Restoration of hair growth by surgical implantation of follicular dermal sheath

The development of hair follicles and nail

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

The promising prospect of human hair follicle regeneration in the shadow of new tissue engineering strategies

Hair loss disorder (alopecia) affects numerous people around the world. The low effectiveness and numerous side effects of common treatments have prompted researchers to investigate alternative and effective solutions. Hair follicle (HF) bioengineering is the knowledge of using hair-inductive (trichogenic) cells. Most bioengineering-based approaches focus on regenerating folliculogenesis through manipulation of regulators of physical/molecular properties in the HF niche. Despite the high potential of cell therapy, no cell product has been produced for effective treatment in the field of hair regeneration. This problem shows the challenges in the functionality of cultured human hair cells. To achieve this goal, research and development of new and practical approaches, technologies and biomaterials are needed. Based on recent advances in the field, this review evaluates emerging HF bioengineering strategies and the future prospects for the field of tissue engineering and successful HF regeneration.

Sympathetic Reinnervation of Intact and Upper Follicle Xenografts into BALB/c-nu/nu Mice

Increasing concerns about hair loss affect people's quality of life. Recent studies have found that sympathetic nerves play a positive role in regulating hair follicle stem cell activity to promote hair growth. However, no study has investigated sympathetic innervation of transplanted follicles. Rat vibrissa follicles were extracted and implanted under the dorsal skin of BALB/c-nu/nu mice using one of two types of follicles: (1) intact follicles, where transplants included bulbs, and (2) upper follicles, where transplants excluded bulbs. Follicular samples were collected for hematoxylin and eosin staining, immunofluorescence staining for tyrosine hydroxylase (TH, a sympathetic marker) and enzyme-linked immunosorbent assays. At 37 days after implantation in both groups, follicles had entered anagen, with the growth of long hair shafts; tyrosine-hydroxylase-positive nerves were innervating follicles (1.45-fold); and norepinephrine concentrations (2.03-fold) were significantly increased compared to 5 days, but did not return to normal. We demonstrate the survival of intact and upper follicle xenografts and the partial restoration of sympathetic reinnervations of both transplanted follicles.

Lrig1-expressing epidermal progenitors require SCD1 to maintain the dermal papilla niche

Niche cells are widely known to regulate stem/progenitor cells in many mammalian tissues. In the hair, dermal papilla niche cells are well accepted to regulate hair stem/progenitor cells. However, how niche cells themselves are maintained is largely unknown. We present evidence implicating hair matrix progenitors and the lipid modifying enzyme, Stearoyl CoA Desaturase 1, in the regulation of the dermal papilla niche during the anagen-catagen transition of the mouse hair cycle. Our data suggest that this takes place via autocrine Wnt signalling and paracrine Hedgehog signalling. To our knowledge, this is the first report demonstrating a potential role for matrix progenitor cells in maintaining the dermal papilla niche.

Changes in the Expression of Smooth Muscle Cell–Related Genes in Human Dermal Sheath Cup Cells Associated with the Treatment Outcome of Autologous Cell–Based Therapy for Male and Female Pattern Hair Loss

In a clinical study of autologous cell–based therapy using dermal sheath cup (DSC) cells, the treatment of hair loss showed improvements. However, the outcomes were variable. Here, correlations between marker gene expression in DSC cells and treatment outcomes were assessed to predict therapeutic efficacy. Overall, 32 DSC cell lines were used to evaluate correlations between marker gene expression and treatment outcomes. Correlations between vascular pericyte and preadipocyte marker expression and treatment outcomes were inconsistent. As smooth muscle cell markers, MYOCD correlated negatively with treatment outcomes and SRF consistently demonstrated an inverse correlation. Additionally, CALD1 correlated negatively and ACTA2 correlated inversely with treatment outcomes. DSC cell lines were divided into good and moderate/poor responders to further investigate the correlations. SRF and CALD1 were lower in a good responder compared with a moderate responder. Next, DSC cells were differentiated toward dermal papilla cells. Dermal papilla markers SOX2 and LEF1 before differentiation had moderate positive and inverse correlations with the treatment outcome, respectively. SOX2 after differentiation more consistently demonstrated a positive correlation. Significant downregulation of smooth muscle–related genes was also observed after differentiation. These findings revealed putative markers for preclinical evaluation of DSC cells to improve hair loss.

Decomposing a deterministic path to mesenchymal niche formation by two intersecting morphogen gradients

Organ formation requires integrating signals to coordinate proliferation, specify cell fates, and shape tissue. Tracing these events and signals remains a challenge, as intermediate states across many critical transitions are unresolvable over real time and space. Here, we designed a unique computational approach to decompose a non-linear differentiation process into key components to resolve the signals and cell behaviors that drive a rapid transition, using the hair follicle dermal condensate as a model. Combining scRNA sequencing with genetic perturbation, we reveal that proliferative Dkk1+ progenitors transiently amplify to become quiescent dermal condensate cells by the mere spatiotemporal patterning of Wnt/β-catenin and SHH signaling gradients. Together, they deterministically coordinate a rapid transition from proliferation to quiescence, cell fate specification, and morphogenesis. Moreover, genetically repatterning these gradients reproduces these events autonomously in "slow motion" across more intermediates that resolve the process. This analysis unravels two morphogen gradients that intersect to coordinate events of organogenesis.

Dermal Fibroblast Heterogeneity and Its Contribution to the Skin Repair and Regeneration

Significance: Dermal fibroblasts are the major cell type in the skin's dermal layer. These cells originate from distinct locations of the embryo and reside in unique niches in the dermis. Different dermal fibroblasts exhibit distinct roles in skin development, homeostasis, and wound healing. Therefore, these cells are becoming attractive candidates for cell-based therapies in wound healing. Recent Advances: Human skin dermis comprises multiple fibroblast subtypes, including papillary, reticular, and hair follicle-associated fibroblasts, and myofibroblasts after wounding. Recent studies reveal that these cells play distinct roles in wound healing and contribute to diverse healing outcomes, including nonhealing chronic wound or excessive scar formation, such as hypertrophic scars (HTS) and keloids, with papillary fibroblasts having antiscarring and reticular fibroblast scar-forming properties. Critical Issues: The identities and functions of dermal fibroblast subpopulations in many respects remain unknown. In this review, we summarize the current understanding of dermal fibroblast heterogeneity, including their defined cell markers and dermal niches, dynamic changes, and contributions to skin wound healing, with the emphasis on scarless healing, healing with excessive scars (HTS and keloids), chronic wounds, and the potential application of this heterogeneity for developing cell-based therapies that allow wounds to heal faster with less scarring. Future Directions: Heterogeneous dermal fibroblast populations and their functions are poorly characterized. Refining and advancing our understanding of dermal fibroblast heterogeneity and their participation in skin homeostasis and wound healing may create potential therapeutic applications for nonhealing chronic wounds or wounds that heal with excessive scarring.

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.

Cyclic growth of dermal papilla and regeneration of follicular mesenchymal components during feather cycling

How dermis maintains tissue homeostasis in cyclic growth and wounding is a fundamental unsolved question. Here, we study how dermal components of feather follicles undergo physiological (molting) and plucking injury-induced regeneration in chickens. Proliferation analyses reveal quiescent, transient-amplifying (TA) and long-term label-retaining dermal cell (LRDC) states. During the growth phase, LRDCs are activated to make new dermal components with distinct cellular flows. Dermal TA cells, enriched in the proximal follicle, generate both peripheral pulp, which extends distally to expand the epithelial-mesenchymal interactive interface for barb patterning, and central pulp, which provides nutrition. Entering the resting phase, LRDCs, accompanying collar bulge epidermal label-retaining cells, descend to the apical dermal papilla. In the next cycle, these apical dermal papilla LRDCs are re-activated to become new pulp progenitor TA cells. In the growth phase, lower dermal sheath can generate dermal papilla and pulp. Transcriptome analyses identify marker genes and highlight molecular signaling associated with dermal specification. We compare the cyclic topological changes with those of the hair follicle, a convergently evolved follicle configuration. This work presents a model for analyzing homeostasis and tissue remodeling of mesenchymal progenitors.

H syndrome: A review of treatment options and a hypothesis of phenotypic variability

H syndrome is a rare autosomal recessive disorder with clinical features comprising: hyperpigmentation, hypertrichosis, hearing loss, heart anomalies, low height, hypogonadism and hepatosplenomegaly. H syndrome results from loss-of-function mutations in SLC29A3 which leads to abnormal proliferation and function of histiocytes. Herein, we discuss the considerable phenotypic heterogeneity detected in a consanguineous Egyptian family comprising of four affected siblings, two of which are monozygotic twin and the possible therapeutics. The phenotypic variability may be attributed to the role of histiocytes in the tissue response to injury. Such variable expressivity of H syndrome renders the diagnosis challenging and delays the management. The different treatment approaches used for this rare entity are reviewed.

The dermal sheath: An emerging component of the hair follicle stem cell niche

Hair follicles cyclically regenerate throughout adult mammalian life, owing to a resident population of epithelial hair follicle stem cells. Stem cell (SC) activity drives bouts of follicle growth, which are periodically interrupted by follicle regression and rest. These phases and the transitions between them are tightly spatiotemporally coordinated by signalling crosstalk between stem/progenitor cells and the various cell types of the microenvironment, or niche. The dermal papilla (DP) is a cluster of specialized mesenchymal cells that have long been recognized for important niche roles in regulating hair follicle SC activation, as well as progenitor proliferation and differentiation during follicle growth. In addition to the DP, the mesenchyme of the murine pelage follicle is also comprised of a follicle-lining smooth muscle known as the dermal sheath (DS), which has been far less studied than the DP yet may be equally specialized and important for hair cycling. In this review, we define the murine pelage DS in comparison with human DS and discuss recent work that highlights the emergent importance of the DS in the hair follicle SC niche. Last, we examine potential therapeutic applications for the DS in hair regeneration and wound healing.

Design of a Multifunctional Biomaterial Inspired by Ancient Chinese Medicine for Hair Regeneration in Burned Skin

In deep burn injuries, the dermis of the skin is often severely damaged, and hair follicles are also lost and lose the potential for regeneration. Therefore, the development of wound dressings that promote hair follicle regeneration has important clinical significance. In this study, inspired by an ancient Chinese medicine prescription, a novel fibrous membrane (P/Qu/Cup; P, PCL; Qu, quercetin; Cup, cuprorivaite, CaCuSi₄O₁₀) containing quercetin-copper (Qu-Cu) chelates was fabricated by using quercetin and a highly bioactive bioceramic (CaCuSi₄O₁₀) incorporated in PCL/gelatin electrospun fibers. The fibrous membrane can effectively release Qu and Cu ions to induce proliferation, migration, and differentiation of skin and hair follicle related cells, and the Qu, Cu ions, and Si ions released from the composite membrane revealed synergistic activity to stimulate hair follicle regeneration and wound healing. Our study demonstrated that the analysis of the common components in ancient Chinese prescription is an effective approach to design novel bioactive materials for regenerative medicine.

Use of human intra-tissue stem/progenitor cells and induced pluripotent stem cells for hair follicle regeneration

Background

The hair follicle (HF) is a unique miniorgan, which self-renews for a lifetime. Stem cell populations of multiple lineages reside within human HF and enable its regeneration. In addition to resident HF stem/progenitor cells (HFSPCs), the cells with similar biological properties can be induced from human-induced pluripotent stem cells (hiPSCs). As approaches to regenerate HF by combining HF-derived cells have been established in rodents and a huge demand exists to treat hair loss diseases, attempts have been made to bioengineer human HF using HFSPCs or hiPSCs.

Main body of the abstract

The aim of this review is to comprehensively summarize the strategies to regenerate human HF using HFSPCs or hiPSCs. HF morphogenesis and regeneration are enabled by well-orchestrated epithelial-mesenchymal interactions (EMIs). In rodents, various combinations of keratinocytes with mesenchymal (dermal) cells with trichogenic capacity, which were transplanted into in vivo environment, have successfully generated HF structures. The regeneration efficiency was higher, when epithelial or dermal HFSPCs were adopted. The success in HF formation most likely depended on high receptivity to trichogenic dermal signals and/or potent hair inductive capacity of HFSPCs. In theory, the use of epithelial HFSPCs in the bulge area and dermal papilla cells, their precursor cells in the dermal sheath, or trichogenic neonatal dermal cells should elicit intense EMI sufficient for HF formation. However, technical hurdles, represented by the limitation in starting materials and the loss of intrinsic properties during in vitro expansion, hamper the stable reconstitution of human HFs with this approach. Several strategies, including the amelioration of culture condition or compartmentalization of cells to strengthen EMI, can be conceived to overcome this obstacle. Obviously, use of hiPSCs can resolve the shortage of the materials once reliable protocols to induce wanted HFSPC subsets have been developed, which is in progress. Taking advantage of their pluripotency, hiPSCs may facilitate previously unthinkable approaches to regenerate human HFs, for instance, via bioengineering of 3D integumentary organ system, which can also be applied for the treatment of other diseases.

Short conclusion

Further development of methodologies to reproduce bona fide EMI in HF formation is indispensable. However, human HFSPCs and hiPSCs hold promise as materials for human HF regeneration.

Fibroblast heterogeneity and its implications for engineering organotypic skin models in vitro

Advances in cell culture methods, multidisciplinary research, clinical need to replace lost skin tissues and regulatory need to replace animal models with alternative test methods has led to development of three dimensional models of human skin. In general, these in vitro models of skin consist of keratinocytes cultured over fibroblast-populated dermal matrices. Accumulating evidences indicate that mesenchyme-derived signals are essential for epidermal morphogenesis, homeostasis and differentiation. Various studies show that fibroblasts isolated from different tissues in the body are dynamic in nature and are morphologically and functionally heterogeneous subpopulations. Further, these differences seem to be dictated by the local biological and physical microenvironment the fibroblasts reside resulting in "positional identity or memory". Furthermore, the heterogeneity among the fibroblasts play a critical role in scarless wound healing and complete restoration of native tissue architecture in fetus and oral mucosa; and excessive scar formation in diseased states like keloids and hypertrophic scars. In this review, we summarize current concepts about the heterogeneity among fibroblasts and their role in various wound healing environments. Further, we contemplate how the insights on fibroblast heterogeneity could be applied for the development of next generation organotypic skin models.

Enhanced restoration of in situ-damaged hairs by intradermal transplantation of trichogenous dermal cells

We developed a nude rat model for determining the capacity of trichogenous cells to restore in vivo-damaged hair follicles (HFs). A surgical scalpel was inserted into the rat's dermis to generate the in vivo-damaged pelage HFs, the HFs whose lower parts were lost, but the upper parts containing sebaceous and bulge regions remained intact. Dermal papilla cells (DPCs) and dermal sheath cells (DSCs) from EGFP transgenic rat vibrissae were propagated in culture, and each alone (DPC or DSC) or a mixture (DPC/DSC) was transplanted into the intradermal path made by a scalpel. It was found that the in vivo-damaged HFs had hair self-restoration ability, and the transplanted trichogenic dermal cells prominently enhanced this ability, DPC/DSC transplants being more effective in enhancement than DPC or DSC alone. The restored bulbs contained EGFP-positive cells, shed their original straight shafts, generated new shafts, and further developed into hairs with a sebaceous gland and bulge structures by ~6 weeks post-transplantation. Compared to the preceding animal models, this model is less invasive, requires fewer donor cells and allows repeated operations with higher reproducibility and accuracy. The present study suggests that conditions causing in situ-damaged HFs, such as androgenic alopecia, in which HFs are damaged and miniaturized, can be restored by functional trichogenous dermal cell transplantation therapy. Copyright © 2015 John Wiley & Sons, Ltd.

A review of adipocyte lineage cells and dermal papilla cells in hair follicle regeneration

Alopecia is an exceedingly prevalent problem effecting men and women of all ages. The standard of care for alopecia involves either transplanting existing hair follicles to bald areas or attempting to stimulate existing follicles with topical and/or oral medication. Yet, these treatment options are fraught with problems of cost, side effects, and, most importantly, inadequate long-term hair coverage. Innovative cell-based therapies have focused on the dermal papilla cell as a way to grow new hair in previously bald areas. However, despite this attention, many obstacles exist, including retention of dermal papilla inducing ability and maintenance of dermal papilla productivity after several passages of culture. The use of adipocyte lineage cells, including adipose-derived stem cells, has shown promise as a cell-based solution to regulate hair regeneration and may help in maintaining or increasing dermal papilla cells inducing hair ability. In this review, we highlight recent advances in the understanding of the cellular contribution and regulation of dermal papilla cells and summarize adipocyte lineage cells in hair regeneration.

Effect of intense pulsed light treatment on human skin in vitro: analysis of immediate effects on dermal papillae and hair follicle stem cells

BACKGROUND

Hair follicles house a permanent pool of epithelial stem cells. Intense pulsed light (IPL) sources have been successfully used for hair removal, but long-term hair reduction may require several treatments. Many questions remain regarding the impact of IPL treatment on the structure of the hair follicle, more specifically on hair follicular stem cells and dermal papilla cells, a group of specialized cells that orchestrate hair growth.

OBJECTIVES

To characterize the destruction of human hair follicles and surrounding tissues following IPL treatment, with more attention paid to the bulge and the bulb regions.

METHODS

Human scalp specimens of Fitzpatrick skin phototype II were exposed ex vivo to IPL pulses and were then processed for histological analysis, immunodetection of stem cell-associated keratin 19, and revelation of the endogenous alkaline phosphatase activity expressed in dermal papilla cells.

RESULTS

Histological analysis confirmed that pigmented structures, such as the melanin-rich matrix cells of the bulb in anagen follicles and the hair shaft, are principally targeted by IPL treatment, while white hairs and epidermis remained unaffected. Damage caused by heat sometimes extended over the dermal papilla cells, while stem cells were mostly spared.

CONCLUSIONS

IPL epilation principally targets pigmented structures. Our results suggest that, under the tested conditions, collateral damage does not deplete stem cells. Damage at the dermal papilla was observed only with high-energy treatment modalities. Extrapolated to frequently treated hairs, these observations explain why some hairs grow back after a single IPL treatment.

Hair curvature: a natural dialectic and review

Although hair forms (straight, curly, wavy, etc.) are present in apparently infinite variations, each fibre can be reduced to a finite sequence of tandem segments of just three types: straight, bent/curly, or twisted. Hair forms can thus be regarded as resulting from genetic pathways that induce, reverse or modulate these basic curvature modes. However, physical interconversions between twists and curls demonstrate that strict one-to-one correspondences between them and their genetic causes do not exist. Current hair-curvature theories do not distinguish between bending and twisting mechanisms. We here introduce a multiple papillary centres (MPC) model which is particularly suitable to explain twisting. The model combines previously known features of hair cross-sectional morphology with partially/completely separated dermal papillae within single follicles, and requires such papillae to induce differential growth rates of hair cortical material in their immediate neighbourhoods. The MPC model can further help to explain other, poorly understood, aspects of hair growth and morphology. Separate bending and twisting mechanisms would be preferentially affected at the major or minor ellipsoidal sides of fibres, respectively, and together they exhaust the possibilities for influencing hair-form phenotypes. As such they suggest dialectic for hair-curvature development. We define a natural-dialectic (ND) which could take advantage of speculative aspects of dialectic, but would verify its input data and results by experimental methods. We use this as a top-down approach to first define routes by which hair bending or twisting may be brought about and then review evidence in support of such routes. In particular we consider the wingless (Wnt) and mammalian target of rapamycin (mTOR) pathways as paradigm pathways for molecular hair bending and twisting mechanisms, respectively. In addition to the Wnt canonical pathway, the Wnt/Ca(2+) and planar cell polarity (PCP) pathways, and others, can explain many alternatives and specific variations of hair bending phenotypes. Mechanisms for hair papilla budding or its division by bisection or fission can explain MPC formation. Epithelial-to-mesenchymal (EMT) and mesenchymal-to-epithelial (MET) transitions, acting in collaboration with epithelial-mesenchymal communications are also considered as mechanisms affecting hair growth and its bending and twisting. These may be treated as sub-mechanisms of an overall development from neural-crest stem cell (NCSC) lineages to differentiated hair follicle (HF) cell types, thus providing a unified framework for hair growth and development.

Stem cell dynamics in the hair follicle niche

Hair follicles are appendages of the mammalian skin that have the ability to periodically and stereotypically regenerate in order to continuously produce new hair over our lifetime. The ability of the hair follicle to regenerate is due to the presence of stem cells that along with other cell populations and non-cellular components, including molecular signals and extracellular material, make up a niche microenvironment. Mounting evidence suggests that the niche is critical for regulating stem cell behavior and thus the process of regeneration. Here, we review the literature concerning past and current studies that have utilized mouse genetic models, combined with other approaches to dissect the molecular and cellular composition of the hair follicle niche. We also discuss our current understanding of how stem cells operate within the niche during the process of tissue regeneration and the factors that regulate their behavior.

Strategies to enhance epithelial-mesenchymal interactions for human hair follicle bioengineering

Hair follicle morphogenesis and regeneration depend on intensive but well-orchestrated interactions between epithelial and mesenchymal components. Accordingly, the enhancement of this crosstalk represents a promising approach to achieve successful bioengineering of human hair follicles. The present article summarizes the techniques, both currently available and potentially feasible, to promote epithelial-mesenchymal interactions (EMIs) necessary for human hair follicle regeneration. The strategies include the preparation of epithelial components with high receptivity to trichogenic dermal signals and/or mesenchymal cell populations with potent hair inductive capacity. In this regard, bulge epithelial stem cells, keratinocytes predisposed to hair follicle fate or keratinocyte precursor cells with plasticity may provide favorable epithelial cell populations. Dermal papilla cells sustaining intrinsic hair inductive capacity, putative dermal papilla precursor cells in the dermal sheath/neonatal dermis or trichogenic dermal cells derived from undifferentiated stem/progenitor cells are promising candidates as hair inductive dermal cells. The most established protocol for in vivo hair follicle reconstitution is co-grafting of epithelial and mesenchymal components into immunodeficient mice. In theory, combination of individually optimized cellular components of respective lineages should elicit most intensive EMIs to form hair follicles. Still, EMIs can be further ameliorated by the modulation of non-cell autonomous conditions, including cell compartmentalization to replicate the positional relationship in vivo and humanization of host environment by preparing human stromal bed. These approaches may not always synergistically intensify EMIs, however, step-by-step investigation probing optimal combinations should maximally enhance EMIs to achieve successful human hair follicle bioengineering.

Pattern hair loss in men: diagnosis and medical treatment

Androgenetic alopecia is a common cause of hair loss in both men and women. The exact pathogenesis of androgenetic alopecia is not well understood. As the name implies, the role of androgens and genetic susceptibility predisposes to pattern hair loss due to gradual conversion of terminal hair into vellus hair. Male and female pattern hair loss are clinically distinct entities but histologically indistinguishable. The role of sex hormones in females is less understood. This article discusses current understanding of the etiopathogenesis of hair loss in men, diagnostic tests available, and its medical management.

Sprouty/FGF signaling regulates the proximal-distal feather morphology and the size of dermal papillae

In a feather, there are distinct morphologies along the proximal-distal axis. The proximal part is a cylindrical stalk (calamus), whereas the distal part has barb and barbule branches. Here we focus on what molecular signaling activity can modulate feather stem cells to generate these distinct morphologies. We demonstrate the drastic tissue remodeling during feather cycling which includes initiation, growth and resting phases. In the growth phase, epithelial components undergo progressive changes from the collar growth zone to the ramogenic zone, to maturing barb branches along the proximal-distal axis. Mesenchymal components also undergo progressive changes from the dermal papilla, to the collar mesenchyme, to the pulp along the proximal-distal axis. Over-expression of Spry4, a negative regulator of receptor tyrosine kinases, promotes barb branch formation at the expense of the epidermal collar. It even induces barb branches from the follicle sheath (equivalent to the outer root sheath in hair follicles). The results are feathers with expanded feather vane regions and small or missing proximal feather shafts (the calamus). Spry4 also expands the pulp region while reducing the size of dermal papillae, leading to a failure to regenerate. In contrast, over-expressing Fgf10 increases the size of the dermal papillae, expands collar epithelia and mesenchyme, but also prevents feather branch formation and feather keratin differentiation. These results suggest that coordinated Sprouty/FGF pathway activity at different stages is important to modulate feather epidermal stem cells to form distinct feather morphologies along the proximal-distal feather axis.

Histidine decarboxylase expression influences the neofolliculogenesis of newborn mouse dermal cells

BACKGROUND

With the introduction of hair regeneration techniques using epidermal and dermal cells, hair follicle regeneration became much easier and faster. Current success has been dependent on the availability of cells from newborn or embryonic mice. We recently observed that the hair-inducing ability of newborn mouse dermal cells disappeared in the first few days of life and there was a drastic decrease of histidine decarboxylase (HDC) gene expression from postnatal day 0 (p0) to day 7 (p7).

OBJECTIVE

The aim of this study was to study the role of HDC in hair follicle induction.

METHODS

The mRNA levels of HDC in p0, p7 and p48 C57BL/6 mouse skin were checked with a real time reverse transcription polymerase chain reaction and immunohistochemistry. To test the effect of HDC, HDC expression in p0 mouse dermal cells was suppressed with small interfering RNA (siRNA) transfection. Mock treated and HDC siRNA treated cells were then injected with adult epidermal cells into nude mice skin. Three weeks later, the number, length and thickness of induced hairs were compared.

RESULTS

Compared with p0, the mRNA level of HDC was much lower at p7 and p48. Immunohistochemical staining also revealed a marked decrease of HDC expression in p7 mice skin, compared with p0 skin. Hair patch assays showed that the HDC siRNA treated p0 dermal cells induced less hair follicle structures and shorter and thinner hair shafts than mock treated cells.

CONCLUSION

HDC, whose expression is remarkably downregulated during the first few days after birth in dermal cells of mice, plays essential roles in the hair-inducing ability of newborn mouse dermal cells.

Polyclonal origin and hair induction ability of dermal papillae in neonatal and adult mouse back skin

Hair follicle development and growth are regulated by Wnt signalling and depend on interactions between epidermal cells and a population of fibroblasts at the base of the follicle, known as the dermal papilla (DP). DP cells have a distinct gene expression signature from non-DP dermal fibroblasts. However, their origins are largely unknown. By generating chimeric mice and performing skin reconstitution assays we show that, irrespective of whether DP form during development, are induced by epidermal Wnt activation in adult skin or assemble from disaggregated cells, they are polyclonal in origin. While fibroblast proliferation is necessary for hair follicle formation in skin reconstitution assays, mitotically inhibited cells readily contribute to DP. Although new hair follicles do not usually develop in adult skin, adult dermal fibroblasts are competent to contribute to DP during hair follicle neogenesis, irrespective of whether they originate from skin in the resting or growth phase of the hair cycle or skin with β-catenin-induced ectopic follicles. We propose that during skin reconstitution fibroblasts may be induced to become DP cells by interactions with hair follicle epidermal cells, rather than being derived from a distinct subpopulation of cells.

Reprogramming adult dermis to a neonatal state through epidermal activation of β-catenin

Hair follicle formation depends on reciprocal epidermal-dermal interactions and occurs during skin development, but not in adult life. This suggests that the properties of dermal fibroblasts change during postnatal development. To examine this, we used a PdgfraEGFP mouse line to isolate GFP-positive fibroblasts from neonatal skin, adult telogen and anagen skin and adult skin in which ectopic hair follicles had been induced by transgenic epidermal activation of β-catenin (EF skin). We also isolated epidermal cells from each mouse. The gene expression profile of EF epidermis was most similar to that of anagen epidermis, consistent with activation of β-catenin signalling. By contrast, adult dermis with ectopic hair follicles more closely resembled neonatal dermis than adult telogen or anagen dermis. In particular, genes associated with mitosis were upregulated and extracellular matrix-associated genes were downregulated in neonatal and EF fibroblasts. We confirmed that sustained epidermal β-catenin activation stimulated fibroblasts to proliferate to reach the high cell density of neonatal skin. In addition, the extracellular matrix was comprehensively remodelled, with mature collagen being replaced by collagen subtypes normally present only in developing skin. The changes in proliferation and extracellular matrix composition originated from a specific subpopulation of fibroblasts located beneath the sebaceous gland. Our results show that adult dermis is an unexpectedly plastic tissue that can be reprogrammed to acquire the molecular, cellular and structural characteristics of neonatal dermis in response to cues from the overlying epidermis.

Dynamic signals for hair follicle development and regeneration

Hair follicles form during embryonic development and, after birth, undergo recurrent cycling of growth, regression, and relative quiescence. As a functional mini-organ, the hair follicle develops in an environment with dynamic and alternating changes of diverse molecular signals. Over the past decades, genetically engineered mouse models have been used to study hair follicle morphogenesis and significant advances have been made toward the identification of key signaling pathways and the regulatory genes involved. In contrast, much less is understood in signals regulating hair follicle regeneration. Like hair follicle development, hair follicle regeneration probably relies on populations of stem cells that undergo a highly coordinated and stepwise program of differentiation to produce the completed structure. Here, we review recent advances in the understanding of the molecular signals underlying hair follicle morphogenesis and regeneration, with a focus on the initiation of the primary hair follicle structure placode. Knowledge about hair follicle morphogenesis may help develop novel therapeutic strategies to enhance cutaneous regeneration and improve wound healing.

Large-scale cultivation of transplantable dermal papilla cellular aggregates using microfabricated PDMS arrays

In this work we have developed a strategy for cultivating dermal papilla (DP) cells to form multiple arrayed spheroidal microtissues for transplantation on a micropatterned polydimethylsiloxane (PDMS)-based tissue culture polystyrene (TCPS) plate system. We also describe the behavior of dermal papilla cells on this platform and the spontaneous formation of spheroidal microtissues by DP cells. We used a hydrophobic PDMS arrayed chip as a master to separate the seeded cells in the TCPS culture plate. By controlling the cell seeding densities, a microwell with arrayed DP spheroidal microtissues was easily formed. Formation of DP microtissues was associated with overlapping multilayered cells on the microwells and low cell-substrate adhesivity on the PDMS film. The microwell environment enhanced the aggregation of DP cells into spheroidal microtissues on the TCPS culture plate. The spheroidal microtissues preserved their hair induction potential in vitro and in vivo. A large quantity of DP spheroidal microtissues could be obtained rapidly and simply using this platform. We could harvest hundreds of DP microtissues (352 microtissues) with a cell seeding density of 1×10⁶ cells well⁻¹ after 3 days cultivation in one well of a 24-well plate. This is the first demonstration of the formation of DP spheres in large quantitites.

Contact between dermal papilla cells and dermal sheath cells enhances the ability of DPCs to induce hair growth

We previously showed that cultured rat dermal papilla cells (DPCs) retain their hair-inducing capacity on afollicular epidermal cell (EPCs). Here, we examined the hair growth-inducing capacity of differently subcultured DPCs by transplanting them, along with rat EPCs, onto the backs of nude mice (graft chamber assay). DPCs at passage (p) 6 (DPCs(p6) or, more generally, low-passage DPCs) induced hair formation. However, DPCs(p>30) (high-passage DPCs) had no such activity and induced only subepidermal hair follicles (HFs) that were not encapsulated by the dermal sheath (DS). Thus, we examined the effect of DS cells (DSCs(p=1)) on the ability of DPCs(p=60) to induce hair growth by testing a mixture of these two cell types (cotransplant) in the graft chamber assay, in which DSCs(p=1) and DPCs(p=60) were labeled with enhanced green fluorescent protein (EGFP) and 1,1-dioctadecyl-3,3,3,3-tetramethylindocarbocyanine perchlorate (DiI), respectively. These cotransplants generated hairs as actively as did DPCs(p=6) transplants. Their HFs were encapsulated with EGFP(+)-DS and had DPs consisting largely of EGFP(+)-DPCs (47%) and DiI(+)-DPCs (43%), indicating a major contribution of DSC(p=1)-derived DPCs to HF induction. In addition, the results of in vitro coculture of DPCs(p=60) and DSCs(p=1) suggest that high-passage DPCs stimulate the expression of certain trichogenic genes in DSCs.

Review of hair follicle dermal cells

Hair follicle stem cells in the epithelial bulge are responsible for the continual regeneration of the hair follicle during cycling. The bulge cells reside in a niche composed of dermal cells. The dermal compartment of the hair follicle consists of the dermal papilla and dermal sheath. Interactions between hair follicle epithelial and dermal cells are necessary for hair follicle morphogenesis during development and in hair reconstitution assays. Dermal papilla and dermal sheath cells express specific markers and possess distinctive morphology and behavior in culture. These cells can induce hair follicle differentiation in epithelial cells and are required in hair reconstitution assays either in the form of intact tissue, dissociated freshly prepared cells or cultured cells. This review will focus on hair follicle dermal cells since most therapeutic efforts to date have concentrated on this aspect of the hair follicle, with the idea that enriching hair-inductive dermal cell populations and expanding their number by culture while maintaining their properties, will establish an efficient hair reconstitution assay that could eventually have therapeutic implications.

Hair follicle neogenesis induced by cultured human scalp dermal papilla cells

AIM

To develop a method by which human hair follicle dermal papilla (DP) cells can be expanded in vitro while preserving their hair-inductive potential for use in follicular cell implantation, a cellular therapy for the treatment of hair loss.

MATERIALS & METHODS

DP cells were isolated from scalp hair follicles in biopsies from human donors. DP cell cultures were established under conditions that preserved their hair-inductive potential and allowed for significant expansion. The hair-inductive potential of cells cultured for approximately 36 doublings was tested in an in vivo flap-graft model. In some experiments, DiI was used to label cells prior to grafting.

RESULTS

Under the culture conditions developed, cultures established from numerous donors reproducibly resulted in an expansion that averaged approximately five population doublings per passage. Furthermore, the cells consistently induced hair formation in an in vivo graft assay. Grafted DP cells appeared in DP structures of newly formed hairs, as well as in the dermal sheath and in the dermis surrounding follicles. Induced hair follicles persisted and regrew after being plucked 11 months after grafting.

CONCLUSION

A process for the propagation of human DP cells has been developed that provides significant expansion of cells and maintenance of their hair-inductive capability, overcoming a major technical obstacle in the development of follicular cell implantation as a treatment for hair loss.

The mesenchymal component of hair follicle neogenesis: background, methods and molecular characterization

Hair follicle morphogenesis and regeneration occur by an extensive and collaborative crosstalk between epithelial and mesenchymal skin components. A series of pioneering studies, which revealed an indispensable role of follicular dermal papilla and dermal sheath cells in this crosstalk, has led workers in the field to study in detail the anatomical distribution, functional properties, and molecular signature of the trichogenic dermal cells. The purpose of this paper was to provide a practical summary of the development and recent advances in the study of trichogenic dermal cells. Following a short review of the relevant literature, the methods for isolating and culturing these cells are summarized. Next, the bioassays, both in vivo and in vitro, that enable the evaluation of trichogenic properties of tested dermal cells are described in detail. A list of trichogenic molecular markers identified by those assays is also provided. Finally, this methods review is completed by defining some of the major questions needing resolution.

Differing responses of human follicular and nonfollicular scalp cells in an in vitro wound healing assay: effects of estrogen on vascular endothelial growth factor secretion

Improved wound healing of hairy skin may involve mesenchymal hair follicle cells with stem cell potential and enhancement by estrogen therapy. How estrogen affects follicular dermal papilla (DP) and dermal sheath (DS) cells in wound healing is unknown. Therefore, a comparison of estradiol action on DP, DS, and corresponding interfollicular dermal fibroblasts (DF) in a scratch-wound assay was performed using matching primary cultures established from female temporo-occipital scalp. All three cell types expressed mRNA transcripts and protein for estrogen receptors alpha (ERalpha) and beta (ERbeta). DF ERalpha transcripts were half that of DP and one-third of DS cells, while DF ERbeta transcripts were two-thirds of DP and DS cells. In the scratch-wound assay all three cells types migrated at similar rates, but only the rate of DF was enhanced by estradiol. Mechanical wounding increased DNA synthesis rates of all three cell types and increased the secretion of collagen by DF and DS cells. All three secreted similar basal levels of vascular endothelial growth factor (VEGF), which was increased by wounding DF and DS cells, but not DP cells. DP cells required estradiol to increase VEGF secretion; by contrast VEGF secretion was decreased by estradiol in wounded DS cells. These results highlight differences in the responses of DF, DP, and DS cells to estradiol in a scratch-wound assay, providing further support for the dichotomy of cellular functions in the hair follicle. Further understanding of the role of estrogen in cutaneous wound healing may have important implications for the management of chronic wounds and scarring.

Hair cycle-dependent changes of alkaline phosphatase activity in the mesenchyme and epithelium in mouse vibrissal follicles

Alkaline phosphatase (ALP) activity was detected in the restricted mesenchymal and epithelial regions in mouse vibrissal follicles. Its localization and strength dramatically changed during the hair cycle. Activity in the dermal papilla (DP) was moderate in very early anagen, reached a maximal level in early anagen, decreased at the proximal region of DP after mid anagen, and was kept at a low level during catagen. The bulbar dermal sheath showed intense ALP activity only in early anagen. Although most bulbar epithelium did not show ALP activity, germinative epidermal cells that were adjacent to the ALP-negative DP cells became ALP-positive in mid anagen and rearranged in a single layer so as to encapsulate the DP in mid catagen. During catagen, the outermost layer of bulbar epithelium became ALP-positive, which could be follicular epithelial precursors migrating from the bulge. Before the initiation of hair formation, ALP activity in the bulbar epithelium rapidly decreased and that in DP increased. These dynamic changes of ALP expression might be related to DP's functions in hair induction and also to reconstruction of the bulbar structure during the hair cycle.

Hair follicle reformation induced by dermal papilla cells from human scalp skin

To investigate the possibility of hair follicle reformation induced by dermal papilla cells in vivo and in vitro. Dermal papilla cells, dermal sheath cells obtained from human scalp skin by enzyme digestion were mixed with collagen to form mesenchymal cell-populated collagen gels. Superior and inferior epithelial cells and bulb matrical cells were then cultured on these gels by organotypic culture to recombine bilayer artificial skins. Dermal papilla cells and outer root sheath keratinocytes were mingled together and transplanted under subcutaneous tissue of the dorsal skin of nude mice. The results of histologic examination was observed with HE stain. These recombinants by organotypic culture all reformed bilayer structure like nature skin. Hair follicle-like structure reformation was found in dermal sheath cell-populated collagen gel when combined with superior or inferior epithelial cells. Dermal papilla cells also induced superior and inferior epithelial cells to form hair follicle on nude mice. Low passage dermal papilla cells mixed with hair follicle epithelial cells reformed many typical hair follicle structures and produced hair fibres after transplantation on nude mice. The dermal part of hair follicle, such as dermal papilla cells and dermal sheath cells, has the ability to induce hair follicle formation by interaction with the epithelial cells of hair follicle.

Cultured peribulbar dermal sheath cells can induce hair follicle development and contribute to the dermal sheath and dermal papilla

Green fluorescent protein (GFP)-expressing wild-type, and nontransgenic mouse vibrissa follicle cells were cultured and implanted to mouse ears and footpads. Dermal papiller (DP)-derived cells and cells from the peribulbar dermal sheath "cup" (DSC) induced new hair follicles in both implanted ears and footpads, while nonbulbar dermal sheath cells did not. Confocal microscopy revealed that GFP-expressing DP and DSC cells induced hair growth associated with the formation of DP exclusively comprised of fluorescent cells. In mouse ears, but not footpads, fluorescent DP and DSC cells could also be identified in DP along with nonfluorescent cells. DSC cells were characterized in vivo and in vitro by low alkaline phosphatase activity in contrast to high alkaline phosphatase in DP cells. The results indicate transplanted DP and DSC cells were equally capable of DP formation and hair follicle induction. This suggests the DP and peribulbar DSC may be functionally similar. In addition to observing papillae exclusively composed of GFP-expressing cells, DP and DSC cells may also have combined with resident cells to form papillae composed of implanted GFP-expressing cells and host-derived non-GFP-expressing cells. Alkaline phosphatase expression may be utilized as a simple marker to identify hair follicle mesenchyme derived cells with hair follicle inductive abilities.

Plasticity and cytokinetic dynamics of the hair follicle mesenchyme during the hair growth cycle: implications for growth control and hair follicle transformations

Hair fiber production is the macroscopic end-point of a highly complex set of interactions between the hair follicle's epithelial and mesenchymal components. The nature of this relationship is largely set during hair follicle morphogenesis, but is dramatically revisited in the adult during the unique tissue remodeling events required for hair follicle cycling. Whereas significant attention has focused on the fate of the hair follicle epithelium during these events, associated changes in hair follicle fibroblast subpopulations remain unclear. Here, we present a speculative review that represents a critical and innovative synthesis of the current literature and summarizes a recently submitted original study by the authors, on the nature of hair cycle-dependent fibroblast dynamics and on how perturbations thereof may lead to several clinical manifestations of altered human hair growth.

Plasticity and cytokinetic dynamics of the hair follicle mesenchyme: implications for hair growth control

The continuously remodeled hair follicle is a uniquely exploitable epithelial-mesenchymal interaction system. In contrast to the cyclical fate of the hair follicle epithelium, the dynamics of the supposedly stable hair follicle mesenchyme remains enigmatic. Here we address this issue using the C57BL/6 hair research model. During hair growth, increase in total follicular papilla size was associated with doubling of papilla cell numbers, much of which occurred before intra-follicular papilla cell proliferation, and subsequent to mitosis in the proximal connective tissue sheath. This indicates that some papilla cells originate in, and migrate from, the proliferating pool of connective tissue sheath fibroblasts. Follicular papilla cell number and total papilla size were maximal by anagen VI, but intriguingly, decreased by 25% during this period of sustained hair production. This cell loss, which continued during catagen, was not associated with intra-follicular papilla apoptosis, strongly indicating that fibroblasts migrate out of the late anagen/early catagen papilla and re-enter the proximal connective tissue sheath. Low-level apoptosis occurred only here, along with the "detachment" of cells from the regressing connective tissue sheath. Thus, the hair follicle mesenchyme exhibits significant hair cycle-associated plasticity. Modulation of these cell interchanges is likely to be important during clinically important hair follicle transformations, e.g. vellus-to-terminal and terminal-to-vellus during androgenetic alopecia.

Communication network in the follicular papilla and connective tissue sheath through gap junctions in human hair follicles

Epithelial-mesenchymal interactions play a crucial role in the induction of life-long cyclic transformations of hair follicles. Many studies have already demonstrated several candidates for the soluble factors secreted from the mesenchymal components of the hair follicle, i.e. the follicular papilla (FP) and connective tissue sheath (CTS), which may be responsible for hair cycling. In this paper, we focused on cell-cell contact between FP cells (FPCs), between CTS cells (CTSCs), and between FPCs and CTSCs that may allow these mesenchymal components to function as a syncytium during hair cycling. Electron microscopic examination of the FP and the CTS obtained from human scalp revealed a tri-lamellar structure of the plasma membranes, which is a characteristic of gap junctions at the cell-cell contacting area. The immunohistochemical study with anticonnexin 43 Ab using a confocal laser scanning microscope demonstrated numerous spotted positive signals scattered throughout the FP. In the CTS, spotted positive signals were arranged linearly along the basement membrane of the hair follicle. In particular, these positive spots were aggregated in the transitional region between the FP and the CTS. By Western blot analysis of total protein extracts from the cultured FPCs and neonatal human dermal fibroblasts using anticonnexin 43 antibody, a positive band corresponding to connexin 43 was detected at 43 kDa on both the FPC lane and fibroblast lane. These findings suggest that the FP and the CTS form a communicating network through gap junctions, which may play a role in controlling the dynamic structural changes of hair follicles during hair cycling.

Plasticity of hair follicle dermal cells in wound healing and induction

The capacity of adult hair follicle dermal cells to participate in new follicle induction and regeneration, and to elicit responses from diverse epithelial partners, demonstrates a level of developmental promiscuity and influence far exceeding that of interfollicular fibroblasts. We have recently suggested that adult follicle dermal cells have extensive stem or progenitor cell activities, including an important role in skin dermal wound healing. Given that up to now tissue engineered skin equivalents have several deficiencies, including the absence of hair follicles, we investigated the capacity of follicle dermal cells to be incorporated into skin wounds; to form hair follicles in wound environments; and to create a hair follicle-derived skin equivalent. In our study, we implanted rat follicle dermal cells labelled with a vital dye into ear and body skin wounds. We found that they were incorporated into the new dermis in a manner similar to skin fibroblasts, but that lower follicle dermal sheath also assimilated into hair follicles. Using different combinations of follicle dermal cells and outer root sheath epithelial cells in punch biopsy wounds, we showed that new hair follicles were formed only with the inclusion of intact dermal papillae. Finally by combining follicle dermal sheath and outer root sheath cells in organotypic chambers, we created a skin equivalent with characteristic dermal and epidermal architecture and a normal basement membrane - the first skin to be produced entirely from hair follicle cells. These data support the hypothesis that follicle dermal cells may be important in wound healing and demonstrate their potential usefulness in human skin equivalents and skin substitutes. While we have made progress towards producing skin equivalents that contain follicles, we suggest that the failure of cultured dermal papilla cells to induce follicle formation in wounds illustrates the complex role the follicle dermis may play in skin. We believe that it demonstrates a genuine dichotomy of activity for follicle cells within skin.

Histologic study of the regeneration process of human hair follicles grafted onto SCID mice after bulb amputation

This study examines histologically the degeneration and subsequent regeneration processes of human hair follicles whose bulb is severely damaged. Human scalp hair follicles were isolated and grafted onto immunodeficient mice after their bulb was amputated. On day 14, thickening and corrugation of the vitreous membrane, apoptosis of follicular keratinocytes, and regression of the lower portion of the follicles were observed. By day 20, mesenchymal cells had accumulated around the lower end of the follicles. From day 14 through 50, the follicular regression and apoptosis continued, and between days 30 and 40 the follicles became maximally shortened, and the vitreous membrane disappeared. By day 50 the lower end of the follicles had become cup-shaped, and the cup surrounded an aggregate of mesenchymal cells that corresponded to the dermal papilla. By day 60, all the grafted follicles had developed into anagen VI follicles, and the apoptosis had ceased. These results indicate that human scalp hair follicles whose bulb is completely destroyed enter into dystrophic telogen after restoration of the dermal papilla, then into anagen, and that the duration of the dystrophic telogen is shorter than that of the normal hair cycle.

In vivo induction of hair growth by dermal cells isolated from hair follicles after extended organ culture

Successful hair follicle organ culture has been established for some time, but hair growth in vitro is limited and generally terminates prematurely in comparison with in vivo. The reasons why growth stops in culture are as yet unknown. In this investigation, adult rat vibrissa follicles for which growth in culture is limited to about 10 d, were maintained in vitro for a minimum of 20 d after the hair shaft stopped growing. The pattern of fiber growth and long-term follicle pathology reflected the initial hair cycle stage at the time of isolation. Furthermore, there was evidence that a group of follicles put into culture when in late anagen were attempting to cycle in vitro. Microscopy showed that, in spite of widespread pathologic changes to the follicle epithelium, dermal cells in the follicle showed remarkable resilience. Their viability was confirmed when primary cell cultures were established from isolated dermal tissue. These cells labeled positively for alpha-smooth muscle actin, an established marker of hair follicle dermal cell phenotype in vitro. Moreover, isolated dermal tissue induced hair growth when implanted into inactivated hair follicles in vivo. These data confirm that the cessation in hair growth is not due to a loss of the inductive capacity in the dermal component. Long-term organ culture may provide opportunities to investigate factors that are expressed or lost during hair growth cessation. In addition it may be possible to develop this method further to obtain a reliable and predictable model of hair follicle cycling in vitro.

Controls of hair follicle cycling

Nearly 50 years ago, Chase published a review of hair cycling in which he detailed hair growth in the mouse and integrated hair biology with the biology of his day. In this review we have used Chase as our model and tried to put the adult hair follicle growth cycle in perspective. We have tried to sketch the adult hair follicle cycle, as we know it today and what needs to be known. Above all, we hope that this work will serve as an introduction to basic biologists who are looking for a defined biological system that illustrates many of the challenges of modern biology: cell differentiation, epithelial-mesenchymal interactions, stem cell biology, pattern formation, apoptosis, cell and organ growth cycles, and pigmentation. The most important theme in studying the cycling hair follicle is that the follicle is a regenerating system. By traversing the phases of the cycle (growth, regression, resting, shedding, then growth again), the follicle demonstrates the unusual ability to completely regenerate itself. The basis for this regeneration rests in the unique follicular epithelial and mesenchymal components and their interactions. Recently, some of the molecular signals making up these interactions have been defined. They involve gene families also found in other regenerating systems such as fibroblast growth factor, transforming growth factor-beta, Wnt pathway, Sonic hedgehog, neurotrophins, and homeobox. For the immediate future, our challenge is to define the molecular basis for hair follicle growth control, to regenerate a mature hair follicle in vitro from defined populations, and to offer real solutions to our patients' problems.

Ruby laser-assisted hair removal: an ultrastructural evaluation of cutaneous damage

Ruby laser-assisted hair removal is thought to act via selective photothermolysis of melanin in the hair follicles. Although initial clinical trials of permanent hair removal using ruby lasers are promising, the exact mechanisms of hair destruction and the potential damage to other structures of skin are not known. The aim of this study was to evaluate the cutaneous ultrastructural changes following ruby laser hair removal. Nineteen healthy Caucasian patients with dark (brown/black) hair were treated with the ruby laser and biopsies taken after 0, 2, 3, 5, 7, 14 and 21 days. Specimens were examined by light and electron microscopy. Laser-treated specimens showed widespread coagulation and charring of subcutaneous hair shafts. These obviously damaged follicles were randomly dispersed amongst intact follicles within the same treatment sites. Microscopic changes were also seen in the basal epidermis where melanin was concentrated, irrespective of any obvious macroscopic damage. A low level of inflammatory response seen up to 2 weeks after treatment always followed laser treatment. Suprabasal epidermal necrosis was only seen in patients with blister formation after treatment. Ruby laser irradiation results in selective damage to the hair follicles, with microscopic changes to the basal epidermis. The damage is probably compounded by the inflammatory response to the damaged hair. The normal appearance and distribution of collagen in the dermal layer supported the clinical evidence that laser-assisted hair removal, if performed correctly, does not lead to scar formation.

Hair matrix germinative epidermal cells confer follicle-inducing capabilities on dermal sheath and high passage papilla cells

Low passage cultured dermal papilla cells from adult rats stimulate complete hair follicle neogenesis when re-implanted into heterotypic skin. In contrast, cultured sheath cells are non-inductive despite sharing other behavioural characteristics (a common lineage and in situ proximity) with papilla cells. However, since sheath cells can behave inductively in amputated follicles after regenerating the papilla, this poses the question of what influences the sheath to papilla cell transition? During reciprocal tissue interactions specific epidermal cues are crucial to skin appendage development, and while in vivo assays to date have focussed on dermal interactive influence, our aim was to investigate epidermal potential. We have previously observed that hair follicle epidermal cells display exceptional interactive behaviour when combined with follicle dermal cells in vitro. Thus in the present study, hair follicle germinative, outer root sheath or skin basal epidermal cells were separately combined with each of three non-inductive dermal cell types (high passage papilla, low passage sheath or fibroblast) and then implanted into small ear skin wounds. The sheath/germinative and papilla/germinative cell implants repeatedly induced giant vibrissa-type follicles and fibres. In complete contrast, any single cell type and all other forms of recombination were consistently non-inductive. Hence, the adult germinative epidermal cells enable non-inductive adult dermal cells to stimulate hair follicle neogenesis, effectively, by altering their ‘status’, causing the sheath cells to ‘specialise’ and the ‘aged’ papilla cells to ‘rejuvenate’.