Causes and therapeutic limitations of clinical alopecia and the advent of human pluripotent stem cell follicular transplantation

Abstract

Hair loss is a prevalent condition that affects many people worldwide and is often associated with self-neglect and anxiety. Hair transplantation is emerging as a viable therapeutic option for addressing several forms of hair loss that are resistant to pharmacological treatments. The etiology of hair loss is multifactorial and includes genetic predisposition, psychological factors, exposure to toxic agents, nutritional deficiencies, and mechanical stress. However, existing treatments, including topical medications, oral therapies, and phototherapy, often fail to provide comprehensive solutions. As a result, hair transplantation is currently the primary option for patients seeking a permanent solution to their hair loss. In this study, we systematically evaluated different hair follicle transplantation techniques and analyzed their respective advantages and limitations. Our results show that challenges such as limited hair follicle supply, suboptimal follicle survival rates, and recurrent hair loss hinder the effectiveness of transplantation, leaving many patients unsatisfied with the available options. Fortunately, advances in regenerative medicine, particularly the development of single-cell-integrated hair follicles or skin organoids derived from induced pluripotent stem cells, offer promising alternatives. These innovations have the potential to provide a substantial supply of uniform hair follicles tailored to the specific needs of individual patients, thereby expanding treatment options for those inadequately served by conventional methods. In summary, apart from a limited range of topical and systemic treatments, hair transplantation is often a last resort for people with hair loss. The potential to generate new hair follicles or organoids heralds a new era for large-scale clinical applications and the establishment of standardized treatment protocols.

Introduction

Hair loss is an emotionally distressing condition affecting many individuals worldwide. According to Feng’s survey [1], 60.3% of men and 46.7% of women in Wuhan (Hubei Province, China) experience hair loss, highlighting its significant prevalence across genders. These findings suggest that nearly half of the population is affected by hair loss. Individuals suffering from hair loss often face challenges such as hair shedding, an exposed scalp, an altered appearance, and potential ridicule, all of which may contribute to mental health issues. Research indicates that individuals with hair loss frequently experience stigmatization [2], chronic stress, and depression [3, 4]. Furthermore, depression associated with hair loss can significantly reduce a person’s quality of life, with prevalent symptoms including sleep disturbance, fatigue, and loss of appetite [5].

Despite medical interventions, some cases of hair loss remain resistant to treatments such as corticosteroids, minoxidil, Janus kinase (JAK) inhibitors, and phosphodiesterase 4 inhibitors [6–9]. In such cases, hair transplantation (HT) becomes necessary to achieve cosmetic restoration. For individuals with extensive hair loss, pharmacological treatments often prove inadequate, making HT the most viable option for permanent aesthetic improvement [10]. Studies have shown that HT can significantly improve patients’ quality of life and self-esteem [11]. Additionally, evaluations of loneliness, anxiety, and depression levels in patients undergoing HT indicate that successful procedures can lead to positive psychosocial outcomes [12]. However, HT remains inadequate for many patients due to limitations such as insufficient donor hair follicles, low graft survival rates, poor blood supply, and recurrent hair loss [13, 14].

Fortunately, human induced pluripotent stem cells (iPSCs) and their derived organoids have shown substantial potential in biological research and clinical applications, including the development of brain, retina, kidney, liver, and lung organoids [15]. Significant advancements have also been made in the study of hair follicle organoids [16, 17], with researchers successfully integrating skin organoids derived from planar hair-bearing skin into nude mice. These breakthroughs suggest that iPSC-derived single-cell-integrated hair follicles or skin organoids could offer new solutions for patients. By providing hair follicles tailored to an individual’s needs, these innovative approaches may overcome the limitations of current transplantation techniques.

In this review, we summarize diseases associated with hair loss, analyze recent advancements and shortcomings in HT technology, and integrate regenerative medicine research into the field of hair follicles. We aim to provide new insights and potential research directions for other researchers in this field.

Overview of hair loss

Hair loss is a symptom that can arise from various underlying causes, including chemical toxins, inadequate nutritional intake, hormonal dysregulation, tumors, neurological or psychological disorders, infections, and mechanical forces [18]. Broadly, hair loss can be categorized into two types: non-scarring and scarring. Moreover, there are varying degrees of hair loss; therefore, the treatment plan should focus not only on the extent of hair loss but also on identifying its underlying causes, which may be key to effective treatment. Due to the diverse causes of hair loss, different treatment options may be required.

Chemical toxins

Many immune-related diseases result from the dysregulation of the autoimmune system, which causes it to indiscriminately attack its own antigens. If this attack targets the hair follicles, it can lead to hair loss [19–21]. Chemicals or toxins can accumulate in the hair follicles, leading to follicle damage and hair loss. Drug-induced hair loss is not uncommon; in addition to cancer therapies, monoclonal antibodies, as well as certain antibiotics and antiviral agents, can contribute to conditions such as alopecia areata, scarring alopecia, and psoriatic alopecia, among others [22]. Chemotherapy agents, which target highly proliferative cells, often affect hair in the anagen phase. The typical chemotherapy agents include alkylating agents, anthracyclines, antibiotics, antimetabolites, vinca alkaloids, and taxanes [23]. In patients undergoing chronic hemodialysis, excessive hair loss may be associated with the use of dalteparin for anticoagulation; however, the underlying mechanism remains unclear, and discontinuing dalteparin may facilitate recovery [24].

Inadequate nutritional supply

There is a rich network of capillaries under the scalp, and hair growth requires a supply of nutrients from these blood vessels. However, some diseases can lead to occlusion or atrophy of the blood vessels in the scalp, ultimately resulting in hair loss. Microspheres used for selective endovascular embolization are widely employed in the treatment of intracranial hemorrhage; however, rare complications such as skin necrosis and alopecia may occur [25]. Post-burn scarring alopecia, characterized by poorly vascularized and fibrotic scar tissue, presents a challenging treatment scenario. Combining nano-fat injection with follicular unit extraction (FUE) HT presents a promising therapeutic approach [26]. The utilization of nano-fat as a regenerative source has shown potential benefits. However, there remains considerable scope for further enhancement and optimization of this technique. A study points out that the top etiologies of female alopecia include nutrient deficiencies, with iron deficiency (ID) identified as the most important factor, accounting for 70.3% of cases. After iron supplementation, patients reported subjective improvement in hair regrowth [27].

Hormonal dysregulation

Hormones are substances that regulate the physiological activity of the entire body, maintaining the function of organs by influencing the activity of cells. Abnormalities in endocrine organs can disrupt multiple bodily functions; such dysfunctions can impact various physiological processes and may result in hair loss. For example, female pattern hair loss (FPHL) can occur in women with polycystic ovary syndrome (PCOS) [28]. The dermal component cells within hair follicles contain type II 5α-reductase, which can convert circulating testosterone in the blood into dihydrotestosterone in that area [29]. Then the dihydrotestosterone can bind to intracellular androgen receptors, resulting in progressive miniaturization of the hair follicles and hair loss. Although guidelines for treating FPHL have been well established, there are no standardized treatment protocols for patients with PCOS who experience hair loss.

Tumor-induced hair loss

A primary tumor may metastasize to the scalp and cause neoplastic alopecia (NA) [30]. The pathogenetic mechanism of NA may involve the destruction of pilosebaceous units due to a desmoplastic reaction caused by tumor cells. This reaction leads to an abundance of collagen fibers that compress the surrounding normal tissues, thereby affecting their blood supply. An alternative hypothesis suggests that the tumor mass exerts pressure on the tissue, compromising the survival and development of the pilosebaceous units. A common treatment for cancer, oncological radiotherapy, could also cause alopecia [31]. Traditional medicine prioritizes saving lives; however, the psychological needs of patients may be ignored, leaving them in pain and feeling helpless. Therefore, a proper HT program could improve their quality of life.

Nervous or mental disorders

Neurological or psychiatric disorders may also affect hair follicles. Cephalgia alopecia is a rare headache disorder in which headaches cause recurrent activation of the trigeminal and upper cervical branches, leading to the cessation of hair growth [32]. Trichotillomania involves patients repetitively pulling out their hair, which results in hair loss and functional impairment [33].

Infections

Pathogenic infections can also cause hair loss. Syphilitic alopecia is a rare type of hair loss characteristic of secondary syphilis. The lesions manifest as multiple, irregular, non-scarring patches of hair loss that appear in a characteristic moth-eaten pattern. The key point is that serology tests are positive for treponemal and anti-treponemal antibodies [34, 35]. Numerous spirochetes around blood vessels and the perifollicular epithelium may trigger an inflammatory response that leads to hair loss. Early treatment with anti-syphilis therapy can result in complete recovery.

Mechanical forces

Mechanical tension can cause traction alopecia; it may be related to tight braids, locks, and other hairstyles [36]. This is a reversible condition that resolves when the tension is reduced. However, long-term traction alopecia can develop into permanent scarring hair loss, in which case HT may be helpful.

Here, we summarize some characteristics of different types of alopecia (Table 1). In addition to primary alopecia, many other conditions can accompany hair loss. We have summarized several diseases that can cause hair loss (Table 2), illustrating that hair lo·ss is never a straightforward issue. As the last line of defense against alopecia, HT faces many difficulties and challenges.

Table 1.

Characteristics of primary cicatricial alopecia

Scarring or non-Scarring	Type	Subtype	Pattern hair loss
Non-scarring alopecia	AGA	Female pattern hair loss	Yes
		Male pattern hair loss	Yes
	AA	Patchy alopecia areata	Yes
		Alopecia reticularis	Yes
		Alopecia totalis	Yes
		AU	Yes
		Diffuse hair loss	Yes
		Ophiasis	Yes
		Central alopecia Areata (sisaipho)	Yes
		perinevoid alopecia	Yes
		Beard alopecia	Yes
		Eyebrow and eyelash alopecia	Yes
	TE		No
	AE		Probably
	Congenital triangular alopecia		Yes
Scarring alopecia	Lymphocytic cicatricial alopecia	Frontal fibrosing alopecia	Yes
		CCCA	Yes
		LPP	Probably
	Neutrophilic cicatricial alopecia	Folliculitis decalvans	Probably
		Dissecting cellulitis	Probably

AGA: androgenetic alopecia; AA: alopecia areata; AU: alopecia universalis; TE: telogen effluvium; AE: anagen effluvium; CCCA: central centrifugal cicatricial alopecia; LLP: lichen planopilaris

Table 2.

Diseases causing hair loss

Hair loss-related diseases	Mechanisms	Treatments	Reference
GD	Autoimmune thyroid disorder	Hair loss would self-heal after the underlying disease is cured	2025 [19],
Hair loss with nutrition	Nutrient deficiency	Iron supplements or other nutrition supplements	2023 [27],
CIA	Cytotoxic chemotherapy agents unintendedly targeting hair matrix cells	Limited, no effective treatment available	2012 [23],
RIA	Ionizing radiation	Limited, effective RIA-preventive therapy is unavailable	2023 [31],
Psoriatic alopecia	Autoimmune disease	Modulation of immunity	2015 [21],
impetigo	inflammation	anti-infective response	1914 [75],
Lupus erythematosus alopecia	Autoimmune response	Modulation of immunity	2019 [20],
Drug-induced hair loss	Side effect of medication	Reducing the dose or discontinuation, discontinuing the causative drug, and monitoring hair regrowth	2023 [22],
Ischemic alopecia	Insufficient blood supply	Medication to improve blood supply	2023 [25],
TA	Mechanical damage	Topical corticosteroid medications (liquids, ointments, or oils), tension-related hairstyles should be discontinued	2023 [36],
PCOS	Androgen activity	Anti-androgen medication, minoxidil treatment	2023 [28],
NA	Tumor metastasizes to the skin of the scalp	Limited, no effective treatment available	2021 [30],
Alopecia in anticoagulated patients	Unclear	Regular hair growth can be restored by replacing low-molecular-weight heparin with regional citrate anticoagulation	2001 [24],
Cephalgia alopecia	Activation of trigeminal and upper cervical branches	Onabotulinumtoxin A can be used to treat both headache and hair loss	2020 [32],
Trichotillomania	Obsessive-compulsive disorder	Habit reversal therapy and medication, tricyclic antidepressant clomipramine, behavioral therapy, glutamatergic agents, antipsychotic medications, and cannabinoid agonists	2016 [33],
Post-burn scarring alopecia	Poorly vascularized and fibrotic scar	Treat burns underlying disease, FUE hair transplantation	2023 [26],
Syphilitic alopecia	Inflammation	Modulation of immunity	2013, 2018 [34, 35],
APMR	Autosomal recessive disorder	Limited global cases, unclear treatment methods, focus on intellectual rehabilitation for the underlying disease, genetic counseling prevention, supportive therapy	2008, 2021 [76, 77],
Pressure-induced alopecia	Pressure exerted by the volume of injectables	Self-limited and reversible	2024 [78],

GD: Graves’ disease; CIA: chemotherapy-induced alopecia; RIA: radiotherapy-induced alopecia; TA: traction alopecia; PCOS: polycystic ovarian syndrome; NA: neoplastic alopecia; APMR: alopecia mental retardation syndrome

The challenges and limitations of HT

Alopecia is a common phenomenon, but the mechanisms behind it are complex and varied, presenting significant challenges for understanding and addressing the disorder. Regardless of the cause, HT can help address the disfigurement caused by hair loss in the mid to late stages. While pharmacological treatments can only stop the progression of hair loss, regrowing lost hair on a bald scalp is nearly impossible [8, 9]. There is currently no effective treatment to prevent or halt hair loss due to anagen effluvium (AE). For example, exposure to X-rays exceeding 30 Gy can permanently damage hair follicles, resulting in injury to the anagen hair shafts and destruction of hair follicle stem cells [37]. In such cases, HT may be necessary to improve the quality of life. Here, we present a summary of HT options from recent studies, along with an analysis of their respective strengths and weaknesses (Table 3). Despite the gradual advancements in medical conditions, HT continues to encounter several limitations and challenges.

Table 3.

Evolution of HT schemes

	HT	Recipients	Advantages	Weak points	Reference
Clinical trials	4-mm punch grafts	Humans	Long-term growth	“Pluggy” appearance	2021 [79],
	FUT	Humans	mimicking Normal scalp growth	Leaves long thin scars	2021 [79],
	FUE	Humans	No sutures and no linear scars	Susceptible to damage to donor hair follicles	2021 [79],
	Combining nano-fat injection and FUE hair transplantation	Humans	Improvement of blood supply to the graft area	Contraindicated in patients with total baldness	2023 [26],
	Injection of DSC cells	Humans	Less traumatic, promotes hair regrowth,	Positive effect was temporary	2020 [80],
Research experiments	Robotic hair transplantation	Spherical phantom of the scalp	reduces time Reduces the duration that grafts remain without a blood supply,	Individual differences not yet taken into account	2024 [81],
	Hair-bearing human skin generated entirely from pluripotent stem cells	Nude mice	self-assembles in vitro, and can be used to reconstitute skin in vivo Produces many spare follicles that can be accessed at any time, permits gene therapy	Long culture cycles and inconsistent graft success rate	2020 [16],
	Bioprinting of hair follicle germs	Mice	Automation, mass production, ability to customize scalp patches in personalized shapes	Simple structure, imperfect functionality	2023 [82, 83],
	MSC therapy	Mice	Modulation of immunity and reduced inflammation	Can only be used as an adjunctive therapy	2021 [84],
	Vascularized DP spheroids	Mice	Improves hair induction efficiency following engraftment	No hair growth seen after transplantation	2022 [85],

HT: hair transplantation; FUT: follicular unit transplantation; FUE: follicular unit excision; DSC: dermal sheath cup; MSCs: mesenchymal stem cells

Lack of hair follicle source

The traditional method of HT involves moving healthy hair follicles from a relatively healthy scalp to a bald scalp. However, the suitability of HT is assessed based on several factors, including the extent and pattern of hair loss, the density of hair in the donor area, the age of the patients, their overall health, and their expectations from the procedure [38]. Sometimes, the most troublesome issue is the absence of healthy hair follicles in the patient. HT is not recommended for diffuse unpatterned alopecia (DUPA) due to the lack of a donor area [38]. However, this does not mean that such cases are beyond treatment. DUPA is not an isolated case; the terminal stage of male pattern baldness is also a condition that may lack sufficient self-sourced hair follicles for transplantation. Traditional follicular transplantation involves removing healthy hair follicles, which can create scarring or keloids in the area of the original healthy hair, and these areas may never grow new hair follicles [39, 40].

To solve the problem of deficiency in hair follicle sources, many solutions have been proposed; unfortunately, the issue remains unsettled. In the case of extensive scalp hair loss, some doctors consider body hair transplantation (BHT) as an alternative when there may be insufficient remaining hair follicles to be transplanted to the bald area [41]. Transplanting hair from other parts of the body is a more challenging procedure that requires greater surgical proficiency, which can lead to uncertainty about the outcome. Moreover, body hair tends to differ in texture and growth patterns compared with scalp hair, resulting in a hairstyle that may not match the natural appearance due to the inherent characteristics of body hair, such as being softer, curlier, and limited in length. The transplantation of artificial hair has been proposed as a supplement for insufficient hair; however, the current state of science and technology yields sub-optimal results. Furthermore, the studies supporting the application of artificial hair fibers are not reliable, as important design elements, such as randomization and control groups, are absent and some measurement methods seem inappropriate [42].

Low survival rate of hair follicles and deficient blood supply

Patient-sourced hair follicles cannot be kept in vitro for too long. To increase the survival rate of hair follicles, cleaning them may help [43]. Additionally, the use of a derma roller can improve surgical results [44]. However, the survival rate of hair follicles in vitro remains a challenge; it decreases sharply when the hair follicles are preserved for more than 48 h before implantation [45]. The survival rate of hair follicles is a significant issue that reduces the effectiveness of HT.

Adequate blood supply is essential for hair follicles; however, in many cases, it cannot be guaranteed. Skin thickness and firmness are major factors that affect how difficult transplantation will be, which in turn influences the rate of graft attrition [46]. For example, HT for lichen planopilaris (LPP) and frontal fibrosing alopecia (FFA) is feasible, but the results may be less favorable than those for HT for non-scarring alopecia [47]. Due to poor vascularization in fibrotic scar tissue, the viability of grafts is limited [26]. A study reported cases of scalp necrosis in the recipient area, which is a complication that reduces graft survival rates and can cause irreversible damage. It may also lead to osteomyelitis of the cranium, a life-threatening condition [48]. Insufficient blood supply is often cited as a contributing factor.

To assess the difficulty of different hair transplants, doctors have developed rating scales based on hair types and skin types [49]. To minimize damage and optimize outcomes, surgeons have explored blade design, depth, and puncture angle [50]. To achieve better results, recommendations for operation procedures have been proposed, including the choice of implanter, graft placement and insertion techniques, hemostasis protocols, staff training, and medical collaboration [51]. Although doctors have made extensive efforts to improve outcomes, “seeds cannot sprout in barren soil.”

Complications of HT

As a surgical procedure, HT is inevitably associated with certain risks. Common postoperative effects of the procedure include donor area erythema, perifollicular crusts, perifollicular whitish halos, perifollicular erythema, white circles, recipient area erythema, repilation black dots, dystrophic hairs, folliculitis, and yellow dots [52]. Two to four days after the HT procedure, edema may appear on the forehead and upper eyelids [53]. Moreover, some rare complications can also occur, such as pseudolymphomatous folliculitis [54] and Mycobacterium abscessus scalp infections [55].

A common complication of surgical procedures is infection, and the scalp, which is constantly exposed to the air and sheltered by hair, is susceptible to pathogenic microorganisms. Consequently, various inflammatory reactions can arise post-transplantation. Inflammation-related complications after HT may include LPP, erosive pustulosis of the scalp (EPS), and superficial folliculitis (SF) [56]. More severe cases can occur; for example, there have been reports of nodulocystic lesions that have been mistakenly transplanted from the donor area to the recipient area, resulting in numerous chronic recurrent and aggressive cyst-induced scars on the recipient scalp [57]. Additionally, traumatic arteriovenous fistulae can occur after HT, causing progressively enlarged swelling, pain, and functional impairment [58].

Although HT often yields satisfactory results, it is important to note that the majority of patients will experience shedding within four years [59]. Additionally, non-scarring alopecia patches may also appear in the donor area after FUE, showing classic signs of hair breakage, including exclamation marks and black dots [60]. Patients without enough donor hair follicles may only achieve sparser follicle grafts. Despite advancements in mechanization and automation within various medical fields, these technologies have not yet surpassed traditional surgical methods in HT, and manual techniques remain essential.

Potential of iPSCs in improving HT outcomes

While HT has offered hope to some alopecia patients, its development faces significant challenges. These include the insufficient availability of hair follicles, inadequate blood supply, hair shedding, and complications such as infections. However, with the emergence of iPSCs in medical research, new hope has arisen for HT, offering renewed possibilities for patients who had previously been given up. Studies have confirmed that progenitor-cell-enriched micrografts can increase hair count and density and, to some extent, mitigate the shortened growth phases and prolonged telogen phases [61].

Cell therapy may be a possibility

In our laboratory, we discovered that isolated cells from the skin of newborn mice could develop into robust hair follicles when transplanted into nude mice (Fig. 1). We processed neonatal mouse skin into keratinocytes and dermal cells and then injected these into the dermis of nude mice, where healthy hair growth was observed, demonstrating the potential for reconstructing hair follicles from these cells. However, the underlying mechanism of this phenomenon remains unclear.

Fig. 1.

Reconstruction of hair follicles for hair regeneration in mice. Skin tissues were harvested from neonatal mice, followed by the separation of dermal and epidermal cells to disrupt the original hair follicle structure. The mixed cells were then transplanted into nude mice, and hair growth was monitored post-operation. The data presented in this figure were generated by Hang Zhou

Coincidentally, other researchers are also interested in this phenomenon. They found that enhancing specific gene expressions may provide more robust hair follicles and improve the efficacy of HT. Lei et al. [62] isolated cells from newborn mice, remixed them, and cultured them to form organ tissues containing hair primordia. During the integration of hair follicles into mouse models, they explored the patterns involved and revealed several factors that may promote hair regrowth and remodeling [62]. They found that continuously inhibiting protein kinase C signaling and the timely supply of growth factors, Wnts, and matrix metalloproteinases (MMPs) may restore morphological transitions and rescue the hair-forming ability. Moreover, Chen et al. [63] found that epidermal stem cells (Epi-SCs) and skin-derived progenitor cells (SKPs) could integrate into a bilayer structure resembling the epidermis and dermis, with the PI3K-Akt signaling pathway significantly upregulated in Epi-SCs. It was suggested that the PI3K-Akt signaling pathway plays a crucial role in hair follicle regeneration, using mice as a model, and that this discovery may herald potential therapeutic applications that can enhance hair regeneration. Culturing hair follicle stem cells and identifying optimal additives to maintain favorable conditions for cell transplantation require further investigation. Additional research is essential to refine clinical solutions for hair loss and mitigate patient suffering.

iPSC-derived organoids provide robust hair follicle candidates

Follicles derived from iPSCs have the potential to supply sufficient follicles for patients with no remaining hair follicles, offering renewed hope to those who had previously despaired. Numerous iPSC-derived cell or organoid products have been used in clinical therapy trials, and their safety has been substantiated (Table 4), which provides confidence for the future application of hair follicle organoids. Our laboratory is also engaged in constructing hair follicle organoids, with microscopic examination revealing hair growth (Fig. 2).

Table 4.

Clinical trials of iPSC- and hESC-derived cells

Disease	Application of iPSCs or hESCs	Outcomes	Clinical trial phase	Reference
B-cell lymphoma	iPSC-derived CD19-directed CAR NK cells	A potent platform for cancer treatment	I	2025 [86],
Advanced GA	Human-embryonic-stem cell-derived RPE	Safe and well tolerated in treating advanced dry age-related macular degeneration	I/ IIa	2024 [87],
Type 1 diabetes	CiPSC islets	Stable glycemic control, time-in-target glycemic range at > 98%, and glycated hemoglobin at around 5%	I	2024 [88],
alloPTR	iPSC-derived platelets	Safe during an observation period of 1 year	I	2022 [89],
IC	hESC-derived MSCs	Visual pain analog scale improved	I	2022 [90],
Type 1 diabetes	Pluripotent-stem-cell-derived pancreatic endoderm cells	Fasting C-peptide levels, glucose-responsive C-peptide levels, and meal-stimulated C-peptide secretion improved	I/ II	2021 [91, 92],
SR-aGvHD	iPSC-derived MSCs	Safe and well tolerated	I	2020 [93],

iPSC: induced pluripotent stem cell; hESCs: human embryonic stem cells; CAR: chimeric antigen receptor; NK: natural killer; RPE: retinal pigmented epithelium; GA: geographic atrophy; CiPSC islets: chemically induced pluripotent-stem-cell-derived islets; alloPTR: alloimmune platelet transfusion refractoriness; IC: interstitial cystitis; MSCs: mesenchymal stem cells; SR-aGvHD: steroid-resistant acute graft versus host disease

Fig. 2.

Morphological changes of skin organoids (SKOs) during culture. (A) Morphological changes in SKOs cultured from days 3 to 119 were examined using a light microscope. (B) The morphology of SKOs on day 155 was observed using a polarized light microscope. (C) The morphology of SKOs on day 256 was also observed using a polarized light microscope. The data presented in this figure were generated by Yu-Xuan Zhang

Organoids derived from iPSCs may address issues related to inadequate blood supply. The low survival rate of follicles transplanted into fibrotic scar tissue is significantly influenced by poor vascularization [17, 26]. Hair follicle organoids have the potential to establish a self-assembled vascular network, thereby improving the success rate of surgical interventions. Research has demonstrated that iPSCs can differentiate into human organoid-derived endothelial cells (ECs), which have been used to reorganize perfused capillaries and form a chimeric vascular network with blood vessels derived from chicken embryos [64]. This study suggests that iPSC-derived organoids could potentially create a chimeric vascular network with human skin.

Many research teams are working to overcome the challenges of hair follicle organoid construction. Lee et al. [16] constructed a human cyst-like skin organoid consisting of a stratified epidermis, a fat-rich dermis, and pigmented hair follicles equipped with sebaceous glands. When transplanted onto nude mice, these organoids can form planar hair-bearing skin. The hair follicle organoid they constructed possesses nearly all the cutaneous appendages that a follicle should have, but it has a very long production time of about 4 to 5 months. Some scholars have generated hair-skin tissue exclusively from a homogeneous population of human pluripotent stem cells in a three-dimensional in vitro culture system, and this tissue can be maintained in vitro for 150 days [65]. In their experiments, the culture system allowed for the gradual differentiation of a population of human pluripotent stem cells into ectodermal and cranial neural crest cells, which gave rise to the epidermis and dermis and subsequently to hair follicles, ultimately forming a stratified layer of skin with pigmented hair follicles, sebaceous glands, Merkel cells, and sensory neurons. Another research team generated skin organoids (SKOs) from human skin fibroblasts or placental CD34⁺-cell-derived iPSCs [66].

Currently, researchers are highly optimistic about the application prospects of hair follicle organoids. As the cultivation and study of these organoids continue to advance, their clinical translation is expected to offer significant benefits for individuals suffering from hair loss.

Bioengineering and microfluidic technologies optimizing the growth process of hair follicle organoids

Integrating biological and engineering principles may advance theoretical research into practical applications. Cultures of pluripotent-stem-cell-derived skin organoids (PSOs) are often generated in fluid 3D environments. However, due to fluid tension or other factors, the dermis is located outside the sphere while the epidermis is inside, constituting an analog of clinically termed epidermal cysts. This arrangement leads to internal necrosis, which restricts growth. Quílez et al. [67] increased the probability of hair follicle formation by limiting the inward growth of skin organoids and enhancing the efficiency of keratinocyte differentiation through spindle microfluidics, which improved the formation of dermal organoids. By using stem cells to self-organize organoids, Wang et al. [68] discovered the mechanism behind epidermal-dermal interaction. They found that the native contraction force of dermal cells provides a stretching force to the corresponding epidermal cells, activating the stretching force sensor Piezo1. Subsequently, epidermal Piezo1 triggers the initial mesenchymal-epithelial interaction (MEI). Undoubtedly, this mechanism represents merely a fraction of the numerous potential mechanisms involved. Enhancing cultivation efficiency and stability necessitates further comprehensive investigation.

Biomimetic co-culture methods have been investigated to produce higher-quality human dermal papilla cell aggregates (hDPA) with enhanced hair-inducing properties, potentially improving organoid regeneration efficiency and facilitating large-scale clinical applications [69]. A microfluidic-assisted technique has also been developed that allows for the encapsulation of both mouse mesenchymal stromal cells (MSCs) and epidermal cells (EPCs) in gelatin methacrylate (GelMA) nuclei and light-cured catechol-grafted hyaluronic acid (HAD) shells. This technique supports cell proliferation and maintains a slow release of growth factors, allowing for stable growth and survival for a period of time after transplantation. It may bear similarities to the capsules used in clinical practice in that it protects the contents over time [70]. All these techniques provide the potential for large-scale HT and border applications.

With the aid of bioengineering research, it is possible to efficiently obtain a large quantity of uniform-quality hair follicle grafts. Therefore, the rational utilization of bioengineering and microfluidic technologies can also significantly contribute to the advancement of hair follicle transplantation.

Conclusion and future perspectives

Hair loss manifests in various forms and can result from a wide range of underlying causes, many of which have historically been under-researched. Although hair loss itself is rarely life-threatening, it significantly impacts individuals’ quality of life, affecting self-esteem, work performance, and social interactions. Therefore, effectively addressing hair loss is crucial for enhancing patients’ overall well-being. Currently, the prognosis for some patients has improved through careful assessment of hair loss and the optimization of surgical protocols. However, further research and innovative approaches are needed to address the diverse causes of hair loss and to develop more effective treatments.

In addressing the key limitations of HT, including the scarcity of donor hair follicles, insufficient blood supply, and the high rate of hair loss recurrence, iPSC-derived products hold significant potential. In the context of hair regeneration, iPSCs have the potential to differentiate into various cell types, including dermal papilla cells, melanocytes, follicular epithelial cells, and keratinocytes [71–73]. These cells can serve as supplementary components for HT. By directly injecting a cellular mixture into the alopecic area, these cells can form hair follicle structures under conditions of adequate vascular supply, thereby promoting hair regrowth. Furthermore, recent research has demonstrated the feasibility of constructing vascularized skin organoids [17], which may address the challenge of insufficient blood supply in transplantation.

Notably, iPSCs offer personalized treatment options, such as hair follicle organoids differentiated from patient-derived iPSCs. For patients requiring urgent transplantation, MHC-matched cell lines from iPSC databases can be used to generate follicle organoids suitable for a wide range of transplantation needs. Additionally, gene editing techniques can produce cells with reduced antigenicity [74], potentially improving the survival rate of allografts. As follicle-like organoids increasingly resemble human hair follicles, large-scale production of these structures may provide a continuous supply of follicles for transplantation, addressing the current limitations in hair restoration therapies.

In conclusion, aside from a limited range of topical and oral medications, HT is often considered the last resort for managing hair loss. However, current methods fall short due to donor follicle scarcity, inconsistent results, and limited use, highlighting the need for new solutions. The development of iPSC-based technologies to generate hair follicles and organoids represents a transformative advancement in the field. With the potential for large-scale production, standardized protocols, and personalized treatments, iPSCs offer a promising pathway toward more effective and accessible therapies for hair loss in the future (Fig. 3).

Fig. 3.

Stem cells play a crucial role in hair regeneration. Alopecia, which is categorized into various types, often exhibits low responsiveness to conventional medical treatments and phototherapy, making hair transplantation a last-resort option in current healthcare practices. Fortunately, induced pluripotent stem cells (iPSCs) and mesenchymal stem cells (MSCs) have the potential to differentiate into hair follicle stem cells, offering promising prospects for cell-based therapies. Additionally, the development of hair follicle organoids with vascular structures addresses the challenge of limited hair resources and inadequate blood supply. Furthermore, aseptic cultivation techniques reduce the risk of infection, potentially providing an effective solution for treating refractory alopecia

Abbreviations

FPHL

Female pattern hair loss

JAK

Janus kinase

AGA

Androgenetic alopecia

HT

Hair transplantation

RSES

Rosenberg self-esteem scale

DLQI

Dermatology life quality index

iPSCs

Induced pluripotent stem cells

AA

Alopecia areata

FUE

Follicular unit extraction

FPHL

Female pattern hair loss

PCOS

Polycystic ovary syndrome

NA

Neoplastic alopecia

DUPA

Diffuse unpatterned alopecia

BHT

Body hair transplantation

LPP

Lichen planopilaris

FFA

Frontal fibrosing alopecia

EPS

Erosive pustulosis of the scalp

SF

Superficial folliculitis

MMPs

Matrix metalloproteinases

Epi-SCs

Epidermal stem cells

SKPs

Skin-derived progenitor cells

ECs

Endothelial cells

SKOs

Skin organoids

PSOs

Pluripotent-stem-cell-derived skin organoids

MEI

Mesenchymal-epithelial interaction

hDPA

Human dermal papilla cell aggregates

MSCs

Mesenchymal stromal cells

EPCs

Epidermal cells

GelMA

Gelatin methacrylate

HAD

Hyaluronic acid

AU

Alopecia universalis

TE

Telogen effluvium

AE

Anagen effluvium

CCCA

Central centrifugal cicatricial alopecia

GD

Graves’ disease

CIA

Chemotherapy-induced alopecia

RIA

Radiotherapy-induced alopecia

TA

Traction alopecia

PCOS

Polycystic ovarian syndrome

NA

Neoplastic alopecia

APMR

Alopecia-mental retardation syndrome

FUT

Follicular unit transplantation

DSC

Dermal sheath cup

MSCs

Mesenchymal stem cells

Author contributions

Conceptualization, YWZ; writing - original draft preparation, HZ; writing - review and discussion, YWZ, HZ, YXZ, and QKL; supervision and supply of resources, YML and YWZ; acquisition of funding, YWZ, YML, and HZ. All authors read and approved the final manuscript.

Funding

This research was partially funded by the National Natural Science Foundation of China (82270697), Jiangsu Provincial Key Discipline Cultivation Unit (JSDW202229), Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX22_3717), Science and Technology Planning Project of Guangdong Province (2021B1212040016), and Guangdong Basic and Applied Basic Research Foundation (2023A1515012574).and Haihe Laboratory of Cell Ecosystem Innovation Fund (HH24KYZX0008).

Data availability

Not applicable.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.