Epidermal stem cells

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					R E V I E W A RT I C L E                                                                              BJD      British Journal of Dermatology

Epidermal stem cells: practical perspectives and
potential uses
O. Abbas and M. Mahalingam*
Department of Dermatology, American University of Beirut Medical Center, Beirut, Lebanon
*Dermatopathology Section, Department of Dermatology, Boston University School of Medicine, Boston, MA, U.S.A.


Correspondence                                     Throughout adult life, the epidermis and the hair follicle undergo a perpetual
Meera Mahalingam.                                  cycle of growth, regression and rest. Stem cells in the epidermis not only ensure
E-mail:                            the maintenance of epidermal homeostasis and hair regeneration, but also con-
                                                   tribute to repair of the epidermis after injury. These stem cells lie within specific
Accepted for publication
7 April 2009
                                                   niches in the hair follicle and the epidermis. The availability of monoclonal anti-
                                                   bodies that can be used on formalin-fixed paraffin-embedded tissue has greatly
Key words                                          facilitated the use of this methodology as an adjunct to uncovering stem cell
epidermal, melanocytic, stem cells                 niches. In this review, we attempt to provide an overview of the potential mark-
                                                   ers available to identify and study stem cells in an effort to providing a better
Conflicts of interest
                                                   understanding of the pathogenesis of skin diseases including disorders of hair
None declared.
                                                   loss and malignancies. The potential uses of these markers in prognosis and in
DOI 10.1111/j.1365-2133.2009.09250.x               expanding the therapeutic options in several disorders will also be addressed.

There are two broad categories of mammalian stem cells: stem                       mammals, the epidermis originates from neuroectodermal
cells that are capable of differentiating into all of the special-                 cells that remain at the surface of the embryo after gastrula-
ized embryonic tissues and stem cells that are found in differ-                    tion.4 The epidermis usually begins as a single undifferentiated
ent regenerative adult tissues and are of importance in the                        progenitor cell layer which then gives rise to the interfollicular
maintenance of normal tissue turnover and repair by replen-                        epidermis, the hair follicle and sebaceous glands.4 Dermal
ishing specialized differentiated cells.1,2 In mammals, adult or                   fibroblasts, vessels, arrector pili muscles, mature adipocytes in
tissue-specific stem cells have been identified in various tis-                      the subcutis and immune cells residing in the skin originate
sues, including the haematopoietic system, central nervous                         from mesoderm-derived cells while melanocytes and sensory
system, corneal epithelium, thymic epithelium and neural                           nerve endings of the skin are derived from the neural crest.2,5
crest, among others. Within these tissues, the stem cells are                      During adult life, the maintenance of these various cell types
usually found in a specialized environment or niche that                           is the function of the different stem cells residing in the skin.
provides important signals to guide their function.2
   All these tissue-specific stem cells are thought to share two
                                                                                   Epidermal stem cells
main characteristics. Firstly, self-renewal or the ability to
renew indefinitely, and, secondly, multipotency, or the capac-                      In mammals, the epidermis is a multilayered epithelium that
ity to differentiate into multiple specialized cell lineages of the                is composed of hair follicles, sebaceous glands and interfollic-
specific tissue.2 The former is believed to be maintained sec-                      ular epidermis. The interfollicular epidermis, defined as the
ondary to asymmetrical division of stem cells which gives rise                     portion of the epidermis located between the orifices of hair
to a stem cell that remains in the niche and to a lineage-                         follicles, regenerates throughout adult life in order to replace
restricted transient amplifying cell or progenitor cell which                      terminally differentiated cells that are continuously shed from
exits the niche and undergoes several limited rounds of prolif-                    the surface of the skin and also to renew the hair follicle.1
eration before undergoing terminal differentiation.3 Compared                      The regeneration of the epidermis and the hair follicle is sus-
with stem cells, the transient amplifying cells are more differ-                   tained by many different types of epidermal stem cell, which
entiated and committed unipotent cells that have, as a conse-                      also participate in the repair of the skin after injuries. In add-
quence, lost the ability to self-renew.1,3                                         ition to their self-renewing capacity and multipotency, these
                                                                                   cells are quiescent with a low tendency to divide, but upon
                                                                                   injury are characterized by an extensive and sustained self-
Embryology of the skin
                                                                                   renewal capacity.1 The bulge region of the hair follicle repre-
There are many different types of cell residing in the skin and                    sents the best characterized epidermal stem cell population
these originate from multiple different embryonic sources. In                      described to date, but there is evidence of other stem cell

                                                                                                                                     Ó 2009 The Authors
228                                  Journal Compilation Ó 2009 British Association of Dermatologists • British Journal of Dermatology 2009 161, pp228–236
                                                                    Potential uses of epidermal stem cells, O. Abbas and M. Mahalingam 229

populations in the interfollicular epidermis and, potentially,                 The adult hair follicle consists of an upper portion that is
the sebaceous gland.1,6,7                                                   permanent and a lower portion that constantly remodels dur-
                                                                            ing the hair cycle. The hair cycle consists of three phases: ana-
                                                                            gen (the growth phase in which the hair shaft, inner and
Identification of epidermal stem cells
                                                                            outer root sheaths, and new hair matrix are generated), cat-
Several methods have been used to identify epidermal stem                   agen (a phase of epithelial regression driven by apoptosis) and
cells and to try to differentiate them from other types of cell             telogen (a phase of relative quiescence).9,15–17 Studies have
residing in the epidermis including transient amplifying cells              shown that maintenance of the hair follicle cycle is largely
and terminally differentiated cells.1,6                                     dependent on different stem cell populations capable of giving
   The first method makes use of the slow-cycling nature of                  rise to the different epithelial components of the hair follicle.
epidermal stem cells. Using this method all actively dividing               The bulge region of the hair follicle, defined as the portion of
cells within the epidermis are pulse-labelled with injections               the outer root sheath of the hair follicle at the insertion site of
of a DNA precursor, such as tritiated thymidine or bromo-                   the arrector pili muscle, is currently the best characterized site
deoxyuridine.8–10 This is then followed by a chase period                   of epidermal stem cell populations.
(4–10 weeks) during which the label is lost from rapidly
proliferating cells such as the transient amplifying cells as a
                                                                            Markers of hair follicle stem cell populations
consequence of proliferation-associated dilution, while the
rarely dividing stem cells retain the label for prolonged peri-             Bulge keratinocyte stem cells, in addition to being quiescent,
ods and are therefore called label-retaining cells.8–10 Based               have been shown to have all of the characteristics of stem
on this method, label-retaining cells in mice were found to                 cells.1,17 Although previous studies indicate that bulge stem
be present in the bulge region of the hair follicle, in the                 cells give rise to all components of the epidermis, there is
basal layer of the epidermis and even in select basal sebo-                 more recent evidence to indicate that in normal states, they
cytes.9,11                                                                  do not contribute to the reconstitution of the interfollicular
   The second method makes use of the high proliferative                    epidermis.17,18 Injury to the epidermis results in migration of
capacity of epidermal stem cells.3 Using this method the pro-               the bulge cells to the epidermis where they then contribute to
liferative potential of cultured cells is assessed by examining             wound repair.18,19 Identification of this stem cell population
the clonogenicity of individual cells through serial passage or             in both mice and human hair follicles has been made possible
colony-forming efficacy.12,13 Based on this method, three                    by the use of markers with differing specificities (Table 1,
types of epidermal cells with differing capacities to proliferate           Figs 1, 2).
have been identified: cells with no proliferative capacity (ter-
minally differentiated cells), cells with a limited proliferative
capacity (transient amplifying cells) and cells with a clonogenic
or high proliferative capacity (stem cells).12                              Integrins are adhesion molecules that mediate keratinocyte
   Although these two methods help in the identification of                  adhesion to the underlying basement membrane and also play
epidermal stem cells, they do not allow for the easy isolation              a role in controlling epidermal differentiation and morpho-
of living stem cells for further analysis.1 Several epidermal               genesis.1,13,20–24 Expression of a6-integrin was consistently
stem cell markers have been identified during the past few                   found in the outermost layer of the outer root sheath of the
years through the use of a candidate approach or, more                      hair follicle with no specific delineation of the bulge.24 Other
recently, by global gene expression profiling. However, reli-                areas showing increased expression included the basement
able and specific stem cell markers for epidermal stem cells                 membrane of the entire follicle and the dermal papilla.1,22–24
and their transient amplifying cell progeny are still lacking. In           Expression of b1-integrin expression, on the other hand,
this paper, we review the current data available on stem cell               appears to be confined to the bulge region of the hair follicle,
markers of the different stem cell populations present in the               in select studies, although others have indicated that it displays
epidermis with emphasis on the most reliable and specific,                   a similar, albeit less specific, expression to that of a6-integrin
and to discuss their potential uses to uncover new stem cell                with upregulation in the outermost layer of the outer root
populations and potential targets for gene therapy.                         sheath, basement membrane of the hair follicles and the
                                                                            dermal papilla.13,20–24 Based on these expression patterns, the
                                                                            utility of integrins as stem cell markers, for now at least,
Hair follicle stem cells
                                                                            appears limited.
Development of the hair follicle occurs through a temporal
series of epithelial–mesenchymal interactions which com-
mence with the formation of the hair placode at sites of
underlying dermal mesenchymal cell condensate that form the                 Keratinocytes are characterized by the differential expression
dermal papilla.14 The overlying hair placode proliferates and               of keratin intermediate filaments. In the epidermis, keratino-
differentiates to form the hair follicle, a direct consequence of           cytes in the basal cell layer express keratins 5 and 14, while
underlying dermal papilla cells.                                            keratins 1 and 10 are expressed by cells in the suprabasal

Ó 2009 The Authors
Journal Compilation Ó 2009 British Association of Dermatologists • British Journal of Dermatology 2009 161, pp228–236
230 Potential uses of epidermal stem cells, O. Abbas and M. Mahalingam

Table 1 Spectrum of markers available to identify bulge keratinocyte stem cells

                     Murine studies                          Human studies                                  Comments
  a6-Integrin        ORS mostly bulge IFE and SG11           ORS of bulge and of all HF21,24                Nonspecific24
                                                             BM of the whole HF24
  b1-Integrin        Most of ORS11                           ORS of bulge and of all HF21,24                Nonspecific24
                     Bulb11                                  BM of the whole HF24
                     IFE and SG11                            DP24
  CK15               Bulge11                                 Bulge24,25,27                                  In mice, colocalize with LRCs11
                                                             Part or all of ORS24,27
                                                             Basal layer of the epidermis24
                                                             Eccrine glands24,27
  CK19               Bulge30                                 Bulge24,30                                     CK-positive cells also expressed
                                                             Most of ORS of entire HF24                      b1-integrin30
                                                             Basal layer of the epidermis30
  p63                –                                       Basal and suprabasal epidermal cells32,34      Homologue of the p53 tumour
                                                             ORS and hair matrix of HF34                     suppressor gene34
                                                                                                            May be a marker of TAC1,34
  CD34               Bulge region34                          Low or absent in human bulge17,24              Most specific marker of HF bulge in
                                                             ORS lower to the bulge24,27                     mice34
  CD200              –                                       Increased in bulge cells24,26                  Best marker of the human bulge
                                                             ORS of isthmus, DP, sweat glands and            keratinocyte SCs26
                                                              companion layer24                             CD200 may suppress immune
  Tenascin-C         –                                       Homogeneous expression along the CTS           May be an important functional
                                                              of HF, with upregulated expression in          component of SC niches40
                                                              the bulge mesenchyme24
  Lhx2               Postnatal bulge compartment41           Companion layer24                              In mice, may play a role in the
                                                             ORS distant (both proximal and distal)          maintenance of the undifferentiated
                                                              from the bulge24                               state of HF progenitor cells41
  MTS24              Upregulated in an area between          –                                              These cells did not express CD34 or
                      SGs and the HF bulge42                                                                 CK1543
                                                                                                            In mice, identifies epithelial
                                                                                                             progenitor cells in thymus43
  Bmi-1              –                                       HF bulge43                                     Maintains SCs by downregulating
                                                             Basal and suprabasal cells43                    p16ink4a43
                                                             ORS of HFs43                                   Not specific
                                                             Sebaceous and sweat glands

  ORS, outer root sheath; IFE, interfollicular epithelium; SG, sebaceous gland; HF, hair follicle; BM, basement membrane; DP, dermal papillae;
  LRC, label-retaining cells; TAC, transient amplifying cell; CK, cytokeratin; SC, stem cells; CTS, connective tissue sheath.

Fig 1. Most consistent immunoreactivity of the human hair follicle           Fig 2. Most consistent immunoreactivity of the murine hair follicle
and interfollicular epidermis for the stem cell markers a6b1, CK15,          and interfollicular epidermis for stem cell markers a6b1, CK15, CK19,
CK19, CD200, CD34, nestin and MITF. ORS, outer root sheath.                  CD34, nestin, MTS24 and Blimp1. ORS, outer root sheath.

                                                                                                                               Ó 2009 The Authors
                               Journal Compilation Ó 2009 British Association of Dermatologists • British Journal of Dermatology 2009 161, pp228–236
                                                                    Potential uses of epidermal stem cells, O. Abbas and M. Mahalingam 231

layer.17,24 In addition to the above, the hair follicle expresses           suppression of immune responses that help protect keratinocytes
keratins 6, 16 and 17.17,24 Initial observations of the presence            in the hair follicle from destruction by inflammation.37–39
of cytokeratin (CK)-15 positive cells in the bulge region of                Other human epithelial structures that showed upregulated
the murine and human hair follicle indicate that CK15 might                 expression of CD200 included the outermost layer of the outer
be targeting a population of stem cells restricted to the bulge             root sheath of the isthmus, the dermal papilla including its
region.11,25 However, other studies have shown that CK15                    blood vessels, the sweat glands and the companion layer of
expression is also seen in the outermost layer of the outer root            the human hair follicle.24,26 In contrast to the preferential
sheath of the human hair follicle, the basal layer of the epider-           expression of CD200 in the human bulge stem cells,26 murine
mis and the eccrine glands.24,26,27 Based on CK15 expression                studies have shown that CD200 is expressed only in the outer
in the mitotically active basal cell layers of the hair follicle, it        root sheath of hair follicles.37
is believed that CK15 may play a pivotal role in early keratino-               Tenascin-C, a key extracellular matrix protein, is thought to
cyte differentiation and one that pre-dates the fate of a cell              be an important functional component of stem cell niches.40
becoming epidermal or hair-like.28 Other keratins believed to               Favouring this, a recent study documented homogeneous
be restricted to the bulge region include CK19 although more                expression of tenascin-C along the connective tissue sheath of
recent studies indicate that CK19-positive cells may also be                human scalp hair follicles (this is comparable to the basement
seen in the outermost layer of the outer root sheath proximal               membrane, so is not part of the epithelium) and upregulated
and distal to the bulge.24,29,30 In vivo and in vitro studies have          expression in the bulge mesenchyme.24
shown that CK19 may be important in the commitment of                          Although Lhx2 (Lim-homeodomain transcription factor)
stem cells to an epidermal cell fate and differentiation.24,31              may play a role in the maintenance of the growth and undif-
                                                                            ferentiated nature of murine hair follicle progenitor cells,41 a
                                                                            recent study on human anagen hair follicles showed that Lhx2
Transcription factors
                                                                            is not a useful marker of stem cells in the human bulge as its
Structurally related proteins belonging to the family of tran-              expression appeared to be most prominent in the companion
scription factors include p53, p63, considered to be a homo-                layer (the layer between the outer root sheath and the inner
logue of the p53 tumour suppressor gene, and p73.32–34                      root sheath), with reduced expression in the outer root sheath
Although early in vitro studies identified p63 as a stem cell                distant from the bulge region.24
marker, more recent in vivo studies indicate that p63 expression               More recently, MTS24, a cell surface marker labelling a
is not restricted to epidermal stem cells, but involves basal and           membrane-bound antigen present in the early stages of hair
suprabasal epidermal cells as well as the outer root sheath and             follicle development in adult mice has identified a new reser-
hair matrix of hair follicles. Given this pattern of expression,            voir of hair follicle keratinocytes between sebaceous glands
it is believed to be a marker of transient amplifying cells                 and the bulge region with a proliferative capacity and gene
rather than stem cells.32–34                                                expression profile indicating that they may be identifying pro-
                                                                            genitor or stem cells. Of interest, these cells do not express
                                                                            the stem cell markers CD34 or CK15.42
Haematopoietic progenitor cells
                                                                               Another marker tested for its role as a stem cell marker is
Expression of CD34, a marker purportedly specific for haemato-               Bmi-1 (B-cell-specific Moloney murine leukaemia virus inte-
poietic progenitor cells, has also been shown in the bulge                  gration site 1), which normally plays a role in the mainte-
region of murine hair follicles.35 Currently, CD34 is believed              nance of stem cells by downregulating p16ink4a, a tumour
to be the best marker to delineate stem cells in the bulge                  suppressor gene.43 However, its variable expression in basal
region in murine models.35 Contrasting sharply with this, one               and suprabasal keratinocytes, outer root sheath of hair folli-
study demonstrated the absence of CD34 expression in the                    cles, sebaceous glands and sweat glands indicates that is not
human bulge.17 More recent evidence indicates that CD34 is                  restricted to the bulge region and may therefore not be of any
expressed in the outermost layers of the outer root sheath,                 utility as a specific epidermal stem cell marker.43
below the bulge region, of the human hair follicle.24,27,36
Of particular interest, these CD34-positive cells were CK15-
                                                                            Bulge melanocyte stem cells
negative indicating that they may represent transient amplifying
cells or progeny of bulge stem cells.27,36                                  In each hair cycle, at the transition from the anagen to catagen
                                                                            phase, melanocytes in the hair bulb matrix undergo apoptosis
                                                                            with their reconstitution occurring at the beginning of the
Other markers
                                                                            next anagen phase.1 Evidence from murine and human studies
More recently, using laser capture microdissection and micro-               indicates that this reconstitution process is made possible by a
array analysis for global gene expression profiles, CD200-                   population of follicular bulge stem cells committed to melano-
positive cells have been isolated from label-retaining human                cyte differentiation.1 These melanocyte stem cells are usually
bulge cells.26 Evidence from murine studies indicates that                  quiescent but become activated and proliferate at the onset of
CD200, through its interaction with immunologically active                  the anagen phase leading to the repopulation of the hair folli-
cells expressing the CD200 receptor, may play a role in the                 cle matrix with melanocytes that generate melanin leading to

Ó 2009 The Authors
Journal Compilation Ó 2009 British Association of Dermatologists • British Journal of Dermatology 2009 161, pp228–236
232 Potential uses of epidermal stem cells, O. Abbas and M. Mahalingam

Table 2 Spectrum of markers available to identify bulge melanocyte stem cells

                         Murine studies                  Human studies                    Comments
  Pax3                   Highlight cells in the          Highlight cells in the           Melanocyte SC maintenance and differentiation47
                          bulge region47,48,50            bulge region47,48               Controls MITF expression50
  MITF                   Highlight cells in the          Highlight cells in the           Regulate balance between SC maintenance and
                          bulge region48,50               bulge region45,48                differentiation45,48,50
  Dct (TRP-2)            Highlight cells in the          –                                Early marker to be discovered for this SC population44,50
                          bulge region44,50

  MITF, melanocyte master transcriptional regulator; SC, stem cells.

pigmentation of the hair shaft.44 In addition, the defective                 similar population of cells in the human hair follicle epithe-
self-maintenance of these melanocytes stem cells, which is                   lium have been contradicted by more recent studies showing
thought to be part of physiological ageing, may be the under-                a population of nestin-positive cells residing in the upper
lying cause of hair greying.45                                               two-thirds of the hair follicle, hair follicle connective tissue
   The melanocyte stem cell markers include Pax3 and MITF,                   sheath, dermal papilla, sweat gland epithelium and in the
also known as melanocyte master transcriptional regulator                    inner aspect of the outer root sheath below the bulge.24,27,53–56
(Table 2).46–49 The former, Pax3, has been shown to maintain
the undifferentiated state of stem cells while simultaneously
                                                                             Sebaceous gland progenitor cells
functioning in initiation of the melanogenic cascade.46,47,49
More recently, MITF, which may play a role in stem cell                      Being a hair follicle appendage located above the bulge and
maintenance within the bulge through an antiapoptotic effect                 below the hair shaft orifice, sebaceous glands function in the
mediated by induction of Bcl-2 expression, has been shown to                 generation of terminally differentiated sebocytes.57 While de-
be highly expressed in the human bulge and is believed to                    velopment of sebaceous glands starts with the formation of
serve as a potential marker of this stem cell population.47,48               progenitor cells towards the end of embryogenesis, maturation
                                                                             of the sebaceous gland occurs only after birth.57 Shortly there-
                                                                             after, they go into a resting phase to become activated again
Bulge neural crest-derived stem cells
                                                                             at puberty. Although there is evidence to indicate that, when
Recently, a new population of stem cells, neural crest-derived               the skin is wounded, bulge sebaceous glands contribute to all
stem cells, has been identified within the murine hair follicle               components of the epidermis including sebaceous glands,
bulge.50–53 These stem cells, with markers that differentiate                recent studies have shown that, under homeostatic conditions,
them from other stem cells in the bulge, apparently have the                 bulge cells do not contribute to the formation of the seba-
ability to differentiate in vitro to keratinocytes, neurons, mela-           ceous glands.17 Thus, sebaceous gland homeostasis necessitates
nocytes, glial cells, smooth muscle cells and adipocytes.50–53               the presence of a progenitor population of cells that gives rise
   One of the markers which helped in the identification of                   to a continual flux of proliferating, differentiating and disinte-
this stem cell population is nestin, an intermediate filament                 grating sebocytes. In fact, recent murine studies identified a
protein expressed in the neuroepithelial stem cell cytoplasm                 resident basal sebocyte population with characteristics of pro-
and known to be a marker for neural stem cells (Table 3).                    genitor cells suggesting that the sebaceous glands are capable
Although these nestin-positive cells do not contribute to the                of self-maintainance.58
keratinocyte compartment in homeostatic conditions, they                        The recent identification of Blimp1 (B lymphocyte-induced
have been shown to enhance blood vessel formation during                     maturation protein 1) has helped characterize the progenitor
hair follicle growth.50 Of interest, in the murine bulge region,             cell population in the sebaceous glands of mice. Normally,
nestin-positive cells were also CD34-positive but CK15-                      Blimp1 represses c-myc, which plays an important role in
negative.50–53 Previous studies indicating an absence of a                   sebaceous gland differentiation, thus inhibiting proliferation

Table 3 Spectrum of markers available to identify bulge neural crest-derived stem cells

              Murine studies              Human studies                                       Comments
  Nestin      Highlight cells in          Inner aspect of the ORS below the bulge24,27        In mice, these cells were also CD34-positive but
               the bulge region50–53      Also, upper 2 ⁄ 3 of HF, HF CTS, DP and              CK15-negative52
                                           sweat gland epithelium                             In mice, these cells can form keratinocytes, neurons,
                                                                                               melanocytes, glia, muscle cells and adipocytes

  HF, hair follicle; ORS, outer root sheath; CTS, connective tissue sheath; DP, dermal papillae; CK, cytokeratin.

                                                                                                                                Ó 2009 The Authors
                               Journal Compilation Ó 2009 British Association of Dermatologists • British Journal of Dermatology 2009 161, pp228–236
                                                                       Potential uses of epidermal stem cells, O. Abbas and M. Mahalingam 233

Table 4 Spectrum of markers available to identify sebaceous gland progenitor cells

                       Murine studies                              Human studies                             Comments
  Blimp1               Basal sebocytes with                        –                                         These cells can give rise to HF under
                        characteristics of progenitor                                                         effect of b-catenin59
  CK15                 Few basal sebocytes11                       Basal sebocytes of both                   Sebaceous tumours were also positive
                                                                    sebaceous gland and duct7                 for CK157

  HF, hair follicle; CK, cytokeratin.

and differentiation of sebocytes.58 That these Blimp1-positive                studies on other animal models indicate a different organiza-
progenitor cells have stem cell potential is based on studies                 tion in specific anatomical sites such as the palms and soles.63
indicating that under the effect of b-catenin, a hair follicle                These include the localization of label-retaining cells, repre-
inductive signal, they give rise to the hair follicle.59 More                 senting stem cells, in the deep rete ridges of the monkey pal-
recently, CK15 has been shown to highlight basal cells in the                 mar epidermis.63 In humans, the task of identifying stem cells
sebaceous gland and duct.7 Whether or not these cells repre-                  in the interfollicular epidermis has been more difficult owing
sent the same population of cells identified by Blimp1 remains                 to the inability to use label-retaining studies (there are ethical
to be elucidated by further studies (Table 4).                                and technical limitations as the label may be harmful). Initial
                                                                              human studies on neonatal foreskin and breast skin tissue
                                                                              have suggested the presence of a population with stem cell
Stem cells in the interfollicular epidermis
                                                                              properties in the shallow rete ridges.64,65 More recently, a
The only mitotically active layer in the interfollicular epider-              population with the molecular signature of stem cells and
mis, a layer of stratified squamous epithelium, is the basal                   transient amplifying cells has been shown to reside at the tips
layer.57 The outermost layer consists of cells that are continu-              of deep rete ridges in the adult breast, palms and soles, sites
ously shed from the surface and, given recent evidence                        relatively protected from external injury.20,66,67 Markers of
indicating that bulge stem cells do not contribute to the                     potential utility in the identification of these cells include
regeneration of interfollicular epidermis under normal                        a6-intgrin, b1-integrin and CK15, CK10, CD71, and desmo-
homeostatic conditions, it seems plausible that the interfolli-               somal proteins (Table 5).66–68 A recent study on human
cular epidermis has its own stem cell population.18,19 Labelling              neonatal foreskin demonstrated that although CD200 is useful
studies on murine DNA have demonstrated that the interfol-                    in the identification of bulge keratinocyte stem cells,26 its use
licular epidermis is dependent on multiple, functionally inde-                in the identification of interfollicular epidermal stem cells
pendent, hexagonal units, called the epidermal proliferative                  appears limited.69
units.60–62 These epidermal proliferative units ensure lifelong
cell production to compensate for the continual loss of cells
                                                                              Potential clinical uses of stem cell markers
from the surface of the skin.60–62 Each epidermal proliferative
unit consists of a single centrally located stem cell, its imme-              In addition to uncovering multiple stem cell populations in
diate transient amplifying cell progeny adjacent, with more                   the epidermis with differing potentials for proliferation and
differentiated keratinocytes lying directly above and mature,                 differentiation, stem cell markers have led to advances in
albeit enucleated, squames at the surface.60–62 However,                      epidermal stem cell research by providing insights into

Table 5 Spectrum of markers available to identify interfollicular epidermis stem cells

                  Murine studies           Human studies                                        Comments
  b1-Integrin     Upregulated in basal     Shallow rete ridges64,65                             These cells had low to negative levels of CK10
                   IFE keratinocytes11     Tips of the deep rete ridges66–68                     and desmosomal proteins68
  a6-Integrin     Upregulated in basal     Upregulated in few basal IFE keratinocytes66,67      These cells had low CD71 expression20,66,67
                   IFE keratinocytes11
  CK15            Upregulated in basal     Upregulated in few basal IFE keratinocytes at        These cells were also a6 positive, CK10 negative20
                   IFE keratinocytes11      tips of rete ridges20,27
  CK19            Bulge30                  Deep rete ridges of glabrous skin27                  CK-positive cells also expressed b130
                                           Bulge and most of ORS24,30
  CD200           –                        Few basal keratinocytes were positive68              These cells did not have the properties of SCs68

  IFE, interfollicular epithelium; CK, cytokeratin; ORS, outer root sheath; SC, stem cells.

Ó 2009 The Authors
Journal Compilation Ó 2009 British Association of Dermatologists • British Journal of Dermatology 2009 161, pp228–236
234 Potential uses of epidermal stem cells, O. Abbas and M. Mahalingam

understanding stem cell biology and behaviour. Their use has             or Bowen disease.74 Nestin and CD133 show significantly
also proven to be of importance in shedding light on the aetio-          higher expression in melanomas compared with benign naevi,
pathogenesis of genetic, neoplastic and inflammatory derma-               and in metastatic compared with primary melanomas.77 Thus,
toses, as well as in the prognosis and expansion of therapeutic          progression from benign to malignant to metastatic disease
options of these disorders.                                              appears to be mediated by pathways that restore the stem cell
                                                                         features and characteristics.
Shedding light on aetiopathogenesis
                                                                         Expanding therapeutic options
Recent reports confirm involvement of the hair follicle bulge
region in scarring alopecias.27,70,71 Briefly, Mobini et al.70            By the identification and isolation of different stem cell popu-
hypothesized that the pathogenesis of lichen planopilaris                lations, stem cell markers may provide a bank of stem cells of
involves cytotoxic-mediated destruction of bulge stem cells by           potential utility in cutaneous regenerative medicine. For exam-
an inflammatory infiltrate composed predominantly of                       ple, bulge keratinocyte stem cells can theoretically be used to
CD8-positive T lymphocytes. Pozdnyakova and Mahalingam71                 produce bioengineered hair follicles to treat alopecia.
confirmed involvement of the bulge region in primary                      Although an in vivo method to reconstitute human hair follicles
scarring alopecias by demonstrating the absence of bulge                 has not, to date, been done,78 de novo hair follicle generation
CK15-positive stem cells during early active stages of a hetero-         using in vivo hair follicle reconstitution assays has indeed been
geneous group of scarring alopecias characterized by moderate            established in mice.79 Another example is the use of the mur-
to heavy inflammation. This effect on the bulge stem cells                ine bulge neural crest stem cells in the regeneration of
in scarring alopecias is further supported by studies showing            nerves.52 Through the use of new and more specific markers,
that CD200, a recently identified marker for human bulge                  it may be possible in the future to identify a similar stem cell
cells, plays an important role in the regulation of immune               population in humans that will serve in the regeneration of
response.37,39 CD200 is a transmembrane protein which                    nerves in humans.
through its interaction with CD200 receptors on immune cells                The exhaustion or incomplete maintenance of melanocyte
provides inhibitory immunoregulatory signals.37,39 Based on              stem cells is believed to be the cause of loss of hair shaft
this, upregulated expression of CD200 within the human                   pigmentation and grey hairs.45 Using specific markers, better
bulge region is thought to provide a degree of immune privi-             isolation and study of this population of stem cells may pro-
lege to stem cells.                                                      vide clues that are of help in the prevention of hair greying.
   Cancers contain cells with variable differentiation lineages             Using specific stem cell markers may also uncover stem cell
among which there seems to be a small population of cancer               populations that could serve as targets for gene therapy. Gene
stem cells that are needed to maintain the tumour mass.72                delivery to specific stem cell populations in the skin may in
As cancer stem cells share characteristics of self-renewal,              the future serve as a therapeutic modality for the correction of
increased proliferative capacity and multipotency with normal            several congenital disorders including hair diseases such as
stem cells, it has been hypothesized that cancer stem cells              ectodermal dysplasias, monilethrix, Netherton syndrome and
originate from mutated normal stem cells.72 Supporting this is           Menkes disease, as well as other genodermatoses such as
evidence demonstrating stem cell populations in cutaneous                hereditary epidermolysis bullosa.80,81
neoplasms, such as increased CK19 expression in squamous                    In addition, the identification and characterization of cancer
cell carcinoma, CK15 expression in hair follicle and seba-               stem cells within different cutaneous malignancies may
ceous gland carcinomas and nestin and CD34 expression in                 uncover key molecules as putative targets in therapies aimed
melanoma.7,73–77                                                         at inhibiting tumour growth.72,73
   The use of MITF has helped in the delineation of the migra-              Finally, the potential of CD200 expression within the
tory pathway of stem cells of melanocytic lineage with evi-              human bulge in providing immune privilege to bulge stem
dence indicating that melanocytes demonstrate progression                cells may have important therapeutic implications in scarring
from an intradermal to an intraepidermal to an intrafollicular           alopecias as inducing overexpression of CD200 in the bulge
localization.48 This may be of significance in understanding              area might be protective and therapeutic.37,39
the pathogenesis of intradermal melanocytic proliferation or
genodermatoses such as Waardenburg syndrome, in which
incomplete migration and persistence of melanocytes in the
dermis occurs.48                                                         Overall, some markers appear to be better than others in reli-
                                                                         ably identifying stem cells, an issue further compounded by
                                                                         the potential presence of more that one stem cell population
Prognostic implications
                                                                         in the skin. The use of stem cell markers is also proving to be
Expression of stem cells in cutaneous malignancies is not only           an important tool in better understanding the aetiopathogene-
increased, but may have prognostic implications.73 Signifi-               sis of many dermatological disorders as well as in providing
cantly higher CK19 expression has recently been demonstrated             prognostic information and more revolutionary therapeutic
in squamous cell carcinoma compared with actinic keratosis               options.

                                                                                                                           Ó 2009 The Authors
                           Journal Compilation Ó 2009 British Association of Dermatologists • British Journal of Dermatology 2009 161, pp228–236
                                                                       Potential uses of epidermal stem cells, O. Abbas and M. Mahalingam 235

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                                                                                                                               Ó 2009 The Authors
                              Journal Compilation Ó 2009 British Association of Dermatologists • British Journal of Dermatology 2009 161, pp228–236

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