The hair follicle enigma

Changing human hair fibre colour and shape from the follicle

INTRODUCTION

Natural hair curvature and colour are genetically determined human traits, that we intentionally change by applying thermal and chemical treatments to the fibre. Presently, those cosmetic methodologies act externally and their recurrent use is quite detrimental to hair fibre quality and even to our health.

OBJECTIVES

This work represents a disruptive concept to modify natural hair colour and curvature. We aim to model the fibre phenotype as it is actively produced in the follicle through the topical delivery of specific bioactive molecules to the scalp.

METHODS

Transcriptome differences between curly and straight hairs were identified by microarray. In scalp samples, the most variable transcripts were mapped by in situ hybridization. Then, by using appropriate cellular models, we screened a chemical library of 1200 generic drugs, searching for molecules that could lead to changes in either fibre colour or curvature. A pilot-scale, single-centre, investigator-initiated, prospective, blind, bilateral (split-scalp) placebo-controlled clinical study with the intervention of cosmetics was conducted to obtain a proof of concept (RNEC n.92938).

RESULTS

We found 85 genes transcribed significantly different between curly and straight hair, not previously associated with this human trait. Next, we mapped some of the most variable genes to the inner root sheath of follicles, reinforcing the role of this cell layer in fibre shape moulding. From the drug library screening, we selected 3 and 4 hits as modulators of melanin synthesis and gene transcription, respectively, to be further tested in 33 volunteers. The intentional specific hair change occurred: 8 of 14 volunteers exhibited colour changes, and 16 of 19 volunteers presented curvature modifications, by the end of the study.

CONCLUSION

The promising results obtained are the first step towards future cosmetics, complementary or alternative to current methodologies, taking hair styling to a new level: changing hair from the inside out.

Incorporation of hair follicles in 3D bioprinted models of human skin

Current approaches fail to adequately introduce complex adnexal structures such as hair follicles within tissue engineered models of skin. Here, we report on the use of 3D bioprinting to incorporate these structures in engineered skin tissues. Spheroids, induced by printing dermal papilla cells (DPCs) and human umbilical vein cells (HUVECs), were precisely printed within a pregelled dermal layer containing fibroblasts. The resulting tissue developed hair follicle-like structures upon maturation, supported by migration of keratinocytes and melanocytes, and their morphology and composition grossly mimicked that of the native skin tissue. Reconstructed skin models with increased complexity that better mimic native adnexal structures can have a substantial impact on regenerative medicine as grafts and efficacy models to test the safety of chemical compounds.

Exploration of potential lipid biomarkers for age-induced hair graying by lipidomic analyses of hair shaft roots with follicular tissue attached

BACKGROUND

Age-induced hair graying (AIHG)is one of the visual hallmarks of aging, but its biological mechanism remains unclear. Changes in the hair-follicle lipid profiles associated with AIHG have not been defined.

OBJECTIVES

To define the differences in the hair follicle lipid profiles of female black and gray/white hair follicles.

METHODS

The lipid profile of hair follicles was determined by ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS). Multivariate data analysis was used to determine changes in the lipid profiles in hair follicle roots.

RESULTS

We identified the different lipids in hair follicle roots between black hair (HB) and white hair (HW) and analyzed the key lipids that contribute to the development of HW. The results showed that the total lipid content of the HW was significantly reduced. There were significant differences in sphingolipid content, with HB higher than HW. Two subclasses, glucosylceramide (GlcCer) and galactosylceramide, were significantly different. GlcCer, phosphatidylserine, and phosphatidic acid levels were higher in the HB group. The sphingolipid metabolism involved in GlcCer(d18:1/24:1[15Z]) is a statistically significant lipid metabolic pathway.

CONCLUSION

Five major individual lipid candidates are involved in the production of AIHG. GlcCer shows a significant reduction in HW and is a potential target for further research into AIHG.

Cucurbitacin promotes hair growth in mice by inhibiting the expression of fibroblast growth factor 18

BACKGROUND

The inhibition of fibroblast growth factor 18 (FGF18) promotes the transition of hair follicles (HFs) from the telogen phase to the anagen phase. Cucurbitacin has been shown to have a good effect in promoting hair cell growth. This study explored the potential effect of cucurbitacin on hair growth and its effect on FGF18 expression in mice.

METHODS

Male C57BL/6J mice were randomly divided into the following two groups: (I) the vehicle group; and (II) the cucurbitacin group. Matrix cream and cucurbitacin cream were applied to the depilated skin on the back of the vehicle group mice and the cucurbitacin group mice, respectively. On days 3, 6, 9, 12, 15, and 18, the hair growth in the depilated dorsal skin of the mice was recorded with a digital camera and a HF detector, and the HF cycle status of the mice was observed by hematoxylin and eosin (H&E) staining. In addition, the level of FGF18 messenger ribonucleic acid (mRNA) in the dorsal skin was measured on days 15 and 18 by quantitative real-time polymerase chain reaction (qRT-PCR), while the level of FGF18 protein was measured by western blot and immunofluorescence staining.

RESULTS

The dorsal skin to which the cucurbitacin cream was applied began to darken on day 6 and grew hairs on day 9, which was 3 days earlier than the dorsal skin to which the matrix cream was applied. The H&E staining revealed a transition from the telogen phase to the anagen phase 3 days earlier for the cucurbitacin cream-treated skin than the matrix cream-treated skin. In addition, the skin treated with cucurbitacin cream also showed a significant decrease in FGF18 mRNA as seen by qRT-PCR, and reduced FGF18 protein levels as detected by western blot and immunofluorescence staining compared to the skin treated with matrix cream only.

CONCLUSIONS

Cucurbitacin significantly reduced the levels of FGF18 mRNA and protein in the dorsal skin of mice to accelerate the HFs to enter the anagen phase earlier, thereby promoting the regeneration of hair. Thus, cucurbitacin can be considered a new and valuable agent for the development of anti-hair loss products.

Exosome Therapy in Hair Regeneration: A literature review of the evidence, challenges, and future opportunities

BACKGROUND

Alopecia is a common chief complaint and is challenging to treat. As such, regenerative treatments to promote hair growth are an emerging area of research. Exosomes, which are extracellular vesicles involved in cell communication, homeostasis, differentiation, and organogenesis, have been shown to play a central role in hair morphogenesis and regeneration with potential for use as alopecia treatment.

AIMS

This review summarizes and assesses the body of literature surrounding exosomes as regenerative therapeutics for alopecia and identifies areas for improvement in future research.

METHODS

A review was conducted using a comprehensive list of keywords including "exosome," "alopecia," and "hair loss" on PubMed, EMBASE, and Google Scholar databases published from inception to February 2022. Reference lists of identified articles were included. 47 studies were included. Clinical trial databases were searched using the term "exosome," however no trials relevant to hair growth were identified.

RESULTS

Our updated and comprehensive review details the history of exosome use in medicine, postulated underlying mechanisms in treating hair loss, and current clinical studies. Preclinical studies demonstrate clear benefits of exosome therapeutics in regenerative medicine and for hair loss treatment. Clinical trials demonstrate safety of exosome use in medicine, but data showing efficacy and safety of exosome therapy for alopecia are lacking. We identified several gaps in knowledge required for effective clinical translation including safety, exosome source, and optimal treatment delivery mechanism and dosage.

CONCLUSION

Exosomes are on the horizon as an exciting therapeutic for the treatment of alopecia. Further studies and clinical trials are required.

[Research progress of hair follicle and related stem cells in scar-free wound healing]

Objective

To explore the research progress of hair follicle and related stem cells in scar-free skin healing in recent years.

Methods

The literature related to hair follicle and related stem cells, wound healing, and scar formation in recent years was extensively reviewed and summarized from the aspects of cell function and molecular mechanism.

Results

Scar tissue after wound healing treated with hair follicle transplantation and related stem cell therapy is more mild or even without scar formation. The cell types and molecular mechanisms of the above phenomena are complex, and the bone morphogenetic protein signal transduction pathway and Wnt signal transduction pathway are strongly correlated.

Conclusion

The research of hair follicle and related stem cells in scar-free skin healing is at the initial stage at present. Strengthening the mechanism research may provide new ideas for the treatment of wound and scar.

The novel mechanisms and applications of exosomes in dermatology and cutaneous medical aesthetics

Exposure to the external environment may lead to instability and dysfunction of the skin, resulting in refractory wound, skin aging, pigmented dermatosis, hair loss, some immune-mediated dermatoses, and connective tissue diseases. Nowadays, many skin treatments have not achieved a commendable balance between medical recovery and cosmetic needs. Exosomes are cell-derived nanoscale vesicles carrying various biomolecules, including proteins, nucleic acids, and lipids, with the capability to communicate with adjacent or distant cells. Recent studies have demonstrated that endogenic multiple kinds of exosomes are crucial orchestrators in shaping physiological and pathological development of the skin. Besides, exogenous exosomes, such as stem cell exosomes, can serve as novel treatment options to repair, regenerate, and rejuvenate skin tissue. Herein, we review new insights into the role of endogenic and exogenous exosomes in the skin microenvironment and recent advances in applications of exosomes related to dermatology and cutaneous medical aesthetics. The deep understanding of the mechanisms by which exosomes perform biological functions in skin is of great potential to establish attractive therapeutic methods for the skin.

The biology of human hair greying

Hair greying (canities) is one of the earliest, most visible ageing-associated phenomena, whose modulation by genetic, psychoemotional, oxidative, senescence-associated, metabolic and nutritional factors has long attracted skin biologists, dermatologists, and industry. Greying is of profound psychological and commercial relevance in increasingly ageing populations. In addition, the onset and perpetuation of defective melanin production in the human anagen hair follicle pigmentary unit (HFPU) provides a superb model for interrogating the molecular mechanisms of ageing in a complex human mini-organ, and greying-associated defects in bulge melanocyte stem cells (MSCs) represent an intriguing system of neural crest-derived stem cell senescence. Here, we emphasize that human greying invariably begins with the gradual decline in melanogenesis, including reduced tyrosinase activity, defective melanosome transfer and apoptosis of HFPU melanocytes, and is thus a primary event of the anagen hair bulb, not the bulge. Eventually, the bulge MSC pool becomes depleted as well, at which stage greying becomes largely irreversible. There is still no universally accepted model of human hair greying, and the extent of genetic contributions to greying remains unclear. However, oxidative damage likely is a crucial driver of greying via its disruption of HFPU melanocyte survival, MSC maintenance, and of the enzymatic apparatus of melanogenesis itself. While neuroendocrine factors [e.g. alpha melanocyte-stimulating hormone (α-MSH), adrenocorticotropic hormone (ACTH), ß-endorphin, corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH)], and micropthalmia-associated transcription factor (MITF) are well-known regulators of human hair follicle melanocytes and melanogenesis, how exactly these and other factors [e.g. thyroid hormones, hepatocyte growth factor (HGF), P-cadherin, peripheral clock activity] modulate greying requires more detailed study. Other important open questions include how HFPU melanocytes age intrinsically, how psychoemotional stress impacts this process, and how current insights into the gerontobiology of the human HFPU can best be translated into retardation or reversal of greying.

Hair follicle regional specificity in different parts of bay Mongolian horse by histology and transcriptional profiling

Background

Different morphological structures of hairs having properties like defense and camouflage help animals survive in the wild environment. Horse is one of the rare kinds of animals with complex hair phenotypes in one individual; however, knowledge of horse hair follicle is limited in literature and their molecular basis remains unclear. Therefore, the investigation of horse hair follicle morphogenesis and pigmentogenesis attracts considerable interest.

Result

Histological studies revealed the morphology and pigment synthesis of hair follicles are different in between four different parts (mane, dorsal part, tail, and fetlock) of the bay Mongolian horse. Hair follicle size, density, and cycle are strongly associated with the activity of alkaline phosphatase (ALP). We observed a great difference in gene expression between the mane, tail, and fetlock, which had a greater different gene expression pattern compared with the dorsal part through transcriptomics. The development of the hair follicle in all four parts was related to angiogenesis, stem cells, Wnt, and IGF signaling pathways. Pigmentogenesis-related pathways were involved in their hair follicle pigment synthesis.

Conclusions

Hair follicle morphology and the activity of ALP differ among four body parts in bay Mongolian horse. Hair follicles of the different body parts of the are not synchronized in their cycle stages. GO terms show a regional specificity pattern between different skin parts of the bay Mongolian horse. These results provide an insight into the understanding of the biological mechanism of the hair follicle in other mammals.

Induction of Hair Keratins Expression by an Annurca Apple-Based Nutraceutical Formulation in Human Follicular Cells

Hair disorders may considerably impact the social and psychological well-being of an individual. Recent advances in the understanding the biology of hair have encouraged the research and development of novel and safer natural hair growth agents. In this context, we have previously demonstrated—at both preclinical and clinical level—that an Annurca apple-based dietary supplement (AMS), acting as a nutraceutical, is endowed with an intense hair-inductive activity (trichogenicity), at once increasing hair tropism and keratin content. Herein, in the framework of preclinical investigations, new experiments in primary human models of follicular keratinocytes and dermal papilla cells have been performed to give an insight around AMS biological effects on specific hair keratins expression. As well as confirming the biocompatibility and the antioxidant proprieties of our nutraceutical formulation, we have proven an engagement of trichokeratins production underlying its biological effects on human follicular cells. Annurca apples are particularly rich in oligomeric procyanidins, natural polyphenols belonging to the broader class of bioflavonoids believed to exert many beneficial health effects. To our knowledge, none of the current available remedies for hair loss has hitherto shown to stimulate the production of hair keratins so clearly.

The what, why and how of curly hair: a review

An attempt to understand and explain a peculiarity that was observed for curly fibres during experimentation revealed disparate literature reporting on several key issues. The phenotypical nature of curly fibres is only accurately understood within the larger scope of hair fibres, which are highly complex biological structures. A brief literature search produced thousands of research items. Besides the large amount of information on the topic, there was also great variability in research focus. From our review, it appeared that the complexity of hair biology, combined with the variety of research subtopics, often results in uncertainty when relating different aspects of investigation. During the literature investigation, we systematically categorized elements of curly hair research into three basic topics: essentially asking why fibres curl, what the curly fibre looks like and how the curly fibre behaves. These categories were subsequently formalized into a curvature fibre model that is composed of successive but distinctive tiers comprising the elements in curly hair research. The purpose of this paper is twofold: namely to present (i) a literature review that explores the different aspects of curly human scalp hair and (ii) the curvature fibre model as a systemized approach to investigating curly hair.

Adipose-derived cellular and cell-derived regenerative therapies in dermatology and aesthetic rejuvenation

Cellular and cell-derived components of adipose-derived tissue for the purposes of dermatologic and aesthetic rejuvenation applications have become increasingly studied and integrated into clinical practice. These components include micro-fragmented fat (nanofat), the stromal vascular fraction (SVF), adipose-derived mesenchymal stem cells (ASC), and extracellular vesicles (EVs), which have all shown capability to repair, regenerate, and rejuvenate surrounding tissue. Various aesthetic applications including hair growth, scar reduction, skin ischemia-reperfusion recovery, and facial rejuvenation are reviewed. In particular, results from preclinical and clinical studies are discussed, with a focus on clarification of nomenclature.

Functional anatomy of the hair follicle: The Secondary Hair Germ

The secondary hair germ (SHG)-a transitory structure in the lower portion of the mouse telogen hair follicle (HF)-is directly involved in anagen induction and eventual HF regrowth. Some crucial aspects of SHG functioning and ontogenetic relations with other HF parts, however, remain undefined. According to recent evidence (in contrast to previous bulge-centric views), the SHG is the primary target of anagen-inducing signalling and a source of both the outer root sheath (ORS) and ascending HF layers during the initial (morphogenetic) anagen subphase. The SHG is comprised of two functionally distinct cell populations. Its lower portion (originating from lower HF cells that survived catagen) forms all ascending HF layers, while the upper SHG (formed by bulge-derived cells) builds up the ORS. The predetermination of SHG cells to a specific morphogenetic fate contradicts their attribution to the "stem cell" category and supports SHG designation as a "germinative" or a "founder" cell population. The mechanisms of this predetermination driving transition of the SHG from "refractory" to the "competent" state during the telogen remain unknown. Functionally, the SHG serves as a barrier, protecting the quiescent bulge stem cell niche from the extensive follicular papilla/SHG signalling milieu. The formation of the SHG is a prerequisite for efficient "precommitment" of these cells and provides for easier sensing and a faster response to anagen-inducing signals. In general, the formation of the SHG is an evolutionary adaptation, which allowed the ancestors of modern Muridae to acquire a specific, highly synchronized pattern of hair cycling.

How chemotherapy and radiotherapy damage the tissue: Comparative biology lessons from feather and hair models

Chemotherapy and radiotherapy are common modalities for cancer treatment. While targeting rapidly growing cancer cells, they also damage normal tissues and cause adverse effects. From the initial insult such as DNA double-strand break, production of reactive oxygen species (ROS) and a general stress response, there are complex regulatory mechanisms that control the actual tissue damage process. Besides apoptosis, a range of outcomes for the damaged cells are possible including cell cycle arrest, senescence, mitotic catastrophe, and inflammatory responses and fibrosis at the tissue level. Feather and hair are among the most actively proliferating (mini-)organs and are highly susceptible to both chemotherapy and radiotherapy damage, thus provide excellent, experimentally tractable model systems for dissecting how normal tissues respond to such injuries. Taking a comparative biology approach to investigate this has turned out to be particularly productive. Started in chicken feather and then extended to murine hair follicles, it was revealed that in addition to p53-mediated apoptosis, several other previously overlooked mechanisms are involved. Specifically, Shh, Wnt, mTOR, cytokine signalling and ROS-mediated degradation of adherens junctions have been implicated in the damage and/or reparative regeneration process. Moreover, we show here that inflammatory responses, which can be prominent upon histological examination of chemo- or radiotherapy-damaged hair follicle, may not be essential for the hair loss phenotype. These studies point to fundamental, evolutionarily conserved mechanisms in controlling tissue responses in vivo, and suggest novel strategies for the prevention and management of adverse effects that arise from chemo- or radiotherapy.

Dermal white adipose tissue undergoes major morphological changes during the spontaneous and induced murine hair follicle cycling: a reappraisal

In murine skin, dermal white adipose tissue (DWAT) undergoes major changes in thickness in synchrony with the hair cycle (HC); however, the underlying mechanisms remain unclear. We sought to elucidate whether increased DWAT thickness during anagen is mediated by adipocyte hypertrophy or adipogenesis, and whether lipolysis or apoptosis can explain the decreased DWAT thickness during catagen. In addition, we compared HC-associated DWAT changes between spontaneous and depilation-induced hair follicle (HF) cycling to distinguish between spontaneous and HF trauma-induced events. We show that HC-dependent DWAT remodelling is not an artefact caused by fluctuations in HF down-growth, and that dermal adipocyte (DA) proliferation and hypertrophy are HC-dependent, while classical DA apoptosis is absent. However, none of these changes plausibly accounts for HC-dependent oscillations in DWAT thickness. Contrary to previous studies, in vivo BODIPY uptake suggests that increased DWAT thickness during anagen occurs via hypertrophy rather than hyperplasia. From immunohistomorphometry, DWAT thickness likely undergoes thinning during catagen by lipolysis. Hence, we postulate that progressive, lipogenesis-driven DA hypertrophy followed by dynamic switches between lipogenesis and lipolysis underlie DWAT fluctuations in the spontaneous HC, and dismiss apoptosis as a mechanism of DWAT reduction. Moreover, the depilation-induced HC displays increased DWAT thickness, area, and DA number, but decreased DA volume/area compared to the spontaneous HC. Thus, DWAT shows additional, novel HF wounding-related responses during the induced HC. This systematic reappraisal provides important pointers for subsequent functional and mechanistic studies, and introduces the depilation-induced murine HC as a model for dissecting HF-DWAT interactions under conditions of wounding/stress.

Hair disorders in patients with cancer

Cytotoxic chemotherapies, molecularly targeted therapies, immunotherapies, radiotherapy, stem cell transplants, and endocrine therapies may lead to hair disorders, including alopecia, hirsutism, hypertrichosis, and pigmentary and textural hair changes. The mechanisms underlying these changes are varied and remain incompletely understood, hampering the development of preventive or therapeutic guidelines. The psychosocial impact of chemotherapy-induced alopecia has been well documented primarily in the oncology literature; however, the effect of other alterations, such as radiation-induced alopecia, hirsutism, and changes in hair color or texture on quality of life have not been described. This article reviews clinically significant therapy-related hair disorders in oncology patients, including the underlying pathophysiological mechanisms, severity grading scales, patient-reported quality of life questionnaires, management strategies, and future translational research opportunities.

Characterisation of cell cycle arrest and terminal differentiation in a maximally proliferative human epithelial tissue: Lessons from the human hair follicle matrix

Human hair follicle (HF) growth and hair shaft formation require terminal differentiation-associated cell cycle arrest of highly proliferative matrix keratinocytes. However, the regulation of this complex event remains unknown. CIP/KIP family member proteins (p21^CIP1, p27^KIP1 and p57^KIP2) regulate cell cycle progression/arrest, endoreplication, differentiation and apoptosis. Since they have not yet been adequately characterized in the human HF, we asked whether and where CIP/KIP proteins localise in the human hair matrix and pre-cortex in relation to cell cycle activity and HF-specific epithelial cell differentiation that is marked by keratin 85 (K85) protein expression. K85 expression coincided with loss or reduction in cell cycle activity markers, including in situ DNA synthesis (EdU incorporation), Ki-67, phospho-histone H3 and cyclins A and B1, affirming a post-mitotic state of pre-cortical HF keratinocytes. Expression of CIP/KIP proteins was found abundantly within the proliferative hair matrix, concomitant with a role in cell cycle checkpoint control. p21^CIP1, p27^KIP1 and cyclin E persisted within post-mitotic keratinocytes of the pre-cortex, whereas p57^KIP2 protein decreased but became nuclear. These data imply a supportive role for CIP/KIP proteins in maintaining proliferative arrest, differentiation and anti-apoptotic pathways, promoting continuous hair bulb growth and hair shaft formation in anagen VI. Moreover, post-mitotic hair matrix regions contained cells with enlarged nuclei, and DNA in situ hybridisation showed cells that were >2N in the pre-cortex. This suggests that CIP/KIP proteins might counterbalance cyclin E to control further rounds of DNA replication in a cell population that has a propensity to become tetraploid. These data shed new light on the in situ-biography of human hair matrix keratinocytes on their path of active cell cycling, arrest and terminal differentiation, and showcase the human HF as an excellent, clinically relevant model system for cell cycle physiology research of human epithelial cells within their natural tissue habitat.