Status of Research on the Development and Regeneration of Hair Follicles

Abstract

Hair loss, or alopecia, is a prevalent condition in modern society that imposes substantial mental and psychological burdens on individuals. The types of hair loss include androgenetic alopecia, alopecia areata, and telogen effluvium; among these, androgenetic alopecia is the most common. Traditional treatment modalities mainly involve medical options, such as minoxidil and finasteride, and surgical interventions, such as hair transplantation. However, these treatments still have many limitations. Therefore, exploring the pathogenesis of hair loss, specifically focusing on the development and regeneration of hair follicles (HFs), and developing new strategies for promoting hair regrowth are essential. Some emerging therapies for hair loss have gained prominence, including low-level laser therapy, micro-needling, fractional radiofrequency, platelet-rich plasma, and stem cell therapy. The aforementioned therapeutic strategies appear promising for hair loss management. This review investigates the mechanisms underlying HF development and regeneration, examining the structure, development, cycle, and cellular function of HFs. Additionally, it analyzes the symptoms, types, and causes of hair loss as well as its current conventional treatments. This study provides an overview of the most effective regenerative medicine-based therapies for hair loss.

1. Introduction

With the rapid increase in social demands and work-life pressure, hair loss has become a common and increasingly severe problem. Various factors, such as trauma, mental stimulation, genetics, endocrine imbalance, physical stress, and chemical exposure, can lead to severe hair loss, affecting patients' appearance and mental health.

Currently, the first-line treatment for hair loss involves the use of minoxidil and finasteride. However, these two drugs require long-term usage, which may result in drug tolerance and other side effects. Moreover, hair loss often recurs upon discontinuation of the drugs. Although hair follicle (HF) transplantation has been gradually gaining acceptance, the low availability of HF donor sites limits patient satisfaction. With the advancement of regenerative medicine, various treatment modalities have been introduced to safely and effectively resolve the problem of hair loss. These modalities include low-level laser therapy (LLLT), micro-needling, platelet-rich plasma (PRP) therapy, and stem cell therapy. Modern treatments have opened new avenues for addressing challenges associated with traditional hair loss treatment. This review analyzes the types and causes of hair loss by exploring the molecular mechanisms underlying the development and regeneration of HFs. Herein, we discuss new treatment strategies for enhancing hair loss treatments. Our review provides a theoretical basis for the future application of stem cell therapy for hair loss.

2. Histomorphology of the Hair Follicle

Hair follicles are small skin organs that are distinctive traits of mammals. Hair growth is driven by cellular activity within HFs, which serve as the fundamental unit of hair. Although different mammalian HFs exhibit different morphological forms, they share similar structural features. The HF is divided into two portions by a boundary known as the bulge. The upper portion of the HF includes the infundibulum and isthmus, while the lower portion consists of the bulb, including protrusions, hairballs, and papillae. Notably, the lower portion constitutes approximately one-third of the HF and participates in the hair growth cycle. In contrast, the upper portion does not participate in the cycle, as it undergoes minimal apoptosis and regeneration in the follicular phase. Thus, the upper portion is known as the permanent component, whereas the lower portion is called the cycling component.

Structure of a Hair Follicle

The layers of the HF, extending from the outside to the inside, include the outer hair root sheath (ORS), companion layer, inner hair root sheath (IRS), and hair shaft (HS). HF structures, such as the IRS and HS, are formed by hair matrix cells that wrap around the HFs in the body to create the hair dermal papilla (DP). The infundibulum, isthmus, bulge, and bulb of the HF are part of the HF epidermis and originate from the ectoderm. The DP, a raised structure formed by mesoderm-derived dermal tissue connected to the hairball, determines the hairball size, HS diameter and length, and hair growth duration. The HS is at the center of the HF epidermis, and the entire epidermis is surrounded by a connective tissue sheath of mesodermal origin.

Structure of a Hair Follicle

3. HF Morphogenesis and Development

The morphogenesis and development of HFs depend on epidermis-interstitial interactions. This process is regulated by various signaling pathways, particularly the synergistic actions of the Wnt, bone morphogenetic protein (BMP), hedgehog, transforming growth factor (TGF)-β, fibroblast growth factor (FGF), and Notch signaling pathways. The Wnt/β-catenin signaling pathway is considered pivotal for HFs transitioning from the telogen phase to the anagen phase. This pathway is involved in all the stages of HF development and determines the differentiation fate of HF cells during development.

The morphogenesis and development of HFs proceed through three sequential stages: induction (production of hair placodes), organogenesis (downward growth of hair placodes), and cytodifferentiation (morphogenesis of HFs). The dermis provides the initiation signal for HF development. In the embryonic stage, epidermal stem cells rapidly proliferate and differentiate to form hair placodes under the influence of signaling pathways such as the Wnt and BMP pathways. Hair placodes are a series of regular plate-like structures, and their formation indicates the commencement of HF development.

Stages of Hair Follicle Development

4. Hair Follicle Cycle

The morphology of mammalian HFs changes periodically, resulting in the division of HF growth into anagen, catagen, and telogen phases. Each phase is tightly regulated and characterized by substantial changes in gene expression, cell proliferation, and cell differentiation. During the anagen phase, the HF produces a complete shaft from the top to the root. The anagen phase determines the length of hair and depends on the continued proliferation and differentiation of stromal cells at the base of the follicle.

Phases of Hair Growth Cycle

5. Mechanisms Underlying Hair Follicle Regeneration

HF regeneration is based on complex signal interactions between the HF stem cell pool and the hair DP. Ectoderm stem cells serve as the primary cell source for HF regeneration. Mesenchymal hair papilla cells regulate this process by secreting signaling molecules. HF stem cells typically remain quiescent but can proliferate rapidly when stimulated by damage or growth signals, producing numerous transit amplifying cells and postmitotic differentiating cells, which play essential roles in skin damage repair and HF reconstruction.

6. Current Hair Loss Situation

Hair loss is a common and treatable condition that has evolved into a chronic problem, affecting a substantial portion of the population. The demand for hair loss treatments has continually increased, reducing the incidence of mental and physical health disorders and the financial burden associated with hair loss. However, very few drug regimens have been approved by regulatory agencies for clinical use, primarily corticosteroids, minoxidil, and 5-alpha-reductase inhibitors (finasteride and dutasteride).

7. Causes of Hair Loss

Common types of hair loss include androgenetic alopecia (AGA), alopecia areata (AA), and telogen effluvium (TE). Among these types, AGA is the most prevalent, characterized by hair thinning and receding hairlines. AA is an autoimmune-related hair loss disease characterized by sudden localized patchy alopecia, while TE results from the disruption of the HF cycle, causing a large number of hairs in the anagen phase to enter the telogen phase simultaneously.

8. Conventional Treatments for Hair Loss

Hair loss treatment is a crucial aspect of clinical dermatology. In recent years, many treatments have been proven to promote hair regrowth. Traditional drug and surgical interventions continue to play crucial roles in addressing hair loss.

Drug Therapy

To date, minoxidil and finasteride are the main drugs approved by the US Food and Drug Administration (FDA) for the treatment of AGA. Minoxidil is a potassium channel opener that exerts a vasodilating effect, extending the duration of the anagen phase and inducing angiogenesis around the HFs. Finasteride, on the other hand, inhibits the conversion of testosterone to dihydrotestosterone (DHT), which is implicated in hair loss.

Conclusion

The research on hair follicle development and regeneration is advancing rapidly, providing new insights into the mechanisms underlying hair loss and potential therapeutic strategies. Emerging therapies, particularly those based on regenerative medicine, offer promising avenues for effective hair loss treatment. Continued exploration of these strategies may lead to more effective solutions for individuals struggling with hair loss, ultimately improving their quality of life.

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References

1. Status of research on the development and regeneration of hair follicles. NCBI Link