Can Plant Extracts Help Prevent Hair Loss or Promote Hair Growth? A Review Comparing Their Therapeutic Efficacies, Phytochemical Components, and Modulatory Targets

This narrative review aims to examine the therapeutic potential and mechanism of action of plant extracts in preventing and treating alopecia (baldness). We searched and selected research papers on plant extracts related to hair loss, hair growth, or hair regrowth, and comprehensively compared the therapeutic efficacies, phytochemical components, and modulatory targets of plant extracts. These studies showed that various plant extracts increased the survival and proliferation of dermal papilla cells in vitro, enhanced cell proliferation and hair growth in hair follicles ex vivo, and promoted hair growth or regrowth in animal models in vivo. The hair growth-promoting efficacy of several plant extracts was verified in clinical trials. Some phenolic compounds, terpenes and terpenoids, sulfur-containing compounds, and fatty acids were identified as active compounds contained in plant extracts. The pharmacological effects of plant extracts and their active compounds were associated with the promotion of cell survival, cell proliferation, or cell cycle progression, and the upregulation of several growth factors, such as IGF-1, VEGF, HGF, and KGF (FGF-7), leading to the induction and extension of the anagen phase in the hair cycle. Those effects were also associated with the alleviation of oxidative stress, inflammatory response, cellular senescence, or apoptosis, and the downregulation of male hormones and their receptors, preventing the entry into the telogen phase in the hair cycle. Several active plant extracts and phytochemicals stimulated the signaling pathways mediated by protein kinase B (PKB, also called AKT), extracellular signal-regulated kinases (ERK), Wingless and Int-1 (WNT), or sonic hedgehog (SHH), while suppressing other cell signaling pathways mediated by transforming growth factor (TGF)-β or bone morphogenetic protein (BMP). Thus, well-selected plant extracts and their active compounds can have beneficial effects on hair health. It is proposed that the discovery of phytochemicals targeting the aforementioned cellular events and cell signaling pathways will facilitate the development of new targeted therapies for alopecia.


Introduction
Hair, a filament-like structure composed of keratin proteins and melanin pigments, grows from the dermis and goes out of the epidermis [1].Its upper part is called the hair shaft and the lower part is called the hair root [2].The hair and various cells and matrices around and below it form a mini-organ called a hair follicle [2,3].The lateral sides of the hair root are surrounded by the inner and outer root sheath cells [4].The underside of the hair root is bulb-shaped, and the hair root is in contact with the papilla cells of the dermis, which are surrounded by matrix cells (keratinocytes) [5,6].The capillaries in the subcutaneous tissue beneath the papilla provide the nutrients, oxygen, and growth factors necessary for hair growth.Stem cells reside in the outer root sheath, located in the bulge of the hair follicle [7,8].Dermal papilla cells release hormones that stimulate the differentiation of stem cells into different cell types via progenitor cells.Matrix cells act as germ cells and differentiate into the inner root sheath and keratin-producing cells.These cells continue dividing, proliferating, differentiating, and keratinizing, leading to hair production and growth.Melanocytes within the layer of matrix cells produce and supply melanin pigments, which are incorporated into the hair.
Hairs contribute to various skin functions, such as physical protection, insulation, sebum dispersal, sensory perception, etc. [9].Additionally, in human society, hair greatly impacts self-esteem, quality of life, attractiveness, and social interactions [10].Various factors, such as genetics, immune reactions, hormonal imbalances, inflammation, increased stress, poor nutrition, and medications, can cause hair loss accompanied by anagen to telogen transition [11][12][13][14].Although hair loss is not a major disease that threatens life or entails serious functional disability, some people are saddened and dissatisfied with hair loss since it affects human appearance [15].
The hair cycle consists of three distinct phases: anagen (growth) phase, catagen (regression, intermediate, or transition) phase, and telogen (resting) phase [7].The anagen phase lasts 3 to 5 years and more than 80% of human hair is in this phase.The catagen phase lasts about a month and 3% of human hair is in this phase.In the catagen phase, hair growth stops, and the hair bulb recedes toward the surface of the scalp.The telogen phase lasts, on average, 2 to 7 months, and 10 to 20% of human hair is in this phase.In the telogen phase, hairs are loosely attached to the hair follicle while its bulb is dormant.At the end of the telogen phase, when a new hair cycle begins, new hair shafts push out existing hairs, causing them to fall out.This stage is also classified separately as the exogen (shedding) phase [16].
Hair loss types are classified into scarring alopecia, non-scarring alopecia, and structural hair disorders [17].Scarring alopecia is caused by tissue damage that leads to the irreversible and permanent loss of hair follicles.In non-scarring alopecia, the function of hair follicles is temporally suppressed but may be recoverable using certain treatments, leading to hair regrowth.The fragility of the hair shafts causes structural hair disorders.Non-scarring alopecia includes focal hair loss, diffused hair loss, and patterned hair loss, such as androgenetic alopecia in men (male pattern hair loss), female pattern hair loss, and trichotillomania [18].
Several medicines can treat hair loss in humans [19].Minoxidil, originally developed as a drug to lower blood pressure by dilating blood vessels, was unexpectedly found to stimulate hair growth, and thus was later developed as a hair growth promoter [20][21][22].Minoxidil has been described to stimulate cell proliferation, vascular endothelial growth factor (VEGF) expression, and prostaglandin synthesis while inhibiting collagen synthesis in various skin and hair follicle cell types [23].Finasteride and dutasteride, inhibitors of steroid 5α-reductase enzyme, which converts testosterone into dihydrotestosterone (DHT), were originally developed to treat the symptoms of benign prostatic hyperplasia [24] and are also used to treat male androgenetic alopecia [25].Finasteride selectively inhibits steroid 5α-reductase type II isozyme and dutasteride inhibits both type I and II isozymes [26].Various other strategies including cell-based treatments [27] and natural product-based treatments [28] are being attempted to treat hair loss.
Plants have unique survival strategies and synthesize and utilize various metabolites that animals do not have, and these are called phytochemicals [29].Phytochemicals are broadly classified into phenolic compounds, terpenes/terpenoids, nitrogen-containing compounds, sulfur-containing compounds, etc., and have various physicochemical, biochemical, and biological activities depending on their chemical structures [30].Plant extracts have been applied to treat human diseases in traditional medicine, and single compounds derived from plants have been developed into medicines or provided a basis for the development of other new drugs [31].Plant-derived extracts and compounds have been used to protect the skin against environmental factors, such as ultraviolet rays [32] and air pollution [33], and to alleviate several skin conditions, such as inflammation [34] and keloid scar [35].The biological activity and pharmacological effects of various plantderived extracts and compounds have also been studied for their potential application in promoting hair health [28,36,37].
Although several medicines already serve good roles in hair loss prevention and hair growth promotion, natural products can provide an alternative option for hair care, offering ease and comfort to people who do not prefer chemically manufactured oral pills or topical agents.The primary purpose of this review is to examine the therapeutic potential of plant extracts in preventing hair loss or promoting hair growth or regrowth.Given the presence of other review papers on similar topics [28,36,37], this review focuses on comparing the therapeutic efficacies, phytochemical components, and modulatory targets of plant extracts evaluated in recent studies.We hope that this review will contribute to understanding the current status and prospects of research in this field and developing new therapeutic strategies for hair loss.

Methods
We accessed the PubMed database (https://pubmed.ncbi.nlm.nih.gov/,accessed on 30 April 2024) to search for research articles related to the topic of this narrative review.A preliminary literature search using various keywords, such as 'hair loss', 'hair growth', 'hair regrowth', 'extract', 'plant', 'herb', 'root', 'leaf', 'leaves', 'stem', and 'flower', and Boolean search commands, such as 'AND' and 'OR', resulted in hundreds of research articles that were too many to be explored in-depth in a single review paper.We refined the search results by limiting the search ranges for some keywords to title words only to select more highly focused studies.We used the following key terms: (hair loss[Title] OR hair growth[Title] OR hair regrowth[Title]) AND extract[Title] AND (plant OR herb OR root OR leaf OR leaves OR stem OR flower).This search identified 57 research articles written in English.Additionally, we accessed the Web of Science (https://www.webofscience.com/,accessed on 30 April 2024) and Google Scholar (https://scholar.google.com/,accessed on 30 April 2024) databases for an additional literature search, identifying 38 more research articles that examined plant extracts including several marine plants.Most identified research articles are cited and explored in the appropriate chapter(s) according to their contents, excluding a few articles that investigated the extracts of animals or fungi (4 articles), or only pure compounds (2 articles).

Effects of Plant Extracts on Dermal Papilla Cells In Vitro
The fates of dermal papilla cells are closely related to the hair growth cycle.Therefore, the viability and proliferation of dermal papilla cells are useful targets to prevent hair loss and promote hair growth.
Table 1 summarizes the extracts derived from a plant or several plants that have been reported to enhance the proliferation of human follicle dermal papilla cells (HFDPCs) or related cells in vitro.Table 2 summarizes the plant extracts that enhanced cell viability reduced by testosterone or DHT.H]-thymidine or bromodeoxyuridine (BrdU) during DNA synthesis in cells were also used to measure cell proliferation in some studies.Ki-67 nuclear protein is associated with ribosomal RNA transcription [69] and its immunostaining has been used to evaluate cell proliferation in some studies.
Tables 1 and 2 show the effective concentrations of plant extracts that enhanced the proliferation or viability of dermal papilla cells, as reported in previous studies.These data will be helpful in roughly comparing the relative activities of various plant extracts and selecting plant extracts with high application potential.More accurate and reliable information can be obtained through studies that directly measure and compare the activities of various extracts under the same conditions.
It is interesting to observe that male hormones reduced the viability of dermal papilla cells and that several plant extracts restored cell viability [66][67][68].The camellia (Camellia japonica) extract promoted cell proliferation and alleviated the decline in cell viability caused by androgenic hormones [61,67].

Effects of Plant Extracts on Hair Follicles Ex Vivo
In several previous studies, the effects of various plant extracts on hair growth, hair cycle, and proliferation of the associated cells were evaluated in experiments ex vivo using hair follicles obtained from human or animal donors, as summarized in Table 3.

Human hair follicles
The extract (100 mg mL −1 ) recovered the number of Ki-67-positive hair matrix keratinocytes reduced by DHT. Park et al., 2015 [43] Water extract from oriental melon (Cucumis melo) leaves

Human hair follicles
The extract (100 µg mL −1 ) enhanced the elongation of hair (entire hair length).

Human hair follicles
The extract (500 µg mL −1 ) enhanced the elongation of hair.

Human hair follicles
The extract (10 mg mL −1 ) enhanced the elongation of hair.Hashimoto et al., 2022 [72] Extract of Panax ginseng

Human hair follicles
The extract (100 µg mL −1 ) enhanced the elongation of the hair shaft.
Iwabuchi et al., 2024 [63] Extracts from various plants, such as Cucumis melo, Orthosiphon stamineus, and Panax ginseng, promoted hair shaft growth in organ-cultured hair follicles [47, 63,70].The extract from Cucumis melo promoted the proliferation of keratinocytes in the hair bulb and matrix constituting the hair follicles [70].Additionally, extracts from some plants, such as Cucumis melo, Houttuynia cordata, and Polygonum multiflorum, prolonged the anagen phase of the hair cycle [49,51,70].Brassica oleracea and Panax ginseng extracts restored hair shaft growth and proliferation of constituent cells in hair follicles, respectively, which were suppressed by testosterone or DHT [43,66].The ex vivo experimental results suggest the therapeutic potential of these plant extracts to improve hair growth.

Effects of Plant Extracts on Hair Growth in Animal Models In Vivo
Table 4 summarizes the effects of various plant extracts on hair growth in animal models.The test substance, animal model, vehicle or formula of the test substance, route and period of administration, measurement items, and comparison data between groups are shown.The list includes the extracts of marine plants, such as Eucheuma cottonii [73] and Sargassum fusiforme [74].<, ≤, and ∼ = represent big differences, little differences, and no difference, respectively.↑ represents increases.
Mice and rats have often been used as animal models to evaluate hair growth whereas rabbits or guinea pigs have rarely been used [107,111,112,115,116].Many studies have used C57BL/6 mice, which have the advantage of being easy to observe with the naked eye due to their dark fur color.Some studies have used different substrains of C57BL/6 mice, such as C57BL/6N [79], C57BL/6NCrSlc [84], and C57BL/6J [58], although this does not mean that a particular substrain is more suitable for hair growth studies.Animals of different colors also have been used in hair growth research without major problems.Previous studies have used C3H mice with brown fur [38,39,82,84], albino mice with white fur [103,109], and albino Wistar rats or Sprague-Dawley rats with white fur [78,95].These animal models commonly require hair removal in hair growth research, but athymic BALB/c nude mice with natural hair growth defects do not require hair removal, providing an alternative model [90].
When mice are about 7 weeks old, most of the hair on their skin is synchronized in the telogen phase [117], so removing hair from mice at this age can help reduce inter-individual variation in the hair cycle.Hair removal methods include shaving with clippers or applying a kind of hair-removing solution or product followed by wiping to remove [58].Some previous studies have developed animal models that mimic hormonal hair loss conditions by topical application or subcutaneous injection of testosterone in mice [68,84] or that mimic menopause conditions by ovariectomy in female Sprague-Dawley rats [95].The chronic restraint stress model has also been used in hair research [96].
The plant extract has been applied topically in many studies, but it has also been administered via subcutaneous injection [39,42] or oral feeding [68,95].When applying a test substance topically, it is necessary to optimize the vehicle by considering the solubility of the drug, skin irritation, and skin absorption.Typically, propylene glycol, ethanol (EtOH), glycerin, and water have been used alone or in combination as a vehicle.Test substances were administered once a day in most cases, yet there were also cases where they were administered twice a day or once every few days.Many studies used minoxidil as the positive control, while finasteride has also been used [68].
The entire period of test substance administration after hair removal varied depending on the study, from 2 weeks [68,82] to 7 weeks [43], and the measurement of hair growth often continued until the hairs in the hair removal area had grown to the length of the surrounding area.However, in a study that counted the number of hair follicles per unit skin area or hair shafts per follicle, the test substance was administered for 10 days [77] or 3 months [95].Overall, the test period can vary depending on the test purpose and measurement items.
Various plant extracts promoted hair growth or alleviated the delay in hair growth caused by androgen hormones in animal models.Some plant extracts promoted telogento-anagen conversion in the hair cycle.Therefore, many of these extracts have potential applications in preventing and treating human alopecia.
It is difficult to compare the hair growth-promoting efficacy of plant extracts evaluated separately in different studies.However, suppose individual studies include negative or positive controls or multiple test groups administered various doses of the test substance.In that case, it is possible to interpret the reliability of the experimental results and the relative efficacy of the test substance.It is also necessary to conduct follow-up studies by prioritizing plant extracts that showed relatively strong efficacy in reliable studies compared to positive controls.

Clinical Studies on the Hair Growth Promotion or Suppression Efficacy of Plant Extracts
In clinical trials examining hair loss and hair growth, a combination of instrumental analysis and visual evaluation is used [118,119].Table 5 summarizes several double-blind, randomized, placebo-controlled trials on human subjects that evaluated the efficacy of a solution, tonic, lotion, cream, or shampoo containing different plant extracts promoting or suppressing hair growth.The shampoo increased the hair density and total hair count compared with those in the placebo group.
Topical application of the products containing Stryphnodendron adstringens bark extract and Curcuma aeruginosa extract reduced the growth of terminal hairs and axillary hairs, respectively, in women [120,122].In contrast, topical application of the products containing the extract of Thuja occidentalis, Oryza sativa, Curcuma aeruginosa, Centipeda minima, or Silybum marianum increased hair density in all human subjects [65,72,118,121,124,125]. Topical application of a product containing herbal mixture extracts also promoted hair growth and reduced hair loss in human subjects [45,123].These results suggest that plant extracts may have different effects of enhancing or inhibiting hair growth in various body parts depending on their types, contents, and formulas.Therefore, in developing hair care products using plant extracts, multiple factors must be considered to realize the purpose of use.Some plant extracts have been reported to help increase hair density when taken orally [62,126], so research on the route of administration is also needed.

Phytochemical Components and Active Compounds in Plant Extracts
As shown in Table 6, the main phytochemical components and active compounds of plant extracts have been presented in several studies.In this chapter, we will examine these compounds by dividing them into phenolic compounds, terpenes and terpenoids, sulfur-containing compounds, fatty acids, and other compounds.60% EtOH extract of seed cakes of Camellia japonica Active compounds with experimental evidence are indicated with bold letters.
Extract of Geranium sibiricum contains gallic acid and corilagin (an ellagitannin) [46].Extracts of Perilla frutescens and Lycopus lucidus contain rosmarinic acid as the main component [59,71], and extract of Allium ascalonicum contains rosmarinic acid, p-coumaric acid, and quercetin [57].Rosmarinic acid was shown to attenuate cell death caused by testosterone and promote VEGF gene expression in cells [57,59,71].Of the various phytochemical components in Leea indica extract, phthalic acid and other several compounds have been proposed as potential inhibitors of prostaglandin D 2 synthase based on in silico ligand binding analysis [103].

Terpenes and Terpenoids
The chemical structures of some terpenes and terpenoids are shown in From the extract of Rosmarinus officinalis, 12-methoxycarnosic acid, a diterpenoid, was isolated as an active compound and this compound enhanced the proliferation of cultured LNCaP cells [84].Extract of Curcuma aeruginosa contains high amounts of germacrone and other volatile sesquiterpenoids, such as dehydrocurdione, zederone, cucumenone, curcumenol, and furanodiene [122].Extract of Centipeda minima contains high From the extract of Rosmarinus officinalis, 12-methoxycarnosic acid, a diterpenoid, was isolated as an active compound and this compound enhanced the proliferation of cultured LNCaP cells [84].Extract of Curcuma aeruginosa contains high amounts of germacrone and other volatile sesquiterpenoids, such as dehydrocurdione, zederone, cucumenone, curcumenol, and furanodiene [122].Extract of Centipeda minima contains high amounts of brevilin A and several other sesquiterpene lactones, such as arnicolide C, arnicolide D, and microhelenin C [125].Extract of Stachytarpheta jamaicensi contains genipin (a monoterpene iridoid compound, phytol (a hydrogenated diterpene alcohol), and fatty acids (e.g., αlinolenic acid, palmitic acid, and tridecanoic acid) [89].Extract of Inula helenium contains costunolide, a sesquiterpene lactone [124].
Panax ginseng extracts contain unique triterpenoid saponins, such as ginsenosides Rb1, Rg1, Rg3, and Re [43,63].Testosterone suppressed the proliferation of hair matrix keratinocytes in hair follicle explants while upregulating androgen receptors in cultured hDPCs, and all these changes were inhibited by ginsenosides Rb1 and Rg3 [43].Ginsenosides Rb1, Rg1, and Re enhanced the proliferation of iDPCs while decreasing the mRNA level of BMP4 [63].A purified extract of Lycopersicon esculentum contains high amounts of tetraterpene carotenoids, such as all-trans-lycopene and 5-cis-lycopene, which are the main active components associated with hair growth-promoting effects [85].
Panax ginseng extracts contain unique triterpenoid saponins, such as ginsenosides Rb1, Rg1, Rg3, and Re [43,63].Testosterone suppressed the proliferation of hair matrix keratinocytes in hair follicle explants while upregulating androgen receptors in cultured hDPCs, and all these changes were inhibited by ginsenosides Rb1 and Rg3 [43].Ginsenosides Rb1, Rg1, and Re enhanced the proliferation of iDPCs while decreasing the mRNA level of BMP4 [63].A purified extract of Lycopersicon esculentum contains high amounts of tetraterpene carotenoids, such as all-trans-lycopene and 5-cis-lycopene, which are the main active components associated with hair growth-promoting effects [85].

Sulfur-Containing Compounds, Fatty Acids, and Other Compounds
The chemical structures of some sulfur-containing compounds, fatty acids, and other miscellaneous compounds are shown in Figure 3. Extract of Brassica oleracea contains sulforaphane and glucoraphanin (a glucosinolate of sulforaphane) [66].These components promoted hair shaft growth in hair follicles derived from C57/BL6 mice [66].Dimethyl sulfone has been isolated as an active compound from extract of Blumea eriantha, and the isolated compound increased the length of the hair follicle [109].
A fat-soluble extract of Boehmeria nipononivea contains large amounts of α-linolenic acid, linoleic acid, and palmitic acid [75].When comparing the hair growth-promoting Extract of Brassica oleracea contains sulforaphane and glucoraphanin (a glucosinolate of sulforaphane) [66].These components promoted hair shaft growth in hair follicles derived from C57/BL6 mice [66].Dimethyl sulfone has been isolated as an active compound from extract of Blumea eriantha, and the isolated compound increased the length of the hair follicle [109].
A fat-soluble extract of Boehmeria nipononivea contains large amounts of α-linolenic acid, linoleic acid, and palmitic acid [75].When comparing the hair growth-promoting effects of various fatty acids in C57/BL6 mice, α-linolenic acid, elaidic acid, and stearic acid were more effective than others [75].

Antioxidant, Anti-Inflammatory, and Anti-Senescence Effects of Plant Extracts
Oxidative stress induced by external and internal factors is expressed as an increase in prooxidants, a decrease in antioxidants, and an increase in oxidative damage [127].It acts as a causative mechanism disrupting the homeostasis of the skin, scalp, and hair [128,129].Reactive oxygen species (ROS), which mediate oxidative stress, can cause an inflammatory response and cellular senescence, hindering hair growth and triggering hair loss [129,130].Ultraviolet rays and air pollution have been shown to cause oxidative stress in dermal papilla cells and increase cell death [131,132].Various types of antioxidants have been studied as a defense for scalp and hair [133,134].

Leea indica
In silico Molecular docking analysis identified some phytochemicals, such as including phthalic acid, that showed high ligand efficiencies towards prostaglandin D 2 synthase.

In vitro
The extract had a DPPH radical-scavenging capacity. Park et al., 2021 [55] Pinus thunbergii Male C57/ BL6 mice The extract reduced pro-inflammatory cytokines, such as TNF-α and IL-1β, while increasing anti-inflammatory cytokines, such as IL-4 and IL-13, in the dorsal skin.

Camellia japonica HFDPCs
The extract suppressed the production of IL-6 and IL-1α in cells stimulated with DHT.It also reduced the expression of senescence-associated β-galactosidase (SA-β-gal) in DHT-treated cells.

Lycopus lucidus
HFDPCs The extract reduced IL-1β levels in cells exposed to hydrogen peroxide (H 2 O 2 ).

In vitro
The extract had DPPH radical-scavenging capacity.

Effects of Plant Extracts on the Apoptotic Cell Death Pathway
Apoptosis is a type of programmed cell death that is executed to remove unnecessary, unhealthy, or unrecoverable cells.In its intrinsic mitochondria-dependent pathway, the ratios of proapoptotic members (e.g., BCL-2-associated X protein (BAX), Bcl-2 homologous antagonist/killer (BAK), and BCL-2 associated agonist of cell death (BAD)) to antiapoptotic members (e.g., B-cell lymphoma 2 (BCL-2), B-cell lymphoma-extra-large (BCL-xL), and myeloid cell leukemia 2 (MCL-2)) of the BCL-2 family increase [136,137].Incorporating dimers of proapoptotic members into the mitochondrial membrane makes it leaky.Then, cytochrome C is released from the mitochondria and binds to apoptotic protease-activating factor 1 (APAF-1) in the cytoplasm to recruit caspase 9, which in turn activates caspase 3, 6, 7 (called executioner caspases), and other proteases involved in the degradation of cellular components.The extrinsic receptor-dependent apoptosis pathway is mediated by death receptors, such as tumor necrosis factor receptor 1 (TNFR-1) and FAS, and an adaptor, FAS-associated protein with death domain (FADD) [138,139].The activated receptor and adaptor cooperatively recruit caspase 8, which in turn activates executioner caspases.

Effects of Plant Extracts on Male Hormones
Table 9 shows the effects of some plant extracts on the expression of male hormones and their receptors in cells and animals.It is recognized that an increase in male hormones is highly correlated with hair loss [140] and studies have reported the effects of plant extracts on the expression of male hormones and their receptors in cell and animal models [43,51,62,67,78].Steroid 5α-reductase type II catalyzes the transformation of testosterone to DHT in cells, and its inhibitor can have therapeutic potential in treating male pattern hair loss [141].Extracts of several plants and a herbal mixture have been shown to reduce the expression level of steroid 5α-reductase type II in cells [50,53,57,62,67,110,135].Further, Sophora flavescens and Rosmarinus officinalis extracts have been shown to inhibit the catalytic activity of steroid 5α-reductase type II in vitro [61,76,84].↓ represents decreases.

Effects of Plant Extracts on Cell Cycle
The cell cycle consists of the gap (G) 1 phase, synthesis (S) phase, G2 phase, mitosis (M) phase, and G0 phase.In the G1 phase, retinoblastoma (Rb) protein sequesters E2F transcription factors and arrests the cell cycle, yet when Rb is hyper-phosphorylated, it releases E2F and the cell cycle enters the S phase [142].p53 induces the transcription of p21 CIP1 that inhibits CDK-mediated hyper-phosphorylation of Rb, stabilizing the Rb/E2F complex and causing cell cycle arrest [142].p16 INK4 inhibits CDK4 activity and reduces Rb phosphorylation, suppressing cell cycle progression [143].
Table 10 shows several plant extracts that promoted the cell cycle in HFDPCs.The extracts of Erica multiflora and Camellia japonica increased the percentage of cells in the S or G2/M phase [39,58].Houttuynia cordata and Camellia japonica extracts induced the cell cycle G1-S phase transition by upregulating CDK4 or downregulating p16 INK4 or p53 [49,67].HFDPCs ↑, ↓, and = represent increases, decreases, and no changes, respectively.Abbreviations: CDK-cyclin-dependent kinase; INK-inhibitors of CDK.

Effects of Plant Extracts on the Expression Levels of Growth Factors
As reported in many previous studies, various growth factors, such as insulin-like growth factor (IGF) [144], VEGF [145], hepatocyte growth factor (HGF) [146], and ker-atinocyte growth factor (KGF) (also called fibroblast growth factor 7, FGF-7) [147], can affect dermal papilla cell physiology or hair growth.
As summarized in Table 11, various plant extracts have been reported to affect the mRNA or protein levels of several growth factors in HFDPCs and animal models.Plant extracts promoting cell proliferation or hair growth generally increased IGF-1, VEGF, HGF, and KGF (FGF-7) levels, with some exceptions.↑, ↓, and = represent increases, decreases, and no changes, respectively.Abbreviations: IGF-insulin-like growth factor; VEGF-vascular endothelial growth factor; HGF-hepatocyte growth factor; KGF-keratinocyte growth factor; FGF-7-fibroblast growth factor 7.

Effects of Plant Extracts on the AKT and Mitogen-Activated Protein Kinase (MAPK) Signaling Pathways
The activation of phosphoinositide 3-kinases (PI3Ks) and the subsequent phosphorylation and activation of protein kinase B (PKB, also called AKT) by 3-phosphoinositidedependent kinase 1 (PDK1) or other protein kinases promote cell cycle progression and enhance cell survival [150].AKT-mediated phosphorylation (inactivation) of glycogen synthase kinase 3 beta (GSK3β) prevents phosphorylation and degradation of cyclin D1, promoting G1-S phase transition [151].AKT can inhibit apoptosis by phosphorylating and inactivating several proapoptotic proteins, such as BAD and caspase 9 [152].

Effects of Plant Extracts on the Wingless and Int-1 (WNT) Signaling Pathways
The canonical and non-canonical WNT signaling pathways are involved in regulating cell proliferation, polarity, or migration [156].In the canonical WNT pathway mediated by β-catenin, the stability of β-catenin is negatively regulated by its phosphorylation at multiple sites by several protein kinases, such as casein kinase 1 (CK1) and GSK3β [156].When WNT signaling is activated, GSK3β is inactivated through phosphorylation by several protein kinases, such as AKT, or other mechanisms.Then, β-catenin that has avoided proteasomal degradation enters the nucleus, where it acts as a transcriptional coactivator, interacting with several transcription factors, such as lymphoid enhancerbinding factor 1 (LEF1), and regulates the transcription of various target genes, including cyclin D1 and c-Myc [157].The target genes also include dickkopf 1 (DKK1), which inhibits the WNT pathway in a negative feedback loop [158].The DKK1 expression level is associated with hair loss; thus, DKK1 inhibition represents an attractive strategy to promote hair growth in androgenetic alopecia [159,160].

Effects of Plant Extracts on the Sonic Hedgehog (SHH) Signaling Pathways
Hedgehog ligands, including sonic hedgehog (SHH), desert hedgehog (DHH), and Indian hedgehog (IHH), are paracrine signaling factors that mediate cell-to-cell communication [161].The SHH signaling pathway is involved in regulating hair follicle morphogenesis [162].The interaction between SHH and the transmembrane protein patched (PTC) triggers the release of smoothened (SMO) from suppressing PTC, which leads to the dissociation of glioma-associated oncogene transcription factor (GLI) from a cytosolic complex [163].GLI proteins enter the nucleus and act as transcription factors regulating the expression of target genes [164].
Table 14 summarizes plant extracts that affected the SHH signaling pathway.Several plant extracts increased SHH protein levels in hair follicles in animal models.The extract of Allium ascalonicum and Coffea arabica promoted gene expression of SHH, SMO, and GLI1 at the cellular level [57,135].↑ represents increases.Abbreviations: SMO-smoothened; GLI-glioma-associated oncogene transcription factor.
5.9.Effects of Plant Extracts on the Transforming Growth Factor (TGF)-β and Bone Morphogenetic Protein (BMP) Signaling Pathways TGF-βs and BMPs are members of the TGF-β superfamily.In the canonical TGF-β signaling pathway, binding of TGF-βs to their receptors induces the phosphorylation of small mothers against decapentaplegic (SMAD) 2 and SMAD3 (called receptor-regulated SMADs or R-SMADs) followed by the formation of a trimeric complex with SMAD4 (called a common partner SMAD or co-SMAD), which enters the nucleus and induces the transcription of target genes [165].The target genes include SMAD7 (called an inhibitory SMAD or I-SMAD), which blocks TGF-β signaling in a negative feedback loop [166].In the canonical BMP signaling pathway, SMADs 1, 5, and 8 act as R-SMADs, and SMAD 6 acts as an I-SMAD, whereas SAMD4 acts as a co-SMAD [167].TGFs and BMPs can also trigger the non-canonical signaling pathways mediated by multiple protein kinases independently of SMADs [167,168].TGF-βs and BMPs are known to negatively affect hair growth by suppressing hair follicle function and causing hair cycle progression into the telogen phase [169,170].

Discussion
Research has been actively conducted to develop effective and safe treatments for human hair loss using natural products, especially plant-based materials.As explained in the previous sections, the hair growth-promoting potential of plant extracts has been supported in many in vitro experiments using cells (Tables 1 and 2), ex vivo experiments using hair follicle explants (Table 3), in vivo experiments using mice or rats (Table 4), and clinical trials in humans (Table 5).Experimental groups treated with certain plant extracts had cell proliferation and hair growth significantly higher than negative control groups and comparable to positive control groups treated with minoxidil or finasteride.These results suggest that a beneficial effect on hair growth is expected when plant extracts are administered appropriately.
While hair follicles are mini-organs in which several types of cells interact and cooperate to produce and grow hair, many studies have evaluated the effects of test substances using single-cell models in which only specific cells, such as dermal papilla cells, are cultured (Tables 1 and 2).Considering that interactions between various constituent cells are important for the function of hair follicles, it is necessary to develop technologies for co-culturing multiple cells or three-dimensional cultures, and further artificially creating hair follicles.Ex vivo experiments using excised hair follicles help to overcome some of the limitations of cell models, and the effect of test substances on hair growth has been successfully evaluated in several ex vivo studies (Table 3).However, there are limitations in the supply of human tissue.
Various animal models have been used for primary efficacy testing of plant extracts (Table 4).Animal hair removal models have been most often used in hair growth research although these models have the disadvantage of having little similarity to natural human hair loss.It is worth noting that several plant extracts showed hair growth promotion efficacy equivalent to or higher than minoxidil, a positive control.These include extracts from Rumex japonicus [44], Cucumis melo [70], Perilla frutescens [71], Leea indica [103], Blumea eriantha [109], etc.
Animal models in which hair removal is combined with male hormone administration [68,71,84] or ovariectomy [95] have high physiological relevance as models of androgenetic alopecia in men and postmenopausal alopecia in women, respectively.Extracts of Terminalia bellirica, Perilla frutescens, and Rosmarinus officinalis recovered hair growth suppressed by testosterone or DHT [68,71,84].Extract of Ribes nigrum promoted hair growth in ovariectomized female Sprague-Dawley rats [95].
Athymic animals with a congenital tendency for hair loss provide a model for natural hair loss without needing hair removal [90].In a study using male athymic BALB/c nude mice, extract of Chrysanthemum zawadskii promoted hair growth more effectively than extract of Polygonum multiflorum [90].Examining which type of human hair loss is most similar to an animal model is necessary, since it increases the utility of the animal model in hair growth research.An animal model in which hair loss is induced by spatially confined stress may be utilized in studying similar stress-induced alopecia in humans [96].
Although many extracts have shown high potential for hair growth-promoting effects in animal models, only a few have advanced to the level of clinical trials (Table 5).We do not take any position supporting or disputing previously reported clinical trial results.Currently, no matter what the purpose of the use or the route of administration, we do not recommend the human application of any plant extract without its prior confirmed safety.Expansion of clinical trials is necessary to verify the effectiveness and safety of the final product containing plant extracts.
Plants were often extracted using hot water or various organic solvents, such as methanol (MeOH), EtOH, acetone [75], ethyl acetate [85], and n-hexane [122].Supercritical CO 2 extraction [85,86], cold vacuum extraction [113], and emulsion-assisted extraction methods [125] have also been used to prepare a special type of plant extract.Solvent partition [71,87,103] and chromatography [71,109] have been used to partially purify or isolate pure active compounds from a crude plant extract.Plant extracts have been formulated in a solution [45], tonic [121,125], lotion [122], cream [120], shampoo [65,123], or nanoparticles [102,171] for topical application.Tablets and other types of food products have also been manufactured for oral administration [62,126].The improvement in quality control and extraction and purification methods to increase the content of active ingredients in plant extracts and the development of optimized formulas and drug carriers to improve the biological availability and delivery of active compounds to the point of action are needed to prompt the development of effective hair care products using plant extracts.
The biological activity of plant extracts enhancing cell proliferation or hair growth has been attributed to their main phytochemical components (Table 6), such as phenolic compounds (Figure 1), terpenes and terpenoids (Figure 2), sulfur-containing compounds, fatty acids, and other compounds (Figure 3).In some studies, the biological activity of single active compounds has been verified at the cellular level or in vivo.Representative examples of compounds with proven activity include decursin [98], rosmarinic acid [57,59,71], 12-methoxycarnosic acid [84], ginsenosides [43,63], sulforaphane, glucoraphanin [66], dimethyl sulfone [109], α-linolenic acid [75], and linoleic acid [86].The experimental evidence accumulated so far is insufficient to derive the structure-activity relationship, and we look forward to additional research on this task for optimized drug discovery.
Several plant extracts have been shown to prevent alopecia by inducing or prolonging the anagen phase of the hair cycle and inhibiting entry into the telogen phase (Tables 3-5).The pharmacological effects of plant extracts that induced and extended the anagen phase in the hair cycle could be associated with the promotion of cell proliferation (Table 1), cell survival (Table 2), or cell cycle progression (Table 10); the upregulation of several growth factors, such as IGF-1, VEGF, HGF, and KGF (FGF-7) (Table 11); and the stimulation of several cell signaling pathways mediated by AKT, ERK, WNT, or SHH (Tables 12-14).In addition, the pharmacological effects of plant extracts that prevented the entry into the telogen phase in the hair cycle could be attributed to the alleviation of oxidative stress, inflammatory response, cellular senescence (Table 7), or apoptosis (Table 8); the downregulation of male hormones and their receptors (Table 9); and the suppression of several cell signaling pathways mediated by TGF-β or BMP (Table 15).These findings suggest a potential mechanism of action of plant extracts in promoting hair growth and preventing hair loss, which is schematized in Figure 4.
Because the hair cycle depends on the health and function of various cells in the hair follicles, which are in turn affected by multiple physiological factors, such as hormones and stresses [2,14,[172][173][174], it is necessary to analyze in detail the etiology and pathology of alopecia for each patient and develop a customized treatment strategy accordingly.To achieve this, effective medications targeting specific cellular events and cell signaling pathways involved in hair growth and loss are needed.Exploration of plant-based natural products against these modulatory targets will provide a promising opportunity to discover natural remedies or lead compounds for targeted therapies for different types of hair loss.
Overall, research in this field has not only expanded the list of plant extracts and phytochemicals with the potential to promote hair health but has also deepened our understanding of their mechanisms of action.However, there are not many studies that comprehensively explore pharmacological effects, active compounds, and molecular targets of the plant extracts.More integrated and expanded research that reflects the latest knowledge presented in this review is needed to promote the development of improved treatments for alopecia.
These findings suggest a potential mechanism of action of plant extracts in promoting hair growth and preventing hair loss, which is schematized in Figure 4.Because the hair cycle depends on the health and function of various cells in the hair follicles, which are in turn affected by multiple physiological factors, such as hormones and stresses [2,14,[172][173][174], it is necessary to analyze in detail the etiology and pathology of alopecia for each patient and develop a customized treatment strategy accordingly.To achieve this, effective medications targeting specific cellular events and cell signaling pathways involved in hair growth and loss are needed.Exploration of plant-based natural products against these modulatory targets will provide a promising opportunity to discover natural remedies or lead compounds for targeted therapies for different types of hair loss.
Overall, research in this field has not only expanded the list of plant extracts and phytochemicals with the potential to promote hair health but has also deepened our understanding of their mechanisms of action.However, there are not many studies that comprehensively explore pharmacological effects, active compounds, and molecular targets of the plant extracts.More integrated and expanded research that reflects the latest knowledge presented in this review is needed to promote the development of improved treatments for alopecia.

Conclusions
Accumulated evidence from in vitro, in vivo, and clinical studies suggests that several plant extracts and phytochemicals can help prevent hair loss or promote hair growth and regrowth.Well-selected plant extracts can provide additional or alternative hair loss treatment options to people reluctant to use medicines.In addition, the active compounds can serve as lead compounds for new drug discovery and development.Their effects on the hair cycle were associated with the modulation of cell proliferation, cell survival, cell cycle progression, growth factors, hormones, oxidative stress, inflammatory response, cellular senescence, apoptosis, and several cell signaling pathways mediated by AKT, ERK, WNT, SHH, TGF-β, or BMP.Therefore, it is proposed that the discovery of phytochemicals modulating these targets will lead to the development of new targeted therapies for alopecia.

Figure 3 .
Figure 3.Chemical structures of sulfur-containing compounds, fatty acids, and other compounds.

Figure 3 .
Figure 3.Chemical structures of sulfur-containing compounds, fatty acids, and other compounds.

Figure 4 .
Figure 4.The modulatory targets of plant extracts for promoting hair growth and preventing hair loss.Several plant extracts containing various active phytochemicals can initiate or extend the anagen phase of the hair cycle by stimulating the expression of several growth factors; the AKT, ERK, WNT, and SHH signaling pathways; or inducing the cell cycle progression.Some plant extracts can prevent entry into the telogen phase of the hair cycle by inhibiting androgen expression and the TGF-β and BMP signaling pathways or alleviating ROS-mediated oxidative stress, inflammatory response, cellular senescence, and apoptosis.Plant extracts with different mechanisms of action can show differentiated efficacy according to the type of hair loss with different etiology.Black arrows indicate the hair cycle progression associated with hair growth and loss.Sharp red arrows indicate upregulation, stimulation, or promotion, and blunted blue arrows indicate downregulation, inhibition, or suppression by plant extracts.

Figure 4 .
Figure 4.The modulatory targets of plant extracts for promoting hair growth and preventing hair loss.Several plant extracts containing various active phytochemicals can initiate or extend the anagen phase of the hair cycle by stimulating the expression of several growth factors; the AKT, ERK, WNT, and SHH signaling pathways; or inducing the cell cycle progression.Some plant extracts can prevent entry into the telogen phase of the hair cycle by inhibiting androgen expression and the TGF-β and BMP signaling pathways or alleviating ROS-mediated oxidative stress, inflammatory response, cellular senescence, and apoptosis.Plant extracts with different mechanisms of action can show differentiated efficacy according to the type of hair loss with different etiology.Black arrows indicate the hair cycle progression associated with hair growth and loss.Sharp red arrows indicate upregulation, stimulation, or promotion, and blunted blue arrows indicate downregulation, inhibition, or suppression by plant extracts.

Table 1 .
Effects of plant extracts on the proliferation of dermal papilla cells in vitro.
* Concentrations at which the plant extract enhanced cell proliferation compared to the vehicle control.

Table 2 .
Effects of plant extracts on the viability of HFDPCs treated with androgens in vitro. 3

Table 3 .
Effects of plant extracts on hair follicles ex vivo.

Table 4 .
Effects of plant extracts on hair growth in animal models.

Table 5 .
Effects of plant extracts on hair growth in clinical trials.

Table 6 .
Main phytochemical components and active compounds in plant extracts.

Table 7 .
Antioxidant, anti-inflammatory, and anti-senescence effects of plant extracts.

Table 8 .
Effects of plant extracts on apoptosis pathway.

Table 9 .
Effects of plant extracts on androgens, their receptors, and steroid 5α-reductase type II.

Table 10 .
Effects of plant extracts on cell cycle.

Table 11 .
Effects of plant extracts on the mRNA and protein levels of several growth factors.

Table 12 .
Effects of plant extracts on the AKT and mitogen-activated protein kinase (MAPK) signaling pathways.

Table 13 .
Effects of plant extracts on the mediators of the WNT signaling pathways.

Table 14 .
Effects of plant extracts on the mediators of the sonic hedgehog (SHH) signaling pathways.

Table 15 .
Effects of plant extracts on the TGF-β and BMP signaling pathways.