Reconstructed human skin shows epidermal invagination towards integrated neopapillae indicating early hair follicle formation in vitro

Hair Regeneration Methods Using Cells Derived from Human Hair Follicles and Challenges to Overcome

The hair follicle is a complex of mesenchymal and epithelial cells acquiring different properties and characteristics responsible for fulfilling its inductive and regenerative role. The epidermal and dermal crosstalk induces morphogenesis and maintains hair follicle cycling properties. The hair follicle is enriched with pluripotent stem cells, where dermal papilla (DP) cells and dermal sheath (DS) cells constitute the dermal compartment and the epithelial stem cells existing in the bulge region exert their regenerative role by mediating the epithelial-mesenchymal interaction (EMI). Many studies have developed and focused on various methods to optimize the EMI through in vivo and in vitro approaches for hair regeneration. The culturing of human hair mesenchymal cells resulted in the loss of trichogenicity and inductive properties of DP cells, limiting their potential application in de novo hair follicle generation in vivo. Epithelial stem cells derived from human hair follicles are challenging to isolate and culture, making it difficult to obtain enough cells for hair regeneration purposes. Mesenchymal stem cells and epithelial stem cells derived from human hair follicles lose their ability to form hair follicles during culture, limiting the study of hair follicle formation in vivo. Therefore, many attempts and methods have been developed to overcome these limitations. Here, we review the possible and necessary cell methods and techniques used for human hair follicle regeneration and the restoration of hair follicle cell inductivity in culture.

Environmentally Controlled Microfluidic System Enabling Immune Cell Flow and Activation in an Endothelialised Skin-On-Chip

Integration of reconstructed human skin (RhS) into organ-on-chip (OoC) platforms addresses current limitations imposed by static culturing. This innovation, however, is not without challenges. Microfluidic devices, while powerful, often encounter usability, robustness, and gas bubble issues that hinder large-scale high-throughput setups. This study aims to develop a novel re-usable multi-well microfluidic adaptor (MMA) with the objective to provide a flexible tool for biologists implementing complex 3D biological models (e.g., skin) while enabling simultaneous user control over temperature, medium flow, oxigen (O2), nitrogen (N2), and carbon dioxide (CO2) without the need for an incubator. The presented MMA device is designed to be compatible with standard, commercially available 6-well multi-well plates (6MWPs) and 12-well transwells. This MMA-6MWP setup is employed to generate a skin-on-chip (SoC). RhS viability is maintained under flow for three days and their morphology closely resembles that of native human skin. A proof-of-concept study demonstrates the system's potential in toxicology applications by combining endothelialised RhS with flowing immune cells. This dynamic setting activates the monocyte-like MUTZ-3 cells (CD83 and CD86 upregulation) upon topical exposure of RhS to a sensitizer, revealing the MMA-6MWP's unique capabilities compared to static culturing, where such activation is absent.

Full-Thickness Perfused Skin-on-a-Chip with In Vivo-Like Drug Response for Drug and Cosmetics Testing

In this study, we present a novel 3D perfused skin-on-a-chip model fabricated using micro-precision 3D printing, which offers a streamlined and reproducible approach for incorporating perfusion. Perfused skin models are well-regarded for their advantages, such as improved nutrient supply, enhanced barrier function, and prolonged tissue viability. However, current models often require complex setups, such as self-assembled endothelial cells or sacrificial rods, which are prone to variability and time-consuming. Our model uses projection micro-stereolithography 3D printing to create precise microcapillary-like channels using a biocompatible resin, overcoming the drug-absorbing properties of PDMS. A customized chip holder allows for the simultaneous culture of six perfused chips, enabling high-throughput testing. The engineered skin-on-a-chip features distinct dermis and epidermis layers, confirmed via H&E staining and immunostaining. To evaluate drug screening capabilities, inflammation was induced using TNF-α and treated with dexamethasone, with cytokine levels compared to 2D cultures and human skin biopsies. Our 3D model exhibited drug response trends similar to human skin, while showing reduced cytotoxicity over time compared to biopsies. This perfused skin-on-a-chip provides a reliable, physiologically relevant alternative for drug and cosmetics screening, simplifying perfusion setup while preserving key benefits.

Oral host-microbe interactions investigated in 3D organotypic models

The oral cavity is inhabited by abundant microbes which continuously interact with the host and influence the host's health. Such host-microbe interactions (HMI) are dynamic and complex processes involving e.g. oral tissues, microbial communities and saliva. Due to difficulties in mimicking the in vivo complexity, it is still unclear how exactly HMI influence the transition between healthy status and disease conditions in the oral cavity. As an advanced approach, three-dimensional (3D) organotypic oral tissues (epithelium and mucosa/gingiva) are being increasingly used to study underlying mechanisms. These in vitro models were designed with different complexity depending on the research questions to be answered. In this review, we summarised the existing 3D oral HMI models, comparing designs and readouts, discussing applications as well as future perspectives.

Modelling Human Hair Follicles-Lessons from Animal Models and Beyond

The hair follicle is a specialized appendage of the skin that is critical for multiple functions, including thermoregulation, immune surveillance, and sebum production. Mammals are born with a fixed number of hair follicles that develop embryonically. Postnatally, these hair follicles undergo regenerative cycles of regression and growth that recapitulate many of the embryonic signaling pathways. Furthermore, hair cycles have a direct impact on skin regeneration in homeostasis, cutaneous wound healing, and disease conditions such as alopecia. Here, we review the current knowledge of hair follicle formation during embryonic development and the post-natal hair cycle, with an emphasis on the molecular signaling pathways underlying these processes. We then discuss efforts to capitalize on the field's understanding of in vivo mechanisms to bioengineer hair follicles or hair-bearing skin in vitro and how such models may be further improved to develop strategies for hair regeneration.

Reconstructed Human Skin with Hypodermis Shows Essential Role of Adipose Tissue in Skin Metabolism

BACKGROUND

Dysregulation of skin metabolism is associated with a plethora of diseases such as psoriasis and dermatitis. Until now, reconstructed human skin (RhS) models lack the metabolic potential of native human skin, thereby limiting their relevance to study human healthy and diseased skin. We aimed to determine whether incorporation of an adipocyte-containing hypodermis into RhS improves its metabolic potential and to identify major metabolic pathways up-regulated in adipose-RhS.

METHODS

Primary human keratinocytes, fibroblasts and differentiated adipose-derived stromal cells were co-cultured in a collagen/fibrin scaffold to create an adipose-RhS. The model was extensively characterized structurally in two- and three-dimensions, by cytokine secretion and RNA-sequencing for metabolic enzyme expression.

RESULTS

Adipose-RhS showed increased secretion of adipokines. Both RhS and adipose-RhS expressed 29 of 35 metabolic genes expressed in ex vivo native human skin. Addition of the adipose layer resulted in up-regulation of 286 genes in the dermal-adipose fraction of which 7 were involved in phase I (CYP19A1, CYP4F22, CYP3A5, ALDH3B2, EPHX3) and phase II (SULT2B1, GPX3) metabolism. Vitamin A, D and carotenoid metabolic pathways were enriched. Additionally, pro-inflammatory (IL-1β, IL-18, IL-23, IL-33, IFN-α2, TNF-α) and anti-inflammatory cytokine (IL-10, IL-12p70) secretion was reduced in adipose-RhS.

CONCLUSIONS

Adipose-RhS mimics healthy native human skin more closely than traditional RhS since it has a less inflamed phenotype and a higher metabolic activity, indicating the contribution of adipocytes to tissue homeostasis. Therefore it is better suited to study onset of skin diseases and the effect of xenobiotics.

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.

Mimicking fat grafting of fibrotic scars using 3D-organotypic skin cultures

Wound healing of deep burn injuries is often accompanied by severe scarring, such as hypertrophic scar (HTS) formation. In severe burn wounds, where the subcutis is also damaged, the scars adhere to structures underneath, resulting in stiffness of the scar and impaired motion. Over the recent years, a promising solution has emerged: autologous fat grafting, also known as lipofilling. Previous clinical reports have shown that the anti-fibrotic effect has been attributed to the presence of adipose-derived stromal cells (ADSC). In the proposed study, we aim to investigate the effect of fat grafting in 3D organotypic skin cultures mimicking an HTS-like environment. To this end, organotypic skin cultures were embedded with normal skin fibroblasts (NF) or HTS-derived fibroblasts with or without incorporation of human adipose subcutaneous tissue (ADT) and one part was thermally wounded to examine their effect on epithelialization. The developed skin cultures were analysed on morphology and protein level. Analysis revealed that ADT-containing organotypic skin cultures comprise an improved epidermal homeostasis, and a fully formed basement membrane, similar to native human skin (NHS). Furthermore, the addition of ADT significantly reduced myofibroblast presence, which indicates its anti-fibrotic effect. Finally, re-epithelialization measurements showed that ADT reduced re-epithelialization in skin cultures embedded with NFs, whereas HTS-fibroblast-embedded skin cultures showed complete wound closure. In conclusion, we succeeded in developing a 3D organotypic HTS-skin model incorporated with subcutaneous tissue that allows further investigation on the molecular mechanism of fat grafting.

The dermal papilla dilemma and potential breakthroughs in bioengineering hair follicles

The generation and growing of de novo hair follicles is the most daring hair replacement approach to treat alopecia. This approach has been explored at least since the 1960s without major success. Latest in the 1980s, the realization that the mesenchymal compartment of hair follicles, the dermal papilla (DP), is the crucial signaling center and element required for fulfilling this vision of hair follicle engineering, propelled research into the fibroblasts that occupy the DP. However, working with DP fibroblasts has been stubbornly frustrating. Decades of work in understanding the nature of DP fibroblasts in vitro and in vivo have led to the appreciation that hair follicle biology is complex, and the dermal papilla is an enigma. Functional DP fibroblasts tend to aggregate in 2D culture, while impaired DP cells do not. This fact has stimulated recent approaches to overcome the hurdles to DP cell culture by mimicking their natural habitat, such as growing DP fibroblasts in three dimensions (3D) by their self-aggregation, adopting 3D matrix scaffold, or bioprinting 3D microstructures. Furthermore, including keratinocytes in the mix to form hair follicle-like composite structures has been explored but remains a far cry from a useful and affordable method to generate human hair follicles in sufficient quantity and quality in a practical time frame for patients. This suggests that the current strategies may have reached their limitations in achieving successful hair follicle bioengineering for clinical applications. Novel approaches are required to overcome these barriers, such as focusing on embryonic cell types and processes in combination with emerging techniques.

A Humanized In Vitro Model of Innervated Skin for Transdermal Analgesic Testing

Sensory innervation of the skin is essential for its function, homeostasis, and wound healing mechanisms. Thus, to adequately model the cellular microenvironment and function of native skin, in vitro human skin equivalents (hSE) containing a sensory neuron population began to be researched. In this work, a fully human 3D platform of hSE innervated by induced pluripotent stem cell-derived nociceptor neurospheres (hNNs), mimicking the native mode of innervation, is established. Both the hSE and nociceptor population exhibit morphological and phenotypical characteristics resembling their native counterparts, such as epidermal and dermal layer formation and nociceptor marker exhibition, respectively. In the co-culture platform, neurites develop from the hNNs and navigate in 3D to innervate the hSE from a distance. To probe both skin and nociceptor functionality, a clinically available capsaicin patch (Qutenza) is applied directly over the hSE section and neuron reaction is analyzed. Application of the patch causes an exposure time-dependent neurite regression and degeneration. In platforms absent of hSE, axonal degeneration is further increased, highlighting the role of the skin construct as a barrier. In sum, an in vitro tool of functional innervated skin with high interest for preclinical research is established.

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.

Bioengineering of Hair Follicle-like Structure for Validation of Hair Growth Promoting Compounds

We aimed to establish screening and efficacy test techniques for use in the development of hair-promoting agents. To this end, we used the dermal papilla cell (DPc)-derived immortalized cell line (SV40T-hTERT DPc) and neonatal foreskin-derived keratinocyte cell line (Ker-CT) to form an immortalized cell-based hair follicle-like structure. The SV40T-hTERT DPc spheroids exhibited a higher cell ratio in the spheroids than primary DPc spheroids, and SV40T-hTERT DPc aggregated with spheroids larger in diameter than primary DPc when the same cell number was seeded into the low-adhesion plate. Microscopic imaging and fluorescence staining results indicated that both primary and immortalized cell combinations form a hair follicle-like structure with a long-stretched keratinocyte layer under the condition that the spheroids have the same diameter as that of in vivo dermal papillary tissue in the hair follicle. The hair follicle-like structure elongation was increased upon treatment with three known hair follicle growth-promoting compounds (minoxidil, tofacitinib, and ascorbic acid) compared with that in the control group. Therefore, using immortalized cells to generate a coherent follicle-like structure, we have developed models for screening and evaluating hair-care materials commonly used in the industry.

Biofabrication of Human Skin with Its Appendages

Much effort has been made to generate human skin organ in the laboratory. Yet, the current models are limited due to the lack of many critical biological and structural features of the skin. Importantly, these in vitro models lack appendages and fail to recapitulate the whole human skin construction. Thus, engineering a human skin with the capacity to generate all components, including appendages, is a major challenge. This review intends to provide an update on the recent efforts underway to regenerate appendage-bearing skin organs based on scaffold-free and scaffold-based bioengineering approaches. Although the mouse skin equivalents containing hair follicles, sebaceous glands, and sweat glands have been established in vitro, there has been limited success in humans. A combination of biofabricated matrices and cell aggregates, such as organoids, can pave the way for generating skin substitutes with human-like biological, structural, and physical features. Accordingly, the formation of human skin organoids and reconstruction of vascularized skin equipped with immune cells prompt calls for more scientific research. The generation of appendage-bearing skin substitutes can be applied in practice for wound healing, hair restoration, and scar treatment.

Generation and integration of hair follicle-primed spheroids in bioengineered skin constructs

Skin is a complex organ made up of different cell layers, appendages, connective tissues, and immune repertoires. These different components interact extensively to maintain the overall functions of the integumentary system. In particular, appendages such as hair follicles critically contribute to the skin's function in thermoregulation, sensory perception, and homeostatic regeneration. Despite a strong need for better skin regenerative therapeutics, efforts to bio-engineer highly functional appendage-containing human reconstituted skinin vitrohave not yielded much success. Here, we report methods in generating and incorporating hair follicle-primed heterotypic spheroids into epidermal-dermal skin constructs that induced invaginating outgrowths with follicle-like organization and lineage gene expression. By co-culturing epithelial keratinocytes (KCs) with dermal papilla (DP) cells in low attachment plates, we established the media and culture conditions that best supported the viability, signalling and remodelling of the cell aggregates to form 3D KC-DP spheroids with the expression of both DP inductiveness and hair follicle lineage genes. We show that long-term growth and maturation of KC cells in these spheroids was supported by incorporation into epidermal-dermal constructs but not in scaffold-less media. When cultured, the bio-fabricated constructs developed invaginations from the integrated spheroids with follicle-forming potential. The generation of these constructs is a step towards the development of functional hair-bearing skin mimetics.

Proof-of-Concept Organ-on-Chip Study: Topical Cinnamaldehyde Exposure of Reconstructed Human Skin with Integrated Neopapillae Cultured under Dynamic Flow

Pharmaceutical and personal care industries require human representative models for testing to ensure the safety of their products. A major route of penetration into our body after substance exposure is via the skin. Our aim was to generate robust culture conditions for a next generation human skin-on-chip model containing neopapillae and to establish proof-of-concept testing with the sensitizer, cinnamaldehyde. Reconstructed human skin consisting of a stratified and differentiated epidermis on a fibroblast populated hydrogel containing neopapillae spheroids (RhS-NP), were cultured air-exposed and under dynamic flow for 10 days. The robustness of three independent experiments, each with up to 21 intra-experiment replicates, was investigated. The epidermis was seen to invaginate into the hydrogel towards the neopapille spheroids. Daily measurements of lactate dehydrogenase (LDH) and glucose levels within the culture medium demonstrated high viability and stable metabolic activity throughout the culture period in all three independent experiments and in the replicates within an experiment. Topical cinnamaldehyde exposure to RhS-NP resulted in dose-dependent cytotoxicity (increased LDH release) and elevated cytokine secretion of contact sensitizer specific IL-18, pro-inflammatory IL-1β, inflammatory IL-23 and IFN-γ, as well as anti-inflammatory IL-10 and IL-12p70. This study demonstrates the robustness and feasibility of complex next generation skin models for investigating skin immunotoxicity.

Bringing hydrogel-based craniofacial therapies to the clinic

This review explores the evolution of the use of hydrogels for craniofacial soft tissue engineering, ranging in complexity from acellular injectable fillers to fabricated, cell-laden constructs with complex compositions and architectures. Addressing both in situ and ex vivo approaches, tissue restoration secondary to trauma or tumor resection is discussed. Beginning with relatively simple epithelia of oral mucosa and gingiva, then moving to more functional units like vocal cords or soft tissues with multilayer branched structures, such as salivary glands, various approaches are presented toward the design of function-driven architectures, inspired by native tissue organization. Multiple tissue replacement paradigms are presented here, including the application of hydrogels as structural materials and as delivery platforms for cells and/or therapeutics. A practical hierarchy is proposed for hydrogel systems in craniofacial applications, based on their material and cellular complexity, spatial order, and biological cargo(s). This hierarchy reflects the regulatory complexity dictated by the Food and Drug Administration (FDA) in the United States prior to commercialization of these systems for use in humans. The wide array of available biofabrication methods, ranging from simple syringe extrusion of a biomaterial to light-based spatial patterning for complex architectures, is considered within the history of FDA-approved commercial therapies. Lastly, the review assesses the impact of these regulatory pathways on the translational potential of promising pre-clinical technologies for craniofacial applications. STATEMENT OF SIGNIFICANCE: While many commercially available hydrogel-based products are in use for the craniofacial region, most are simple formulations that either are applied topically or injected into tissue for aesthetic purposes. The academic literature previews many exciting applications that harness the versatility of hydrogels for craniofacial soft tissue engineering. One of the most exciting developments in the field is the emergence of advanced biofabrication methods to design complex hydrogel systems that can promote the functional or structural repair of tissues. To date, no clinically available hydrogel-based therapy takes full advantage of current pre-clinical advances. This review surveys the increasing complexity of the current landscape of available clinical therapies and presents a framework for future expanded use of hydrogels with an eye toward translatability and U.S. regulatory approval for craniofacial applications.

Recreation of a hair follicle regenerative microenvironment: Successes and pitfalls

The hair follicle (HF) is an exquisite skin appendage endowed with cyclical regenerative capacity; however, de novo follicle formation does not naturally occur. Consequently, patients suffering from extensive skin damage or hair loss are deprived of the HF critical physiological and/or aesthetic functions, severally compromising skin function and the individual's psychosocial well-being. Translation of regenerative strategies has been prevented by the loss of trichogenic capacity that relevant cell populations undergo in culture and by the lack of suitable human-based in vitro testing platforms. Here, we provide a comprehensive overview of the major difficulties associated with HF regeneration and the approaches used to overcome these drawbacks. We describe key cellular requirements and discuss the importance of the HF extracellular matrix and associated signaling for HF regeneration. Finally, we summarize the strategies proposed so far to bioengineer human HF or hair-bearing skin models and disclose future trends for the field.

Hidradenitis suppurativa: Disease pathophysiology and sex hormones

Hidradenitis suppurativa is a cutaneous chronic inflammatory disease that is estimated to affect about 1% of the population and caused pain, malodorous discharge, disfigurement, and poor quality of life with psychosocial problems. The typical features are recurrent painful nodules, abscesses, and sinus tracts on the axillae, groins, gluteal areas, and anogenital regions since postpuberty. Smoking and obesity are two major triggering factors of hidradenitis suppurativa. Women are prone to have hidradenitis suppurativa than men in Western countries, but the male-to-female ratio is reversed in oriental countries. The disease severity can be affected by menstruation, pregnancy, and menopause. Furthermore, the phenotypes are different among men and women with hidradenitis suppurativa. Men are prone to have buttock involvement while women are prone to have axillary, groins, and submammary lesions. This review introduces the skin appendages and pathophysiology of hidradenitis suppurativa and then focuses on the sex difference and the effects of sex hormones on hidradenitis suppurativa and current hormone-associated treatments.

Recent advances in the induced pluripotent stem cell-based skin regeneration

Skin regeneration has been a challenging clinical problem especially in cases of chronic wounds such as diabetic foot ulcers, and epidermolysis bullosa-related skin blisters. Prolonged non-healing wounds often lead to bacterial infections increasing the severity of wounds. Current treatment strategies for chronic wounds include debridement of wounds along with antibiotics, growth factors, and stem cell transplantation therapies. However, the compromised nature of autologous stem cells in patients with comorbidities such as diabetes limits the efficacy of the therapy. The discovery of induced pluripotent stem cell (iPSC) technology has immensely influenced the field of regenerative therapy. Enormous efforts have been made to develop integration-free iPSCs suitable for clinical therapies. This review focuses on recent advances in the methods and reprogramming factors for generating iPSCs along with the existing challenges such as genetic alterations, tumorigenicity, immune rejection, and regulatory hurdles for the clinical application of iPSCs. Furthermore, this review also highlights the benefits of using iPSCs for the generation of skin cells and skin disease modeling over the existing clinical therapies for skin regeneration in chronic wounds and skin diseases.

Microscopy-guided laser ablation for the creation of complex skin models with folliculoid appendages

Engineering complex tissues requires the use of advanced biofabrication techniques that allow the replication of the tissue's 3D microenvironment, architecture and cellular interactions. In the case of skin, the most successful strategies to introduce the complexity of hair follicle (HF) appendages have highlighted the importance of facilitating direct interaction between dermal papilla (DP) cells and keratinocytes (KCs) in organotypic skin models. In this work, we took advantage of microscopy-guided laser ablation (MGLA) to microfabricate a fibroblast-populated collagen hydrogel and create a subcompartment that guides the migration of KCs and lead their interaction with DP cells to recreate follicular structures. Upon definition of the processing parameters (laser incidence area and power), MGLA was used to create 3D microchannels from the surface of a standard organotypic human skin model up to the aggregates containing DP cells and KCs, previously incorporated into the dermal-like fibroblast-collagen layer. Analysis of the constructs showed that the fabricated microfeatures successfully guided the fusion between epidermal and aggregates keratinocytes, which differentiated into follicular-like structures within the organotypic human skin model, increasing its functionality. In summary, we demonstrate the fabrication of a highly structured 3D hydrogel-based construct using MGLA to attain a complex skin model bearing folliculoid structures, highlighting its potential use as an in vitro platform to study the mechanisms controlling HF development or for the screening of bioactive substances.

Back to the Future: From Appendage Development Toward Future Human Hair Follicle Neogenesis

Hair disorders such as alopecia and hirsutism often impact the social and psychological well-being of an individual. This also holds true for patients with severe burns who have lost their hair follicles (HFs). HFs stimulate proper wound healing and prevent scar formation; thus, HF research can benefit numerous patients. Although hair development and hair disorders are intensively studied, human HF development has not been fully elucidated. Research on human fetal material is often subject to restrictions, and thus development, disease, and wound healing studies remain largely dependent on time-consuming and costly animal studies. Although animal experiments have yielded considerable and useful information, it is increasingly recognized that significant differences exist between animal and human skin and that it is important to obtain meaningful human models. Human disease specific models could therefore play a key role in future therapy. To this end, hair organoids or hair-bearing skin-on-chip created from the patient’s own cells can be used. To create such a complex 3D structure, knowledge of hair genesis, i.e., the early developmental process, is indispensable. Thus, uncovering the mechanisms underlying how HF progenitor cells within human fetal skin form hair buds and subsequently HFs is of interest. Organoid studies have shown that nearly all organs can be recapitulated as mini-organs by mimicking embryonic conditions and utilizing the relevant morphogens and extracellular matrix (ECM) proteins. Therefore, knowledge of the cellular and ECM proteins in the skin of human fetuses is critical to understand the evolution of epithelial tissues, including skin appendages. This review aims to provide an overview of our current understanding of the cellular changes occurring during human skin and HF development. We further discuss the potential implementation of this knowledge in establishing a human in vitro model of a full skin substitute containing hair follicles and the subsequent translation to clinical use.

Development of Skin-On-A-Chip Platforms for Different Utilizations: Factors to Be Considered

There is increasing interest in miniaturized technologies in diagnostics, therapeutic testing, and biomedicinal fundamental research. The same is true for the dermal studies in topical drug development, dermatological disease pathology testing, and cosmetic science. This review aims to collect the recent scientific literature and knowledge about the application of skin-on-a-chip technology in drug diffusion studies, in pharmacological and toxicological experiments, in wound healing, and in fields of cosmetic science (ageing or repair). The basic mathematical models are also presented in the article to predict physical phenomena, such as fluid movement, drug diffusion, and heat transfer taking place across the dermal layers in the chip using Computational Fluid Dynamics techniques. Soon, it can be envisioned that animal studies might be at least in part replaced with skin-on-a-chip technology leading to more reliable results close to study on humans. The new technology is a cost-effective alternative to traditional methods used in research institutes, university labs, and industry. With this article, the authors would like to call attention to a new investigational family of platforms to refresh the researchers’ theranostics and preclinical, experimental toolbox.

Functional hair follicle regeneration: an updated review

The hair follicle (HF) is a highly conserved sensory organ associated with the immune response against pathogens, thermoregulation, sebum production, angiogenesis, neurogenesis and wound healing. Although recent advances in lineage-tracing techniques and the ability to profile gene expression in small populations of cells have increased the understanding of how stem cells operate during hair growth and regeneration, the construction of functional follicles with cycling activity is still a great challenge for the hair research field and for translational and clinical applications. Given that hair formation and cycling rely on tightly coordinated epithelial-mesenchymal interactions, we thus review potential cell sources with HF-inducive capacities and summarize current bioengineering strategies for HF regeneration with functional restoration.

Generation of pigmented skin grafts from human hair follicles and dermal fibroblasts

Skin equivalents are able to mimic key features of human skin and they can be used for a very broad range of applications, such as fundamental studies of skin biology, disease and toxicological models, as well as an alternative for animal testing. The high end of their use is in therapy of wound healing and repigmentation, disorders that massively affect individual health as well as quality of life and pose considerable burden to healthcare systems worldwide. Tissue-engineered skin grafts often originate from invasively obtained cell material (i.e. biopsy). Hereby, an unmet need for non-invasively gained autologous biological starting material has been created. The hair follicle, entirely non-invasively available by plucking, harbors a heterogeneous cell pool including stem cells with an immense differentiation capacity, hereby representing an attractive source of cells, especially for purposes of regenerative medicine. In this study, we engineered three-dimensional pigmented epidermal and dermoepidermal grafts using human keratinocytes and melanocytes from the outer root sheath of hair follicles combined with dermal fibroblasts. The grafts were generally anatomically correct and functional regarding stratification, formation of epidermal melanin units as well as extracellular matrix deposition, exhibiting moderate differences to the skin anatomy and function, typical for the in vitro culture.

Hair follicle germs containing vascular endothelial cells for hair regenerative medicine

Hair regenerative medicine has emerged as a promising approach for the treatment of severe hair loss. Recent advances in three-dimensional tissue engineering, such as formation of hair follicle germs (HFGs), have considerably improved hair regeneration after transplantation in animal models. Here, we proposed an approach for fabricating HFGs containing vascular endothelial cells. Epithelial, dermal papilla, and vascular endothelial cells initially formed a single aggregate, which subsequently became a dumbbell-shaped HFG, wherein the vascular endothelial cells localized in the region of dermal papilla cells. The HFGs containing vascular endothelial cells exhibited higher expression of hair morphogenesis-related genes in vitro, along with higher levels of hair shaft regeneration upon transplantation to the dorsal side of nude mice, than those without vascular endothelial cells. The generated hair follicles represented functional characteristics, such as piloerection, as well as morphological characteristics comparable to those of natural hair shafts. This approach may provide a promising strategy for fabricating tissue grafts with higher hair inductivity for hair regenerative medicine.

Advances in generation of three-dimensional skin equivalents: pre-clinical studies to clinical therapies

The inability of two-dimensional cell culture systems to adequately map the structure and function of complex organs like skin necessitates the development of three-dimensional (3D) skin models. A diverse range of 3D skin equivalents have been developed over the last few decades for studying complex properties of skin as well as for drug discovery and clinical applications for skin regeneration in chronic wounds, such as diabetic foot ulcers, where the normal mechanism of wound healing is compromised. These 3D skin substitutes also serve as a suitable alternative to animal models in industrial applications and fundamental research. With the emergence of tissue engineering, new scaffolds and matrices have been integrated into 3D cell culture systems, along with gene therapy approaches, to increase the efficacy of transplanted cells in skin regeneration. This review summarizes recent approaches to the development of skin equivalents as well as different models for studying skin diseases and properties and current therapeutic applications of skin substitutes.

Organ-on-a-chip engineering: Toward bridging the gap between lab and industry

Organ-on-a-chip (OOC) is a very ambitious emerging technology with a high potential to revolutionize many medical and industrial sectors, particularly in preclinical-to-clinical translation in the pharmaceutical arena. In vivo, the function of the organ(s) is orchestrated by a complex cellular structure and physiochemical factors within the extracellular matrix and secreted by various types of cells. The trend in in vitro modeling is to simplify the complex anatomy of the human organ(s) to the minimal essential cellular structure "micro-anatomy" instead of recapitulating the full cellular milieu that enables studying the absorption, metabolism, as well as the mechanistic investigation of drug compounds in a "systemic manner." However, in order to reflect the human physiology in vitro and hence to be able to bridge the gap between the in vivo and in vitro data, simplification should not compromise the physiological relevance. Engineering principles have long been applied to solve medical challenges, and at this stage of organ-on-a-chip technology development, the work of biomedical engineers, focusing on device engineering, is more important than ever to accelerate the technology transfer from the academic lab bench to specialized product development institutions and to the increasingly demanding market. In this paper, instead of presenting a narrative review of the literature, we systemically present a synthesis of the best available organ-on-a-chip technology from what is found, what has been achieved, and what yet needs to be done. We emphasized mainly on the requirements of a "good in vitro model that meets the industrial need" in terms of the structure (micro-anatomy), functions (micro-physiology), and characteristics of the device that hosts the biological model. Finally, we discuss the biological model-device integration supported by an example and the major challenges that delay the OOC technology transfer to the industry and recommended possible options to realize a functional organ-on-a-chip system.

Reconstructed human skin shows epidermal invagination towards integrated neopapillae indicating early hair follicle formation in vitro

Application of reconstructed human Skin (RhS) is a promising approach for the treatment of extensive wounds and for drug efficacy and safety testing. However, incorporating appendages, such as hair follicles, into RhS still remains a challenge. The hair follicle plays a critical role in thermal regulation, dispersion of sweat and sebum, sensory and tactile functions, skin regeneration, and repigmentation. The aim of this study was to determine whether human neopapilla could be incorporated into RhS (differentiated epidermis on fibroblast and endothelial cell populated dermis) and whether the neopapillae maintain their inductive follicular properties in vitro. Neopapillae spheroids, constructed from expanded and self‐aggregating dermal papilla cells, synthesized extracellular matrix typically found in follicular papillae. Compared with dermal fibroblasts, neopapillae showed increased expression of multiple genes (Wnt5a, Wnt10b, and LEF1) known to regulate hair development and also increased secretion of CXCL1, which is a strong keratinocyte chemoattractant. When neopapillae were incorporated into the dermis of RhS, they stimulated epidermal down‐growth resulting in engulfment of the neopapillae sphere. Similar to the native hair follicle, the differentiated invaginating epidermis inner side was keratin 10 positive and the undifferentiated outer side keratin 10 negative. The outer side was keratin 15 positive confirming the undifferentiated nature of these keratinocytes aligning a newly formed collagen IV, laminin V positive basement membrane within the hydrogel. In conclusion, we describe a RhS model containing neopapillae with hair follicle‐inductive properties. Importantly, epidermal invagination occurred to engulf the neopapillae, thus demonstrating in vitro the first steps towards hair follicle morphogenesis in RhS.