6.

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Vol. 5, No. 2 2008
Drug Discovery Today: Disease Mechanisms

Editors-in-Chief
Toren Finkel – National Heart, Lung and Blood Institute, National Institutes of Health, USA
DRUG DISCOVERY Charles Lowenstein – The John Hopkins School of Medicine, Baltimore, USA
TODAY
DISEASE
MECHANISMS Common skin conditions and disorders

Hair physiology and its disorders

Kevin J. McElwee1,2,*, Rodney Sinclair3
1
Department of Dermatology and Skin Science, The University of British Columbia, 835 West Tenth Avenue, Vancouver, BC V5Z 4E8, Canada
2
Vancouver Coastal Health Research Institute, The University of British Columbia, Vancouver, BC, Canada
3
Department of Dermatology, St. Vincent’s Hospital, Melbourne, Fitzroy, Australia

Hair follicles are complex skin appendages, the pertur-

Section Editor:
bations of which have an impact on human health and Michael Roberts – School of Medicine, University of
emotional welfare disproportionate to their small Queensland, Australia
dimensions. Changes to the parameters of hair follicle
size, numbers per unit area of skin, and growth cycle
responsive vellus hair follicles are promoted into terminal
time duration determine hair coverage and fundamen- follicles [5].
tally underlie the diagnosis of an individual with alope- The perturbation and loss of hair follicles and alterations in
cia or hypertrichosis. Here we review the hair follicle’s hair fiber production in humans is generally not a life-threa-
tening event, but does have a dramatic impact on quality of
physiology, its disorders and principles of hair disorder
life and emotional wellbeing [6,7]. As such, hair disorders are
treatment development. a significant issue for many individuals. The hair follicle
physiology, changes during the development of hair disor-
ders and potential modes of treatment development are
Introduction
examined here.
The skin, the second largest organ in the body after the
skeleton, is of primary importance to the survival of mam-
Hair follicle development
malian life [1]. The skin structure is derived from the embryo-
Hair follicles first start to form in the skin of a human embryo
nic ectoderm and mesoderm, which give rise to the epidermis
from the 8th to 12th week of gestation. Hair follicle formation
and dermis, respectively [2]. Within these generalized layers
typically begins on the face and then progressively spreads
of the integument are specialized structures also derived from
ventrally and caudally over the body. The development and
the ectoderm and/or mesoderm including sensory nerves,
differentiation of hair follicles during embryogenesis is clas-
sweat glands and hair follicles [3].
sically divided into eight stages characterized by distinct
Humans have over 2 million hair follicles, which
morphologies [8].
collectively may have significant positive and negative
This formation of hair follicle appendages requires a com-
influence on skin health [4]. Most follicles produce short,
plex and currently poorly understood sequence of autocrine,
rapidly cycling, fine and nonpigmented vellus hairs. Eye-
paracrine and endocrine signals to occur both within and
brow, eyelash and scalp follicles produce long, slowly
between the epithelium and dermis. How these regulators
cycling, coarse, pigmented, terminal hairs from birth. How-
interact with each other, their relative significance, the
ever, follicles retain a degree of plasticity with respect to
degree of redundancy in the signaling system, and how these
the type of hair produced and a number can transform
signals determine the development of such a complex struc-
from vellus into terminal hair production and vice versa.
ture, its size, its distribution and subsequent growth cycle
This is particularly seen during puberty when androgen-
characteristics are still largely unknown [9]. However, multi-
*Corresponding author: K.J. McElwee (kevin@keratin.com) ple signaling and signal transduction pathways have been

1740-6765/$ ß 2008 Elsevier Ltd. All rights reserved. DOI: 10.1016/j.ddmec.2008.04.001 e163
 Drug Discovery Today: Disease Mechanisms | Common skin conditions and disorders Vol. 5, No. 2 2008

proven significant for the correct development and geogra- condensate of specialized mesenchymal cells has important
phical distribution of hair follicle formation through the skin inductive and hair growth regulatory properties [3]. In fact,
[10]. surgical removal of the dermal papilla and the lower dermal
Of particular note, Wnt gene products and associated sheath prevents hair growth, which indicates the importance
intracellular mediators of cell signaling, b-catenin and of these specialized mesenchymal cells as a key signaling
LEF1 have been shown to be fundamental to hair follicle center in hair follicles [16].
embryogenesis [11,12]. The subsequent cascade of activated
signaling pathways mediated by b-catenin and LEF1 include Hair follicle growth cycling
ectodysplasin A (EDA), noggin, transforming growth factor Three main phases of the growth cycle are recognized – a
beta 2 (TGFb2), TGFbR-II, b1 integrin and neural cell adhe- growth phase (anagen phase I–VI), regression phase (catagen)
sion molecule (NCAM) among others. Experimental induc- and resting phase (telogen) [4]. The time duration of each
tion of their respective expression induces hair follicle phase depends on the type and location of the hair follicle.
formation suggesting a significant role for these factors in Under physiological conditions, approximately 85% of scalp
hair follicle development. By contrast, multiple inhibitors of hair follicles are in anagen while 15% are in a telogen phase
hair follicle placode formation, such as bone morphogenic [5]. The duration of anagen in healthy scalp hair follicles is
protein (BMP)-2, BMP-4, p75NTR and activin ba are also typically two to six years and is the principle determinant of
expressed in the embryonic skin and may serve to regulate hair length. The anagen phase is followed by a short resting
the distribution pattern and size of hair follicles [10]. phase, catagen. Catagen is characterized by a cessation of
It has previously been asserted that no new follicles protein and pigment production, involution of the hair
develop after birth and that the immense regenerative capa- follicle and a fundamental restructuring of the extracellular
city within hair follicles serves only to induce anagen forma- matrix. In telogen, the hair follicle regresses to less than half
tion and epithelial repair following wounding. Recently some its anagen phase size. Morphologically, all that remains is a
hair follicle inductive capacity has been identified following peg of epithelial cells overlying a cluster of quiescent dermal
wounding and again Wnt expression seems important in papilla cells. Hair cycle synchronization is lost after the
determining cell fate as epithelial cells migrate out of hair second moult wave shortly after birth. Hereafter hairs cycle
follicles to participate in wound repair [13]. independently and seasonal moulting is not seen in humans
[3].
Hair follicle morphology Recent research suggests that hair fiber shedding is an
Hair follicles are formed with multiple epithelium- and active and highly controlled process, which differs from the
mesenchyme-derived cell layers each performing unique quiescence typically found throughout much of the telogen
functions and together comprising more than 20 distinct cell phase. The term ‘exogen’ has been introduced to identify
populations. The hair follicle, along with the sebaceous gland the hair fiber shedding event as a separate process during
and the arrector pili muscle, form the so-called ‘pilosebaceous hair follicle cycling [17]. The morphology of the hair root
unit’ [14]. The relative proportions of individual components suggests that the exogen process involves a proteolytic event
within the pilosebaceous unit vary between different hair in the cells of the telogen shaft base. The nature of this
follicle types but the fundamental structure essentially shedding process remains to be identified. However, desmo-
remains the same. In the scalp skin, terminal hair follicle glein 3 appears to have a role in maintaining telogen hair
units comprise groups of three to five individual follicles that anchorage within the follicle and loss of desmoglein 3 is
are surrounded by a fibrous sheath and may share a common associated with exogen [18]. Empty hair follicles after shed-
arrector pili muscle. ding of the hair fiber, but before the onset of renewed
The hair follicle can be divided into a permanent super- anagen, are in a stage termed ‘kenogen’. Kenogen can be
ficial structure and a transient cycling region, which includes observed in healthy skin, but the frequency and duration are
the hair bulb. The dividing line between the permanent and significantly greater in individuals with androgenetic alo-
transient parts of a hair follicle lies immediately below the pecia [19].
hair follicle bulge region and the insertion point of the Under normal physiological conditions, each hair follicle
arrector pili muscle. The bulge region has particular impor- will continue to cycle throughout life. These cycles are regu-
tance in hair follicle growth. It is a specialized compartment lated by specific changes in the local signaling milieu, based
of the outer root sheath in which epithelial and neurecto- on changes in the expression of cytokines, hormones, neu-
dermal stem cells are believed to reside [15]. rotransmitters and their receptors as well as transcription
Hair fiber and inner root sheath growth is a consequence of factors and enzymes, which act via endocrine, paracrine or
rapid proliferation of keratinocyte transit amplifying autocrine routes. What determines the clock mechanism and
cells that reside in the bulb region adjacent to the dermal the duration of anagen in individual hair follicles is not
papilla during an anagen growth phase. The dermal papilla known, although many hypotheses have been suggested [4].

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Hair growth and onset of a new anagen growth phase factors (FGFs) and hairless, a mediator of Wnt signaling [10]
recapitulates hair follicle embryogenesis and reutilizes simi- (Fig. 1).
lar molecular signaling pathways. As with hair follicle
embryogenesis, the primary initiators (as currently known) Hair biology applied to hair disorders
for the start of a new hair growth cycle and onset of anagen The biological properties of hair follicles contribute to our
involve Wnt/b-catenin signaling pathway interaction. WNT perceptions and definitions of what is a hair disorder. Alo-
proteins are strongly expressed in hair follicle bulge epithe- pecias or hypertrichoses are, in essence, an observation of too
lial cells adjacent to the dermal papilla at anagen onset. little or too much hair fiber over an area of skin significantly
Several studies have demonstrated that the perturbation of different from the norm as defined by society. In other words,
Wnt signaling inhibits cell proliferation and dermal pene- changes to the parameters of hair follicle number per unit
tration events subsequent to anagen initiation. Overexpres- area of skin, changes in hair follicle size and growth cycle
sion of Wnt and Wnt-associated proteins such as sonic time duration determine cosmetic hair coverage and
hedgehog (Shh) induces anagen in telogen stage hair follicles whether or not an individual has a hair disorder (Figs 2,3).
[20,21]. Many other molecules are believed to be involved in One or more of these factors may lie behind the development
hair cycle regulation including multiple fibroblast growth of a hair growth disorder and the net impact of involvement

初期
毛发生长初期的促进和维持
毛发生长中期到终期的促进和维持

早初期

中期

终期

Figure 1. Selected factors with known hair growth regulatory roles. Selected signaling factors with known anagen promotion or maintenance properties
(left) versus catagen–telogen promotion or maintenance properties (right) in the hair growth cycle. Anagen promoters: b-catenin; FGF2, fibroblast growth
factor 2; FGF7, fibroblast growth factor 7 (keratinocyte growth factor); HGF, hepatocyte growth factor; IGF1, insulin-like growth factor 1; LEF1, lymphoid
enhancer binding factor 1; MSP, macrophage stimulating factor; PDGF, platelet-derived growth factor; SHH, sonic hedgehog; TGFa, transforming growth
factor alpha; NOG, noggin; PRL, prolactin; VEGF, vascular endothelial growth factor; WNTs, multiple WNT factors. Catagen–telogen promoters: BDNF,
brain-derived neurotrophic factor; BMP-2, bone morphogenetic protein 2; BMP-4, bone morphogenetic protein 4; EGF, epidermal growth factor; FGF2,
fibroblast growth factor 2; FGF5, fibroblast growth factor 5; IL1, interleukin 1; IL6, interleukin 6; IFNg, interferon gamma; NT3, neurotrophin 3; OSM,
oncostatin M; PTH, parathyroid hormone; TAC1, substance P; TGFb1, transforming growth factor beta 1; TGFb2, transforming growth factor beta 2.

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Figure 2. Changes in hair follicle parameters associated with hair disorder development. All hair loss disorders can be reduced to three essential
mechanisms that involve hair follicle density, hair follicle size and hair growth cycle duration. Compared to a normal hair follicle density (a), a reduction in
the density of hair follicles per unit area can reduce the apparent coverage of skin by hair (b). A reduction in the size of the hair follicles in an area of skin can
also result in reduced hair coverage (c).

of more than one factor may be greater than the sum of the hair follicle formation does occur at a normal density, but the
parts. hair follicles fail to regenerate following their second or third
catagen. Congenital atrichia and Marie Unna hypotrichosis
Congenital disorders of hair growth are good examples where hair follicles form and grow hair in
Congenital disorders of hair growth are almost always genetic their first full cycle, but fail to survive and successfully enter
rather than environmental. Fundamentally a congenital alo- subsequent hair growth cycles. Such conditions involve
pecia or hypertrichosis is because of a problem in the correct mutations in one or more genes that are functionally sig-
embryogenic formation of hair follicles. As hair coverage is nificant for cohesive hair follicle structure maintenance. The
defined by hair follicle density, size and growth cycle, late onset, patterned destruction of hair follicles in Marie
embryogenic modifications to these parameters can result Unna hypotrichosis makes this condition unique.
in congenital hair growth disorders. Whether hair follicles are terminal or vellus in nature and
The most obvious embryogenic modification possible is a their number and distribution are at the root of congenital
reduction in the number of hair follicles formed per unit area hypertrichosis. In utero, babies are covered from head to toe
of skin. Aplasia cutis congenita, in which regions of skin fail by a uniform coat of fine lanugo hair about 1 cm in length.
to form correctly, is an example. Several forms of congenital Shortly before full-term the scalp hairs elongate into terminal
hypotrichosis may also involve a failure of development of hairs, the eyebrows remain unchanged, while the remaining
the full complement of hair follicles. However, more typically involute into vellus hairs. The expected norm is terminal hair

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Figure 3. Changes in hair growth cycles associated with hair disorder development. Changes to the hair growth cycle that elicit hair growth disorders can
take one or more of three key presentations. Compared to a normal hair growth cycle time course (a), changes to the hair growth cycle can occur such that
the duration of anagen is truncated yielding brief production of short and fine hair fibers (b). Equally, changes to the hair growth cycle can involve a
prolongation of the resting telogen phase after the hair from the previous growth cycle is shed (c).

follicle distribution restricted to the scalp and eyebrows and long, fine, unpigmented hair growth. Again, the density of
vellus hair follicles to cover the face and body in newborns. hair follicles has not been altered and the distribution of
When terminal hair follicles form beyond these limits and terminal and vellus hair follicles is correct. The problem is
take the place of vellus hair follicles on the face and else- one entirely based on an unusually long duration of anagen
where, the consequence is diagnosed as congenital hypertri- growth in facial and body vellus hair follicles. FGF5 knock-
chosis. In this event the density of hair follicles is presumed to out mice have an angora phenotype and are thought to be
be unaltered. Rather, the size of the hair follicle that develops the animal correlate of congenital hypertrichosis lanugi-
is inappropriate for the skin region. nosa [4].
Finally, the hair growth cycle duration can play a role.
Congenital hypertrichosis lanuginosa is a disorder in which Acquired disorders of hair growth
the duration of the anagen growth phase is prolonged Acquired disorders of hair growth are more complex in
beyond the norm in vellus hair follicles, the net result being their nature and the hair biology parameters involved.

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Fundamentally though, the three factors of hair follicle den- Principle biological objectives of hair disorder
sity, size and growth cycle parameters underlie acquired treatments
disorders. Hair disorders are fundamentally caused by the changes in
Which of these biological hair growth factors is predomi- hair follicle density, size and/or changes to the hair growth
nant varies with different diagnoses. In telogen effluvium, for cycle. The principle of any hair disorder treatment should be
example, changes to the hair growth cycle predominate. Hair to directly modulate one or more of these parameters. What
follicle density remains the same and a switch in hair follicle approach is taken depends in part on the nature of the hair
size from terminal to vellus does not occur. By contrast, growth disorder. Where the underlying cause of the changes
cicatricial alopecia development relies more or less exclu- to hair growth density, size and growth cycle is known, the
sively on loss of hair follicle density through follicle destruc- most obvious approach is to target the initiating disease
tion. mechanism. Treatment to remove the disease-initiating
The most common alopecia, androgenetic alopecia, pro- event, while not directly acting on the affected hair follicles,
duces a patterned baldness that is a good example of a may enable the damaged hair follicles to recover through
condition that involves complex changes in more than their inherent regenerative capacity. The range of possible
one biological parameter. Arguably, changes to hair growth hair disease initiating mechanisms involved varies signifi-
cycling are the dominant issue in early pattern baldness. The cantly from hormonal activity and stress, to inflammatory
development of pattern baldness involves a reduction in the responses and genetic mutation. The considerable hetero-
percentage of scalp hair follicles in anagen, a reduction in the geneity of hair disorder pathogeneses cannot be reviewed
anagen growth phase duration, and a corresponding increase here. However, given many hair disorders involve changes to
in the percentage of hair follicles in telogen and an increase in hair follicle distribution, size and growth cycle, it is poten-
the duration of telogen. These alterations, in combination tially possible to treat multiple disorders through the devel-
with a shift in the exogen phase from primarily occurring in opment of treatments modulating one or more of these
early anagen to initiation primarily during telogen, result in parameters.
thinning hair coverage [4].
The predominant biological hair growth factor involved Hair follicle density replication
also varies over time for the same diagnosis. In pattern For disorders in which hair follicle density is the predominant
baldness, while the initial dominant biological factor in concern, such as end stage androgenetic alopecia, scarring
the disorder is a change in hair growth cycling, later the alopecia or congenital hypotrichoses, the primary objective
switch in hair follicle size from terminal to vellus dominates. in new treatment development should be to generate new
Ultimately, some loss of hair follicle density occurs as vellus hair follicles from adult skin. On initial consideration such an
hair follicles are destroyed. This loss of hair follicle density idea might be rejected as absurd; however, a long history of
contributes less to a loss of meaningful hair coverage given evidence suggests such an event is possible. Variously termed
the hair follicles are vellus in nature, but the loss in hair hair replication, hair multiplication, hair cloning and follicular
follicle density reduces options for treating pattern bald- neogenesis, the creation of new hair follicles has been demon-
ness. If hair follicles survive in some form there is the strated. Cultured dermal papilla cells or dermal sheath ‘cup’
potential to correct their aberrant size or growth cycle. Once cells from the bulbar region of hair follicles can be implanted
they have gone, scalp reduction surgery, hair transplanta- and shown to regenerate new hair follicles as well as to
tion, or in the future hair follicle replication, are the only enlarge resident hair follicles [23]. This cell-based therapeutic
options. approach is currently an active area of commercial research
Alopecia areata, the second most common alopecia seen in by several companies.
dermatology clinics, is a particularly complex acquired hair Initiation of hair follicle formation in postnatal skin by
disorder. Loss of hair follicle immune privilege and exposure molecular and genetic manipulation might also be possible.
of hair follicle autoantigens are believed to lead to infiltration For some time it has been speculated that hair follicle for-
of inflammatory cells and premature termination of the mation could occur in response to wounding. Recent research
anagen phase, respectively. Transmission of the signal to has demonstrated that such an event can occur and is Wnt
neighboring follicles sets up a molt wave emanating from signaling dependent [13]. However, given the significant
the central focus that most commonly peters out and results potential danger of initiating unregulated skin neoplasia
in a circular patch of bald scalp. Hair follicle miniaturization growths with such an approach it remains to be seen whether
is a prominent feature of alopecia areata, however, although a molecular- or genetic-based treatment to induce hair folli-
the histology of these miniaturized hairs is indistinguishable cles to form is feasible. In practical terms, the development of
from those seen in androgenetic alopecia; these hairs main- drug-based treatments is more likely to focus on modifying
tain the capacity to rapidly revert into terminal hairs once the the size and growth cycle of resident hair follicles as opposed
inflammatory signals die out [22]. to inducing new hair follicle formation.

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Hair follicle size manipulation follicles to miniaturized vellus-like hair follicles correspond-
Hair follicle size determines the size of the hair fiber produced ingly results in a switch from terminal hair fiber to vellus hair
and hair follicle size has been linked to the size of the dermal fiber production. This switch in size seems to be the conse-
papilla. Larger dermal papillae dictate thicker hair fiber growth. quence of dermal papillae responding to androgen hormones
In androgenetic alopecia, the switch from large terminal hair with a reduction in cell numbers. Consequently, a focus of hair

Table 1. Targets and related therapies 干扰目标后期待的结果 作者 治疗临床试验 参考

目标和相关的治疗方法 Strategic approach Expected outcome Who is working Therapies in trial Refs
to target目标策略方法 of intervention at target on the target
Drug promotion Minoxidil-based treatments
Increased duration of McNeil-PPC, Inc. All trials complete, [24,27]
of hair growth in multiple topical
anagen hair growth 增加初期毛发的生长的持续时间 commercially available
药物促进毛发生长 and increased hair 和增加毛囊的大小
formulations 所有的试验都已经完成
可用于商业用途
米诺地尔治疗多发的局部形成 follicle size
Drug inhibition Oral finasteride inhibitor Reduced action of Merck & Co., Inc. All trials complete, [24,25]
of androgen- of Type II 5 alpha reductase dihydrotestosterone in commercially available
减少双氢睾酮在雄激素性脱发
mediated hair loss II型5α还原酶口服 androgenetic alopecia. 中的活性。增加初期毛发生长 所有的试验都已经完成
非那司提抑制剂 Increased duration of 的持续时间和增加毛囊的大小 可用于商业用途
雄性激素药物抑制剂
介导的毛发缺失 anagen hair growth
and increased hair
follicle size
Drug inhibition Blockade of androgen Reduced action of Multiple Limited clinical studies [29,30]
of androgen- production or androgen testosterone and complete,
mediated hair lossreceptors using dihydrotestosterone in commercially available
antiandrogen drugs androgenetic alopecia. 减少睾丸酮和双氢睾酮在雄激素性脱发 有限的临床研究已经完成，
中的活性。增加初期毛发生长的持续时间 可用于商业用途
(spironolactone, Increased duration of 和增加毛囊的大小
cyproterone acetate) anagen hair growth and
使用抗雄激素药物（螺旋内酯固醇，醋酸环丙氯地孕酮） increased hair follicle size
阻断雄性激素的产生或雄性激素受体
Drug inhibition Oral dutasteride inhibitor Reduced action of Glaxo Smith Kline, Inc. Phase III trials active [26]
of androgen- of Type I and Type II 5 dihydrotestosterone in
mediated hair loss alpha reductase androgenetic alopecia. 减少双氢睾酮在雄激素性脱发 III期临床试验有效
中的活性。增加毛发生长的持
I型和II型5α还原酶口服度他雄胺 Increased duration of 续时间和增加毛囊的大小
抑制剂 anagen hair growth and
increased hair follicle size
使用促效药激活Shh基因通路
Drug modification Activation of the sonic Increased duration of anagen Curis, Inc. Preclinical research [28]
of hair follicle size hedgehog (Shh) gene hair growth and increased 增加初期毛发生长的持续时间 临床前研究
和增加毛囊的大小
药物修饰毛囊的大小 pathway using agonists hair follicle size
Drug modification Ammonium trichloro Increased duration of anagen BioMas Ltd Phase II clinical trials [31]
of hair follicle size (dioxoethylene-O,O0 -) hair growth and increased 增加初期毛发生长的持续时间 planned for 2008
tellurate compounds hair follicle size 和增加毛囊的大小 计划于2008年临床II期试验
Drug modification Tetrapeptide aldehyde Increased duration of anagen Neosil, Inc. Phase II clinical trials ClinicalTrials.
of hair follicle size proteasome inhibitors hair growth and increased 增加初期毛发生长的持续时间 planned for 2008 gov Identifier:
和增加毛囊的大小
四肽乙醛蛋白酶体抑制剂 hair follicle size 计划于2008年临床II期试验 NCT00418730
Hair follicle formation New hair follicle Increased number of new Follica, Inc. Phase I clinical trials [13]
毛囊的形成 induction using hair follicles per unit area planned for 2008
promoters of WNT 利用WNT基因活化促进剂 增加单位区域新毛囊的数目 计划于2008年临床I期试验
gene activity 诱导新毛囊

Hair follicle Hair follicle replication Increased number of new Aderans Research, Inc. Pilot clinical trials [23,32]
regeneration and/or reactivation hair follicles and/or increase complete Pilot临床试验已经完成
毛囊的再生 using cell implantation size of miniaturized hair follicles Intercytex, Inc. Phase II clinical trials 计划于2008年临床II期
利用细胞移植使毛囊 planned for 2008 试验
增加新毛囊数目或者
复制或者是再生 增加小型化毛囊大小 Phoenix Bio Co., Ltd Unknown
Hair follicle Surgical transplantation Modification of hair follicle Multiple Available commercially [33]
redistribution of hair follicles from density in alopecic scalp regions 改善在脱发的头皮区域中 可用于商业用途
毛囊的重新分布 donor regions to 外科手术从供体的部位移植毛囊 毛囊密度
alopecic scalp skin 脱发的头皮皮肤中

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 Drug Discovery Today: Disease Mechanisms | Common skin conditions and disorders Vol. 5, No. 2 2008

disorder development research has been on preventing dermal ceutical industry despite the considerable commercial poten-
papilla cell responses to androgens or overcoming their tial. In part, the reluctance to enter the field may be because of
response by an increase in hair follicle growth activity. Finas- the complexity of the hair follicle unit and the many issues to
teride and dutasteride both block conversion of testosterone be addressed in the development of an effective treatment.
into the more biologically active derivative dihydrotestoester- Our understanding of how hair follicles function and why
one and administration to balding men arrests and partially changes to the hair follicle density, size and growth cycle
reverses hair follicle miniaturization [24–26] (Table 1). occur during disorder development is poorly understood. As a
Minoxidil is a hair loss product with the ability to increase result, no clear avenue of investigation has emerged. How-
the size of miniaturized hair follicles in addition to initiating ever, several biotech companies have been formed in recent
anagen growth in telogen stage hair follicles [24,27]. Its years, primarily driven by academic scientists with an interest
specific mechanism of action is unclear but as a potassium in hair biology. With time these companies may develop new
channel opener, it has a significant positive effect on dermal treatment approaches to hair loss and hypertrichosis.
papilla cells. By promoting increased dermal papilla cell
activity, minoxidil might increase the dermal papilla cell
Acknowledgements
signaling to the rest of the pilosebaceous unit resulting in
KJM is supported by the National Alopecia Areata Foundation
an increase in size. More recently, commercial development
(NAAF), the Canadian Dermatology Foundation (CDF), The
of Shh signaling as a method of increasing hair follicle size
Michael Smith Foundation for Health Research (MSFHR) and
and modifying the growth cycle has also been reported [28]. It
the Canadian Institutes of Health Research (CIHR). RS is
is probable that several other molecular approaches to hair
supported by the National Alopecia Areata Foundation
follicle enlargement remain to be discovered.
(NAAF), the National Health and Medical Research Council
(NHMRC), the Scientific Research Fund of the Australian
Hair follicle growth cycle modulation
College of Dermatologists and the Australian Dermatology
The key event in the hair growth cycle that contributes to
Research and Education Fund.
alopecia is the relationship of exogen hair shedding to anagen
and telogen. If exogen occurs in a typical sequence during early
anagen then old telogen hair fiber is shed but replaced quickly References
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