Self-organizing hair peg-like structures from dissociated skin progenitor cells: New insights for human hair follicle organoid engineering and Turing patterning in an asymmetric morphogenetic field

Organoids in skin wound healing

Stem cells (SCs) can self-replicate and differentiate into multiple lineages. Organoids, 3D cultures derived from SCs, can replicate the spatial structure and physiological characteristics of organs in vitro. Skin organoids can effectively simulate the physiological structure and function of skin tissue, reliably restoring the natural skin ecology in various in vitro environments. Skin organoids have been employed extensively in skin development and pathology research, offering valuable insights for drug screening. Moreover, they play crucial roles in skin regeneration and tissue repair. This in-depth review explores the construction and applications of skin organoids in wound healing, with a focus on their construction process, including skin appendage integration, and significant advancements in wound-healing research.

Beyond skin deep: Revealing the essence of iPS cell-generated skin organoids in regeneration

Various methods have been used for in vivo and in vitro skin regeneration, including stem cell therapy, tissue engineering, 3D printing, and platelet-rich plasma (PRP) injection therapy. However, these approaches are rooted in the existing knowledge of skin structures, which overlook the normal physiological processes of skin development and fall short of replicating the skin's regenerative processes outside the body. This comprehensive review primarily focuses on skin organoids derived from human pluripotent stem cells, which have the capacity to regenerate human skin tissue by restoring the embryonic skin structure, thus offering a novel avenue for producing in vitro skin substitutes. Furthermore, they contribute to the repair of damaged skin lesions in patients with systemic sclerosis or severe burns. Particular emphasis will be placed on the origins, generations, and applications of skin organoids, especially in dermatology, and the challenges that must be addressed before clinical implementation.

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.

Optimization of hair follicle spheroids for hair-on-a-chip

Currently, most models for hair follicle research have the limitation of not replicating some key features of the hair follicle microenvironment. To complement this, we transfected various factors for hair growth into dermal papilla cells (DPCs) by electroporation and cultured the spheroids with keratinocytes (KCs). We optimized the cell number and culture period for applying spheroids to hair-on-a-chip. Furthermore, we investigated the expression of hair growth factors in spheroids depending on the presence or absence of human umbilical vein endothelial cells (HUVECs) and transfection. In spheroids in which DPCs, KCs, and HUVECs were co-cultured for 21 days, the expression of lymphoid enhancer factor 1 (LEF1), T-cell factor 1 (TCF1), and keratin 25 (K25) in the center of the spheroid, the expression of keratin 17 (K17) on the outer surface of the spheroid, and the shape of hair extending outward from the spheroid surface were observed. From these results, it is expected that a hair-on-a-chip experiment in which short-term cultured TKH spheroids are injected into the dermis and co-cultured with KC will enable the production of full-thickness skin equivalents containing hair in vitro without transplantation into animals.

Potential application of PBM use in hair follicle organoid culture for the treatment of androgenic alopecia

Androgenic alopecia is a hereditary condition of pattern hair loss in genetically susceptible individuals. The condition has a significant impact on an individual's quality of life, with decreased self-esteem, body image issues and depression being the main effects. Various conventional treatment options, such as minoxidil, finasteride and herbal supplements, aim to slow down hair loss and promote hair growth. However, due to the chronic nature of the condition the financial cost of treatment for androgenic alopecia is very high and conventional treatment options are not universally effective and come with a host of side effects. Therefore, to address the limitations of current treatment options a novel regenerative treatment option is required. One promising approach is organoids, organoids are 3D cell aggregates with similar structures and functions to a target organ. Hair follicle organoids can be developed in vitro. However, the main challenges are to maintain the cell populations within the organoid in a proliferative and inductive state, as well as to promote the maturation of organoids. Photobiomodulation is a form of light therapy that stimulates endogenous chromophores. PBM has been shown to improve cell viability, proliferation, migration, differentiation and gene expression in dermal papilla cells and hair follicle stem cells. Therefore, photobiomodulation is a potential adjunct to hair follicle organoid culture to improve the proliferation and inductive capacity of cells.

Epidermal-dermal coupled spheroids are important for tissue pattern regeneration in reconstituted skin explant cultures

Tissue patterning is critical for the development and regeneration of organs. To advance the use of engineered reconstituted skin organs, we study cardinal features important for tissue patterning and hair regeneration. We find they spontaneously form spheroid configurations, with polarized epidermal cells coupled with dermal cells through a newly formed basement membrane. Functionally, the spheroid becomes competent morphogenetic units (CMU) that promote regeneration of tissue patterns. The emergence of new cell types and molecular interactions during CMU formation was analyzed using scRNA-sequencing. Surprisingly, in newborn skin explants, IFNr signaling can induce apical-basal polarity in epidermal cell aggregates. Dermal-Tgfb induces basement membrane formation. Meanwhile, VEGF signaling mediates dermal cell attachment to the epidermal cyst shell, thus forming a CMU. Adult mouse and human fetal scalp cells fail to form a CMU but can be restored by adding IFNr or VEGF to achieve hair regeneration. We find different multi-cellular configurations and molecular pathways are used to achieve morphogenetic competence in developing skin, wound-induced hair neogenesis, and reconstituted explant cultures. Thus, multiple paths can be used to achieve tissue patterning. These insights encourage more studies of "in vitro morphogenesis" which may provide novel strategies to enhance regeneration.

The mechano-chemical circuit drives skin organoid self-organization

Stem cells in organoids self-organize into tissue patterns with unknown mechanisms. Here, we use skin organoids to analyze this process. Cell behavior videos show that the morphological transformation from multiple spheroidal units with morphogenesis competence (CMU) to planar skin is characterized by two abrupt cell motility-increasing events before calming down. The self-organizing processes are controlled by a morphogenetic module composed of molecular sensors, modulators, and executers. Increasing dermal stiffness provides the initial driving force (driver) which activates Yap1 (sensor) in epidermal cysts. Notch signaling (modulator 1) in epidermal cyst tunes the threshold of Yap1 activation. Activated Yap1 induces Wnts and MMPs (epidermal executers) in basal cells to facilitate cellular flows, allowing epidermal cells to protrude out from the CMU. Dermal cell-expressed Rock (dermal executer) generates a stiff force bridge between two CMU and accelerates tissue mixing via activating Laminin and β1-integrin. Thus, this self-organizing coalescence process is controlled by a mechano-chemical circuit. Beyond skin, self-organization in organoids may use similar mechano-chemical circuit structures.

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.

Biomaterials and bioengineering to guide tissue morphogenesis in epithelial organoids

Organoids are self-organized and miniatured in vitro models of organs and recapitulate key aspects of organ architecture and function, leading to rapid progress in understanding tissue development and disease. However, current organoid culture systems lack accurate spatiotemporal control over biochemical and physical cues that occur during in vivo organogenesis and fail to recapitulate the complexity of organ development, causing the generation of immature organoids partially resembling tissues in vivo. Recent advances in biomaterials and microengineering technologies paved the way for better recapitulation of organ morphogenesis and the generation of anatomically-relevant organoids. For this, understanding the native ECM components and organization of a target organ is essential in providing rational design of extracellular scaffolds that support organoid growth and maturation similarly to the in vivo microenvironment. In this review, we focus on epithelial organoids that resemble the spatial distinct structure and function of organs lined with epithelial cells including intestine, skin, lung, liver, and kidney. We first discuss the ECM diversity and organization found in epithelial organs and provide an overview of developing hydrogel systems for epithelial organoid culture emphasizing their key parameters to determine cell fates. Finally, we review the recent advances in tissue engineering and microfabrication technologies including bioprinting and microfluidics to overcome the limitations of traditional organoid cultures. The integration of engineering methodologies with the organoid systems provides a novel approach for instructing organoid morphogenesis via precise spatiotemporal modulation of bioactive cues and the establishment of high-throughput screening platforms.

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.

Adaptive multi-degree-of-freedom in situ bioprinting robot for hair-follicle-inclusive skin repair: A preliminary study conducted in mice

Skin acts as an essential barrier, protecting organisms from their environment. For skin trauma caused by accidental injuries, rapid healing, personalization, and functionality are vital requirements in clinical, which are the bottlenecks hindering the translation of skin repair from benchside to bedside. Herein, we described a novel design and a proof-of-concept demonstration of an adaptive bioprinting robot to proceed rapid in situ bioprinting on a full-thickness excisional wound in mice. The three-dimensional (3D) scanning and closed-loop visual system integrated in the robot and the multi-degree-of-freedom mechanism provide immediate, precise, and complete wound coverage through stereotactic bioprinting, which hits the key requirements of rapid-healing and personalization in skin repair. Combined with the robot, epidermal stem cells and skin-derived precursors isolated from neonatal mice mixed with Matrigel were directly printed into the injured area to replicate the skin structure. Excisional wounds after bioprinting showed complete wound healing and functional skin tissue regeneration that closely resembling native skin, including epidermis, dermis, blood vessels, hair follicles and sebaceous glands etc. This study provides an effective strategy for skin repair through the combination of the novel robot and a bioactive bioink, and has a promising clinical translational potential for further applications.

Global Research Status and Trends in Hair Follicle Stem Cells: a Bibliometric Analysis

BACKGROUND

Hair follicle stem cells (HFSCs) are derived from the bulge region and are important autologous stem cell sources. Bibliometric is a statistical method that quantitatively analyses the research papers concerned about one special topic. This study aims to estimate the research status and trends of HFSCs worldwide by bibliometric analyses.

METHODS

Data were obtained from the Web of Science by searching keywords related to HFSCs. Publication distributions stratified by countries/regions, institutions, journals, and authors were systematically assessed. The frequency of keywords was assessed, and bibliometric mapping was employed to describe the development of HFSC research.

RESULTS

A total of 458 publications that met our screening criteria were included in this study, consisting of 423 (92.4%) articles and 35 (7.6%) reviews. The United States of America (USA) ranked first in the number of publications at 146 (31.9%), followed by China at 130 (28.4%), which is consistent with the rank of the H-index. Author keywords were classified into three clusters, namely, basic study, applied study, and biomarker; average publication time of keywords in applied study cluster is later than basic study cluster. The keywords "bulge", "nestin", and "skin" are the top three most frequent keywords in basic studies; "differentiation", "proliferation", and "alopecia" are the top three most frequent keywords in applied studies. With respect to the latest research hotspots, "apoptosis" and "tissue engineering" are relatively new keywords.

CONCLUSIONS

The USA and China were the most productive countries for research on HFSCs. The focus of keywords gradually shifted from basic study to applied study. Research on the differentiation/proliferation of HFSCs and the role of HFSCs in alopecia have been recent research focuses. Apoptosis and tissue engineering are recommended as promising research hotspots. Our study provides profound insights into the research history, current status, and future trend of HFSCs.

Self-assembled complete hair follicle organoids by coculture of neonatal mouse epidermal cells and dermal cells in Matrigel

BACKGROUND

3D organoid cultures of hair follicles (HFs) are powerful models that mimic native HF for both in-depth study of HF disease and precision therapy. However, few studies have investigated the complete structure and properties of HF organoids. To investigate and characterize the complete HF organoids self-assembled by coculture of neonatal mouse epidermal cells (MECs) and dermal cells in Matrigel.

METHODS

Fresh epidermal and dermal cells from newborn mice (n=4) were isolated, and cocultured (1:1 ratio) in Matrigel using DMEM/F12 medium for 1 week. During the culture, an inverted microscope was used to observe the morphology of the 3D constructs. After 1 week, hematoxylin-eosin (HE) and immunofluorescence (IF) staining of HF-related markers (K5, K73, AE13, and K10), HF stem cell markers (K15, CD34, CD49f), skin-derived precursor-related marker (Nestin), and dermal papillae (DP)-specific markers (SOX2 and ALP) was performed in the harvested constructs to identify the HF organoids.

RESULTS

Epidermal and dermal cells self-assembled into HF organoids comprising an infundibular cyst-like structure, a lower segment-like structure, and a bulb-like structure from tail to root. The HF organoid had multiple, well-defined compartments similar to native anagen HF. Of the three segments, K73 was expressed in the inner root sheath-like layer, AE13 was localized in the hair shaft-like structure, K5, K15, CD34, and CD49f were present in the outer root sheath-like layer, Nestin labeled the connective tissue sheath-like layer, and SOX2 and ALP were expressed in the DP-like structure. Furthermore, K10 and K73 were expressed in the infundibular cyst-like structure. The expression of these molecular proteins was consistent with native anagen HF.

CONCLUSIONS

The complete HF organoid regenerated in Matrigel has specific compartments and is an excellent model to study HF disease and precision therapy.

Pattern formation from spatially heterogeneous reaction-diffusion systems

First proposed by Turing in 1952, the eponymous Turing instability and Turing pattern remain key tools for the modern study of diffusion-driven pattern formation. In spatially homogeneous Turing systems, one or a few linear Turing modes dominate, resulting in organized patterns (peaks in one dimension; spots, stripes, labyrinths in two dimensions) which repeats in space. For a variety of reasons, there has been increasing interest in understanding irregular patterns, with spatial heterogeneity in the underlying reaction-diffusion system identified as one route to obtaining irregular patterns. We study pattern formation from reaction-diffusion systems which involve spatial heterogeneity, by way of both analytical and numerical techniques. We first extend the classical Turing instability analysis to track the evolution of linear Turing modes and the nascent pattern, resulting in a more general instability criterion which can be applied to spatially heterogeneous systems. We also calculate nonlinear mode coefficients, employing these to understand how each spatial mode influences the long-time evolution of a pattern. Unlike for the standard spatially homogeneous Turing systems, spatially heterogeneous systems may involve many Turing modes of different wavelengths interacting simultaneously, with resulting patterns exhibiting a high degree of variation over space. We provide a number of examples of spatial heterogeneity in reaction-diffusion systems, both mathematical (space-varying diffusion parameters and reaction kinetics, mixed boundary conditions, space-varying base states) and physical (curved anisotropic domains, apical growth of space domains, chemicalsimmersed within a flow or a thermal gradient), providing a qualitative understanding of how spatial heterogeneity can be used to modify classical Turing patterns. This article is part of the theme issue 'Recent progress and open frontiers in Turing's theory of morphogenesis'.

Systems of pattern formation within developmental biology

Applications of mathematical models to developmental biology have provided helpful insight into various subfields, ranging from the patterning of animal skin to the development of complex organ systems. Systems involved in patterning within morphology present a unique path to explain self-organizing systems. Current efforts show that patterning systems, notably Reaction-Diffusion and specific signaling pathways, provide insight for explaining morphology and could provide novel applications revolving around the formation of biological systems. Furthermore, the application of pattern formation provides a new perspective on understanding developmental biology and pathology research to study molecular mechanisms. The current review is to cover and take a more in-depth overlook at current applications of patterning systems while also building on the principles of patterning of future research in predictive medicine.

UDP-GlcNAc-1-Phosphotransferase Is a Clinically Important Regulator of Human and Mouse Hair Pigmentation

UDP-GlcNAc-1-phosphotransferase, a product of two separate genes (GNPTAB, GNPTG), is essential for the sorting and transportation of lysosomal enzymes to lysosomes. GNPTAB gene defects cause extracellular missorting of lysosomal enzymes resulting in lysosomal storage diseases, namely mucolipidosis type II and mucolipidosis type III alpha/beta, which is associated with hair discoloration. Yet, the physiological functions of GNPTAB in the control of hair follicle (HF) pigmentation remain unknown. To elucidate these, we have silenced GNPTAB in organ-cultured human HFs as a human ex vivo model for mucolipidosis type II. GNPTAB silencing profoundly inhibited intrafollicular melanin production, the correct sorting of melanosomes, tyrosinase activity, and HMB45 expression in the HF pigmentary unit and altered HF melanocyte morphology in situ. In isolated primary human HF melanocytes, GNPTAB knockdown significantly reduced melanogenesis, tyrosinase activity, and correct tyrosinase protein sorting as well as POMC expression and caused the expected lysosomal enzyme missorting in vitro. Moreover, transgenic mice overexpressing an inserted missense mutation corresponding to that seen in human mucolipidosis type II and mucolipidosis type III alpha/beta showed significantly reduced HF pigmentation, thus corroborating the in vivo relevance of our ex vivo and in vitro findings in the human system. This identifies GNPTAB as a clinically important enzymatic control of human HF pigmentation, likely by directly controlling tyrosinase sorting and POMC transcription in HF melanocytes.

Bespoke Turing Systems

Reaction-diffusion systems are an intensively studied form of partial differential equation, frequently used to produce spatially heterogeneous patterned states from homogeneous symmetry breaking via the Turing instability. Although there are many prototypical "Turing systems" available, determining their parameters, functional forms, and general appropriateness for a given application is often difficult. Here, we consider the reverse problem. Namely, suppose we know the parameter region associated with the reaction kinetics in which patterning is required-we present a constructive framework for identifying systems that will exhibit the Turing instability within this region, whilst in addition often allowing selection of desired patterning features, such as spots, or stripes. In particular, we show how to build a system of two populations governed by polynomial morphogen kinetics such that the: patterning parameter domain (in any spatial dimension), morphogen phases (in any spatial dimension), and even type of resulting pattern (in up to two spatial dimensions) can all be determined. Finally, by employing spatial and temporal heterogeneity, we demonstrate that mixed mode patterns (spots, stripes, and complex prepatterns) are also possible, allowing one to build arbitrarily complicated patterning landscapes. Such a framework can be employed pedagogically, or in a variety of contemporary applications in designing synthetic chemical and biological patterning systems. We also discuss the implications that this freedom of design has on using reaction-diffusion systems in biological modelling and suggest that stronger constraints are needed when linking theory and experiment, as many simple patterns can be easily generated given freedom to choose reaction kinetics.

Skin organoids: A new human model for developmental and translational research

Culturing skin cells outside of the body has been a cornerstone of dermatological investigation for many years; however, human skin equivalent systems typically lack the full complexity of native skin. Notably, skin appendages, such as hair follicles and sweat glands, remain a challenge to generate or maintain in cell cultures and reconstruct in damaged skin. Recent work from our lab has demonstrated methods for generating appendage-bearing skin tissue-known as skin organoids-from pluripotent stem cells. Here, we will summarize this work and other related works, and then discuss the potential future applications of skin organoids in dermatological research.

"Two-Cell Assemblage" Assay: A Simple in vitro Method for Screening Hair Growth-Promoting Compounds

Alopecia arises due to inadequate hair follicle (HF) stem cell activation or proliferation, resulting in prolongation of the telogen phase of the hair cycle. Increasing therapeutic and cosmetic demand for alleviating alopecia has driven research toward the discovery or synthesis of novel compounds that can promote hair growth by inducing HF stem cell activation or proliferation and initiating the anagen phase. Although several methods for evaluating the hair growth-promoting effects of candidate compounds are being used, most of these methods are difficult to use for large scale simultaneous screening of various compounds. Herein, we introduce a simple and reliable in vitro assay for the simultaneous screening of the hair growth-promoting effects of candidate compounds on a large scale. In this study, we first established a 3D co-culture system of human dermal papilla (hDP) cells and human outer root sheath (hORS) cells in an ultra-low attachment 96-well plate, where the two cell types constituted a polar elongated structure, named "two-cell assemblage (TCA)." We observed that the long axis length of the TCA gradually increased for 5 days, maintaining biological functional integrity as reflected by the increased expression levels of hair growth-associated genes after treatment with hair growth-promoting molecules. Interestingly, the elongation of the TCA was more prominent following treatment with the hair growth-promoting molecules (which occurred in a dose-dependent manner), compared to the control group (p < 0.05). Accordingly, we set the long axis length of the TCA as an endpoint of this assay, using a micro confocal high-content imaging system to measure the length, which can provide reproducible and reliable results in an adequate timescale. The advantages of this assay are: (i) it is physiologically and practically advantageous as it uses 3D cultured two-type human cells which are easily available; (ii) it is simple as it uses length as the only endpoint; and (iii) it is a high throughput system, which screens various compounds simultaneously. In conclusion, the "TCA" assay could serve as an easy and reliable method to validate the hair growth-promoting effect of a large volume of library molecules.

From one pattern into another: analysis of Turing patterns in heterogeneous domains via WKBJ

Pattern formation from homogeneity is well studied, but less is known concerning symmetry-breaking instabilities in heterogeneous media. It is non-trivial to separate observed spatial patterning due to inherent spatial heterogeneity from emergent patterning due to nonlinear instability. We employ WKBJ asymptotics to investigate Turing instabilities for a spatially heterogeneous reaction-diffusion system, and derive conditions for instability which are local versions of the classical Turing conditions. We find that the structure of unstable modes differs substantially from the typical trigonometric functions seen in the spatially homogeneous setting. Modes of different growth rates are localized to different spatial regions. This localization helps explain common amplitude modulations observed in simulations of Turing systems in heterogeneous settings. We numerically demonstrate this theory, giving an illustrative example of the emergent instabilities and the striking complexity arising from spatially heterogeneous reaction-diffusion systems. Our results give insight both into systems driven by exogenous heterogeneity, as well as successive pattern forming processes, noting that most scenarios in biology do not involve symmetry breaking from homogeneity, but instead consist of sequential evolutions of heterogeneous states. The instability mechanism reported here precisely captures such evolution, and extends Turing's original thesis to a far wider and more realistic class of systems.

Pre-aggregation of scalp progenitor dermal and epidermal stem cells activates the WNT pathway and promotes hair follicle formation in in vitro and in vivo systems

BACKGROUND

Billions of dollars are invested annually by pharmaceutical companies in search of new options for treating hair loss conditions; nevertheless, the challenge remains. One major limitation to hair follicle research is the lack of effective and efficient drug screening systems using human cells. Organoids, three-dimensional in vitro structures derived from stem cells, provide new opportunities for studying organ development, tissue regeneration, and disease pathogenesis. The present study focuses on the formation of human hair follicle organoids.

METHODS

Scalp-derived dermal progenitor cells mixed with foreskin-derived epidermal stem cells at a 2:1 ratio aggregated in suspension to form hair follicle-like organoids, which were confirmed by immunostaining of hair follicle markers and by molecular dye labeling assays to analyze dermal and epidermal cell organization in those organoids. The hair-forming potential of organoids was examined using an in vivo transplantation assay.

RESULTS

Pre-aggregation of dermal and epidermal cells enhanced hair follicle formation in vivo. In vitro pre-aggregation initiated the interactions of epidermal and dermal progenitor cells resulting in activation of the WNT pathway and the formation of pear-shape structures, named type I aggregates. Cell-tracing analysis showed that the dermal and epidermal cells self-assembled into distinct epidermal and dermal compartments. Histologically, the type I aggregates expressed early hair follicle markers, suggesting the hair peg-like phase of hair follicle morphogenesis. The addition of recombinant WNT3a protein to the medium enhanced the formation of these aggregates, and the Wnt effect could be blocked by the WNT inhibitor, IWP2.

CONCLUSIONS

In summary, our system supports the rapid formation of a large number of hair follicle organoids (type I aggregates). This system provides a platform for studying epithelial-mesenchymal interactions, for assessing inductive hair stem cells and for screening compounds that support hair follicle regeneration.