From telogen to exogen: mechanisms underlying formation and subsequent loss of the hair club fiber

Modelling Human Hair Follicles-Lessons from Animal Models and Beyond

The hair follicle is a specialized appendage of the skin that is critical for multiple functions, including thermoregulation, immune surveillance, and sebum production. Mammals are born with a fixed number of hair follicles that develop embryonically. Postnatally, these hair follicles undergo regenerative cycles of regression and growth that recapitulate many of the embryonic signaling pathways. Furthermore, hair cycles have a direct impact on skin regeneration in homeostasis, cutaneous wound healing, and disease conditions such as alopecia. Here, we review the current knowledge of hair follicle formation during embryonic development and the post-natal hair cycle, with an emphasis on the molecular signaling pathways underlying these processes. We then discuss efforts to capitalize on the field's understanding of in vivo mechanisms to bioengineer hair follicles or hair-bearing skin in vitro and how such models may be further improved to develop strategies for hair regeneration.

In vitro hair growth-promoting effect of Lgr5-binding octapeptide in human primary hair cells

BACKGROUND

Hair loss occurs due to various biological and environmental causes, which can have psychosocial consequences. The Wnt/β-catenin signaling is well-known for its role in hair growth and regeneration, as it induces the proliferation and differentiation of hair cells. When the leucine-rich G protein-coupled receptor 5 (Lgr5) interacts with the R-spondins, the frizzled receptor (FZD), a Wnt receptor, becomes stabilized, resulting in an increased β-catenin activity.

AIM

We investigated whether the octapeptide that binds to Lgr5 enhances proliferation and differentiation of human primary hair cells through the activation of Wnt/β-catenin signaling.

METHODS

The binding affinity of the octapeptide to Lgr5 was evaluated using surface plasmon resonance (SPR). We confirmed changes in proliferation and related factors like β-catenin activation and growth factors (GFs) expression in human hair follicle dermal papilla cells (HHFDPCs). Additionally, we observed the proliferation and the expression of differentiation markers in human hair follicle outer root sheath cells (HHFORSCs), human hair follicle germinal matrix cells (HHFGMCs), and human hair follicle stem cells (HHFSCs). We used three-dimensional HHFDPC spheroid culture treated with dihydrotestosterone (DHT) to create in vitro conditions that mimic androgenetic alopecia, and we studied the effects of octapeptide on Wnt expression and HHFSC differentiation.

RESULTS

The binding of the octapeptide to Lgr5 was confirmed using SPR analysis. In HHFDPCs, treatment with octapeptide resulted in a concentration-dependent increase in proliferation. We also observed increased nuclear translocation of β-catenin and increased expression of its downstream targets. HHFDPCs treated with octapeptide exhibited increased expression of growth factors and phosphorylation of Akt and ERK. In addition, we confirmed that octapeptide increased proliferation and induced differentiation in HHFORSCs, HHFGMCs, and HHFSCs. Under the HHFDPC spheroid culture conditions, we found that octapeptide restored the inhibition of Wnt-5a and Wnt-10b expressions by DHT. In HHFSCs treated with HHFDPC spheroid culture media, we observed that octapeptide recovered the inhibition of differentiation by DHT.

CONCLUSION

We found that octapeptides activated the Wnt/β-catenin signaling and induced the proliferation and differentiation of human primary hair cells by acting as an exogenous ligand for Lgr5. In addition, octapeptides recovered inhibited hair regeneration characters by DHT in androgenetic alopecia-mimic in vitro model. These findings suggest that octapeptides may be a promising therapeutic option for treating hair loss.

IGFBP-rP1 is a potential therapeutic target in androgenic alopecia

The available interventions for androgenic alopecia (AGA), the most common type of hair loss worldwide, remain limited. The insulin growth factor (IGF) system may play an important role in the pathogenesis of AGA. However, the exact role of IGF binding protein-related protein 1 (IGFBP-rP1) in hair growth and AGA has not been reported. In this study, we first found periodic variation in IGFBP-rP1 during the hair cycle transition in murine hair follicles (HFs). We further demonstrated that IGFBP-rP1 levels were lower in the serum and scalp HFs of individuals with AGA than in those of healthy controls. Subsequently, we verified that IGFBP-rP1 had no cytotoxicity to human outer root sheath cells (HORSCs) and that IGFBP-rP1 reversed the inhibitory effects of DHT on the migration of HORSCs in vitro. Finally, a DHT-induced AGA mouse model was created. The results revealed that the expression of IGFBP-rP1 in murine HFs was downregulated after DHT treatment and that subcutaneous injection of IGFBP-rP1 delayed catagen occurrence and prolonged the anagen phase of HFs in mice with DHT-induced AGA. The present work shows that IGFBP-rP1 is involved in hair cycle transition and exhibits great therapeutic potential for AGA.

Histological characteristics of hair follicles at different hair cycle and in vitro modeling of hair follicle-associated cells of yak (Bos grunniens)

To adapt to the extreme conditions of plateau environments, yaks have evolved thick hair, making them an ideal model for investigating the mechanisms involved in hair growth. We can gain valuable insights into how hair follicles develop and their cyclic growth in challenging environments by studying yaks. However, the lack of essential data on yak hair follicle histology and the absence of in vitro cell models for hair follicles serve as a limitation to such research objectives. In this study, we investigated the structure of skin tissue during different hair follicle cycles using the yak model. Additionally, we successfully established in vitro models of hair follicle-associated cells derived from yak skin, including dermal papilla cells (DPCs), preadipocytes, and fibroblasts. We optimized the microdissection technique for DPCs culture by simplifying the procedure and reducing the time required. Furthermore, we improved the methodology used to differentiate yak preadipocytes into mature adipocytes, thus increasing the differentiation efficiency. The introduction of yak as a natural model provides valuable research resources for exploring the mechanisms of hair growth and contributes to a deeper understanding of hair follicle biology and the development of regenerative medicine strategies.

Nanotechnology-based techniques for hair follicle regeneration

The hair follicle (HF) is a multicellular complex structure of the skin that contains a reservoir of multipotent stem cells. Traditional hair repair methods such as drug therapies, hair transplantation, and stem cell therapy have limitations. Advances in nanotechnology offer new approaches for HF regeneration, including controlled drug release and HF-specific targeting. Until recently, embryogenesis was thought to be the only mechanism for forming hair follicles. However, in recent years, the phenomenon of wound-induced hair neogenesis (WIHN) or de novo HF regeneration has gained attention as it can occur under certain conditions in wound beds. This review covers HF-specific targeting strategies, with particular emphasis on currently used nanotechnology-based strategies for both hair loss-related diseases and HF regeneration. HF regeneration is discussed in several modalities: modulation of the hair cycle, stimulation of progenitor cells and signaling pathways, tissue engineering, WIHN, and gene therapy. The HF has been identified as an ideal target for nanotechnology-based strategies for hair regeneration. However, some regulatory challenges may delay the development of HF regeneration nanotechnology based-strategies, which will be lastly discussed.

Basic principles of hair follicle structure, morphogenesis, and regeneration

Diseases affecting the hair follicle are common in domestic animals, but despite the importance of an intact skin barrier and a fully functional hair coat, knowledge about the detailed morphological features and the diversity of these complex mini-organs are often limited, although mandatory to evaluate skin biopsies with a history of alopecia. The factors that regulate the innate hair follicle formation and the postnatal hair cycle are still not completely understood in rodents, only rudimentarily known in humans, and are poorly understood in our companion animals. This review aims to summarize the current knowledge about hair follicle and hair shaft anatomy, the arrangement of hair follicles, hair follicle morphogenesis in the embryo, and the lifelong regeneration during the postnatal hair cycle in domestic animals. The role of follicular stem cells and the need for a multitude of interacting signaling events during hair follicle morphogenesis and regeneration is unquestioned. Because of the lack of state of the art methods that can be applied in rodents but are not feasible in companion animals, most of the information in this review is based on rodent studies. However, the few data from domestic animals that are available will be discussed, and it can be assumed that at least the principal molecular mechanisms are similar in rodents and other species.

Bioactive Compound-Loaded Nanocarriers for Hair Growth Promotion: Current Status and Future Perspectives

Hair loss (alopecia) has a multitude of causes, and the problem is still poorly defined. For curing alopecia, therapies are available in both natural and synthetic forms; however, natural remedies are gaining popularity due to the multiple effects of complex phytoconstituents on the scalp with fewer side effects. Evidence-based hair growth promotion by some plants has been reported for both traditional and advanced treatment approaches. Nanoarchitectonics may have the ability to evolve in the field of hair- and scalp-altering products and treatments, giving new qualities to hair that can be an effective protective layer or a technique to recover lost hair. This review will provide insights into several plant and herbal formulations that have been reported for the prevention of hair loss and stimulation of new hair growth. This review also focuses on the molecular mechanisms of hair growth/loss, several isolated phytoconstituents with hair growth-promoting properties, patents, in vivo evaluation of hair growth-promoting activity, and recent nanoarchitectonic technologies that have been explored for hair growth.

Male pattern hair loss: Can developmental origins explain the pattern?

Male pattern hair loss (MPHL), also referred to as male androgenetic alopecia (AGA) is the most common type of non-scarring progressive hair loss, with 80% of men suffering from this condition in their lifetime. In MPHL, the hair line recedes to a specific part of the scalp which cannot be accurately predicted. Hair is lost from the front, vertex, and the crown, yet temporal and occipital follicles remain. The visual effect of hair loss is due to hair follicle miniaturisation, where terminal hair follicles become dimensionally smaller. Miniaturisation is also characterised by a shortening of the growth phase of the hair cycle (anagen), and a prolongation of the dormant phase (kenogen). Together, these changes result in the production of thinner and shorter hair fibres, referred to as miniaturised or vellus hairs. It remains unclear why miniaturisation occurs in this specific pattern, with frontal follicles being susceptible while occipital follicles remain in a terminal state. One main factor we believe to be at play, which will be discussed in this viewpoint, is the developmental origin of the skin and hair follicle dermis on different regions of the scalp.

A Comprehensive Review of Mammalian Pigmentation: Paving the Way for Innovative Hair Colour-Changing Cosmetics

The natural colour of hair shafts is formed at the bulb of hair follicles, and it is coupled to the hair growth cycle. Three critical processes must happen for efficient pigmentation: (1) melanosome biogenesis in neural crest-derived melanocytes, (2) the biochemical synthesis of melanins (melanogenesis) inside melanosomes, and (3) the transfer of melanin granules to surrounding pre-cortical keratinocytes for their incorporation into nascent hair fibres. All these steps are under complex genetic control. The array of natural hair colour shades are ascribed to polymorphisms in several pigmentary genes. A myriad of factors acting via autocrine, paracrine, and endocrine mechanisms also contributes for hair colour diversity. Given the enormous social and cosmetic importance attributed to hair colour, hair dyeing is today a common practice. Nonetheless, the adverse effects of the long-term usage of such cosmetic procedures demand the development of new methods for colour change. In this context, case reports of hair lightening, darkening and repigmentation as a side-effect of the therapeutic usage of many drugs substantiate the possibility to tune hair colour by interfering with the biology of follicular pigmentary units. By scrutinizing mammalian pigmentation, this review pinpoints key targetable processes for the development of innovative cosmetics that can safely change the hair colour from the inside out.

Autoantigen Discovery in the Hair Loss Disorder, Alopecia Areata: Implication of Post-Translational Modifications

Alopecia areata (AA) is a chronic, multifactorial, polygenic, and heterogeneous disorder affecting growing hair follicles in susceptible individuals, which results in a non-scarring and reversible hair loss with a highly unpredictable course. Despite very considerable research effort, the nature of the precipitating factor(s) responsible for initiating AA in any given hair follicle remains unclear, due largely to significant gaps in our knowledge of the precise sequence of the etiopathogenic events in this dermatosis. However, disease-related changes in the immune-competence of the lower growing hair follicle, together with an active immune response (humoral and cellular) to hair follicle-associated antigens, are key associated phenomena. Confirmation of the hair follicle antigen(s) implicated in AA disease onset has remained stubbornly elusive. While it may be considered somewhat philosophical by some, it is also unclear whether immune-mediated hair loss in AA results from a) an ectopic (i.e., in an abnormal location) immune response to native (unmodified) self-antigens expressed by the healthy hair follicle, b) a normal immune response against modified self-antigens (or neoantigens), or c) a normal immune response against self-antigens (modified/non-modified) that were not previously visible to the immune system (because they were conformationally-hidden or sequestered) but become exposed and presentable in an MHC-I/-II molecule-restricted manner. While some candidate hair follicle antigen target(s) in AA are beginning to emerge, with a potential role for trichohyalin, it is not yet clear whether this represents the initial and immunodominant antigenic focus in AA or is simply one of an expanding repertoire of exposed hair follicle tissue damage-associated antigens that are secondary to the disease. Confirmation of autoantigen identity is essential for our understanding of AA etiopathogenesis, and consequently for developing a more informed therapeutic strategy. Major strides have been made in autoantigen discovery in other autoimmune conditions. In particular, some of these conditions may provide insights into how post-translational modifications (e.g., citrullination, deamidation, etc.) of hair follicle-restricted proteins may increase their antigenicity and so help drive the anti-hair follicle immune attack in AA.

An extract of Leontopodium alpinum inhibits catagen development ex vivo and increases hair density in vivo

Objectives

Hair loss and reduction in hair volume are hallmarks of hair disorders, such as telogen effluvium, or male or female pattern hair loss, and hair ageing, which can cause severe distress in both men and women. Common anti‐hair loss drugs carry some side effects; therefore, novel, safer approaches targeting milder phenotypes are highly advocated. In this context, we investigated an extract of the alpine plant Edelweiss, Leontopodium alpinum var. Helvetia, for its ability to modulate hair follicle (HF) growth ex vivo and inhibit hair loss while increasing hair regeneration in vivo.

Methods

Human amputated HFs were microdissected from three donors, two women and one man, and cultured ex vivo for 6 days. After treatment with 0.001% Edelweiss extract (EWDE), we investigated hair shaft production and anagen/catagen conversion, and measured known parameters associated with hair growth, that is hair matrix keratinocyte proliferation and apoptosis, dermal papilla inductivity, and growth factors, by quantitative (immuno)histomorphometry. To assess the anti‐hair loss potential of the alpine plant compound, we performed a randomized, placebo‐controlled human study enrolling Caucasian women and men, aged 18 to 65 years, with normal hair loss. After 5 months’ daily use of an extract containing leave‐on serum, we analysed hair density and anagen‐to‐catagen/telogen ratio by the Trichogram analysis.

Results

Our results revealed a significant prolongation in the anagen phase in HFs treated with 0.001% Edelweiss, as indicated by an increase in HFs remaining in anagen and a significant decrease in hair cycle score. In line with this effect, EWDE significantly stimulated hair matrix (HM) keratinocyte proliferation, and dermal papilla inductivity, as shown by a significant up‐regulation of versican expression and alkaline phosphatase activity, and a tendential increase in FGF7 immunoreactivity in the dermal papilla of all HFs or only anagen VI HFs. Corroborating the ex vivo results, we observed a significant increase in growing hair shaft numbers (hair density) after treatment with Edelweiss extract formulation, and a tendential up‐regulation in the anagen‐to‐catagen/telogen ratio.

Conclusions

We show here, through several lines of evidence, that the selected extract of the alpine plant Leontopodium alpinum var Helvetia (Edelweiss) inhibits premature catagen induction, possibly by stimulating dermal papilla inductivity. It is therefore worth exploiting this extract clinically as an anti‐hair loss agent, both for preventing ageing‐associated hair shedding and as an adjuvant therapy for hair loss disorders.

Deletion of hypoxia-inducible factor prolyl 4-hydroxylase 2 in FoxD1-lineage mesenchymal cells leads to congenital truncal alopecia

Hypoxia-inducible factors (HIFs) induce numerous genes regulating oxygen homeostasis. As oxygen sensors of the cells, the HIF prolyl 4-hydroxylases (HIF-P4Hs) regulate the stability of HIFs in an oxygen-dependent manner. During hair follicle (HF) morphogenesis and cycling, the location of dermal papilla (DP) alternates between the dermis and hypodermis and results in varying oxygen levels for the DP cells. These cells are known to express hypoxia-inducible genes, but the role of the hypoxia response pathway in HF development and homeostasis has not been studied. Using conditional gene targeting and analysis of hair morphogenesis, we show here that lack of Hif-p4h-2 in Forkhead box D1 (FoxD1)-lineage mesodermal cells interferes with the normal HF development in mice. FoxD1-lineage cells were found to be mainly mesenchymal cells located in the dermis of truncal skin, including those cells composing the DP of HFs. We found that upon Hif-p4h-2 inactivation, HF development was disturbed during the first catagen leading to formation of epithelial-lined HF cysts filled by unorganized keratins, which eventually manifested as truncal alopecia. Furthermore, the depletion of Hif-p4h-2 led to HIF stabilization and dysregulation of multiple genes involved in keratin formation, HF differentiation, and HIF, transforming growth factor β (TGF-β), and Notch signaling. We hypothesize that the failure of HF cycling is likely to be mechanistically caused by disruption of the interplay of the HIF, TGF-β, and Notch pathways. In summary, we show here for the first time that HIF-P4H-2 function in FoxD1-lineage cells is essential for the normal development and homeostasis of HFs.

Recreation of a hair follicle regenerative microenvironment: Successes and pitfalls

The hair follicle (HF) is an exquisite skin appendage endowed with cyclical regenerative capacity; however, de novo follicle formation does not naturally occur. Consequently, patients suffering from extensive skin damage or hair loss are deprived of the HF critical physiological and/or aesthetic functions, severally compromising skin function and the individual's psychosocial well-being. Translation of regenerative strategies has been prevented by the loss of trichogenic capacity that relevant cell populations undergo in culture and by the lack of suitable human-based in vitro testing platforms. Here, we provide a comprehensive overview of the major difficulties associated with HF regeneration and the approaches used to overcome these drawbacks. We describe key cellular requirements and discuss the importance of the HF extracellular matrix and associated signaling for HF regeneration. Finally, we summarize the strategies proposed so far to bioengineer human HF or hair-bearing skin models and disclose future trends for the field.

BEARD-A Potential Donor Site in Grade 6 and Grade 7 Alopecia: A Case Series

Background

Autologous hair transplantation has been the convention in cases of androgenic alopecia. Routinely, the occipital area serves as an ideal donor site. The advent of follicular unit extraction (FUE) has made body and beard hair harvest a possibility. Beard hair, in particular, has been far more sought after than other parts of the body.

Materials and Methods

A case series of 20 patients have been documented wherein cases with Grade 6 and 7 androgenic alopecia have been treated with beard hair as an adjunct donor site. The local anatomy, procedural technicalities and method of harvesting have been emphasized. The pre, intra and postoperative records have been maintained.

Discussion

The advent of FUE paved way for minimal downtime, better cosmesis and less scarring facilitating the possibility of using non-scalp hair in hair restoration, thus increasing the overall donor graft availability. Beard hair has its characteristic differences when compared to the scalp hair. Alongside there exists an array of advantages and disadvantages.

Conclusion

Minimal complications and potential advantages have encouraged the usage of beard graft in the recent past. In hindsight, beard to scalp transplantation is a worthwhile alternative in cases demanding an expanded source of donor hair which demands further literary contribution.

Dietary Vitamin A Impacts Refractory Telogen

Hair follicles cycle through periods of growth (anagen), regression (catagen), rest (telogen), and release (exogen). Telogen is further divided into refractory and competent telogen based on expression of bone morphogenetic protein 4 (BMP4) and wingless-related MMTV integration site 7A (WNT7A). During refractory telogen hair follicle stem cells (HFSC) are inhibited. Retinoic acid synthesis proteins localized to the hair follicle and this localization pattern changed throughout the hair cycle. In addition, excess retinyl esters arrested hair follicles in telogen. The purpose of this study was to further define these hair cycle changes. BMP4 and WNT7A expression was also used to distinguish refractory from competent telogen in C57BL/6J mice fed different levels of retinyl esters from two previous studies. These two studies produced opposite results; and differed in the amount of retinyl esters the dams consumed and the age of the mice when the different diet began. There were a greater percentage of hair follicles in refractory telogen both when mice were bred on an unpurified diet containing copious levels of retinyl esters (study 1) and consumed excess levels of retinyl esters starting at 12 weeks of age, as well as when mice were bred on a purified diet containing adequate levels of retinyl esters (study 2) and remained on this diet at 6 weeks of age. WNT7A expression was consistent with these results. Next, the localization of vitamin A metabolism proteins in the two stages of telogen was examined. Keratin 6 (KRT6) and cellular retinoic acid binding protein 2 (CRABP2) localized almost exclusively to refractory telogen hair follicles in study 1. However, KRT6 and CRABP2 localized to both competent and refractory telogen hair follicles in mice fed adequate and high levels of retinyl esters in study 2. In mice bred and fed an unpurified diet retinol dehydrogenase SDR16C5, retinal dehydrogenase 2 (ALDH1A2), and cytochrome p450 26B1 (CYP26B1), enzymes and proteins involved in RA metabolism, localized to BMP4 positive refractory telogen hair follicles. This suggests that vitamin A may contribute to the inhibition of HFSC during refractory telogen in a dose dependent manner.

Understanding Hormonal Therapies: Overview for the Dermatologist Focused on Hair

Hormones have an intimate relationship with hair growth. Hormonal replacement therapy is used to treat menopausal symptoms and to provide protection from chronic diseases for which postmenopausal women may be at risk. Additionally, hormonal therapies are prescribed for contraception and treatment of acne. Considering the widespread use of such therapies, there is a demand for further understanding of their implications in hair disorders. This article reviews the specific properties of current estrogen- and progesterone-containing hormonal treatments and their implications for the patient with hair loss. The complexity of the task comes from the paucity of data and discrepancy in the literature on the effect of the specific hormonal-receptor activities.

ANXA1 affects murine hair follicle growth through EGF signaling pathway

Annexin A1 (ANXA1), a calcium-dependent phospholipid binding protein expressed in animals, plants and microorganisms, participates in various cellular physiological activities. Previous proteomics analysis indicates that the level of ANXA1 in mice dorsal skin changes during hair growth cycle, we speculate that ANXA1 may play an important role in hair follicle (HF) development. Thus, Anxa1 knock-out (KO) and over-expression (OE) mice were constructed to test its function. Our results showed that in addition to the diameter of HF and hair shaft, ANXA1 could participate in hair growth by affecting the density of HF, and the proliferation of hair follicle stem cells (HFSCs). Meanwhile, molecular analysis showed that EGF signaling pathway is involved in the function of ANXA1. The expression of Anxa1 is negatively correlated with the levels of Egf, Notch1, Mkk7, and phosphorylated AKT1 and ERK/2 proteins. The levels of Egf, Notch1, Mkk7 and phosphorylation of AKT1 and ERK/2 increased in Anxa1 KO mice but decreased in Anxa1 OE mice. Taken together, our results suggested that ANXA1 could affect the hair growth by regulating the HFSCs proliferation through EGF signaling pathway.

Glutamine Metabolism Controls Stem Cell Fate Reversibility and Long-Term Maintenance in the Hair Follicle

Stem cells reside in specialized niches that are critical for their function. Upon activation, hair follicle stem cells (HFSCs) exit their niche to generate the outer root sheath (ORS), but a subset of ORS progeny returns to the niche to resume an SC state. Mechanisms of this fate reversibility are unclear. We show that the ability of ORS cells to return to the SC state requires suppression of a metabolic switch from glycolysis to oxidative phosphorylation and glutamine metabolism that occurs during early HFSC lineage progression. HFSC fate reversibility and glutamine metabolism are regulated by the mammalian target of rapamycin complex 2 (mTORC2)-Akt signaling axis within the niche. Deletion of mTORC2 results in a failure to re-establish the HFSC niche, defective hair follicle regeneration, and compromised long-term maintenance of HFSCs. These findings highlight the importance of spatiotemporal control of SC metabolic states in organ homeostasis.

Wanted, dead and alive: Why a multidisciplinary approach is needed to unlock hair treatment potential

Human recorded history is littered with attempts to improve the perceived appearance of scalp hair. Throughout history, treatments have included both biological and chemical interventions. Hair "quality" or "perceived appearance" is regulated by multiple biological intervention opportunities: adding more hairs by flipping follicles from telogen to anagen, or delaying anagen follicles transiting into catagen; altering hair "apparent amount" by modulating shaft diameter or shape; or, in principle, altering shaft physical properties changing its synthesis. By far the most common biological intervention strategy today is to increase the number of hairs, but to date this has proven difficult and has yielded minimal benefits. Chemical intervention primarily consists of active material surface deposition to improve shaft shine, fibre-fibre interactions and strength. Real, perceptible benefits will best be achieved by combining opportunity areas across the three primary sciences: biology, chemistry and physics. Shaft biogenesis begins with biology: proliferation in the germinative matrix, then crossing "Auber's Critical Line" and ceasing proliferation to synthesize shaft components. Biogenesis then shifts to oxidative chemistry, where previously synthesized components are organized and cross-linked into a shaft. We herein term the crossing point from biology to chemistry as "The Orwin Threshold." Historically, hair biology and chemistry have been conducted in different fields, with biological manipulation residing in biomedical communities and hair shaft chemistry and physics within the consumer care industry, with minimal cross-fertilization. Detailed understanding of hair shaft biogenesis should enable identification of factors necessary for optimum hair shaft production and new intervention opportunities.

Investigation into the utility of flying foxes as bioindicators for environmental metal pollution reveals evidence of diminished lead but significant cadmium exposure

Due to their large range across diverse habitats, flying-foxes are potential bioindicator species for environmental metal exposure. To test this hypothesis, blood spots, urine, fur, liver and kidney samples were collected from grey-headed flying-foxes (Pteropus poliocephalus) and black flying-foxes (P. alecto) from the Sydney basin, Australia. Concentrations of arsenic, cadmium, copper, lead, mercury and zinc and 11 other trace metals were determined using inductively coupled plasma mass spectrometry. As predicted, kidney and fur lead concentrations were lower compared to concentrations found in flying-foxes in the early 1990's, due to reduced environmental lead emissions. Tissue cadmium concentrations in flying-foxes were higher compared to previous studies of flying-foxes and other bat species, suggesting that flying-foxes were exposed to unrecognized cadmium sources. Identification of these sources should be a focus of future research. Urine concentrations of arsenic, cadmium, mercury, and lead were proportional to kidney concentrations. Given that urine can be collected from flying-foxes without handling, this demonstrates that many flying-foxes can be assessed for metal exposure with relative ease. The analysis of blood spots was not viable because of variable metal concentrations in the filter paper used. Fur concentrations of metals correlated poorly with tissue concentrations at the low levels of metals found in this study, but fur could still be a useful sample if flying-foxes are exposed to high levels of metals. Lastly, heat inactivation had minimal impact on metal concentrations in kidney and liver samples and should be considered as a tool to protect personnel working with biohazardous samples.

Molecular Mechanisms of Hair Growth and Regeneration: Current Understanding and Novel Paradigms

Hair is a defining feature of mammals and has critical functions, including protection, production of sebum, apocrine sweat and pheromones, social and sexual interactions, thermoregulation, and provision of stem cells for skin homeostasis, regeneration, and repair. The hair follicle (HF) is considered a "mini-organ," consisting of intricate and well-organized structures which originate from HF stem and progenitor cells. Dermal papilla cells are the main components of the mesenchymal compartments in the hair bulb and are instrumental in generating signals to regulate the behavior of neighboring epithelial cells during the hair cycle. Mesenchymal-epithelial interactions within the dermal papilla niche drive HF embryonic development as well as the postnatal hair growth and regeneration cycle. This review summarizes the current understanding of HF development, repair, and regeneration, with special focus on cell signaling pathways governing these processes. In particular, we discuss emerging paradigms of molecular signaling governing the dermal papilla-epithelial cellular interactions during hair growth and maintenance and the recent progress made towards tissue engineering of human hair follicles.

Immune cell regulation of the hair cycle

The ability to manipulate the mammalian hair cycle will lead to novel therapies and strategies to combat all forms of alopecia. Thus, in addition to the epithelial-mesenchymal interactions in the hair follicle, niche and microenvironmental signals that accompany the phases of growth, regression and rest need to be scrutinized. Immune cells are well described in skin homeostasis and wound healing and have recently been shown to play an important role in the mammalian hair cycle. In this review, we will summarize our current knowledge of the role of immune cells in hair cycle control and discuss their relevance to human hair cycling disorders. Increased attention to this aspect of the hair cycle will provide new avenues to manipulate hair regeneration in humans and provide better insight into developing better ex vivo models of hair growth.

Placebo-controlled dose-effect studies with topical minoxidil 2% or 5% in male-patterned hair loss treated with oral finasteride employing an analytical and exhaustive study protocol

Background

Drug trials for male‐pattern hair loss (MPHL) did not investigate hair cycling.

Materials and Methods

Male‐pattern hair loss volunteers (n = 22) took oral finasteride 1 mg daily with randomly either MTS5% or control lotion (1 mL/d). After 12 months on oral drug, 14 were randomized for a dose‐effect study of topical minoxidil 2% or 5%. Each 3‐month “on‐lotion” was followed by a 3‐month “off‐lotion.”

Results

Exogen release and anagen initiation from pre‐existing but functionally deficient follicles occurred mainly during combined dug treatment. Anagen initiation by topical minoxidil 5% could not be maintained by oral finasteride. As compared with control males, the compound index of hair growth raised from 30% at baseline up to 60% within 3 months of combined drug regimen which is better than oral drug only (no change) but still far beyond normalization of productivity (considered as 100%). There was no obvious transformation of miniaturized hair follicles into terminal hair‐producing follicles, and the activation of miniaturized hair follicles was not clinically relevant (slow growth and short duration of anagen).

Conclusions

Benefit with oral finasteride and topical 5% minoxidil (1 mL, 1 per day) resulted from initiation of anagen in deficient terminal follicles without increased growth rates.

Tissue engineering strategies for human hair follicle regeneration: How far from a hairy goal?

The demand for an efficient therapy for alopecia disease has fueled the hair research field in recent decades. However, despite significant improvements in the knowledge of key processes of hair follicle biology such as genesis and cycling, translation into hair follicle replacement therapies has not occurred. Great expectation has been recently put on hair follicle bioengineering, which is based on the development of fully functional hair follicles with cycling activity from an expanded population of hair‐inductive (trichogenic) cells. Most bioengineering approaches focus on in vitro reconstruction of folliculogenesis by manipulating key regulatory molecular/physical features of hair follicle growth/cycling in vivo. Despite their great potential, no cell‐based product is clinically available for hair regeneration therapy to date. This is mainly due to demanding issues that still hinder the functionality of cultured human hair cells. The present review comprehensively compares emergent strategies using different cell sources and tissue engineering approaches, aiming to successfully achieve a clinical cure for hair loss. The hurdles of these strategies are discussed, as well as the future directions to overcome the obstacles and fulfill the promise of a “hairy” feat.

Culture of the human pilosebaceous unit, hair follicle and sebaceous gland

Terence Kealey first pioneered the isolation and organ maintenance of human eccrine and sebaceous glands in the early to mid-1980. This led to subsequent methods describing the isolation and culture of human hair follicles, the human pilosebaceous unit as well as the sebaceous duct. The importance of these models in the study of the biology of human skin glands and appendages has been demonstrated in numerous publications and their importance as models for animal replacement, refinement and reduction (3Rs) is increasingly important. In particular, in vitro (ex vivo) hair follicle culture has played a significant part in helping elucidate the role of signalling molecules in regulating hair growth and hair fibre formation and has been especially useful in understanding metabolic aspects of hair growth. However, obtaining sufficient numbers of hair follicles is becoming increasingly difficult as plastic surgery becomes less invasive and smaller skin samples provided. There is therefore an urgent requirement for the next generation of in vitro models using cell lines and tissue engineering, and this has led to the development of immortalised cell lines as well as attempts to model hair follicle embryogenesis in vitro and development of skin on a chip.

Morroniside regulates hair growth and cycle transition via activation of the Wnt/β-catenin signaling pathway

Hair loss is characterized by a shortened hair anagen phase and hair follicles (HF) miniaturization. Morroniside is the most abundant iridoid glycoside extracted from Cornus officinalis and has various bioactivities in different cell functions and tissue regeneration. In this study, we investigated the effects and the underlying mechanism of morroniside on hair growth and regulation of HF cycle transition. Morroniside treatment significantly enhanced outer root sheath cell (ORSC) proliferation and migration in vitro. Additionally, morroniside upregulated Wnt10b, β-catenin and lef1. The enhanced ORSC proliferation and migration due to morroniside treatment were partly rescued by a Wnt/β-catenin signaling inhibitor, DKK1. Furthermore, in a hair-induced mouse model, morroniside injection accelerated the onset of anagen and delayed HF catagen, as shown by histological examination. Immunohistochemical analyses revealed that Wnt/β-catenin signaling pathway expression was upregulated in the HFs. These findings suggest that morroniside regulates HF growth and development partly through the Wnt/β-catenin signaling pathway and may be a potential treatment for hair loss.

A bald statement - Current approaches to manipulate miniaturisation focus only on promoting hair growth

Hair plays a large part in communication and society with its role changing through time and across cultures. Most people do not leave the house before combing their hair or shaving their beard and for many hair loss or irregular hair growth can have a significant impact on their psychological health. Somewhat unsurprisingly, according to GMR Data, today's global hair care industry is worth an estimated $87 Billion, with hair loss estimated at $2.8 Billion. Considering that no current hair loss-related products can completely reverse hair loss, it is reasonable to believe this market could expand significantly with the discovery of a comprehensive therapy. As such, a great deal of research focuses on overcoming hair loss, and in particular, a common form of hair loss known as androgenetic alopecia (AGA) or male pattern baldness. In AGA, hair follicles miniaturise in a large step change from a terminal to a vellus state. Within this viewpoint article, we discuss how influx and efflux of cells into and out from the dermal papilla (DP) can modulate DP size during the hair cycle. As DP size is positively correlated with the size of the hair fibre produced by a follicle, we argue here that therapies for treating AGA should be developed which can alter DP size, rather than just promote hair growth. We also discuss current therapeutics for AGA and emphasise the importance of using the right model systems to analyse miniaturisation.

Androgens trigger different growth responses in genetically identical human hair follicles in organ culture that reflect their epigenetic diversity in life

Male sex hormones-androgens-regulate male physique development. Without androgen signaling, genetic males appear female. During puberty, increasing androgens harness the hair follicle's unique regenerative ability to replace many tiny vellus hairs with larger, darker terminal hairs ( e.g., beard). Follicle response is epigenetically varied: some remain unaffected ( e.g., eyelashes) or are inhibited, causing balding. How sex steroid hormones alter such developmental processes is unclear, despite high incidences of hormone-driven cancer, hirsutism, and alopecia. Unfortunately, existing development models are not androgen sensitive. Here, we use hair follicles to establish an androgen-responsive human organ culture model. We show that women's intermediate facial follicles respond to men's higher androgen levels by synthesizing more hair over several days, unlike donor-matched, androgen-insensitive, terminal follicles. We demonstrate that androgen receptors-androgen-activated gene transcription regulators-are required and are present in vivo within these follicles. This is the first human organ that involves multiple cell types that responds appropriately to hormones in prolonged culture, in a way which mirrors its natural behavior. Thus, intermediate hair follicles offer a hormone-switchable human model with exceptional, unique availability of genetically identical, but epigenetically hormone-insensitive, terminal follicles. This should enable advances in understanding sex steroid hormone signaling, gene regulation, and developmental and regenerative systems and facilitate better therapies for hormone-dependent disorders.-Miranda, B. H., Charlesworth, M. R., Tobin, D. J., Sharpe, D. T., Randall, V. A. Androgens trigger different growth responses in genetically identical human hair follicles in organ culture that reflect their epigenetic diversity in life.

Novel insights into the pathways regulating the canine hair cycle and their deregulation in alopecia X

Alopecia X is a hair cycle arrest disorder in Pomeranians. Histologically, kenogen and telogen hair follicles predominate, whereas anagen follicles are sparse. The induction of anagen relies on the activation of hair follicle stem cells and their subsequent proliferation and differentiation. Stem cell function depends on finely tuned interactions of signaling molecules and transcription factors, which are not well defined in dogs. We performed transcriptome profiling on skin biopsies to analyze altered molecular pathways in alopecia X. Biopsies from five affected and four non-affected Pomeranians were investigated. Differential gene expression revealed a downregulation of key regulator genes of the Wnt (CTNNB1, LEF1, TCF3, WNT10B) and Shh (SHH, GLI1, SMO, PTCH2) pathways. In mice it has been shown that Wnt and Shh signaling results in stem cell activation and differentiation Thus our findings are in line with the lack of anagen hair follicles in dogs with Alopecia X. We also observed a significant downregulation of the stem cell markers SOX9, LHX2, LGR5, TCF7L1 and GLI1 whereas NFATc1, a quiescence marker, was upregulated in alopecia X. Moreover, genes coding for enzymes directly involved in the sex hormone metabolism (CYP1A1, CYP1B1, HSD17B14) were differentially regulated in alopecia X. These findings are in agreement with the so far proposed but not yet proven deregulation of the sex hormone metabolism in this disease.

The Hair Follicle: An Underutilized Source of Cells and Materials for Regenerative Medicine

The hair follicle is one of only two structures within the adult body that selectively degenerates and regenerates, making it an intriguing organ to study and use for regenerative medicine. Hair follicles have been shown to influence wound healing, angiogenesis, neurogenesis, and harbor distinct populations of stem cells; this has led to cells from the follicle being used in clinical trials for tendinosis and chronic ulcers. In addition, keratin produced by the follicle in the form of a hair fiber provides an abundant source of biomaterials for regenerative medicine. In this review, we provide an overview of the structure of a hair follicle, explain the role of the follicle in regulating the microenvironment of skin and the impact on wound healing, explore individual cell types of interest for regenerative medicine, and cover several applications of keratin-based biomaterials.

Paracrine Secreted Frizzled-Related Protein 4 Inhibits Melanocytes Differentiation in Hair Follicle

Wnt signaling plays crucial role in regulating melanocyte stem cells/melanocyte differentiation in the hair follicle. However, how the Wnt signaling is balanced to be overactivated to control follicular melanocytes behavior remains unknown. Here, by using immunofluorescence staining, we showed that secreted frizzled-related protein 4 (sFRP4) is preferentially expressed in the skin epidermal cells rather than in melanocytes. By overexpression of sFRP4 in skin cells in vivo and in vitro, we found that sFRP4 attenuates activation of Wnt signaling, resulting in decrease of melanocytes differentiation in the regenerating hair follicle. Our findings unveiled a new regulator that involves modulating melanocytes differentiation through a paracrine mechanism in hair follicle, supplying a hope for potential therapeutic application to treat skin pigmentation disorders.

The Hair Shedding Visual Scale: A Quick Tool to Assess Hair Loss in Women

INTRODUCTION

Hair shedding is a common consequence of the normal hair cycle that changes with internal and external factors. Female pattern hair loss (FPHL) is difficult to assess in terms of shedding severity as the conscious perception of hair shedding varies according to each individual, and most utilized methods are semi-invasive or very time consuming. In this study, we establish and validate a hair-shedding scale for women with thick hair of different lengths.

METHODS

A visual analog scale was developed for thick hair of short, medium, and long lengths by dividing a bundle of hairs of each length into nine piles of increasing hair amount that were then photographed and arranged in order of size. Twenty women with no FPHL with each length of hair (60 total) were asked to select the photographed hair bundle that best correlated with the amount of hair they shed on an average day. A total of 94 women with FPHL with excessive shedding were then asked to repeat the same process.

RESULTS

Women with no FPHL and short, medium and long hair had mean shedding scores of 2.5, 2.35 and 2.4, respectively. Women with FPHL and short, medium and long hair had mean shedding scores of 7.25, 7.0 and 7.14, respectively. Statistically significant Spearman's ρ coefficient and κ coefficient demonstrated correlation and inter-observer reliability.

CONCLUSION

Our results show that women with FPHL not only shed considerable hair more than women with no FPHL, but that this hair-shedding visual scale is a fast and effective method of evaluating hair-shedding amounts in an office setting.

A primer for studying cell cycle dynamics of the human hair follicle

The cell cycle is of major importance to human hair follicle (HF) biology. Not only is continuously active cell cycling required to facilitate healthy hair growth in anagen VI HFs, but perturbations in the cell cycle are likely to be of significance in HF pathology (i.e. in scarring, non-scarring, chemotherapy-induced and androgenic alopecias). However, cell cycle dynamics of the human hair follicle (HF) are poorly understood in contrast to what is known in mouse. The current Methods Review aims at helping to close this gap by presenting a primer that introduces immunohistological/immunofluorescent techniques to study the cell cycle in the human HF. Moreover, this primer encourages the exploitation of the human HF as a powerful and clinically relevant tool to investigate mammalian cell cycle biology in situ. To achieve this, we describe methods to study markers of general 'proliferation' (nuclei count, Ki-67 expression), apoptosis (terminal deoxynucleotidyl transferase dUTP nick-end labelling, cleaved caspase 3), mitosis (phospho-histone H3, 'pS780'), DNA synthesis (5-ethynyl-2'-deoxyuridine) and cell cycle regulation (cyclins) in the human HF. In addition, we provide specific examples of dual immunolabelling for instructive cell cycle analyses and for investigating the cell cycle behaviour of specific HF keratinocyte subpopulations, such as keratin 15+ stem/progenitor cells.

The Hair Follicle: A Comparative Review of Canine Hair Follicle Anatomy and Physiology

The hair follicle (HF) has a wide range of functions including thermoregulation, physical and immunological protection against external insults, sensory perception, social interactions, and camouflage. One of the most characteristic features of HFs is that they self-renew during hair cycle (HC) throughout the entire life of an individual to continuously produce new hair. HC disturbances are common in humans and comparable to some alopecic disorders in dogs. A normal HC is maintained by follicular stem cells (SCs), which are predominately found in an area known as the bulge. Due to similar morphological characteristics of the human and canine bulge area, the particularity of compound HFs in humans and dogs as well as similarities in follicular biomarker expression, the dog might be a promising model to study human HC and SC disorders. In this review, we give an overview of normal follicular anatomy, the HC, and follicular SCs and discuss the possible pathogenetic mechanisms of noninflammatory alopecia.

FOXC1 maintains the hair follicle stem cell niche and governs stem cell quiescence to preserve long-term tissue-regenerating potential

Adult tissue stem cells (SCs) reside in niches, which orchestrate SC behavior. SCs are typically used sparingly and exist in quiescence unless activated for tissue growth. Whether parsimonious SC use is essential to conserve long-term tissue-regenerating potential during normal homeostasis remains poorly understood. Here, we examine this issue by conditionally ablating a key transcription factor Forkhead box C1 (FOXC1) expressed in hair follicle SCs (HFSCs). FOXC1-deficient HFSCs spend less time in quiescence, leading to markedly shortened resting periods between hair cycles. The enhanced hair cycling accelerates HFSC expenditure, and impacts hair regeneration in aging mice. Interestingly, although FOXC1-deficient HFs can still form a new bulge that houses HFSCs for the next hair cycle, the older bulge is left unanchored. As the new hair emerges, the entire old bulge, including its reserve HFSCs and SC-inhibitory inner cell layer, is lost. We trace this mechanism first, to a marked increase in cell cycle-associated transcripts upon Foxc1 ablation, and second, to a downstream reduction in E-cadherin-mediated inter-SC adhesion. Finally, we show that when the old bulge is lost with each hair cycle, overall levels of SC-inhibitory factors are reduced, further lowering the threshold for HFSC activity. Taken together, our findings suggest that HFSCs have restricted potential in vivo, which they conserve by coupling quiescence to adhesion-mediated niche maintenance, thereby achieving long-term tissue homeostasis.

A Guide to Studying Human Hair Follicle Cycling In Vivo

Hair follicles (HFs) undergo lifelong cyclical transformations, progressing through stages of rapid growth (anagen), regression (catagen), and relative "quiescence" (telogen). Given that HF cycling abnormalities underlie many human hair growth disorders, the accurate classification of individual cycle stages within skin biopsies is clinically important and essential for hair research. For preclinical human hair research purposes, human scalp skin can be xenografted onto immunocompromised mice to study human HF cycling and manipulate long-lasting anagen in vivo. Although available for mice, a comprehensive guide on how to recognize different human hair cycle stages in vivo is lacking. In this article, we present such a guide, which uses objective, well-defined, and reproducible criteria, and integrates simple morphological indicators with advanced, (immuno)-histochemical markers. This guide also characterizes human HF cycling in xenografts and highlights the utility of this model for in vivo hair research. Detailed schematic drawings and representative micrographs provide examples of how best to identify human HF stages, even in suboptimally sectioned tissue, and practical recommendations are given for designing human-on-mouse hair cycle experiments. Thus, this guide seeks to offer a benchmark for human hair cycle stage classification, for both hair research experts and newcomers to the field.

Skin Diseases in Laboratory Mice: Approaches to Drug Target Identification and Efficacy Screening

A large variety of mouse models for human skin, hair, and nail diseases are readily available from investigators and vendors worldwide. Mouse skin is a simple organ to observe lesions and their response to therapy, but identifying and monitoring the progress of treatments of mouse skin diseases can still be challenging. This chapter provides an overview on how to use the laboratory mouse as a preclinical tool to evaluate efficacy of new compounds or test potential new uses for compounds approved for use for treating an unrelated disease. Basic approaches to handling mice, applying compounds, and quantifying effects of the treatment are presented.

Resting no more: re-defining telogen, the maintenance stage of the hair growth cycle

The hair follicle (HF) represents a prototypic ectodermal-mesodermal interaction system in which central questions of modern biology can be studied. A unique feature of these stem-cell-rich mini-organs is that they undergo life-long, cyclic transformations between stages of active regeneration (anagen), apoptotic involution (catagen), and relative proliferative quiescence (telogen). Due to the low proliferation rate and small size of the HF during telogen, this stage was conventionally thought of as a stage of dormancy. However, multiple lines of newly emerging evidence show that HFs during telogen are anything but dormant. Here, we emphasize that telogen is a highly energy-efficient default state of the mammalian coat, whose function centres around maintenance of the hair fibre and prompt responses to its loss. While actively retaining hair fibres with minimal energy expenditure, telogen HFs can launch a new regeneration cycle in response to a variety of stimuli originating in their autonomous micro-environment (including its stem cell niche) as well as in their external tissue macro-environment. Regenerative responses of telogen HFs change as a function of time and can be divided into two sub-stages: early 'refractory' and late 'competent' telogen. These changing activities are reflected in hundreds of dynamically regulated genes in telogen skin, possibly aimed at establishing a fast response-signalling environment to trauma and other disturbances of skin homeostasis. Furthermore, telogen is an interpreter of circadian output in the timing of anagen initiation and the key stage during which the subsequent organ regeneration (anagen) is actively prepared by suppressing molecular brakes on hair growth while activating pro-regenerative signals. Thus, telogen may serve as an excellent model system for dissecting signalling and cellular interactions that precede the active 'regenerative mode' of tissue remodeling. This revised understanding of telogen biology also points to intriguing new therapeutic avenues in the management of common human hair growth disorders.

Identification of genes important for cutaneous function revealed by a large scale reverse genetic screen in the mouse

The skin is a highly regenerative organ which plays critical roles in protecting the body and sensing its environment. Consequently, morbidity and mortality associated with skin defects represent a significant health issue. To identify genes important in skin development and homeostasis, we have applied a high throughput, multi-parameter phenotype screen to the conditional targeted mutant mice generated by the Wellcome Trust Sanger Institute's Mouse Genetics Project (Sanger-MGP). A total of 562 different mouse lines were subjected to a variety of tests assessing cutaneous expression, macroscopic clinical disease, histological change, hair follicle cycling, and aberrant marker expression. Cutaneous lesions were associated with mutations in 23 different genes. Many of these were not previously associated with skin disease in the organ (Mysm1, Vangl1, Trpc4ap, Nom1, Sparc, Farp2, and Prkab1), while others were ascribed new cutaneous functions on the basis of the screening approach (Krt76, Lrig1, Myo5a, Nsun2, and Nf1). The integration of these skin specific screening protocols into the Sanger-MGP primary phenotyping pipelines marks the largest reported reverse genetic screen undertaken in any organ and defines approaches to maximise the productivity of future projects of this nature, while flagging genes for further characterisation.

Estrone sulfate source of estrone and estradiol formation in isolated human hair roots: identification of a pathway linked to hair growth phase and subject to site-, gender-, and age-related modulations

BACKGROUND

The present study investigated the metabolism of estrone sulfate into bioactive estrogens in the human hair root, including the effects of hair growth phase, anatomical site, gender, and age.

METHODS

Healthy male (n = 18) and female (n = 20) subjects were investigated. Growing (anagen) and resting (telogen) hair roots were collected from selected scalp and body sites.

RESULTS

Estrone sulfate metabolism in the hair root yielded substantial levels of estrone and estradiol. Estrogen synthesis exceeded that associated with aromatization of androgens in a previous study. In subjects <50 years old, estrogen synthesis in scalp hair was lower in men than in women. Comparable levels of estrogen formation were observed in 1) male and female axillary and pubic hair and 2) male beard hair. These levels were higher than the estrogen levels detected in the in scalp hair of men <50 years old. With increasing age, estrogen synthesis increased in men and decreased in women. In telogen hair from all body sites, the capacity to form estrone from estrone sulfate remained unaffected, whereas the ability to form estradiol decreased by 62% and 86% in men and women, respectively.

CONCLUSIONS

Estrogen formation from estrone sulfate in sexually dimorphic hair is linked to the hair growth phase and is subject to gender- and age-related modulations. The magnitude of the in situ estrogen synthesis from estrone sulfate and the selective arrest of estradiol synthesis at the end of the hair cycle suggest that this pathway plays a crucial role in the regulation of human hair growth.

Hair curvature: a natural dialectic and review

Although hair forms (straight, curly, wavy, etc.) are present in apparently infinite variations, each fibre can be reduced to a finite sequence of tandem segments of just three types: straight, bent/curly, or twisted. Hair forms can thus be regarded as resulting from genetic pathways that induce, reverse or modulate these basic curvature modes. However, physical interconversions between twists and curls demonstrate that strict one-to-one correspondences between them and their genetic causes do not exist. Current hair-curvature theories do not distinguish between bending and twisting mechanisms. We here introduce a multiple papillary centres (MPC) model which is particularly suitable to explain twisting. The model combines previously known features of hair cross-sectional morphology with partially/completely separated dermal papillae within single follicles, and requires such papillae to induce differential growth rates of hair cortical material in their immediate neighbourhoods. The MPC model can further help to explain other, poorly understood, aspects of hair growth and morphology. Separate bending and twisting mechanisms would be preferentially affected at the major or minor ellipsoidal sides of fibres, respectively, and together they exhaust the possibilities for influencing hair-form phenotypes. As such they suggest dialectic for hair-curvature development. We define a natural-dialectic (ND) which could take advantage of speculative aspects of dialectic, but would verify its input data and results by experimental methods. We use this as a top-down approach to first define routes by which hair bending or twisting may be brought about and then review evidence in support of such routes. In particular we consider the wingless (Wnt) and mammalian target of rapamycin (mTOR) pathways as paradigm pathways for molecular hair bending and twisting mechanisms, respectively. In addition to the Wnt canonical pathway, the Wnt/Ca(2+) and planar cell polarity (PCP) pathways, and others, can explain many alternatives and specific variations of hair bending phenotypes. Mechanisms for hair papilla budding or its division by bisection or fission can explain MPC formation. Epithelial-to-mesenchymal (EMT) and mesenchymal-to-epithelial (MET) transitions, acting in collaboration with epithelial-mesenchymal communications are also considered as mechanisms affecting hair growth and its bending and twisting. These may be treated as sub-mechanisms of an overall development from neural-crest stem cell (NCSC) lineages to differentiated hair follicle (HF) cell types, thus providing a unified framework for hair growth and development.

Alterations in hair follicle dynamics in women

Endocrine changes supervening after parturition and menopause participate in the control of sebum production and hair growth modulation. The ensuing conditions include some peculiar aspects of hair loss (effluvium), alopecia, and facial hirsutism. The hair cycling is of major clinical relevance because most hair growth disorders result from disturbances in this chronobiological feature. Of note, any correlation between a biologic abnormality and hair cycling disturbance does not prove a relationship of causality. The proportion of postmenopausal women is rising in the overall population. Therefore, the prevalence of these hair follicle disturbances is globally on the rise. Current therapies aim at correcting the underlying hormonal imbalances, and at improving the overall cosmetic appearance. However, in absence of pathogenic diagnosis and causality criteria, chances are low that a treatment given by the whims of fate will adequately control hair effluvium. The risk and frequency of therapeutic inertia are further increased. When the hair loss is not controlled and/or compensated by growth of new hairs, several clinical aspects of alopecia inexorably develop. Currently, there is little evidence supporting any specific treatment for these endocrine hair disorders in post-partum and postmenopausal women. Current hair treatment strategies are symptomatic and nonspecific so current researchers aim at developing new, targeted methods.

Protease activity, localization and inhibition in the human hair follicle

Objective

In humans, the process of hair shedding, referred to as exogen, is believed to occur independently of the other hair cycle phases. Although the actual mechanisms involved in hair shedding are not fully known, it has been hypothesized that the processes leading to the final step of hair shedding may be driven by proteases and/or protease inhibitor activity. In this study, we investigated the presence of proteases and protease activity in naturally shed human hairs and assessed enzyme inhibition activity of test materials.

Methods

We measured enzyme activity using a fluorescence-based assay and protein localization by indirect immunohistochemistry (IHC). We also developed an ex vivo skin model for measuring the force required to pull hair fibres from skin.

Results

Our data demonstrate the presence of protease activity in the tissue material surrounding club roots. We also demonstrated the localization of specific serine protease protein expression in human hair follicle by IHC. These data provide evidence demonstrating the presence of proteases around the hair club roots, which may play a role during exogen. We further tested the hypothesis that a novel protease inhibitor system (combination of Trichogen® and climbazole) could inhibit protease activity in hair fibre club root extracts collected from a range of ethnic groups (UK, Brazil, China, first-generation Mexicans in the USA, Thailand and Turkey) in both males and females. Furthermore, we demonstrated that this combination is capable of increasing the force required to remove hair in an ex vivo skin model system.

Conclusion

These studies indicate the presence of proteolytic activity in the tissue surrounding the human hair club root and show that it is possible to inhibit this activity with a combination of Trichogen® and climbazole. This technology may have potential to reduce excessive hair shedding.

Résumé

Hopx expression defines a subset of multipotent hair follicle stem cells and a progenitor population primed to give rise to K6+ niche cells

The mammalian hair follicle relies on adult resident stem cells and their progeny to fuel and maintain hair growth throughout the life of an organism. The cyclical and initially synchronous nature of hair growth makes the hair follicle an ideal system with which to define homeostatic mechanisms of an adult stem cell population. Recently, we demonstrated that Hopx is a specific marker of intestinal stem cells. Here, we show that Hopx specifically labels long-lived hair follicle stem cells residing in the telogen basal bulge. Hopx(+) cells contribute to all lineages of the mature hair follicle and to the interfollicular epidermis upon epidermal wounding. Unexpectedly, our analysis identifies a previously unappreciated progenitor population that resides in the lower hair bulb of anagen-phase follicles and expresses Hopx. These cells co-express Lgr5, do not express Shh and escape catagen-induced apoptosis. They ultimately differentiate into the cytokeratin 6-positive (K6) inner bulge cells in telogen, which regulate the quiescence of adjacent hair follicle stem cells. Although previous studies have suggested that K6(+) cells arise from Lgr5-expressing lower outer root sheath cells in anagen, our studies indicate an alternative origin, and a novel role for Hopx-expressing lower hair bulb progenitor cells in contributing to stem cell homeostasis.