Molecular principles of hair follicle induction and morphogenesis

Mesenchymal Stem Cell Exosomes Therapy for Acquired Trichorrhexis Nodosa: A Case Series

BACKGROUND

Recent preclinical studies have demonstrated the potential efficacy of stem cell exosomes in the treatment of hair loss. However, there is a paucity of clinical studies investigating the application of exosomes for this purpose. This case series presents three patients treated with exosomes for acquired trichorrhexis nodosa (ATN), a condition characterized by hair shaft abnormalities and breakage.

OBJECTIVES

This study aims to evaluate the effectiveness and safety of utilizing mesenchymal stem cell (MSC) exosomes as a novel therapeutic approach for the management of ATN.

METHODS

A standardized process was employed to prepare 0.1 mL of exosomes, which were subsequently injected into bilateral regions of the patients' scalps at 0.5-1 cm intervals on a monthly basis. Each injection comprised a total volume of 5 mL, and all three patients underwent a minimum of four treatment sessions. The comparative efficacy of the treatment was evaluated using clinical photographs, dermatoscopy, and scanning electron microscopy (SEM) for all three patients post-intervention.

RESULTS

The hair condition of the three patients demonstrated significant improvement, characterized by increased length and density, enhanced pigmentation with a reduced presence of dusty white dots, and the disappearance of dermoscopic black dots and broken hairs. SEM analysis revealed a remarkable recovery in the hair cuticle layers. At the 1-year follow-up, hair growth essentially remained normal.

CONCLUSION

Exosomes derived from mesenchymal stem cells demonstrate efficacy in treating ATN, presenting a novel therapeutic approach for this condition.

BMP4 regulates differentiation of nestin-positive stem cells into melanocytes

Hair follicle (HF) development and pigmentation are complex processes governed by various signaling pathways, such as TGF-β and FGF signaling pathways. Nestin + (neural crest like) stem cells are also expressed in HF stem cells, particularly in the bulge and dermal papilla region. However, the specific role and differentiation potential of these Nestin-positive cells within the HF remain unclear, especially regarding their contribution to melanocyte formation and hair pigmentation. Bone morphogenetic protein 4 (BMP4), members of the TGFβ family, has been implicated in regulating HF growth, coloration, and related cellular behaviors. Its role in directing Nestin-positive cells toward a melanocytic lineage has yet to be fully explored. In this study, mouse HF organoids were constructed and shown to be an ideal model for studying HF growth and development in vitro. Using this model as a basis, we demonstrated that BMP4 controls HF coloration as well as its length, number, and even size. Furthermore, Nestin-positive cells in the HF-especially those in the bulge region-differentiate into melanocytes, which produce the pigments that give HF its color under BMP4 stimulation. The resulting increase in pigmentation within the mouse HF organoids underscores that BMP4 has a major regulatory role in the formation of melanocytes from Nestin-positive stem cells. This research provides insights into the cellular mechanisms underlying hair pigmentation and suggests potential therapeutic applications for pigmentation disorders.

The involvement of Elf5 in regulating keratinocyte proliferation and differentiation processes in skin

Skin and hair development is regulated by multitude of programs of activation and silencing of gene expression to maintain normal skin and hair follicle (HF) development, homeostasis, and cycling. Here, we have identified E74-like factor 5 (Elf5) transcription factor, as a novel regulator of keratinocyte proliferation and differentiation processes in skin. Expression analysis has revealed that Elf5 expression was localised and elevated in stem/progenitor cell populations of both the epidermis (basal and suprabasal) and in HF bulge and hair germ stem cell (SCs) compartments during skin and hair development and cycling. Expressional and functional analysis using RT-qPCR, western blot and colony forming assays, revealed that Elf5 plays an important role in regulating keratinocyte proliferation and differentiation processes as well as potentially determining cell fate by regulating the stem/progenitor cell populations in skin and HFs. These data will provide a platform for pharmacological manipulation of Elf5 in skin, leading to advancements in many areas of research, including stem cell, regenerative medicine, and ageing.

Structure, function and formation of the amniote skin pattern

From feather and hair dotted arrays to pigmented stripes and spots, the spatial distribution of skin appendages and colouration often forms visible ornaments crucial for fitness in the coat of birds and mammals. These geometrical motifs are extremely diverse in nature. Yet, phenotypic surveys evidenced common themes in variation: the orientation, appendage-specificity or pigmentation of a given region may be conserved across groups or species. Here, we review naturalist observations of natural variation in the anatomy and ecological function of the skin pattern in amniotes. We then describe several decades of genetics, mathematical modelling and experimental embryology work aiming at understanding the molecular and morphogenetic mechanisms responsible for pattern formation. We discuss how these studies provided evidence that the morphological trends and differences representative of the phenotypic landscape of skin patterns in wild amniote species is rooted in the mechanisms controlling the production of distinct compartments in the embryonic skin.

Radially patterned morphogenesis of murine hair follicle placodes ensures robust epithelial budding

The bending of simple cellular sheets into complex three-dimensional (3D) forms requires developmental patterning cues to specify where deformations occur, but how positional information directs morphological change is poorly understood. Here, we investigate how morphogen signaling and cell fate diversification contribute to the morphogenesis of murine hair placodes, in which collective cell movements transform radially symmetric primordia into bilaterally symmetric tubes. Through live imaging and 3D volumetric reconstructions, we demonstrate that Wnt and Shh establish radial patterns of cell fate, cell morphology, and movement within developing placodes. Cell fate diversity at different radial positions provides unique and essential contributions to placode morphogenesis. Further, we show that downstream of radial patterning, gradients of classical cadherin expression are required for efficient epithelial rearrangements. Given that the transformation of epithelial discs into 3D tubes is a common morphological motif used to shape diverse organ primordia, mechanisms of radially patterned morphogenesis are likely highly conserved across evolution.

TGFβ family signaling in human stem cell self-renewal and differentiation

Human stem cells are undifferentiated cells with the capacity for self-renewal and differentiation into distinct cell lineages, playing important role in the development and maintenance of diverse tissues and organs. The microenvironment of stem cell provides crucial factors and components that exert significant influence over the determination of cell fate. Among these factors, cytokines from the transforming growth factor β (TGFβ) superfamily, including TGFβ, bone morphogenic protein (BMP), Activin and Nodal, have been identified as important regulators governing stem cell maintenance and differentiation. In this review, we present a comprehensive overview of the pivotal roles played by TGFβ superfamily signaling in governing human embryonic stem cells, somatic stem cells, induced pluripotent stem cells, and cancer stem cells. Furthermore, we summarize the latest research and advancements of TGFβ family in various cancer stem cells and stem cell-based therapy, discussing their potential clinical applications in cancer therapy and regeneration medicine.

Molecular Mechanisms of Hair Follicle Development

Hair is an intricate biological structure that originates from hair follicles (HFs), which are complex mini-organs embedded in the skin. Each HF undergoes continuous cycles of growth (anagen), regression (catagen), and rest (telogen), driven by intricate signaling pathways and interactions between epithelial and mesodermal cells. The development of HFs requires the interplay of several key signaling pathways, including Wnt, Shh, Notch, and BMP. The Wnt pathway is primarily involved in induction, Shh is essential for early organogenesis and later stages of cytodifferentiation, Notch signaling governs the fate of HF stem cells, and BMP plays a role in cytodifferentiation. Hair health is closely associated with psychological well-being and personal distress. While hair loss (alopecia) does not impact biological health, it significantly affects social well-being. Therefore, a deep understanding of the molecular mechanisms underlying HF development is crucial for developing treatments for hair-related problems and improving hair health. This knowledge has led to significant advancements in therapeutic applications, particularly in treating hair loss disorders, enhancing wound healing, and developing cosmetic treatments. This paper aims to review the molecular mechanisms involved in HF development, with an emphasis on their potential impact on human health and well-being.

Comparison of In Vitro Hair Growth Promotion and Anti-Hair Loss Potential of Thai Rice By-Product from Oryza sativa L. cv. Buebang 3 CMU and Sanpatong

The bioactive compounds in herbal extracts may provide effective hair loss treatments with fewer side effects compared to synthetic medicines. This study evaluated the effects of Buebang 3 CMU and Sanpatong rice bran extracts, macerated with dichloromethane or 95% ethanol, on hair growth promotion and hair loss prevention. Overall, Buebang 3 CMU extracts contained significantly higher levels of bioactive compounds, including γ-oryzanol, tocopherols, and various polyphenols such as phytic acid, ferulic acid, and chlorogenic acid, compared to Sanpatong extracts. Additionally, ethanolic extracts demonstrated greater bioactive content and antioxidant activities than those extracted with dichloromethane. These compounds enhanced the proliferation of human hair follicle dermal papilla cells (HFDPCs) by 124.28 ± 1.08% (p < 0.05) and modulated anti-inflammatory pathways by reducing nitrite production to 3.20 ± 0.36 µM (p < 0.05). Key hair growth signaling pathways, including Wnt/β-catenin (CTNNB1), Sonic Hedgehog (SHH, SMO, GLI1), and vascular endothelial growth factor (VEGF), were activated by approximately 1.5-fold to 2.5-fold compared to minoxidil. Also, in both human prostate cancer (DU-145) and HFDPC cells, the ethanolic Buebang 3 CMU extract (Et-BB3-CMU) suppressed SRD5A1, SRD5A2, and SRD5A3 expression-key pathways in hair loss-by 2-fold and 1.5-fold more than minoxidil and finasteride, respectively. These findings suggest that Et-BB3-CMU holds promise for promoting hair growth and preventing hair loss.

Synergistic Phytochemical and Pharmacological Actions of Hair RiseTM Microemulsion: A Novel Herbal Formulation for Androgenetic Alopecia and Hair Growth Stimulation

Androgenetic alopecia (AGA) is a genetic condition characterized by an excessive response to androgens, leading to hairline regression in men and hair thinning at the vertex in women, which can negatively impact self-esteem. Conventional synthetic treatments for AGA are often limited by their side effects. In contrast, Thai medicinal plants offer a promising alternative with fewer adverse effects. This study investigates the synergistic phytochemical and pharmacological effects of a novel Hair RiseTM microemulsion, formulated with bioactive extracts from rice bran (Oryza sativa), shallot bulb (Allium ascalonicum), licorice root (Glycyrrhiza glabra), and corn kernels (Zea mays), for the treatment of hair loss. The microemulsion, in concentrations of 50%, 75%, and 100% (v/v), significantly enhanced the proliferation of human hair follicle dermal papilla cells (HFDPCs) compared to minoxidil. Additionally, it upregulated critical hair growth signaling pathways, including Wnt/β-catenin (CTNNB1), Sonic Hedgehog (SHH, SMO, GLI1), and vascular endothelial growth factor (VEGF), surpassing standard controls such as minoxidil and purmorphamine. The microemulsion also demonstrated potent anti-inflammatory and antioxidant properties by reducing nitric oxide production and oxidative stress, factors that contribute to inflammation and follicular damage in AGA. Furthermore, Hair RiseTM inhibited 5α-reductase (types 1-3), a key enzyme involved in androgen metabolism, in both human prostate cancer cells (DU-145) and HFDPCs. These findings suggest that Hair RiseTM microemulsion presents a promising natural therapy for promoting hair growth and reducing hair loss via multiple synergistic mechanisms, offering a potent, plant-based alternative to synthetic treatments.

Pathway to independence: perspectives on the future

In this Perspective, our 2024 Pathway to Independence Fellows provide their thoughts on the future of their field. Covering topics as diverse as plant development, tissue engineering and adaptation to climate change, and using a wide range of experimental organisms, these talented postdocs showcase some of the major open questions and key challenges across the spectrum of developmental biology research.

1,25-(OH)2D3 promotes hair growth by inhibiting NLRP3/IL-1β and HIF-1α/IL-1β signaling pathways

Vitamin D is a crucial vitamin that participates in various biological processes through the Vitamin D Receptor (VDR). While there are studies suggesting that VDR might regulate hair growth through ligand-independent mechanisms, the efficacy of Vitamin D in treating hair loss disorders has also been reported. Here, through in vivo experiments in mice, in vitro organ culture of hair follicles, and cellular-level investigations, we demonstrate that 1,25-(OH)2D3 promotes mouse hair regeneration, prolongs the hair follicle anagen, and enhances the proliferation and migration capabilities of dermal papilla cells and outer root sheath keratinocytes in a VDR-dependent manner. Transcriptome analysis of VDR-knockout mouse skin reveals the involvement of HIF-1α, NLRP3, and IL-1β in these processes. Finally, we confirm that 1,25-(OH)2D3 can counteract the inhibitory effects of DHT on hair growth. These findings suggest that 1,25-(OH)2D3 has a positive impact on hair growth and may serve as a potential therapeutic agent for androgenetic alopecia (AGA).

Deep Hair Phenomics: Implications in Endocrinology, Development, and Aging

Hair quality is an important indicator of health in humans and other animals. Current approaches to assess hair quality are generally nonquantitative or are low throughput owing to technical limitations of splitting hairs. We developed a deep learning-based computer vision approach for the high-throughput quantification of individual hair fibers at a high resolution. Our innovative computer vision tool can distinguish and extract overlapping fibers for quantification of multivariate features, including length, width, and color, to generate single-hair phenomes of diverse conditions across the lifespan of mice. Using our tool, we explored the effects of hormone signaling, genetic modifications, and aging on hair follicle output. Our analyses revealed hair phenotypes resultant of endocrinological, developmental, and aging-related alterations in the fur coats of mice. These results demonstrate the efficacy of our deep hair phenomics tool for characterizing factors that modulate the hair follicle and developing, to our knowledge, previously unreported diagnostic methods for detecting disease through the hair fiber. Finally, we have generated a searchable, interactive web tool for the exploration of our hair fiber data at skinregeneration.org.

Melatonin promotes hair regeneration by modulating the Wnt/β-catenin signalling pathway

Melatonin (MLT) is a circadian hormone that reportedly influences the development and cyclic growth of secondary hair follicles; however, the mechanism of regulation remains unknown. Here, we systematically investigated the role of MLT in hair regeneration using a hair depilation mouse model. We found that MLT supplementation significantly promoted hair regeneration in the hair depilation mouse model, whereas supplementation of MLT receptor antagonist luzindole significantly suppressed hair regeneration. By analysing gene expression dynamics between the MLT group and luzindole-treated groups, we revealed that MLT supplementation significantly up-regulated Wnt/β-catenin signalling pathway-related genes. In-depth analysis of the expression of key molecules in the Wnt/β-catenin signalling pathway revealed that MLT up-regulated the Wnt/β-catenin signalling pathway in dermal papillae (DP), whereas these effects were facilitated through mediating Wnt ligand expression levels in the hair follicle stem cells (HFSCs). Using a DP-HFSCs co-culture system, we verified that MLT activated Wnt/β-catenin signalling in DPs when co-cultured with HFSCs, whereas supplementation of DP cells with MLT alone failed to activate Wnt/β-catenin signalling. In summary, our work identified a critical role for MLT in promoting hair regeneration and will have potential implications for future hair loss treatment in humans.

The development of hair follicles and nail

The hair follicle and nail unit develop and regenerate through epithelial-mesenchymal interactions. Here, we review some of the key signals and molecular interactions that regulate mammalian hair follicle and nail formation during embryonic development and how these interactions are reutilized to promote their regeneration during adult homeostasis and in response to skin wounding. Finally, we highlight the role of some of these signals in mediating human hair follicle and nail conditions.

Triethylene Glycol Squalene Improves Hair Regeneration by Maintaining the Inductive Capacity of Human Dermal Papilla Cells and Preventing Premature Aging

De novo hair follicle (HF) regeneration, achieved through the replenishment of the dermal papilla (DP), acknowledged as the principal orchestrator of the hair growth cycle, is emerging as a prospective therapeutic intervention for alopecia. Nonetheless, multiple attempts have shown that these cells lose key inductive properties when cultured in a two-dimensional (2D) monolayer, leading to precocious senescence engendered by oxidative stress and inflammatory processes. Consequently, the three-dimensional (3D) spheroid technique is presently widely employed for DP cell culture. Nevertheless, substantiating the regenerative potential of these cells within the hair follicle (HF) milieu remains a challenge. In this current study, we aim to find a new approach to activate the inductive properties of DP cells. This involves the application of hair-growth-stimulating agents that not only exhibit concurrent protective efficacy against the aging process but also induce HF regeneration. To achieve this objective, we initially synthesized a novel highly amphiphilic derivative derived from squalene (SQ), named triethylene glycol squalene (Tri-SQ). Squalene itself is a potent antioxidant and anti-inflammatory compound traditionally employed as a drug carrier for alopecia treatment. However, its application is limited due to its low solubility. Subsequently, we applied this newly synthesized derivative to DP cells. The data obtained demonstrated that the derivative exhibits robust antioxidant and anti-inflammatory activities while concurrently promoting the expression of genes associated with hair growth. Moreover, to further assess the hair regrowth inductive properties of DP cells, we cultured the cells and treated them with Tri-SQ within a 3D spheroid system. Subsequently, these treated cells were injected into the previously depilated dorsal area of six-week-old male C57BL/6 mice. Results revealed that 20 days postinjection, a complete regrowth of hair in the previously hairless area, particularly evident in the case of 3D spheroids treated with the derivative, was observed. Additionally, histological and molecular analyses demonstrated an upregulation of markers associated with hair growth and a concurrent decrease in aging hallmarks, specifically in the 3D spheroids treated with the compound. In summary, our approach, which involves the treatment of Tri-SQ combined with a 3D spheroid system, exhibited a notably robust stimulating effect. This effect was observed in the induction of inductive properties in DP cells, leading to HF regeneration, and concurrently, it demonstrated an inhibitory effect on cellular and follicular aging.

Single-Cell and Spatial Transcriptome Analysis of Dermal Fibroblast Development in Perinatal Mouse Skin: Dynamic Lineage Differentiation and Key Driver Genes

Several single-cell RNA studies of developing mouse skin have elucidated the molecular and cellular processes involved in skin development. However, they have primarily focused on either the fetal or early postnatal period, leaving a gap in our understanding of skin development. In this study, we conducted a comprehensive time-series analysis by combining single-cell RNA-sequencing datasets collected at different stages of development (embryonic days 13.5, 14.5, and 16.5 and postnatal days 0, 2, and 4) and validated our findings through multipanel in situ spatial transcriptomics. Our analysis indicated that embryonic fibroblasts exhibit heterogeneity from a very early stage and that the rapid determination of each lineage occurs within days after birth. The expression of putative key driver genes, including Hey1, Ebf1, Runx3, and Sox11 for the dermal papilla trajectory; Lrrc15 for the dermal sheath trajectory; Zfp536 and Nrn1 for the papillary fibroblast trajectory; and Lrrn4cl and Mfap5 for the fascia fibroblast trajectory, was detected in the corresponding, spatially identified cell types. Finally, cell-to-cell interaction analysis indicated that the dermal papilla lineage is the primary source of the noncanonical Wnt pathway during skin development. Together, our study provides a transcriptomic reference for future research in the field of skin development and regeneration.

Integrating Single-Cell and Spatial Transcriptomics Reveals Heterogeneity of Early Pig Skin Development and a Subpopulation with Hair Placode Formation

The dermis and epidermis, crucial structural layers of the skin, encompass appendages, hair follicles (HFs), and intricate cellular heterogeneity. However, an integrated spatiotemporal transcriptomic atlas of embryonic skin has not yet been described and would be invaluable for studying skin-related diseases in humans. Here, single-cell and spatial transcriptomic analyses are performed on skin samples of normal and hairless fetal pigs across four developmental periods. The cross-species comparison of skin cells illustrated that the pig epidermis is more representative of the human epidermis than mice epidermis. Moreover, Phenome-wide association study analysis revealed that the conserved genes between pigs and humans are strongly associated with human skin-related diseases. In the epidermis, two lineage differentiation trajectories describe hair follicle (HF) morphogenesis and epidermal development. By comparing normal and hairless fetal pigs, it is found that the hair placode (Pc), the most characteristic initial structure in HFs, arises from progenitor-like OGN+/UCHL1+ cells. These progenitors appear earlier in development than the previously described early Pc cells and exhibit abnormal proliferation and migration during differentiation in hairless pigs. The study provides a valuable resource for in-depth insights into HF development, which may serve as a key reference atlas for studying human skin disease etiology using porcine models.

Melanocyte stem cells in the skin: Origin, biological characteristics, homeostatic maintenance and therapeutic potential

Melanocyte stem cells (MSCs), melanocyte lineage-specific skin stem cells derived from the neural crest, are observed in the mammalian hair follicle, the epidermis or the sweat gland. MSCs differentiate into mature melanin-producing melanocytes, which confer skin and hair pigmentation and uphold vital skin functions. In controlling and coordinating the homeostasis, repair and regeneration of skin tissue, MSCs play a vital role. Decreased numbers or impaired functions of MSCs are closely associated with the development and therapy of many skin conditions, such as hair graying, vitiligo, wound healing and melanoma. With the advancement of stem cell technology, the relevant features of MSCs have been further elaborated. In this review, we provide an exhaustive overview of cutaneous MSCs and highlight the latest advances in MSC research. A better understanding of the biological characteristics and micro-environmental regulatory mechanisms of MSCs will help to improve clinical applications in regenerative medicine, skin pigmentation disorders and cancer therapy. KEY POINTS: This review provides a concise summary of the origin, biological characteristics, homeostatic maintenance and therapeutic potential of cutaneous MSCs. The role and potential application value of MSCs in skin pigmentation disorders are discussed. The significance of single-cell RNA sequencing, CRISPR-Cas9 technology and practical models in MSCs research is highlighted.

Sebaceous gland organoid engineering

Sebaceous glands (SGs), as holocrine-secreting appendages, lubricate the skin and play a central role in the skin barrier. Large full-thickness skin defects cause overall architecture disruption and SG loss. However, an effective strategy for SG regeneration is lacking. Organoids are 3D multicellular structures that replicate key anatomical and functional characteristics of in vivo tissues and exhibit great potential in regenerative medicine. Recently, considerable progress has been made in developing reliable procedures for SG organoids and existing SG organoids recapitulate the main morphological, structural and functional features of their in vivo counterparts. Engineering approaches empower researchers to manipulate cell behaviors, the surrounding environment and cell-environment crosstalk within the culture system as needed. These techniques can be applied to the SG organoid culture system to generate functionally more competent SG organoids. This review aims to provide an overview of recent advancements in SG organoid engineering. It highlights some potential strategies for SG organoid functionalization that are promising to forge a platform for engineering vascularized, innervated, immune-interactive and lipogenic SG organoids. We anticipate that this review will not only contribute to improving our understanding of SG biology and regeneration but also facilitate the transition of the SG organoid from laboratory research to a feasible clinical application.

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.

The Biology and Genomics of Human Hair Follicles: A Focus on Androgenetic Alopecia

Androgenetic alopecia is a highly prevalent condition mainly affecting men. This complex trait is related to aging and genetics; however, multiple other factors, for example, lifestyle, are also involved. Despite its prevalence, the underlying biology of androgenetic alopecia remains elusive, and thus advances in its treatment have been hindered. Herein, we review the functional anatomy of hair follicles and the cell signaling events that play a role in follicle cycling. We also discuss the pathology of androgenetic alopecia and the known molecular mechanisms underlying this condition. Additionally, we describe studies comparing the transcriptional differences in hair follicles between balding and non-balding scalp regions. Given the genetic contribution, we also discuss the most significant risk variants found to be associated with androgenetic alopecia. A more comprehensive understanding of this pathology may be generated through using multi-omics approaches.

Mechanical forces across compartments coordinate cell shape and fate transitions to generate tissue architecture

Morphogenesis and cell state transitions must be coordinated in time and space to produce a functional tissue. An excellent paradigm to understand the coupling of these processes is mammalian hair follicle development, which is initiated by the formation of an epithelial invagination-termed placode-that coincides with the emergence of a designated hair follicle stem cell population. The mechanisms directing the deformation of the epithelium, cell state transitions and physical compartmentalization of the placode are unknown. Here we identify a key role for coordinated mechanical forces stemming from contractile, proliferative and proteolytic activities across the epithelial and mesenchymal compartments in generating the placode structure. A ring of fibroblast cells gradually wraps around the placode cells to generate centripetal contractile forces, which, in collaboration with polarized epithelial myosin activity, promote elongation and local tissue thickening. These mechanical stresses further enhance compartmentalization of Sox9 expression to promote stem cell positioning. Subsequently, proteolytic remodelling locally softens the basement membrane to facilitate a release of pressure on the placode, enabling localized cell divisions, tissue fluidification and epithelial invagination into the underlying mesenchyme. Together, our experiments and modelling identify dynamic cell shape transformations and tissue-scale mechanical cooperation as key factors for orchestrating organ formation.

Local and systemic mechanisms that control the hair follicle stem cell niche

Hair follicles are essential appendages of the mammalian skin, as hair performs vital functions of protection, thermoregulation and sensation. Hair follicles harbour exceptional regenerative abilities as they contain multiple somatic stem cell populations such as hair follicle stem cells (HFSCs) and melanocyte stem cells. Surrounding the stem cells and their progeny, diverse groups of cells and extracellular matrix proteins are organized to form a microenvironment (called 'niche') that serves to promote and maintain the optimal functioning of these stem cell populations. Recent studies have shed light on the intricate nature of the HFSC niche and its crucial role in regulating hair follicle regeneration. In this Review, we describe how the niche serves as a signalling hub, communicating, deciphering and integrating both local signals within the skin and systemic inputs from the body and environment to modulate HFSC activity. We delve into the recent advancements in identifying the cellular and molecular nature of the niche, providing a holistic perspective on its essential functions in hair follicle morphogenesis, regeneration and ageing.

MiR-199a-5p inhibits dermal papilla cells proliferation by regulating VEGFA expression in cashmere goat

Hair follicles undergo a renewal cycle consisting of anagen, telogen and catagen stages. MicroRNA (miRNA) plays a crucial role in this process. Recent studies have shown that miR-199a-5p, which exhibits differential expression between anagen and telogen stages in the hair follicle cycle of cashmere goats, inhibits the proliferation of various cell types, including skin keratinocytes and vascular endothelial cells. Since the proliferation of dermal papilla cells (DPCs) is a key factor in the hair follicle cycle, we utilized DPCs to investigate the function and molecular mechanism of miR-199a-5p in cashmere goats. Our functional analysis revealed that miR-199a-5p significantly suppressed cell viability and proliferation of DPCs, as evidenced by MTT, EdU and RT-qPCR methods. Subsequently, we investigated the regulatory mechanism of miR-199a-5p. Through bioinformatics analysis, a potential correlation between lnc102173187 and miR-199a-5p was predicted. However, the dual luciferase reporter assay revealed no interaction between lnc102173187 and miR-199a-5p. Further investigation using dual-luciferase reporter assay, RT-qPCR, and western blot results confirmed that VEGFA was the target gene of miR-199a-5p from. The functional experiment demonstrated that VEGFA promoted the proliferation of DPCs, and antagonized the inhibitory effect of miR-199a-5p on DPCs proliferation. Taken together, this research revealed the role of miR-199a-5p and VEGFA on the proliferation of dermal papilla cells in cashmere goat, which would enrich the theoretical basis for hair follicle development, and could also serve as a marker cofactor to play an important reference and guidance role in the breeding, improvement and optimization of cashmere goat breeds.

A bioartificial and vasculomorphic bone matrix-based organoid mimicking microanatomy of flat and short bones

We engineered an in vitro model of bioartificial 3D bone organoid consistent with an anatomical and vascular microenvironment common to mammalian flat and short bones. To achieve this, we chose the decellularized-decalcified matrix of the adult male rat scapula, implemented with the reconstruction of its intrinsic vessels, obtained through an original intravascular perfusion with polylevolactic (PLLA), followed by coating of the PLLA-fabricated vascularization with rat tail collagen. As a result, the 3D bone and vascular geometry of the native bone cortical and cancellous compartments was reproduced, and the rat tail collagen-PLLA biomaterial could in vitro act as a surrogate of the perivascular extracellular matrix (ECM) around the wall of the biomaterial-reconstituted cancellous vessels. As a proof-of-concept of cell compatibility and site-dependent osteoinductive properties of this bioartificial 3D construct, we show that it in vitro leads to a time-dependent microtopographic positioning of rat mesenchymal stromal cells (MSCs), initiating an osteogenic fate in relation to the bone compartment. In addition, coating of PLLA-reconstructed vessels with rat tail collagen favored perivascular attachment and survival of MSC-like cells (mouse embryonic fibroblasts), confirming its potentiality as a perivascular stroma for triggering competence of seeded MSCs. Finally, in vivo radiographic topography of bone lesions in the human jaw and foot tarsus of subjects with primary osteoporosis revealed selective bone cortical versus cancellous involvement, suggesting usefulness of a human 3D bone organoid engineered with the same principles of our rat organoid, to in vitro investigate compartment-dependent activities of human MSC in flat and short bones under experimental osteoporotic challenge. We conclude that our 3D bioartificial construct offers a reliable replica of flat and short bones microanatomy, and promises to help in building a compartment-dependent mechanistic perspective of bone remodeling, including the microtopographic dysregulation of osteoporosis.

Characterization of the Newborn Epidermis and Adult Hair Follicles Using Whole-Mount Immunofluorescent Staining of Mouse Dorsal Skin

The mammalian integumentary system, including skin and its appendages, serves as a protective barrier for the body. During development, skin epidermis undergoes rapid cell division and differentiation to form multiple stratified layers of keratinocytes. Concurrently the epidermis also gives rise to hair follicles that invaginate into the dermis. In adult skin, the hair follicle undergoes cyclic regeneration fueled by hair follicle stem cells located in the bulge. Three-dimensional and high-resolution imaging of these structures using whole-mount immunofluorescent staining allows to better characterize epidermal progenitors and stem cells.

The Roles of WNT Signaling Pathways in Skin Development and Mechanical-Stretch-Induced Skin Regeneration

The WNT signaling pathway plays a critical role in a variety of biological processes, including development, adult tissue homeostasis maintenance, and stem cell regulation. Variations in skin conditions can influence the expression of the WNT signaling pathway. In light of the above, a deeper understanding of the specific mechanisms of the WNT signaling pathway in different physiological and pathological states of the skin holds the potential to significantly advance clinical treatments of skin-related diseases. In this review, we present a comprehensive analysis of the molecular and cellular mechanisms of the WNT signaling pathway in skin development, wound healing, and mechanical stretching. Our review sheds new light on the crucial role of the WNT signaling pathway in the regulation of skin physiology and pathology.

Hair Growth Promotion and Anti-Hair Loss Effects of By-Products Arabica Coffee Pulp Extracts Using Supercritical Fluid Extraction

Coffee has been a common ingredient in many traditional hair loss remedies, but limited scientific evidence supports its use, particularly in coffee pulp. Androgenetic alopecia (AGA) is caused by androgens, inflammation, and oxidative stress. In the present study, supercritical fluid extraction (SFE) was used under various conditions to obtain six coffee pulp extracts. The SFE-4 extract, using 50% (v/v) ethanol as a co-solvent at conditions of 100 °C and 500 bars for 30 min, exhibited the highest phenolic, flavonoid, and caffeine contents. Additionally, the SFE-4 extract increased the migration and cell proliferation of HFDPCs (human hair follicle dermal papilla cells), which control hair cycle regulation, and had scavenging effects on ABTS and DPPH radicals. Additionally, the SFE-4 extract showed potassium ion channel opener activity in HFDPCs, as well as a stimulation effect on the enzyme matrix metalloproteinase-2 (MMP-2) (28.53 ± 1.08% of control), which may be related to the vascular endothelial growth factor (VEGF) gene upregulation. In human prostate cancer cells (DU-145) and HFDPC cells, the SFE-4 extract significantly decreased the expression of SRD5A1, SRD5A2, and SRD5A3, an essential pathway involved in AGA. Hair growth factor genes in the Wnt/-catenin (CTNNB1) and Sonic Hedgehog (SHH, SMO, and GLI1) pathways could be significantly activated by the SFE-4 extract. These results imply that employing SFE in coffee pulp extraction could help AGA treatment by preventing hair loss and promoting hair growth pathways. This would help small coffee producers gain economic empowerment and ensure the long-term sustainability of agricultural waste utilization.

Recent omics advances in hair aging biology and hair biomarkers analysis

Aging is a complex natural process that leads to a decline in physiological functions, which is visible in signs such as hair graying, thinning, and loss. Although hair graying is characterized by a loss of pigment in the hair shaft, the underlying mechanism of age-associated hair graying is not fully understood. Hair graying and loss can have a significant impact on an individual's self-esteem and self-confidence, potentially leading to mental health problems such as depression and anxiety. Omics technologies, which have applications beyond clinical medicine, have led to the discovery of candidate hair biomarkers and may provide insight into the complex biology of hair aging and identify targets for effective therapies. This review provides an up-to-date overview of recent omics discoveries, including age-associated alterations of proteins and metabolites in the hair shaft and follicle, and highlights the significance of hair aging and graying biomarker discoveries. The decline in hair follicle stem cell activity with aging decreased the regeneration capacity of hair follicles. Cellular senescence, oxidative damage and altered extracellular matrix of hair follicle constituents characterized hair follicle and hair shaft aging and graying. The review attempts to correlate the impact of endogenous and exogenous factors on hair aging. We close by discussing the main challenges and limitations of the field, defining major open questions and offering an outlook for future research.

Molecular and spatial landmarks of early mouse skin development

A wealth of specialized cell populations within the skin facilitates its hair-producing, protective, sensory, and thermoregulatory functions. How the vast cell-type diversity and tissue architecture develops is largely unexplored. Here, with single-cell transcriptomics, spatial cell-type assignment, and cell-lineage tracing, we deconstruct early embryonic mouse skin during the key transitions from seemingly uniform developmental precursor states to a multilayered, multilineage epithelium, and complex dermal identity. We identify the spatiotemporal emergence of hair-follicle-inducing, muscle-supportive, and fascia-forming fibroblasts. We also demonstrate the formation of the panniculus carnosus muscle (PCM), sprouting blood vessels without pericyte coverage, and the earliest residence of mast and dendritic immune cells in skin. Finally, we identify an unexpected epithelial heterogeneity within the early single-layered epidermis and a signaling-rich periderm layer. Overall, this cellular and molecular blueprint of early skin development-which can be explored at https://kasperlab.org/tools-establishes histological landmarks and highlights unprecedented dynamic interactions among skin cells.

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.

Recent Trends in Macromolecule-Based Approaches for Hair Loss Treatment

Macromolecules are large, complex molecules composed of smaller subunits known as monomers. The four primary categories of macromolecules found in living organisms are carbohydrates, lipids, proteins, and nucleic acids; they also encompass a broad range of natural and synthetic polymers. Recent studies have shown that biologically active macromolecules can help regenerate hair, providing a potential solution for current hair regeneration therapies. This review examines the latest developments in the use of macromolecules for the treatment of hair loss. The fundamental principles of hair follicle (HF) morphogenesis, hair shaft (HS) development, hair cycle regulation, and alopecia have been introduced. Microneedle (MN) and nanoparticle (NP) delivery systems are innovative treatments for hair loss. Additionally, the application of macromolecule-based tissue-engineered scaffolds for the in vitro and in vivo neogenesis of HFs is discussed. Furthermore, a new research direction is explored wherein artificial skin platforms are adopted as a promising screening method for hair loss treatment drugs. Through these multifaceted approaches, promising aspects of macromolecules for future hair loss treatments are identified.

Association between DNA Methylation in the Core Promoter Region of the CUT-like Homeobox 1 (CUX1) Gene and Lambskin Pattern in Hu Sheep

CUT-like homeobox 1 (CUX1) has been proven to be a key regulator in sheep hair follicle development. In our previous study, CUX1 was identified as a differential expressed gene between Hu sheep lambskin with small wave patterns (SM) and straight wool patterns (ST); however, the exact molecular mechanism of CUX1 expression has been obscure. As DNA methylation can regulate the gene expression, the potential association between CUX1 core promotor region methylation and lambskin pattern in Hu sheep was explored in the present study. The results show that the core promoter region of CUX1 was present at (-1601-(-1) bp) upstream of the transcription start site. A repressive region (-1151-(-751) bp) was also detected, which had a strong inhibitory effect on CUX1 promoter activity. Bisulfite amplicon sequencing revealed that no significant difference was detected between the methylation levels of CUX1 core promoter region in SM tissues and ST tissues. Although the data demonstrated the differential expression of CUX1 between SM and ST probably has no association with DNA methylation, the identification of the core region and a potential repressive region of CUX1 promoter can enrich the role of CUX1 in Hu sheep hair follicle development.

Impact of Thyroid Dysfunction on Hair Disorders

Hair loss is a problem for everyone, regardless of their age or sex. The three most prevalent types of hair loss, telogen effluvium, alopecia areata, and androgenetic alopecia, have been associated with a variety of risk factors. Strong evidence links thyroid hormones (THs) to hair loss. THs control the growth, differentiation, metabolism, and thermogenesis of body cells. The skin is a significant target organ for THs; however, the cellular and molecular causes of thyroid dysfunction-related skin diseases remain unknown. Hyperthyroidism, hypothyroidism, and drug-induced hypothyroidism can induce widespread hair shedding. Little information is available regarding the incidence and effects of thyroid dysfunction on hair problems. This study aimed to review the impact and prevalence of thyroid disorders on hair loss. The conclusions drawn from this study highlight the underestimated prevalence and impact of thyroid disorders on hair loss. The review of scientific articles, including original research, review articles, and a case report, provides a comprehensive understanding of the topic. This research adds to the existing literature by enhancing our understanding of the relationship between thyroid dysfunction and hair disorders. It contributes to the body of evidence by reviewing relevant studies and summarizing the impact of thyroid disorders on hair loss. The study also highlights the gaps in knowledge and the need for more research in this area to improve the diagnosis and management of hair disorders associated with thyroid dysfunction.

Organ function is preserved despite reorganization of niche architecture in the hair follicle

The ability of stem cells to build and replenish tissues depends on support from their niche. Although niche architecture varies across organs, its functional importance is unclear. During hair follicle growth, multipotent epithelial progenitors build hair via crosstalk with their remodeling fibroblast niche, the dermal papilla, providing a powerful model to functionally interrogate niche architecture. Through mouse intravital imaging, we show that dermal papilla fibroblasts remodel individually and collectively to form a morphologically polarized, structurally robust niche. Asymmetric TGF-β signaling precedes morphological niche polarity, and loss of TGF-β signaling in dermal papilla fibroblasts leads them to progressively lose their stereotypic architecture, instead surrounding the epithelium. The reorganized niche induces the redistribution of multipotent progenitors but nevertheless supports their proliferation and differentiation. However, the differentiated lineages and hairs produced by progenitors are shorter. Overall, our results reveal that niche architecture optimizes organ efficiency but is not absolutely essential for organ function.

The PI3K-Akt-mTOR and Associated Signaling Pathways as Molecular Drivers of Immune-Mediated Inflammatory Skin Diseases: Update on Therapeutic Strategy Using Natural and Synthetic Compounds

The dysregulated phosphatidylinositol-3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) signaling pathway has been implicated in various immune-mediated inflammatory and hyperproliferative dermatoses such as acne, atopic dermatitis, alopecia, psoriasis, wounds, and vitiligo, and is associated with poor treatment outcomes. Improved comprehension of the consequences of the dysregulated PI3K/Akt/mTOR pathway in patients with inflammatory dermatoses has resulted in the development of novel therapeutic approaches. Nonetheless, more studies are necessary to validate the regulatory role of this pathway and to create more effective preventive and treatment methods for a wide range of inflammatory skin diseases. Several studies have revealed that certain natural products and synthetic compounds can obstruct the expression/activity of PI3K/Akt/mTOR, underscoring their potential in managing common and persistent skin inflammatory disorders. This review summarizes recent advances in understanding the role of the activated PI3K/Akt/mTOR pathway and associated components in immune-mediated inflammatory dermatoses and discusses the potential of bioactive natural products, synthetic scaffolds, and biologic agents in their prevention and treatment. However, further research is necessary to validate the regulatory role of this pathway and develop more effective therapies for inflammatory skin disorders.

CRABP2 Promotes the Proliferation of Dermal Papilla Cells via the Wnt/β-Catenin Pathway

In our previous study of Hu sheep hair follicles, we found that CRABP2 was highly expressed in DPCs, which suggested that CRABP2 may influence the number of DPCs. In the present study, we aimed to understand the effect of CRABP2 in Hu sheep dermal papilla cells (DPCs). First, we explored the influence of CRABP2 on the ability of Hu sheep DPCs' proliferation. Based on the results obtained from some experiments, such as CCK-8, EDU, qPCR, and Western blot experiment, we found that the overexpression of CRABP2 facilitated the proliferation of DPCs compared to the negative control group. Then, we also detected the effect of CRABP2 on the Wnt/β-catenin pathway based on the important function of the Wnt/β-catenin pathway in hair follicles. The results showed that CRABP2 could activate the Wnt/β-catenin pathway in DPCs, and it rescues the proliferation of DPCs when the Wnt/β-catenin pathway was inhibited. In summary, our findings indicate that CRABP2 is a vital functional gene in the proliferation of Hu sheep DPCs. Our study will be of great use for revealing the roles of CRABP2 in the hair follicles of Hu sheep.

Gellan gum-based hydrogels support the recreation of the dermal papilla microenvironment

The dermal papilla (DP), a specialized compartment within the hair follicle, regulates hair growth. However, human DP cells rapidly lose their inductivity in 2D-culture given the loss of positional and microenvironmental cues. Spheroids have been capable of recreating the 3D intercellular organization of DP cells, however, DP cell-matrix interactions are poorly represented. Considering the specific nature of the DP's extracellular matrix (ECM), we functionalized gellan gum (GG) with collagen IV-(HepIII) or fibronectin-(cRGDfC) derived peptide sequences to generate a 3D environment in which the phenotype and physiological functions of DP cells are restored. We further tuned the stiffness of the microenvironments by varying GG amount. Biomimetic peptides in stiffer hydrogels promoted the adhesion of DP cells, while each peptide and amount of polymer independently influenced the type and quantity of ECM proteins deposited. Furthermore, although peptides did not seem to have an influence, stiffer hydrogels improved the inductive capacity of DP cells after short term culture. Interestingly, independently of the peptide, these hydrogels supported the recapitulation of basic hair morphogenesis-like events when incorporated in an organotypic human skin in vitro model. Our work demonstrates that tailored GG hydrogels support the generation of a microenvironment in which both cell-ECM and cell-cell interactions positively influence DP cells towards the creation of an artificial DP.

Melatonin promotes the development of the secondary hair follicles by regulating circMPP5

BACKGROUND

The quality and yield of cashmere fibre are closely related to the differentiation and development of secondary hair follicles in the skin of cashmere goats. The higher the density of secondary hair follicles, the higher the quality and yield of cashmere from the fleece. Development of secondary hair follicles commences in the embryonic stage of life and is completed 6 months after birth. Preliminary experimental results from our laboratory showed that melatonin (MT) treatment of goat kids after their birth could increase the density of secondary hair follicles and, thus, improve the subsequent yield and quality of cashmere. These changes in the secondary hair follicles resulted from increases in levels of antioxidant and expression of anti-apoptotic protein, and from a reduction in apoptosis. The present study was conducted to explore the molecular mechanism of MT-induced secondary hair follicle differentiation and development by using whole-genome analysis.

RESULTS

MT had no adverse effect on the growth performance of cashmere kids but significantly improved the character of the secondary hair follicles and the quality of cashmere, and this dominant effect continued to the second year. Melatonin promotes the proliferation of secondary hair follicle cells at an early age. The formation of secondary hair follicles in the MT group was earlier than that in the control group in the second year. The genome-wide data results involved KEGG analysis of 1044 DEmRNAs, 91 DElncRNAs, 1054 DEcircRNAs, and 61 DEmiRNAs which revealed that the mitogen-activated protein kinase (MAPK) signaling pathway is involved in the development of secondary hair follicles, with key genes (FGF2, FGF21, FGFR3, MAPK3 (ERK1)) being up-regulated and expressed. We also found that the circMPP5 could sponged miR-211 and regulate the expression of MAPK3.

CONCLUSIONS

We conclude that MT achieves its effects by regulating the MAPK pathway through the circMPP5 sponged the miR-211, regulating the expression of MAPK3, to induce the differentiation and proliferation of secondary hair follicle cells. In addition there is up-regulation of expression of the anti-apoptotic protein causing reduced apoptosis of hair follicle cells. Collectively, these events increase the numbers of secondary hair follicles, thus improving the production of cashmere from these goats.

Stretching Promotes Wound Contraction Through Enhanced Expression of Endothelin Receptor B and TRPC3 in Fibroblasts

One factor that can contribute to the development of hypertrophic scar contracture is mechanical stress. Mechanical cyclic stretch stimuli enhance the secretion of endothelin-1 (ET-1) from keratinocyte. Cyclical stretching of fibroblasts also increases the expression level of the transient receptor potential ion channel (TRPC3), which is known to couple with the endothelin receptor and induce intracellular Ca2+ signaling via the calcineurin/nuclear factor of activated T cells (NFAT) pathway. The aim of this study was to investigate the relationship between keratinocytes and fibroblasts when they are stretched.

Methods

The conditioned medium from stretched keratinocyte was added to the fibroblast populated collagen lattice. Then, we analyzed the levels of endothelin receptor in the human hypertrophic scar tissue and stretched fibroblasts. To address the function of TRPC3, we have used an overexpression system with the collagen lattice. Finally, the TRPC3 overexpressing fibroblasts were transplanted to mouse dorsal skin, and the rate of skin wound contraction was assessed.

Results

Conditioned medium from stretched keratinocytes increased the rate of contraction of fibroblast populated collagen lattice. In human hypertrophic scar and stretched fibroblasts, endothelin receptor type B was increased. Cyclic stretching of TRPC3 overexpressing fibroblasts activated NFATc4, and stretched human fibroblasts showed more activation of NFATc4 in response to ET-1. The wound treated with TRPC3 overexpressing fibroblasts showed more contraction than control wound.

Conclusion

These findings suggest that cyclical stretching of wounds have an effect on both keratinocytes and fibroblasts, where keratinocytes secret more ET-1, and fibroblasts develop more sensitivity to ET-1 by expressing more endothelin receptors and TRPC3.

Skin-on-a-chip strategies for human hair follicle regeneration

The number of hair loss patients increases every year, and hair loss treatment has several limitations, so research on hair is attracting attention recently. However, most current hair follicle research models are limited by their inability to replicate several key functions of the hair follicle microenvironment. To complement this, an in vitro culture system similar to the in vivo environment must be constructed. It is necessary to develop a hair-on-a-chip that implements a fully functional hair follicle model by reproducing the main characteristics of hair follicle morphogenesis and cycle. In this review, we summarize the gradation of hair follicle morphogenesis and the roles and mechanisms of molecular signals involved in the hair follicle cycle. In addition, we discuss research results of various in vitro organoid products and organ-on-a-chip-based hair follicle tissue chips for the treatment of alopecia and present future research and development directions.

The Enrichment of Specific Hair Follicle-Associated Cell Populations in Plucked Hairs Offers an Opportunity to Study Gene Expression Underlying Hair Traits

Gene expression differences can assist in characterizing important underlying genetic mechanisms between different phenotypic traits. However, when population-dense tissues are studied, the signals from scarce populations are diluted. Therefore, appropriately choosing a sample collection method that enriches a particular type of effector cells might yield more specific results. To address this issue, we performed a polyA-selected RNA-seq experiment of domestic horse (Equus ferus caballus) plucked-hair samples and skin biopsies. Then, we layered the horse gene abundance results against cell type-specific marker genes generated from a scRNA-seq supported with spatial mapping of laboratory mouse (Mus musculus) skin to identify the captured populations. The hair-plucking and skin-biopsy sample-collection methods yielded comparable quality and quantity of RNA-seq results. Keratin-related genes, such as KRT84 and KRT75, were among the genes that showed higher abundance in plucked hairs, while genes involved in cellular processes and enzymatic activities, such as MGST1, had higher abundance in skin biopsies. We found an enrichment of hair-follicle keratinocytes in plucked hairs, but detected an enrichment of other populations, including epidermis keratinocytes, in skin biopsies. In mammalian models, biopsies are often the method of choice for a plethora of gene expression studies and to our knowledge, this is a novel study that compares the cell-type enrichment between the non-invasive hair-plucking and the invasive skin-biopsy sample-collection methods. Here, we show that the non-invasive and ethically uncontroversial plucked-hair method is recommended depending on the research question. In conclusion, our study will allow downstream -omics approaches to better understand integumentary conditions in both health and disease in horses as well as other mammals.

Single-cell chromatin landscapes of mouse skin development

The coat of mammals is produced by hair follicles, and hair follicle is an important and complex accessory organ of skin. As a complex physiological regulation process, hair follicle morphogenesis is regulated by a series of signal pathway factors, involves the interaction of multiple cell types and begins in the early embryonic stage. However, its transcriptional regulatory mechanism is unclear. We have therefore utilized single-cell ATAC sequencing to obtain the chromatin accessibility landscapes of 6,928, 6,961 and 7,374 high-quality cells from the dorsal skins of E13.5, E16.5 and P0 mice (Mus musculus), respectively. Based on marker gene activity clustering, we defined 6, 8 and 5 distinct cell types in E13.5, E16.5 and P0 stages, respectively. Furtherly, we integrated the fibroblasts and keratinocytes clusters, performed further analysis and re-clustered. The single cell map of the chromatin open area was drawn from each cell type and the mechanism of cell transcription regulation was explored. Collectively, our data provide a reference for deeply exploring the epigenetic regulation mechanism of mouse hair follicles development.

Sebaceous immunobiology - skin homeostasis, pathophysiology, coordination of innate immunity and inflammatory response and disease associations

This review presents several aspects of the innovative concept of sebaceous immunobiology, which summarizes the numerous activities of the sebaceous gland including its classical physiological and pathophysiological tasks, namely sebum production and the development of seborrhea and acne. Sebaceous lipids, which represent 90% of the skin surface lipids in adolescents and adults, are markedly involved in the skin barrier function and perifollicular and dermal innate immune processes, leading to inflammatory skin diseases. Innovative experimental techniques using stem cell and sebocyte models have clarified the roles of distinct stem cells in sebaceous gland physiology and sebocyte function control mechanisms. The sebaceous gland represents an integral part of the pilosebaceous unit and its status is connected to hair follicle morphogenesis. Interestingly, professional inflammatory cells contribute to sebocyte differentiation and homeostasis, whereas the regulation of sebaceous gland function by immune cells is antigen-independent. Inflammation is involved in the very earliest differentiation changes of the pilosebaceous unit in acne. Sebocytes behave as potent immune regulators, integrating into the innate immune responses of the skin. Expressing inflammatory mediators, sebocytes also contribute to the polarization of cutaneous T cells towards the Th17 phenotype. In addition, the immune response of the perifollicular infiltrate depends on factors produced by the sebaceous glands, mostly sebaceous lipids. Human sebocytes in vitro express functional pattern recognition receptors, which are likely to interact with bacteria in acne pathogenesis. Sex steroids, peroxisome proliferator-activated receptor ligands, neuropeptides, endocannabinoids and a selective apoptotic process contribute to a complex regulation of sebocyte-induced immunological reaction in numerous acquired and congenital skin diseases, including hair diseases and atopic dermatitis.

Identification of key pathways and genes that regulate cashmere development in cashmere goats mediated by exogenous melatonin

The growth of secondary hair follicles in cashmere goats follows a seasonal cycle. Melatonin can regulate the cycle of cashmere growth. In this study, melatonin was implanted into live cashmere goats. After skin samples were collected, transcriptome sequencing and histological section observation were performed, and weighted gene co-expression network analysis (WGCNA) was used to identify key genes and establish an interaction network. A total of 14 co-expression modules were defined by WGCNA, and combined with previous analysis results, it was found that the blue module was related to the cycle of cashmere growth after melatonin implantation. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that the first initiation of exogenous melatonin-mediated cashmere development was related mainly to the signaling pathway regulating stem cell pluripotency and to the Hippo, TGF-beta and MAPK signaling pathways. Via combined differential gene expression analyses, 6 hub genes were identified: PDGFRA, WNT5A, PPP2R1A, BMPR2, BMPR1A, and SMAD1. This study provides a foundation for further research on the mechanism by which melatonin regulates cashmere growth.

Understanding Mammalian Hair Follicle Ecosystems by Single-Cell RNA Sequencing

Simple Summary

Single-cell sequencing technology can reflect cell population heterogeneity at the single-cell level, leading to a better understanding of the role of individual cells in the microenvironment. Over the past few years, single-cell sequencing technology has not only made more new discoveries in the study of cellular heterogeneity of other rare cells such as stem cells, but has also become the most powerful research method for embryonic development, organ differentiation, cancer occurrence, and cell mapping. In this review, we outline the use of scRNA-seq in hair follicles. In particular, by focusing on landmark studies and the recent discovery of novel subpopulations of hair follicles, we summarize the phenotypic diversity of hair follicle cells and their links to hair follicle morphogenesis. Enhancing our understanding of the progress of hair follicle research will help to elucidate the regulatory mechanisms that determine the fate of different types of cells in the hair follicle, thereby guiding hair loss treatment and hair-producing economic animal breeding research.

Abstract

Single-cell sequencing technology can fully reflect the heterogeneity of cell populations at the single cell level, making it possible for us to re-recognize various tissues and organs. At present, the sequencing study of hair follicles is transiting from the traditional ordinary transcriptome level to the single cell level, which will provide diverse insights into the function of hair follicle cells. This review focuses on research advances in the hair follicle microenvironment obtained from scRNA-seq studies of major cell types in hair follicle development, with a special emphasis on the discovery of new subpopulations of hair follicles by single-cell techniques. We also discuss the problems and current solutions in scRNA-seq observation and look forward to its prospects.

Expression profile of long non-coding RNA in inner Mongolian cashmere goat with putative roles in hair follicles development

The hair follicle is a complex skin accessory organ, which determines hair growth. Long non-coding RNAs (lncRNAs) have been proven to play an important role in hair follicle development, but their specific mechanism is still unclear. In this study, high-throughput sequencing was used to obtain the expression profiles of lncRNA in the hair follicles of Inner Mongolian cashmere goats at different embryonic stages (45, 55, 65, and 75 days), and a total of 6,630 lncRNA were identified. According to the rules of hair follicle development, we combined miRNA and mRNA databases (published) and predicted lncRNA-miRNA, miRNA-mRNA, and lncRNA-mRNA interaction pairs in the 45 vs. 75 comparison group. We obtained 516 lncRNA-mRNA, 1,011 lncRNA-miRNA, and 7,411 miRNA-mRNA relationship pairs. Finally, target genes were analyzed by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG), and it was found that they were mainly enriched in the Wnt signaling pathway and PI3K-Akt signaling pathway related to hair follicle development, indicating that lncRNA may interact with miRNA/mRNA to directly or indirectly regulate the expression of genes related to hair follicle development. Dual-luciferase reporter gene analysis showed that lncRNA MSTRG.1705.1 could bind to Chi-miR-1, while lncRNA MSTRG.11809.1 had no binding site for Chi-miR-433. In conclusion, this study aims to further analyze the molecular regulation mechanism of hair follicle development and to lay a theoretical foundation for revealing the regulation mechanism of cashmere hair follicle growth.

Aging in the sebaceous gland

Sebaceous glands (SGs) originate from hair follicular stem cells and secrete lipids to lubricate the skin. The coordinated effects of intrinsic and extrinsic aging factors generate degradation of SGs at a late age. Senescence of SGs could be a mirror of the late aging of both the human body and skin. The procedure of SG aging goes over an initial SG hyperplasia at light-exposed skin areas to end with SG atrophy, decreased sebum secretion, and altered sebum composition, which is related to skin dryness, lack of brightness, xerosis, roughness, desquamation, and pruritus. During differentiation and aging of SGs, many signaling pathways, such as Wnt/β-catenin, c-Myc, aryl hydrocarbon receptor (AhR), and p53 pathways, are involved. Random processes lead to random cell and DNA damage due to the production of free radicals during the lifespan and neuroendocrine system alterations. Extrinsic factors include sunlight exposure (photoaging), environmental pollution, and cigarette smoking, which can directly activate signaling pathways, such as Wnt/β-catenin, Notch, AhR, and p53 pathways, and are probably associated with the de-differentiation and hyperplasia of SGs, or indirectly activate the abovementioned signaling pathways by elevating the inflammation level. The production of ROS during intrinsic SG aging is less, the signaling pathways are activated slowly and mildly, and sebocytes are still differentiated, yet terminal differentiation is not completed. With extrinsic factors, relevant signaling pathways are activated rapidly and fiercely, thus inhibiting the differentiation of progenitor sebocytes and even inducing the differentiation of progenitor sebocytes into keratinocytes. The management of SG aging is also mentioned.