation of different cutaneous stem cell populations in wound healing, and list corresponding stem cells markers. In summary, this review delineates current strategies, future applications, and limitations of stem cell-based or stem cell-targeted therapy in the management of acute and chronic skin wounds.

Lrig1 expression defines a distinct multipotent stem cell population in mammalian epidermis

Lrig1 is a marker of human interfollicular epidermal stem cells and helps maintain stem cell quiescence. We show that, in mouse epidermis, Lrig1 defines the hair follicle junctional zone adjacent to the sebaceous glands and infundibulum. Lrig1 is a Myc target gene; loss of Lrig1 increases the proliferative capacity of stem cells in culture and results in epidermal hyperproliferation in vivo. Lrig1-expressing cells can give rise to all of the adult epidermal lineages in skin reconstitution assays. However, during homeostasis and on retinoic acid stimulation, they are bipotent, contributing to the sebaceous gland and interfollicular epidermis. beta-catenin activation increases the size of the junctional zone compartment, and loss of Lrig1 causes a selective increase in beta-catenin-induced ectopic hair follicle formation in the interfollicular epidermis. Our results suggest that Lrig1-positive cells constitute a previously unidentified reservoir of adult mouse interfollicular epidermal stem cells.

The tortoise and the hair: slow-cycling cells in the stem cell race

Given the importance of stem cells to adult tissues, it has long been postulated that stem cells divide infrequently to preserve their long-term proliferation potential and to prevent the acquisition of errors during DNA replication. Yet, some stem cells must be able to continually churn out progeny in tissues that rapidly turn over or are subject to sudden injuries or growth spurts. This Review explores the challenges that mammalian stem cells face in balancing the competing demands of proliferation and differentiation in tissues.

Hedgehog signaling is essential for normal wound healing

The hedgehog family of morphogens (sonic [Shh], Indian, and desert hedgehog) are central regulators of embryologic growth and tissue patterning. Although recent work implicates Shh in postnatal tissue repair and development, conclusive evidence is lacking. Here, we demonstrated the importance of Shh in wound repair, by examining the effects of cyclopamine, a specific inhibitor of the Shh signaling cascade, on tissue repair. Using a murine-splinted excisional wound model, which attenuates wound contraction in this loose-skinned rodent, we established that, by all measures (wound closure, epithelialization, granulation formation, vascularity, and proliferation), wound healing was profoundly impaired when Shh signaling was disrupted. Because embryonic disruption of Shh is associated with distinct phenotypic defects, our findings invite investigation of the potential role of Shh signaling under postnatal conditions associated with disregulated wound healing.

The hair follicle as a dynamic miniorgan

Hair is a primary characteristic of mammals, and exerts a wide range of functions including thermoregulation, physical protection, sensory activity, and social interactions. The hair shaft consists of terminally differentiated keratinocytes that are produced by the hair follicle. Hair follicle development takes place during fetal skin development and relies on tightly regulated ectodermal-mesodermal interactions. After birth, mature and actively growing hair follicles eventually become anchored in the subcutis, and periodically regenerate by spontaneously undergoing repetitive cycles of growth (anagen), apoptosis-driven regression (catagen), and relative quiescence (telogen). Our molecular understanding of hair follicle biology relies heavily on mouse mutants with abnormalities in hair structure, growth, and/or pigmentation. These mice have allowed novel insights into important general molecular and cellular processes beyond skin and hair biology, ranging from organ induction, morphogenesis and regeneration, to pigment and stem cell biology, cell proliferation, migration and apoptosis. In this review, we present basic concepts of hair follicle biology and summarize important recent advances in the field.

Chemoattractant activity of degradation products of fetal and adult skin extracellular matrix for keratinocyte progenitor cells

Biological scaffolds composed of naturally occurring extracellular matrix (ECM) have been utilized as templates for the constructive remodelling of numerous tissues in preclinical studies and human clinical applications. The mechanisms by which ECM induces constructive remodelling are not well understood, but it appears that the degradation products of ECM scaffolds may play key roles in cell recruitment. The objective of the present study was to investigate the effects of age and species of the tissue from which ECM is harvested on the chemoattractant activity of degradation products of ECM for human keratinocyte stem and progenitor cells. Adult human skin ECM, fetal human skin ECM and adult porcine skin ECM were prepared, enzymatically digested, characterized by SDS-PAGE and evaluated for in vitro chemoattractant activity for human keratinocyte progenitor and stem cells (HEKn). Degradation products of human fetal skin ECM showed greater chemoattractant activity than human adult skin ECM degradation products for the HEKn. Degradation products of porcine adult skin ECM showed greater chemoattractant activity than human adult skin ECM. The human fetal skin ECM degradation products showed the strongest chemoattractant activity for the HEKn. The findings of this study support the concept that the mechanism of ECM scaffold remodelling involves the recruitment of lineage-directed progenitor cells by scaffold degradation products, and that both the age and species of the tissue from which the ECM is harvested have an effect upon this chemoattractant potential.

Localization of the low-affinity nerve growth factor receptor p75 in human limbal epithelial cells

Biological effects of nerve growth factor (NGF) are mediated through receptors known as nerve growth factor receptors (NGFR), which include p75 and TrkA. This study was initiated after identifying NGFR as an up-regulated gene in the limbus by cDNA microarray analysis and we postulate that its expression may be indicative of a stem/progenitor cell phenotype. Immunohistochemistry was performed on normal human adult (n= 5) and foetal (n= 3) corneal tissue using antibodies directed against p75, TrkA, NGF, p63, ABCG2 and CK3/12. Limbal, conjunctival and pterygium tissue was obtained from patients (n= 10) undergoing pterygium resection and used for immunohistochemical assessment. Paraffin-embedded archival human skin specimens (n= 4) were also evaluated. In vitro expression of NGFR was determined in limbal, conjunctival and pterygium-derived epithelial cells. p75 was selectively expressed by basal epithelial cells in pterygia, conjunctiva and limbus, but was absent in the central cornea. These results were confirmed with two additional p75 specific antibodies. In contrast, TrkA was found in full-thickness pterygium, conjunctival, limbal and corneal epithelium in both adult and foetal eyes. p75 expression was identified in a small percentage, while TrkA was found on the entire population of cultured conjunctival, limbal and pterygium-derived epithelial cells. This receptor was also observed in selective regions of the human epidermis and hair follicle bulge. Our results illustrate the selective expression of p75 in basal pterygium, conjunctival and limbal epithelium, while staining was absent in adult and foetal central cornea. p75 may represent an additional ocular surface epithelial stem/progenitor cell signature gene.

Corneal limbal microenvironment can induce transdifferentiation of hair follicle stem cells into corneal epithelial-like cells

The aim of this study was to investigate the transdifferentiation potential of murine vibrissa hair follicle (HF) stem cells into corneal epithelial-like cells through modulation by corneal- or limbus-specific microenvironmental factors. Adult epithelial stem cells were isolated from the HF bulge region by mechanical dissection or fluorescence-activated cell sorting using antibodies to alpha6 integrin, enriched by clonal expansion, and subcultivated on various extracellular matrices (type IV collagen, laminin-1, laminin-5, fibronectin) and in different conditioned media derived from central and peripheral corneal fibroblasts, limbal stromal fibroblasts, and 3T3 fibroblasts. Cellular phenotype and differentiation were evaluated by light and electron microscopy, real-time reverse transcription-polymerase chain reaction, immunocytochemistry, and Western blotting, using antibodies against putative stem cell markers (K15, alpha6 integrin) and differentiation markers characteristic for corneal epithelium (K12, Pax6) or epidermis (K10). Using laminin-5, a major component of the corneo-limbal basement membrane zone, and conditioned medium from limbal stromal fibroblasts, clonally enriched HF stem and progenitor cells adhered rapidly and formed regularly arranged stratified cell sheets. Conditioned medium derived from limbal fibroblasts markedly upregulated expression of cornea-specific K12 and Pax6 on the mRNA and protein level, whereas expression of the epidermal keratinocyte marker K10 was strongly downregulated. These findings suggest that adult HF epithelial stem cells are capable of differentiating into corneal epithelial-like cells in vitro when exposed to a limbus-specific microenvironment. Therefore, the HF may be an easily accessible alternative therapeutic source of autologous adult stem cells for replacement of the corneal epithelium and restoration of visual function in patients with ocular surface disorders.

Epidermal homeostasis: a balancing act of stem cells in the skin

The skin epidermis and its array of appendages undergo ongoing renewal by a process called homeostasis. Stem cells in the epidermis have a crucial role in maintaining tissue homeostasis by providing new cells to replace those that are constantly lost during tissue turnover or following injury. Different resident skin stem cell pools contribute to the maintenance and repair of the various epidermal tissues of the skin, including interfollicular epidermis, hair follicles and sebaceous glands. Interestingly, the basic mechanisms and signalling pathways that orchestrate epithelial morphogenesis in the skin are reused during adult life to regulate skin homeostasis.

Stem cells and tissue-engineered skin

Advances in tissue engineering of skin are needed for clinical applications (as in wound healing and gene therapy) for cutaneous and systemic diseases. In this paper we review the use of epidermal stem cells as a source of cells to improve tissue-engineered skin. We discuss the importance and limitations of epidermal stem cell isolation using biomarkers, in quest of a pure stem cell preparation, as well as the culture conditions necessary to maintain this purity as required for a qualitatively superior and long-lasting engineered skin. Finally, we review the advantages of using additional multipotent stem cell sources to functionally and cosmetically optimize the engineered tissue.

Tiers of clonal organization in the epidermis: the epidermal proliferation unit revisited

As one of the most proliferative tissues in adult mammals, the epidermis is a good example of the precise regulation necessary between stem cell self-renewal and differentiation. The epidermis is derived from ectodermal progenitor cells and contains three distinct classes of cells: epidermal stem cells which are capable of infinite rounds of cell division; their immediate descendants, transient amplifying cells, which are capable of numerous but finite rounds of cell division; and finally, non-dividing, differentiating cells (Aberdam in Cell and Tissue Research 331:103-107, 2008). This proliferative hierarchy must be tightly regulated both temporally and spatially during epidermal development and homeostasis in order to prevent uncontrolled growth leading to hyperproliferative states and/or tumorigenesis. Historically, the most basic unit of epidermal proliferation has been described as the epidermal proliferation unit (EPU). The EPU, as originally characterized by Christophers, Potten and Mackenzie, is a proliferation unit consisting of approximately 10 basal cells with a clonogenic cell in the center and overlaid by the suprabasal and corneocyte progeny (reviewed in Potten, C. S. (1974). The epidermal proliferative unit: the possible role of the central basal cell. Cell and Tissue Kinetics, 7(1), 77-88). Numerous researchers have identified this classical EPU structure, consisting of approximately 20 cells, in a variety of mammalian skin sources. Recently however, lineage analyses have provided evidence for much larger clonal epidermal units consisting of hundreds to thousands of cells. Furthermore, cutaneous mosaicism as well as a variety of cutaneous pathologies indicate that clonal areas extend to whole patches of mammalian skin many centimeters across. In this review we revisit four decades of experimental evidence and put forward a model of clonal units derived from multiple classes of epidermal progenitors ranging from the largest and most primitive units, clonal ectodermal units, to epidermal stem cell units, and finally, to the most basic structural unit, the EPU.

Building epithelial tissues from skin stem cells

The skin epidermis and its appendages provide a protective barrier that guards against loss of fluids, physical trauma, and invasion by harmful microbes. To perform these functions while confronting the harsh environs of the outside world, our body surface undergoes constant rejuvenation through homeostasis. In addition, it must be primed to repair wounds in response to injury. The adult skin maintains epidermal homeostasis, hair regeneration, and wound repair through the use of its stem cells. What are the properties of skin stem cells, when do they become established during embryogenesis, and how are they able to build tissues with such remarkably distinct architectures? How do stem cells maintain tissue homeostasis and repair wounds and how do they regulate the delicate balance between proliferation and differentiation? What is the relationship between skin cancer and mutations that perturbs the regulation of stem cells? In the past 5 years, the field of skin stem cells has bloomed as we and others have been able to purify and dissect the molecular properties of these tiny reservoirs of goliath potential. We report here progress on these fronts, with emphasis on our laboratory's contributions to the fascinating world of skin stem cells.

Skin and hair: models for exploring organ regeneration

Skin is an excellent model to study the basic biology of organ regeneration and translational approaches to regenerative medicine. Because of the accessibility of the skin, a long history of regenerative approaches already exists. Identifying the commonalities between skin regeneration and the regeneration of other organs could provide major breakthroughs in regenerative medicine. The hair follicle represents a miniature organ with readily accessible stem cells, multiple cell lineages, and signaling centers. During the normal lifespan of a human, this miniature organ regenerates itself more than 10 times. The cells responsible for this remarkable process are called bulge stem cells. A plethora of molecular and genetic tools have been developed to follow their fate and to explore their ontogeny. Major advances have been made toward understanding the normal cell fate of bulge stem cells and their developmental plasticity. Recent studies suggest the epidermis and hair may have an untapped potential to form other organs. Understanding the mechanisms that regulate adult stem-cell proliferation is a major goal for regenerative medicine. In the hair follicle, pharmacologic agents, recombinant proteins, and artificial cell-permeable proteins have been developed to manipulate the proliferation of the quiescent bulge stem cells. These advances illustrate a potential roadmap for regenerative medicine using molecular tools developed for skin biology to promote organ regeneration by manipulating adult stem cells in situ.

Hair follicle stem cells are specified and function in early skin morphogenesis

In adult skin, epithelial hair follicle stem cells (SCs) reside in a quiescent niche and are essential for cyclic bouts of hair growth. Niche architecture becomes pronounced postnatally at the start of the first hair cycle. Whether SCs exist or function earlier is unknown. Here we show that slow-cycling cells appear early in skin development, express SC markers, and later give rise to the adult SC population. To test whether these early slow-cycling cells function as SCs, we use Sox9-Cre for genetic marking and K14-Cre to embryonically ablate Sox9, an essential adult SC gene. We find that the progeny of Sox9-expressing cells contribute to all skin epithelial lineages and Sox9 is required for SC specification. In the absence of early SCs, hair follicle and sebaceous gland morphogenesis is blocked, and epidermal wound repair is compromised. These findings establish the existence of early hair follicle SCs and reveal their physiological importance in tissue morphogenesis.

Differing responses of human follicular and nonfollicular scalp cells in an in vitro wound healing assay: effects of estrogen on vascular endothelial growth factor secretion

Improved wound healing of hairy skin may involve mesenchymal hair follicle cells with stem cell potential and enhancement by estrogen therapy. How estrogen affects follicular dermal papilla (DP) and dermal sheath (DS) cells in wound healing is unknown. Therefore, a comparison of estradiol action on DP, DS, and corresponding interfollicular dermal fibroblasts (DF) in a scratch-wound assay was performed using matching primary cultures established from female temporo-occipital scalp. All three cell types expressed mRNA transcripts and protein for estrogen receptors alpha (ERalpha) and beta (ERbeta). DF ERalpha transcripts were half that of DP and one-third of DS cells, while DF ERbeta transcripts were two-thirds of DP and DS cells. In the scratch-wound assay all three cells types migrated at similar rates, but only the rate of DF was enhanced by estradiol. Mechanical wounding increased DNA synthesis rates of all three cell types and increased the secretion of collagen by DF and DS cells. All three secreted similar basal levels of vascular endothelial growth factor (VEGF), which was increased by wounding DF and DS cells, but not DP cells. DP cells required estradiol to increase VEGF secretion; by contrast VEGF secretion was decreased by estradiol in wounded DS cells. These results highlight differences in the responses of DF, DP, and DS cells to estradiol in a scratch-wound assay, providing further support for the dichotomy of cellular functions in the hair follicle. Further understanding of the role of estrogen in cutaneous wound healing may have important implications for the management of chronic wounds and scarring.

Is the hair follicle necessary for normal wound healing?

The hair follicle contributes cells to the interfollicular epidermis after wounding, but the functional role of these cells has not been resolved. To address this question, Langton et al. (this issue, 2008) take advantage of the Edaradd mutant mouse, which lacks hair follicles on its tail. They discover an initial sluggish response of the hairless tail epidermis to wounding that is rapidly compensated for by recruitment of epidermal cells from outside the normally responsive area. This suggests that the hair follicle is important but not necessary for normal wound healing.

[Human skin stem cells]

The homeostasis of continuously renewing human epidermis relies on the presence of adult stem cells, residing in the basal layer. Epidermal stem cells have been enriched and functionally characterized, but the exact location remained elusive. The human hair follicle and its pigmentation unit also cyclically regenerate from stem cells. Contrary to epidermal stem cells, human hair follicle stem cells have been localized, enriched, functionally and biochemically characterized. Their specific gene expression pattern has been established. The melanocyte stem population has also been localized and characterized. Finally, the hair follicle was found to harbor a number of other multipotent cells, which designates this unique organ as an alternative source of stem cells for tissue regeneration.

Isolation and culture of different epidermal and dermal cell types from human scalp suitable for the development of a therapeutical cell spray

BACKGROUND

Previous studies demonstrated, that cultured epithelial autografts (CEA) can be isolated and skin cell sprays can be produced for application on different types of wounds. The purpose of the present study was to determine which cell types can be isolated from the human scalp and whether these cells can be used for spray transplantation.

METHODS

Outer root sheath cells (ORS), keratinocytes, melanocytes, dermal papilla cells (DP), and dermal sheath cells (DSC) were isolated from human scalp tissue. Isolated cells were characterized, expanded and sprayed in an in vitro model. Growth behaviour, morphology and cell counts were compared with non-sprayed cells.

RESULTS

With acceptable time, equipment and laboratory personnel a sufficient amount of keratinocytes, ORS, melanocytes, DP cells and DSC cells could be achieved. The cells are sufficient for application as a cell spray. Cells, positive for Integrin alpha6, Cytokeratin 19, CD73 and CD105 were identified within the cultures.

CONCLUSIONS

Human scalp is suitable to gain epidermal and dermal cells for the development of therapeutic cell spray transplantation. Further studies have to determine, whether these cells can be combined to produce wound specific skin substitutes.

Epidermal stem cells in skin homeostasis and cutaneous carcinomas

Skin and squamous epithelia regulate water and heat homeostasis and constitute our first barrier of protection against pathogens. Cells from the outermost layer of the skin, the cornified envelope (stratum corneum), are constantly being shed, imposing a constant demand for replenishment to maintain homeostasis. Hair follicles and sebaceous glands provide protective hair growth and skin sebum, and continuously undergo cycles of growth and regression. The outstanding ability of the epidermis, hair follicles and sebaceous glands to self-renew relies on a population of adult stem cells that are maintained throughout our life span. In this review we will provide an overview of our current knowledge about epidermal stem cells, and some of the molecular mechanisms that identify them and dictate their behaviour. We will also summarise our view on the possible link between adult epidermal stem cells and cancer stem cells within skin and squamous neoplasias. The potential of epidermal stem cells in regenerative medicine and for designing targeted antitumoral therapies will be discussed.

Skin stem cells: rising to the surface

The skin epidermis and its appendages provide a protective barrier that is impermeable to harmful microbes and also prevents dehydration. To perform their functions while being confronted with the physicochemical traumas of the environment, these tissues undergo continual rejuvenation through homeostasis, and, in addition, they must be primed to undergo wound repair in response to injury. The skin's elixir for maintaining tissue homeostasis, regenerating hair, and repairing the epidermis after injury is its stem cells, which reside in the adult hair follicle, sebaceous gland, and epidermis. Stem cells have the remarkable capacity to both self-perpetuate and also give rise to the differentiating cells that constitute one or more tissues. In recent years, scientists have begun to uncover the properties of skin stem cells and unravel the mysteries underlying their remarkable capacity to perform these feats. In this paper, I outline the basic lineages of the skin epithelia and review some of the major findings about mammalian skin epithelial stem cells that have emerged in the past five years.

Epidermal homeostasis: do committed progenitors work while stem cells sleep?

Tracking the fate of cells in murine epidermis in vivo has revealed that a committed progenitor cell population can maintain normal adult tissue in the long term without support from a long-lived, self-renewing population of stem cells. Here, we argue that these results challenge the dogma that stem-cell proliferation is required for the cellular homeostasis of the epidermis and other adult tissues, with important implications for tissue physiology and disease.

An extended epidermal response heals cutaneous wounds in the absence of a hair follicle stem cell contribution

Hair follicles have been observed to provide a major cellular contribution to epidermal healing, with emigration of stem-derived cells from the follicles aiding in wound reepithelialization. However, the functional requirements for this hair follicle input are unknown. Here we have characterized the keratinocyte stem cell status of mutant mice that lack all hair follicle development on their tail, and analyzed the consequent alterations in epidermal wound healing rate and mechanisms. In analyzing stem cell behavior in embryonic skin we found that clonogenic keratinocytes are relatively frequent in the ectoderm prior to hair follicle formation. However, their frequency in the interfollicular epidermis drops sharply by birth, at which time the majority of stem cells are present within the hair follicles. We find that in the absence of hair follicles cutaneous wounds heal with an acute delay in reepithelialization. This delay is followed by expansion of the region of activated epidermis, beyond that seen in normal haired skin, followed by appropriate wound closure. JID Journal Club article: for questions, answers, and open discussion about this article please go to http://network.nature.com/group/jidclub.

Intravital insights in skin wound healing using the mouse dorsal skin fold chamber

The skin fold chamber is one of the most accepted animal models for studying the microcirculation both in health and disease. Here we describe for the first time the alternative use of the skin fold chamber in mice for intravital microscopic investigation of skin regeneration after creating a full dermal thickness wound. The dorsal skin fold chamber was implanted in hairless SKH1-hr mice and a full dermal thickness wound (area approximately 4 mm2) was created. By means of intravital fluorescence microscopy, the kinetics of wound healing were analyzed for 12 days post wounding with assessment of epithelialization and nutritive perfusion. The morphology of the regenerating skin was characterized by hematoxylin-eosin histology and immunohistochemistry for proliferation and microvessel density. The model allows the continuous visualization of wound closure with complete epithelialization at day 12. Furthermore, a sola cutis se reficientis could be described by an inner circular ring of vessels at the wound margin surrounded by outer radial passing vessels. Inner circular vessels presented initially with large diameters and matured towards diameters of less than 15 microm for conversion into radial spreading outer vessels. Furthermore, wound healing showed all diverse core issues of skin repair. In summary, we were able to establish a model for the analysis of microcirculation in the healing skin of the mouse. This versatile model allows distinct analysis of new vessel formation and maturation in regenerating skin as well as evaluation of skin healing under different pathologic conditions.

Stem cells: a revolution in therapeutics-recent advances in stem cell biology and their therapeutic applications in regenerative medicine and cancer therapies

Basic and clinical research accomplished during the last few years on embryonic, fetal, amniotic, umbilical cord blood, and adult stem cells has constituted a revolution in regenerative medicine and cancer therapies by providing the possibility of generating multiple therapeutically useful cell types. These new cells could be used for treating numerous genetic and degenerative disorders. Among them, age-related functional defects, hematopoietic and immune system disorders, heart failures, chronic liver injuries, diabetes, Parkinson's and Alzheimer's diseases, arthritis, and muscular, skin, lung, eye, and digestive disorders as well as aggressive and recurrent cancers could be successfully treated by stem cell-based therapies. This review focuses on the recent advancements in adult stem cell biology in normal and pathological conditions. We describe how these results have improved our understanding on critical and unique functions of these rare sub-populations of multipotent and undifferentiated cells with an unlimited self-renewal capacity and high plasticity. Finally, we discuss some major advances to translate the experimental models on ex vivo and in vivo expanded and/or differentiated stem cells into clinical applications for the development of novel cellular therapies aimed at repairing genetically altered or damaged tissues/organs in humans. A particular emphasis is made on the therapeutic potential of different tissue-resident adult stem cell types and their in vivo modulation for treating and curing specific pathological disorders.

Wnt-dependent de novo hair follicle regeneration in adult mouse skin after wounding

The mammalian hair follicle is a complex 'mini-organ' thought to form only during development; loss of an adult follicle is considered permanent. However, the possibility that hair follicles develop de novo following wounding was raised in studies on rabbits, mice and even humans fifty years ago. Subsequently, these observations were generally discounted because definitive evidence for follicular neogenesis was not presented. Here we show that, after wounding, hair follicles form de novo in genetically normal adult mice. The regenerated hair follicles establish a stem cell population, express known molecular markers of follicle differentiation, produce a hair shaft and progress through all stages of the hair follicle cycle. Lineage analysis demonstrated that the nascent follicles arise from epithelial cells outside of the hair follicle stem cell niche, suggesting that epidermal cells in the wound assume a hair follicle stem cell phenotype. Inhibition of Wnt signalling after re-epithelialization completely abrogates this wounding-induced folliculogenesis, whereas overexpression of Wnt ligand in the epidermis increases the number of regenerated hair follicles. These remarkable regenerative capabilities of the adult support the notion that wounding induces an embryonic phenotype in skin, and that this provides a window for manipulation of hair follicle neogenesis by Wnt proteins. These findings suggest treatments for wounds, hair loss and other degenerative skin disorders.