Histophysiology of hair follicles: current concept

Hair follicle histophysiology importance isn't limited by hair role in psychosocial consequences. More scientists consider the hair follicle as an attractive system for studying major biological phenomena because the hair follicle is a regenerating system. In this review we revisit the current information about histophysiology and control of hair follicle cycling. All mature follicles undergo a growth cycle consisting of following phases: growth (anagen), regression (catagen) and rest (telogen). We attempt to integrate the morphology with the physiology and molecular biology. Hair follicles are influenced by environmental, systemic and local factors. The most interesting point of this problem is discussed--an integral regulation of hair follicle cycle by systemic, intertissue and intercellular interactions.     

Modelling Hair Follicle Growth Dynamics as an Excitable Medium

The hair follicle system represents a tractable model for the study of stem cell behaviour in regenerative adult epithelial tissue. However, although there are numerous spatial scales of observation (molecular, cellular, follicle and multi follicle), it is not yet clear what mechanisms underpin the follicle growth cycle. In this study we seek to address this problem by describing how the growth dynamics of a large population of follicles can be treated as a classical excitable medium. Defining caricature interactions at the molecular scale and treating a single follicle as a functional unit, a minimal model is proposed in which the follicle growth cycle is an emergent phenomenon. Expressions are derived, in terms of parameters representing molecular regulation, for the time spent in the different functional phases of the cycle, a formalism that allows the model to be directly compared with a previous cellular automaton model and experimental measurements made at the single follicle scale. A multi follicle model is constructed and numerical simulations are used to demonstrate excellent qualitative agreement with a range of experimental observations. Notably, the excitable medium equations exhibit a wider family of solutions than the previous work and we demonstrate how parameter changes representing altered molecular regulation can explain perturbed patterns in Wnt over-expression and BMP down-regulation mouse models. Further experimental scenarios that could be used to test the fundamental premise of the model are suggested. The key conclusion from our work is that positive and negative regulatory interactions between activators and inhibitors can give rise to a range of experimentally observed phenomena at the follicle and multi follicle spatial scales and, as such, could represent a core mechanism underlying hair follicle growth.     

Melanocyte subpopulation turnover during the human hair cycle: an immunohistochemical study

The human hair cycle is characterized by successive phases of growth and involution that imply tissue regression and regeneration. As a consequence, the hair melanin unit has to be renewed in a cyclic manner. Actually, the behavior of human hair follicle melanocytes throughout the hair cycle has been poorly studied. Thus, the origin of melanocytes present in the bulb after human hair regeneration is still not clarified, and neither are the events that control the melanin biosynthesis activity in the human hair bulb. In this study, we showed at the cellular level that in human pigmented hair follicles, the expression of tyrosinase and tyrosinase-related protein-1 (TRP-1) was detectable during the anagen phases III/IV through VI, only in those melanocytes which were located in the bulb. During the catagen phase, the two evaluated melanogenic enzymes were detectable no more, although melanocytes were still present in the preceding bulbar area. The epithelial column of catagen follicles and the capsule of telogen follicles also contained inactive melanocytes as evidenced by pMel-17 labeling. At the induction of a new anagen hair follicle, some melanocytes were committed to cell division, but only when located in the nascent bulb close to the dermal papilla. Our results emphasize the close relationship between melanogenesis and the hair cycle and suggest that in humans, melanogenesis is restricted to anagen hair follicles not because of the regulation of tyrosinase activity, but because of melanogenic enzyme expression, e.g., tyrosinase and TRP-1. Furthermore, the fact that in the newly developing anagen hair follicles, cell-division commitment and tyrosinase and TRP-1 expression were observed in melanocytes only when located in the nascent bulb suggests a highly regio-specific melanocyte stimulation in early the anagen phase.     

Differential Expression of Proteins Associated with the Hair Follicle Cycle - Proteomics and Bioinformatics Analyses

Hair follicle cycling can be divided into the following three stages: anagen, catagen, and telogen. The molecular signals that orchestrate the follicular transition between phases are still unknown. To better understand the detailed protein networks controlling this process, proteomics and bioinformatics analyses were performed to construct comparative protein profiles of mouse skin at specific time points (0, 8, and 20 days). Ninety-five differentially expressed protein spots were identified by MALDI-TOF/TOF as 44 proteins, which were found to change during hair follicle cycle transition. Proteomics analysis revealed that these changes in protein expression are involved in Ca²⁺-regulated biological processes, migration, and regulation of signal transduction, among other processes. Subsequently, three proteins were selected to validate the reliability of expression patterns using western blotting. Cluster analysis revealed three expression patterns, and each pattern correlated with specific cell processes that occur during the hair cycle. Furthermore, bioinformatics analysis indicated that the differentially expressed proteins impacted multiple biological networks, after which detailed functional analyses were performed. Taken together, the above data may provide insight into the three stages of mouse hair follicle morphogenesis and provide a solid basis for potential therapeutic molecular targets for this hair disease.     

Efficient delivery of transgenes to human hair follicle progenitor cells using topical lipoplex

The topical delivery of transgenes to hair follicles has potential for treating disorders of the skin and hair. Here we show that the topical administration of liposome-DNA mixtures (lipoplex) to mouse skin and to human skin xenografts resulted in efficient in vivo transfection of hair follicle cells. Transfection depended on liposome composition, and occurred only at the onset of a new growing stage of the hair cycle. Manipulating the hair follicle cycle with depilation and retinoic acid treatment resulted in nearly 50% transfection efficiency-defined as the proportion of transfected, newly growing follicles within the xenograft. Transgenes administered in this fashion are selectively expressed in hair progenitor cells and therefore have the potential to affect the characteristics of the follicle. These findings form a foundation for the future use of topical lipoplex applications to alter hair follicle phenotype and treat diseases of the hair and skin.     

Urokinase plasminogen activator (uPA) is a positive regulator of outer root sheath keratinocyte proliferation

Urokinase plasminogen activator (uPA), a serine proteinase, is important in the development and epidermal wound healing, and seems to play a regulatory role in the proliferation of mouse epidermal keratinocytes (KC). In the present study, we found detectable uPA expression in outer root sheath (ORS) KC in the early anagen phase in mouse vibrissa follicles, but not in the late anagen or in the telogen and categen phases. uPA was also detected in ORS KC cultured from neonatal mice vibrissa. Specific exogenous inhibitors of uPA, amiloride and uPA antibody, significantly reduced the proliferation of ORS KC. Thus uPA is consistently elevated in the hyperproliferative hair follicle KC, and inhibition of the enzyme decreases hair follicle KC proliferation. We deduce that uPA is a very important mediator of the hair follicle cycle because its activity correlates with ORS KC proliferation.     

Mesenchymal–epithelial interactions during hair follicle morphogenesis and cycling

Embryonic hair follicle induction and formation are regulated by mesenchymal–epithelial interactions between specialized dermal cells and epidermal stem cells that switch to a hair fate. Similarly, during postnatal hair growth, communication between mesenchymal dermal papilla cells and surrounding epithelial matrix cells coordinates hair shaft production. Adult hair follicle regeneration in the hair cycle again is thought to be controlled by activating signals originating from the mesenchymal compartment and acting on hair follicle stem cells. Although many signaling pathways are implicated in hair follicle formation and growth, the precise nature, timing, and intersection of these inductive and regulatory signals remains elusive. The goal of this review is to summarize our current understanding and to discuss recent new insights into mesenchymal–epithelial interactions during hair follicle morphogenesis and cycling.     

小鼠毛囊不同生长周期中MITF下游色素相关基因的定位表达及相关性分析

小眼畸形转录因子（MITF）不仅是黑色素细胞发育、增殖和存活的必要调节因子,而且对调节相关酶和黑素体蛋白表达来确保黑色素产生具有至关重要的作用。MITF下游色素相关基因在小鼠毛囊生长周期中的表达及相关性仍有待研究。HE染色结果表明不同毛囊时期的小鼠毛囊呈现典型的组织形态学结构;免疫组织化学显示,MITF、GPNMB、OA1、TYR、TYRP2在不同毛囊生长周期中的毛基质及内外毛根鞘均有不同程度的阳性表达。黑色素测定结果表明,在毛囊生长初期和中期,碱性可溶性总黑色素（ASM）、真黑素（EM）以及褐黑素（PM）相对含量高于毛囊生长末期。蛋白免疫印迹结果表明,MITF、GPNMB、OA1、TYR、TYRP2在毛囊生长初期和中期蛋白质相对水平明显高于毛囊生长末期。实时荧光定量PCR结果表明, MITF、GPNMB、OA1、TYR、TYRP2、PMEL在毛囊生长初期和中期,mRNA相对表达量显著高于毛囊生长末期。在不同毛囊生长周期小鼠皮肤的MITF下游色素相关基因表达存在显著差异,表明上述因子在维持黑色素细胞色素生成是不可或缺的因素。     

Disruption of the hedgehog signaling pathway contributes to the hair follicle cycling deficiency in Vdr knockout mice

Mice null for the Vitamin D receptor (VdrKO) have a disrupted first hair follicle cycle and aborted subsequent hair follicle cycling. We examined the expression of different markers and mediators of hair follicle cycling in the hair follicle of the VdrKO mouse during days 13–22 when the hair follicle normally initiates and completes the first catagen. We compared the expression of those genes in mice with a nonsense mutation in hairless (Rhino), which have a similar alopecia phenotype, and to Cyp27b1 null mice which are deficient in the production of 1,25(OH)2D3, the Vdr ligand, but display normal hair follicle cycling. Our results demonstrate the down regulation of hair follicle markers and the alteration of expression of hedgehog (Hh), Wnt, Fgf, and Tgfβ pathways in VdrKO and Rhino mice, but not in Cyp27b1KO mice. Treatment of VdrKO mice with an agonist to the Hh pathway partially restored hair follicle cycling, suggesting a role of this pathway in the regulation of hair follicle cycling by VDR. These results suggest that Vdr regulates directly or indirectly the expression of genes required for hair follicle cycling, including Hh signaling, independent of 1,25(OH)2D3. J. Cell. Physiol. 225: 482–489, 2010. © 2010 Wiley‐Liss, Inc.     

Hair follicle stem cells in the lower bulge form the secondary germ, a biochemically distinct but functionally equivalent progenitor cell population, at the termination of catagen

The lowermost portion of the resting (telogen) follicle consists of the bulge and secondary hair germ. We previously showed that the progeny of stem cells in the bulge form the lower follicle and hair, but the relationship of the bulge cells with the secondary hair germ cells, which are also involved in the generation of the new hair at the onset of the hair growth cycle (anagen), remains unclear. Here we address whether secondary hair germ cells are derived directly from epithelial stem cells in the adjacent bulge or whether they arise from cells within the lower follicle that survive the degenerative phase of the hair cycle (catagen). We use 5-bromo-2'-deoxyuridine to label bulge cells at anagen onset, and demonstrate that the lowermost portion of the bulge collapses around the hair and forms the secondary hair germ during late catagen. During the first six days of anagen onset bulge cells proliferate and self-renew. Bulge cell proliferation at this time also generates cells that form the future secondary germ. As bulge cells form the secondary germ cells at the end of catagen, they lose expression of a biochemical marker, S100A6. Remarkably, however, following injury of bulge cells by hair depilation, progenitor cells in the secondary hair germ repopulate the bulge and re-express bulge cell markers. These findings support the notion that keratinocytes can "dedifferentiate" to a stem cell state in response to wounding, perhaps related to signals from the stem cell niche. Finally, we also present evidence that quiescent bulge cells undergo apoptosis during follicle remodeling in catagen, indicating that a subpopulation of bulge cells is not permanent.     

Melanocyte stem cells: a melanocyte reservoir in hair follicles for hair and skin pigmentation

Most mammals are coated with pigmented hair. Melanocytes in each hair follicle produce melanin pigments for the hair during each hair cycle. The key to understanding the mechanism of cyclic melanin production is the melanocyte stem cell (MelSC) population, previously known as 'amelanotic melanocytes'. The MelSCs directly adhere to hair follicle stem cells, the niche cells for MelSCs and reside in the hair follicle bulge-subbulge area, the lower permanent portion of the hair follicle, to serve as a melanocyte reservoir for skin and hair pigmentation. MelSCs form a stem cell system within individual hair follicles and provide a 'hair pigmentary unit' for each cycle of hair pigmentation. This review focuses on the identification of MelSCs and their characteristics and explains the importance of the MelSC population in the mechanisms of hair pigmentation, hair greying, and skin repigmentation.     

Role of the vitamin D receptor in hair follicle biology

The vitamin D receptor (VDR) is expressed in numerous cells and tissues, including the skin. The critical requirement for cutaneous expression of the VDR has been proven by investigations in mice and humans lacking functional receptors. These studies demonstrate that absence of the VDR leads to the development of alopecia. The hair follicle is formed by reciprocal interactions between an epidermal placode, which gives rise to the hair follicle keratinocytes and the underlying mesoderm which gives rise to the dermal papilla. Hair follicle morphogenesis ends the second week of life in mice. Studies in VDR null mice have failed to demonstrate a cutaneous abnormality during this period of hair follicle morphogenesis. However, VDR null mice are unable to initiate a new hair cycle after the period of morphogenesis is complete, therefore, do not grow new hair. Investigations in transgenic mice have demonstrated that restricted expression of the VDR to keratinocytes is capable of preventing alopecia in the VDR null mice, thus demonstrating that the epidermal component of the hair follicle requires VDR expression to maintain normal hair follicle homeostasis. Studies were then performed to determine which regions of the VDR were required for these actions. Investigations in mice lacking the first zinc finger of the VDR have demonstrated that they express a truncated receptor containing an intact ligand binding and AF2 domain. These mice are a phenocopy of mice lacking the VDR, thus demonstrate the critical requirement of the DNA binding domain for hair follicle homeostasis. Transgenic mice expressing VDRs with mutations in either the ligand-binding domain or the AF2 domain were generated. These investigations demonstrated that mutant VDRs incapable of ligand-dependent transactivation were able to prevent alopecia. Investigations are currently underway to define the mechanism by which the unliganded VDR maintains hair follicle homeostasis.     

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.     

Dietary vitamin A regulates wingless-related MMTV integration site signaling to alter the hair cycle

Alopecia areata (AA) is an autoimmune hair loss disease caused by a cell-mediated immune attack of the lower portion of the cycling hair follicle. Feeding mice 3–7 times the recommended level of dietary vitamin A accelerated the progression of AA in the graft-induced C3H/HeJ mouse model of AA. In this study, we also found that dietary vitamin A, in a dose dependent manner, activated the hair follicle stem cells (SCs) to induce the development and growth phase of the hair cycle (anagen), which may have made the hair follicle more susceptible to autoimmune attack. Our purpose here is to determine the mechanism by which dietary vitamin A regulates the hair cycle. We found that vitamin A in a dose-dependent manner increased nuclear localized beta-catenin (CTNNB1; a marker of canonical wingless-type Mouse Mammary Tumor Virus integration site family (WNT) signaling) and levels of WNT7A within the hair follicle bulge in these C3H/HeJ mice. These findings suggest that feeding mice high levels of dietary vitamin A increases WNT signaling to activate hair follicle SCs.     

Molecular cloning and embryonic expression of the Xenopus Arnt gene

Versican, a proteoglycan recently implicated in hair follicle induction, has been shown to influence axon outgrowth in vitro and in vivo. We used immunohistochemistry to study the relationship between versican expression and innervation, during rat vibrissa follicle development and the adult hair cycle. During development, nerve fibres were commonly associated with areas of weak versican expression, and the path of axons appeared to be delineated by sharp boundaries of versican expression. Versican expression changed in the lower follicle dermis during the adult hair follicle cycle but remained strong around the follicle neck reflecting the constant innervation. Our observations show a correlation between versican expression and peripheral innervation indicating that versican may have a dual role in hair follicle biology.     

Label-retaining cells reside in the bulge area of pilosebaceous unit: implications for follicular stem cells, hair cycle, and skin carcinogenesis

Inconsistent with the view that hair follicle stem cells reside in the matrix area of the hair bulb, we found that label-retaining cells exist exclusively in the bulge area of the mouse hair follicle. The bulge consists of a subpopulation of outer root sheath cells located in the midportion of the follicle at the arrector pili muscle attachment site. Keratinocytes in the bulge area are relatively undifferentiated ultrastructurally. They are normally slow cycling, but can be stimulated to proliferate transiently by TPA. Located in a well-protected and nourished environment, these cells mark the lower end of the "permanent" portion of the follicle. Our findings, plus a reevaluation of the literature, suggest that follicular stem cells reside in the bulge region, instead of the lower bulb. This new view provides insights into hair cycle control and the possible involvement of hair follicle stem cells in skin carcinogenesis.     

Characterization of Wnt gene expression in developing and postnatal hair follicles and identification of Wnt5a as a target of Sonic hedgehog in hair follicle morphogenesis

Mutations in WNT effector genes perturb hair follicle morphogenesis, suggesting key roles for WNT proteins in this process. We show that expression of Wnts 10b and 10a is upregulated in placodes at the onset of follicle morphogenesis and in postnatal hair follicles beginning a new cycle of hair growth. The expression of additional Wnt genes is observed in follicles at later stages of differentiation. Among these, we find that Wnt5a is expressed in the developing dermal condensate of wild type but not Sonic hedgehog (Shh)-null embryos, indicating that Wnt5a is a target of SHH in hair follicle morphogenesis. These results identify candidates for several key follicular signals and suggest that WNT and SHH signaling pathways interact to regulate hair follicle morphogenesis.