Role of macrophages in collagen resorption during hair growth cycle

Dermal macrophage and its potential in inducing hair follicle regeneration

Hair follicle (HF) is an excellent mini-model to study adult tissue regeneration, since it can regenerate itself under appropriate stress settings via interaction with niche components. Dermal macrophages, a group of heterogeneous cell populations, serve as key regulators in this microenvironment. Recent advances in phenotype identification and lineage tracing have unveiled various dermal macrophage subsets involved in stress-induced hair regeneration through different mechanisms, where HF structural integrity is impaired to varying degrees. This review summarized current knowledge regarding the distribution, sources, phenotypes of dermal macrophages in association with HF, as well as the mechanisms underlying macrophage-mediated hair regeneration in response to different internal-stress settings. Further investigation on macrophage dynamics will provide novel cell-targeting therapies for HF engineering and hair loss.

Immune modulation of hair follicle regeneration

The mammalian hair follicle undergoes repeated bouts of regeneration orchestrated by a variety of hair follicle stem cells. The last decade has witnessed the emergence of the immune niche as a key regulator of stem cell behavior and hair follicle regeneration. Hair follicles chemotactically attract macrophages and T cells so that they are in range to regulate epithelial stem cell quiescence, proliferation and differentiation during physiologic and injured states. Disruption of this dynamic relationship leads to clinically significant forms of hair loss including scarring and non-scarring alopecias. In this review, we summarize key concepts behind immune-mediated hair regeneration, highlight gaps in the literature and discuss the therapeutic potential of exploiting this relationship for treating various immune-mediated alopecias.

Immune cell regulation of the hair cycle

The ability to manipulate the mammalian hair cycle will lead to novel therapies and strategies to combat all forms of alopecia. Thus, in addition to the epithelial-mesenchymal interactions in the hair follicle, niche and microenvironmental signals that accompany the phases of growth, regression and rest need to be scrutinized. Immune cells are well described in skin homeostasis and wound healing and have recently been shown to play an important role in the mammalian hair cycle. In this review, we will summarize our current knowledge of the role of immune cells in hair cycle control and discuss their relevance to human hair cycling disorders. Increased attention to this aspect of the hair cycle will provide new avenues to manipulate hair regeneration in humans and provide better insight into developing better ex vivo models of hair growth.

Transcriptomic regulation of seasonal coat color change in hares

Color molts from summer brown to winter white coats have evolved in several species to maintain camouflage year-round in environments with seasonal snow. Despite the eco-evolutionary relevance of this key phenological adaptation, its molecular regulation has only recently begun to be addressed. Here, we analyze skin transcription changes during the autumn molt of the mountain hare (Lepus timidus) and integrate the results with an established model of gene regulation across the spring molt of the closely related snowshoe hare (L. americanus). We quantified differences in gene expression among three stages of molt progression-"brown" (early molt), "intermediate," and "white" (late molt). We found 632 differentially expressed genes, with a major pulse of expression early in the molt, followed by a milder one in late molt. The functional makeup of differentially expressed genes anchored the sampled molt stages to the developmental timeline of the hair growth cycle, associating anagen to early molt and the transition to catagen to late molt. The progression of color change was characterized by differential expression of genes involved in pigmentation, circadian, and behavioral regulation. We found significant overlap between differentially expressed genes across the seasonal molts of mountain and snowshoe hares, particularly at molt onset, suggesting conservatism of gene regulation across species and seasons. However, some discrepancies suggest seasonal differences in melanocyte differentiation and the integration of nutritional cues. Our established regulatory model of seasonal coat color molt provides an important mechanistic context to study the functional architecture and evolution of this crucial seasonal adaptation.

Transcriptome profiling reveals transcriptional and alternative splicing regulation in the early embryonic development of hair follicles in the cashmere goat

The undercoat fiber of the cashmere goat, from the secondary hair follicle (HF), possesses commercial value. However, very few studies have focused on the molecular details of primary and secondary HF initiation and development in goat embryos. In this study, skin samples at embryonic day 45, 55, and 65 (E45, E55, and E65) were collected and prepared for RNA sequencing (RNA-seq). We found that the HF probably initiated from E55 to E65 by analyzing the functional pathways of differentially expressed genes (DEGs). Most key genes in canonical signaling pathways, including WNT, TGF-β, FGF, Hedgehog, NOTCH, and other factors showed clear expression changes from E55 to E65. We, for the first time, explored alternative splicing (AS) alterations, which showed distinct patterns among these three stages. Functional pathways of AS-regulated genes showed connections to HF development. By comparing the published RNA-seq samples from the E60, E120, and newborn (NB) stages, we found the majority of WNT/β-catenin signaling genes were important in the initiation of HF development, while other factors including FOXN1, GATA3, and DLX3 may have a consistent influence on HF development. Our investigation supported the time points of embryonic HF initiation and identified genes that have potential functions of embryonic HF initiation and development. We further explored the potential regulatory roles of AS in HF initiation, which extended our knowledge about the molecular mechanisms of HF development.

Macrophages as a Source and Recipient of Wnt Signals

Macrophages are often viewed through the lens of their core functions, but recent transcriptomic studies reveal them to be largely distinct across tissue types. While these differences appear to be shaped by their local environment, the key signals that drive these transcriptional differences remain unclear. Since Wnt signaling plays established roles in cell fate decisions, and tissue patterning during development and tissue repair after injury, we consider evidence that Wnt signals both target and are affected by macrophage functions. We propose that the Wnt gradients present in developing and adult tissues effectively shape macrophage fates and phenotypes. We also highlight evidence that macrophages, through an ability to dispatch Wnt signals, may couple tissue debridement and matrix remodeling with stem cell activation and tissue repair.

The Role of Macrophages in Acute and Chronic Wound Healing and Interventions to Promote Pro-wound Healing Phenotypes

Macrophages play key roles in all phases of adult wound healing, which are inflammation, proliferation, and remodeling. As wounds heal, the local macrophage population transitions from predominantly pro-inflammatory (M1-like phenotypes) to anti-inflammatory (M2-like phenotypes). Non-healing chronic wounds, such as pressure, arterial, venous, and diabetic ulcers indefinitely remain in inflammation—the first stage of wound healing. Thus, local macrophages retain pro-inflammatory characteristics. This review discusses the physiology of monocytes and macrophages in acute wound healing and the different phenotypes described in the literature for both in vitro and in vivo models. We also discuss aberrations that occur in macrophage populations in chronic wounds, and attempts to restore macrophage function by therapeutic approaches. These include endogenous M1 attenuation, exogenous M2 supplementation and endogenous macrophage modulation/M2 promotion via mesenchymal stem cells, growth factors, biomaterials, heme oxygenase-1 (HO-1) expression, and oxygen therapy. We recognize the challenges and controversies that exist in this field, such as standardization of macrophage phenotype nomenclature, definition of their distinct roles and understanding which phenotype is optimal in order to promote healing in chronic wounds.

Compartmentation of Mitochondrial and Oxidative Metabolism in Growing Hair Follicles: A Ring of Fire

Little is known about the energetics of growing hair follicles, particularly in the mitochondrially abundant bulb. Here, mitochondrial and oxidative metabolism was visualized by multiphoton and light sheet microscopy in cultured bovine hair follicles and plucked human hairs. Mitochondrial membrane potential (ΔΨ), cell viability, reactive oxygen species (ROS), and secretory granules were assessed with parameter-indicating fluorophores. In growing follicles, lower bulb epithelial cells had high viability, and mitochondria were polarized. Most epithelially generated ROS co-localized with polarized mitochondria. As the imaging plane captured more central and distal cells, ΔΨ disappeared abruptly at a transition to a nonfluorescent core continuous with the hair shaft. Approaching the transition, ΔΨ and ROS increased, and secretory granules disappeared. ROS and ΔΨ were strongest in a circumferential paraxial ring at putative sites for formation of the outer cortex/cuticle of the hair shaft. By contrast, polarized mitochondria in dermal papillar fibroblasts produced minimal ROS. Plucked hairs showed a similar abrupt transition of degranulation/depolarization near sites of keratin deposition, as well as an ROS-generating paraxial ring of fire. Hair movement out of the follicle appeared to occur independently of follicular bulb bioenergetics by a tractor mechanism involving the inner and outer root sheaths.

Macrophages contribute to the cyclic activation of adult hair follicle stem cells

Castellana, Paus, and Perez-Moreno discover that skin resident macrophages signal to skin stem cells via Wnt ligands to activate the hair follicle life cycle.

Skin epithelial stem cells operate within a complex signaling milieu that orchestrates their lifetime regenerative properties. The question of whether and how immune cells impact on these stem cells within their niche is not well understood. Here we show that skin-resident macrophages decrease in number because of apoptosis before the onset of epithelial hair follicle stem cell activation during the murine hair cycle. This process is linked to distinct gene expression, including Wnt transcription. Interestingly, by mimicking this event through the selective induction of macrophage apoptosis in early telogen, we identify a novel involvement of macrophages in stem cell activation in vivo. Importantly, the macrophage-specific pharmacological inhibition of Wnt production delays hair follicle growth. Thus, perifollicular macrophages contribute to the activation of skin epithelial stem cells as a novel, additional cue that regulates their regenerative activity. This finding may have translational implications for skin repair, inflammatory skin diseases and cancer.

The cyclic life of hair follicles consists of recurring phases of growth, decay, and rest. Previous studies have identified signals that prompt a new phase of hair growth through the activation of resting hair follicle stem cells (HF-SCs). In addition to these signals, recent findings have shown that cues arising from the neighboring skin environment, in which hair follicles dwell, also participate in controlling hair follicle growth. Here we show that skin resident macrophages surround and signal to resting HF-SCs, regulating their entry into a new phase of hair follicle growth. This process involves the death and activation of a fraction of resident macrophages— resulting in Wnt ligand release —that in turn activate HF-SCs. These findings reveal additional mechanisms controlling endogenous stem cell pools that are likely to be relevant for modulating stem cell regenerative capabilities. The results provide new insights that may have implications for the development of technologies with potential applications in regeneration, aging, and cancer.

What causes alopecia areata?

The pathobiology of alopecia areata (AA), one of the most frequent autoimmune diseases and a major unsolved clinical problem, has intrigued dermatologists, hair biologists and immunologists for decades. Simultaneously, both affected patients and the physicians who take care of them are increasingly frustrated that there is still no fully satisfactory treatment. Much of this frustration results from the fact that the pathobiology of AA remains unclear, and no single AA pathogenesis concept can claim to be universally accepted. In fact, some investigators still harbour doubts whether this even is an autoimmune disease, and the relative importance of CD8(+) T cells, CD4(+) T cells and NKGD2(+) NK or NKT cells and the exact role of genetic factors in AA pathogenesis remain bones of contention. Also, is AA one disease, a spectrum of distinct disease entities or only a response pattern of normal hair follicles to immunologically mediated damage? During the past decade, substantial progress has been made in basic AA-related research, in the development of new models for translationally relevant AA research and in the identification of new therapeutic agents and targets for future AA management. This calls for a re-evaluation and public debate of currently prevalent AA pathobiology concepts. The present Controversies feature takes on this challenge, hoping to attract more skin biologists, immunologists and professional autoimmunity experts to this biologically fascinating and clinically important model disease.

Etiopathogenesis of alopecia areata: Why do our patients get it?

Alopecia areata (AA) is a nonscarring, inflammatory skin disease that results in patchy hair loss. AA is unpredictable in its onset, severity, and duration making it potentially very stressful for affected individuals. Currently, the treatment options for AA are limited and the efficacy of these treatments varies from patient to patient. The exact etiology of AA is unknown. This article provides some insights into the etiopathogenesis of AA and why some people develop it. The current knowledge on the pathogenesis of AA is summarized and some of the recent hypotheses and studies on AA are presented to allow for a fuller understanding of the possible biological mechanisms of AA.

Alopecia areata update: part I. Clinical picture, histopathology, and pathogenesis

Alopecia areata (AA) is an autoimmune disease that presents as nonscarring hair loss, although the exact pathogenesis of the disease remains to be clarified. Disease prevalence rates from 0.1% to 0.2% have been estimated for the United States. AA can affect any hair-bearing area. It often presents as well demarcated patches of nonscarring alopecia on skin of overtly normal appearance. Recently, newer clinical variants have been described. The presence of AA is associated with a higher frequency of other autoimmune diseases. Controversially, there may also be increased psychiatric morbidity in patients with AA. Although some AA features are known poor prognostic signs, the course of the disease is unpredictable and the response to treatment can be variable. Part one of this two-part series on AA describes the clinical presentation and the associated histopathologic picture. It also proposes a hypothesis for AA development based on the most recent knowledge of disease pathogenesis.

LEARNING OBJECTIVES

After completing this learning activity, participants should be familiar with the most recent advances in AA pathogenesis, recognize the rare and recently described variants of AA, and be able to distinguish between different histopathologic stages of AA.

The turnover and degradation of collagen

The interstitial collagens are degraded predominantly extracellularly, by specific collagenases (metalloproteinases) capable of cleaving the helical region across the three chains at a similar locus, solubilizing the cleaved products from the fibril. Other neutral proteinases may also function in this role by cleaving near cross-links in the fibril. Collagen type, molecular aggregation and small changes in temperature all markedly affect rates of collagenolysis in the fibril. Regulation of collagenolysis is also modulated at the levels of (1) cellular production of latent collagenase (procollagenase), (2) activation of latent collagenase, and (3) production of collagenase inhibitors. Fibroblastic cells and certain macrophages are probably the predominant sources of collagenases in inflammation; an enzyme in polymorphonuclear leucocytes (neutrophils) is distinct from the tissue enzyme. Molecules such as mononuclear cell factor (MCF), homologous with interleukin 1, which augment cellular collagenase production in inflammation, are derived from monocytes. The mechanisms of augmented collagenase production involve new protein synthesis and, if this augmentation is analogous to that produced by urate crystals, it is probably associated with increased levels of procollagenase mRNA. MCF production is itself controlled by products of lymphocytes as well as by interactions of monocytes with the Fc portion of immunoglobulins and components of the extracellular matrix. Activation of latent (pro)collagenase probably occurs in vivo through the action of neutral proteinases such as plasmin (through plasminogen activator). These effects may be indirect and exerted through proteolytic activation of a procollagenase activator. Tissue inhibitors act to regulate the active collagenase.

Alopecia areata susceptibility in rodent models

With our current view of alopecia areata as an autoimmune disease, it is probable that disease development in an individual is dependent on multiple genetic and environmental factors interacting in a complex system. Rodent models afford the opportunity to investigate alopecia areata development and to define the significance of the different factors involved. Recently, rodent model characterization has been conducted using flow cytometry, microarray analysis, and functional studies. From these a pattern of events in alopecia areata development has emerged. Although the preliminary activation events for the onset of alopecia areata remain unknown, the response of the immune system is characterized by antigen presentation and costimulation of lymphocytes in the lymph nodes and skin, a deficiency of CD4+/CD25+ regulatory cells, and an action of activated lymphocytes on hair follicles via Fas/FasL signaling and cytokines. Thus, onset of disease may require appropriate (or inappropriate) expression of stimulatory antigens within the hair follicle, the breakdown of the putative hair follicle immune privilege, the presentation of antigens to the immune system, a failure of immune system regulation, and the ability of the activated immune system to disrupt anagen-stage hair follicles. Once the sequence of events is initiated, it may become a self-perpetuating cycle, with epitope spreading leading to a wider range of targets in chronic alopecia areata. Rodent model studies have provided significant insight into alopecia areata, but much more remains to be explained about the mechanisms of disease development.

A comprehensive guide for the accurate classification of murine hair follicles in distinct hair cycle stages

Numerous strains of mice with defined mutations display pronounced abnormalities of hair follicle cycling, even in the absence of overt alterations of the skin and hair phenotype; however, in order to recognize even subtle, hair cycle-related abnormalities, it is critically important to be able to determine accurately and classify the major stages of the normal murine hair cycle. In this comprehensive guide, we present pragmatic basic and auxiliary criteria for recognizing key stages of hair follicle growth (anagen), regression (catagen) and quiescence (telogen) in C57BL/6NCrlBR mice, which are largely based on previous work from other authors. For each stage, a schematic drawing and representative micrographs are provided in order to illustrate these criteria. The basic criteria can be employed for all mouse strains and require only routine histochemical techniques. The auxiliary criteria depend on the immunohistochemical analysis of three markers (interleukin-1 receptor type I, transforming growth factor-beta receptor type II, and neural cell-adhesion molecule), which allow a refined analysis of anatomical hair follicle compartments during all hair cycle stages. In contrast to prior staging systems, we suggest dividing anagen III into three distinct substages, based on morphologic differences, onset and progression of melanogenesis, and the position of the dermal papilla in the subcutis. The computer-generated schematic representations of each stage are presented with the aim of standardizing reports on follicular gene and protein expression patterns. This guide should become a useful tool when screening new mouse mutants or mice treated with pharmaceuticals for discrete morphologic abnormalities of hair follicle cycling in a highly reproducible, easily applicable, and quantifiable manner.

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.

Intercellular adhesion molecule-1 and hair follicle regression

Although the intercellular adhesion molecule-1 (ICAM-1) is recognized for its pivotal role in inflammation and immune responses, its role in developmental systems, such as the cyclic growth (anagen) and regression (catagen) of the hair follicle, remains to be explored. Here we demonstrate that ICAM-1 expression in murine skin is even more widespread and more developmentally regulated than was previously believed. In addition to endothelial cells, selected epidermal and follicular keratinocyte subpopulations, as well as interfollicular fibroblasts, express ICAM-1. Murine hair follicles express ICAM-1 only late during morphogenesis. Thereafter, morphologically identical follicles markedly differ in their ICAM-1 expression patterns, which become strikingly hair cycle-dependent in both intra- and extrafollicular skin compartments. Minimal ICAM-1 and leukocyte function-associated (LFA-1) protein and mRNA expression is observed during early anagen and maximal expression during late anagen and catagen. Keratinocytes of the distal outer root sheath, fibroblasts of the perifollicular connective tissue sheath, and perifollicular blood vessels exhibit maximal ICAM-1 immunoreactivity during catagen, which corresponds to changes of LFA-1 expression on perifollicular macrophages. Finally, ICAM-1-deficient mice display significant catagen acceleration compared to wild-type controls. Therefore, ICAM-1 upregulation is not limited to pathological situations but is also important for skin and hair follicle remodeling. Collectively, this suggests a new and apparently nonimmunological function for ICAM-1-related signaling in cutaneous biology.

A comprehensive guide for the recognition and classification of distinct stages of hair follicle morphogenesis

Numerous spontaneous and experimentally induced mouse mutations develop a hair phenotype, which is often associated with more or less discrete abnormalities in hair follicle development. In order to recognize these, it is critically important to be able to determine and to classify accurately the major stages of normal murine hair follicle morphogenesis. As an aid, we propose a pragmatic and comprehensive guide, modified after previous suggestions by Hardy, and provide a list of easily recognizable classification criteria, illustrated by representative micrographs. Basic and more advanced criteria are distinguished, the former being applicable to all mouse strains and requiring only simple histologic stains (hematoxylin and eosin, Giemsa, periodic acid Schiff, alkaline phosphatase activity), the latter serving as auxiliary criteria, which require a pigmented mouse strain (like C57BL/6J) or immunohistochemistry (interleukin-1 receptor type I, transforming growth factor-beta receptor type II). In addition, we present simplified, computer-generated schematic drawings for the standardized recording and reporting of gene and antigen expression patterns during hair follicle development. This classification aid serves as a basic introduction into the field of hair follicle morphogenesis, aims at standardizing the presentation of related hair research data, and should become a useful tool when screening new mouse mutants for discrete abnormalities of hair follicle morphogenesis (compared with the respective wild type) in a highly reproducible, easily applicable, and quantifiable manner.

Immunopathology of the human hair follicle

Although patients are told that, in many instances, their hair loss is precipitated by stress, they are certainly stressed and saddened by their alopecia. They would be elated with the ability to regrow their hair. Ideally, therapy would be specific and targeted at the cascade of inflammatory, cytokine-mediated, and mesenchymal events which lead to hair loss. Such is the case with infectious folliculitides: Pityrosporum folliculitis is cleared with antifungal agents, bacterial folliculitis is cleared with antibiotics, and herpetic folliculitis is treated with antiviral agents. Future studies of the hair follicle will perhaps unlock the mechanisms that drive and maintain normal hair growth. Until that time scientists will, no doubt, continue to be fascinated by the intricate developmental and immunologic mechanisms that drive this micro-organ of the skin.

Generation and cyclic remodeling of the hair follicle immune system in mice

In this immunohistomorphometric study, we have defined basic characteristics of the hair follicle (HF) immune system during follicle morphogenesis and cycling in C57BL/6 mice, in relation to the skin immune system. Langerhans cells and gammadelta T cell receptor immunoreactive lymphocytes were the predominant intraepithelial hematopoietic cells in neonatal mouse skin. After their numeric increase in the epidermis, these cells migrated into the HF, although only when follicle morphogenesis was almost completed. In contrast to Langerhans cells, gammadelta T cell receptor immunoreactive lymphocytes entered the HF only via the epidermis. Throughout HF morphogenesis and cycling, both cell types remained strikingly restricted to the distal outer root sheath. On extremely rare occasions, CD4+ or CD8+ alphabetaTC were detected within the HF epithelium or the sebaceous gland. Major histocompatibility complex class II+, MAC-1+ cells of macrophage phenotype and numerous mast cells appeared very early on during HF development in the perifollicular dermis, and the percentage of degranulated mast cells significantly increased during the initiation of synchronized HF cycling (first catagen). During both depilation- and cyclosporine A-induced HF cycling, the numbers of intrafollicular Langerhans cells, gammadelta T cell receptor immunoreactive lymphocytes, and perifollicular dermal macrophages fluctuated significantly. Yet, no numeric increase of perifollicular macrophages was detectable during HF regression, questioning their proposed role in catagen induction. In summary, the HF immune system is generated fairly late during follicle development, shows striking differences to the extrafollicular skin immune system, and undergoes substantial hair cycle-associated remodeling. In addition, synchronized HF cycling is accompanied by profound alterations of the skin immune system.

Clusters of perifollicular macrophages in normal murine skin: physiological degeneration of selected hair follicles by programmed organ deletion

In back skin sections from adolescent C57BL/6 mice, regularly distributed, perifollicular inflammatory cell clusters (PICC) were found located around the distal noncycling portion of about 2% of all hair follicles examined. The PICC and the affected hair follicles were characterized during spontaneously developed or induced hair cycle stages, using antibodies against MHC Class II, F4/80, ER-MP23, NLDC 145, CD4, CD8, gammadeltaTCR, IL-1 receptor, and ICAM-1. PICC consisted predominantly of macrophages (MAC), accompanied by a few CD4+ cells, whereas gammadeltaTCR+ and CD8+ cells were absent. During anagen and catagen, some of the PICC+ hair follicles showed variable degenerative phenomena reminiscent of scarring alopecia: thickened basement membrane, ectopic MHC II expression, MAC infiltration into the follicle epithelium, and signs of keratinocyte apoptosis. Loss of distal outer root sheath keratinocytes was detected in 10% of PICC+ hair follicles (0.2% of all hair follicles). Because PICC were located in the vicinity of the bulge region, MAC-dependent damage to follicle stem cells might eventually lead to follicle degeneration. These perifollicular MAC clusters around selected hair follicles may indicate the existence of a physiological program of MAC-dependent controlled follicle degeneration by which damaged or malfunctioning follicles are removed by programmed organ deletion (POD).

Utilization of transgenic mice in the study of matrix degrading proteinases and their inhibitors

The extracellular matrix (ECM) acts as both a structural scaffold and an informational medium. Its dynamic status is determined by cells that secrete its constituent molecules and, in most cases, also secrete enzymes that catalyze degradation of these molecules. A stasis between ECM degrading enzymes and their inhibitors maintains the integrity of the matrix. While controlled ECM remodelling is fundamental to several normal processes, uncontrolled disruption underlies diverse pathological conditions. Transgenic mice with specific modulations or a total lack of expression of certain metalloproteinases, serine proteinases or their inhibitors have been generated to elucidate endogenous expression patterns, identify regulatory elements of these genes, and study the physiological consequences of their deregulated expression. With these models we enhance our understanding of the role of proteinases and their inhibitors in diverse normal processes and pathologies including mammary gland development, hemostasis, emphysema and cancer.

A murine model for inducing and manipulating hair follicle regression (catagen): effects of dexamethasone and cyclosporin A

Most cases of hair loss are based on premature induction of follicle regression (catagen). Deciphering the unknown regulation of catagen is therefore clinically important, but catagen is also an excellent model for organ involution by rapid terminal differentiation and for epithelial cell death (apoptosis). We here report an assay for the controlled pharmacologic induction and manipulation of catagen follicles. Dexamethasone-21-acetate (0.1%) was applied once daily to depilation-induced, growing follicles (anagen VI) on the backs of C57 B1-6 mice. Characteristic catagen-associated changes in skin color were photodocumented and assessed by morphometric histology. Topical dexamethasone induced catagen-like follicles significantly earlier, more homogeneously, and also more extensively than vehicle. This process was inhibited by high intraperitoneal doses of cyclosporin A. In addition to its clinical relevance as a screening assay for catagen-blocking drugs, this simple murine model is an attractive tool for dissecting the molecular, cellular, and developmental biology of catagen.

Expression of classical and non-classical MHC class I antigens in murine hair follicles

Not all keratinocytes in human and rat hair follicles express MHC class I antigens (MHC I). In the present study, we report the first immunohistological profile of classical and non-classical MHC I expression in the skin of adolescent C57 BL-6 mice during the induced hair cycle. MHC I immunoreactivity (H-2b, H-2Db) is absent in the matrix and inner root sheath of growing (= anagen) hair follicles, and the dermal papillae are H-2b negative during catagen and telogen. This lack of normal MHC I expression may serve to sequester potentially damaging autoantigens from immune recognition. In addition, we present the first evidence of non-classical MHC class I antigen expression in normal mammalian skin: during the entire hair cycle, the distal hair follicle shows strong Qa-2 immunoreactivity, which appears to be restricted to an epithelial follicle compartment densely populated by gamma-delta T cells with which Qa-2 molecules may interact as part of a primitive antibacterial defense system of the follicle. The murine hair cycle is an attractive model for dissecting the functional roles of H-2b and Qa-2 molecules in hair biology and in related tissue-interaction systems.

Human hair growth in vitro: a model for the study of hair follicle biology

The factors that regulate hair follicle growth are still poorly understood. In vitro models may be useful in elucidating some aspects of hair follicle biology. We have developed an in vitro human hair growth model that enables us to maintain isolated human hair follicles for up to 10 days, during which time they continue to grow at an in vivo rate producing a keratinised hair fibre. We have shown that epidermal growth factor (EGF) in our system mimics the in vivo depilatory action of EGF in sheep, and suggest that this occurs as a result of EGF stimulating outer root sheath (ORS) cell proliferation which results in the disruption of normal mechanisms of cell-cell interaction in the hair follicle. We identify transforming growth factor-beta (TGF-beta) as a possible negative regulator of hair follicle growth and show that physiological levels of insulin-like growth factor-I (IGF-I) can support the same rates of hair follicle growth as supraphysiological levels of insulin. Furthermore, in the absence of insulin hair follicles show premature entry into a catagen-like state. This is prevented by physiological levels of IGF-I. Finally we demonstrate that the hair follicle is an aerobic glycolytic, glutaminolytic tissue and discuss the possible implications of this metabolism.

Effects of EGF on the morphology and patterns of DNA synthesis in isolated human hair follicles

We have previously reported that human hair grows at a normal rate in vitro for up to 10 d. We have also reported that, on gross observation, epidermal growth factor appears to induce a catagen-like effect on cultured hair follicles, but we have not characterized the details of this. We now report that when isolated human hair follicles are maintained in the presence of epidermal growth factor, the rate of hair follicle elongation is significantly stimulated but hair fiber production is inhibited. Light microscopy showed that epidermal growth factor stimulated a thickening and vacuolation of the cells of the lower outer root sheath of the hair follicle and that the matrix cells of the hair follicle underwent an upward migration resulting in the formation of a 'club hair'-like structure that remained connected to the dermal papilla by a thin strand of epithelial cells. [Methyl-3H] thymidine autoradiography was carried out to investigate the patterns of DNA synthesis and showed that epidermal growth factor inhibited DNA synthesis in the hair follicle matrix cells but dramatically stimulated DNA synthesis in the outer root sheath. We conclude from these studies that epidermal growth factor may be inducing an artificial 'catagen-like' effect by stimulating outer root sheath proliferation, which uncouples the normal patterns of proliferation and migration that occur in the anagen hair follicle and that result in an anagen-to-catagen-like transition. Moreover, these results also suggest that, under certain conditions, outer root sheath cells in the hair follicle may be capable of downward migration.

Hair shaft elongation, follicle growth, and spontaneous regression in long-term, gelatin sponge-supported histoculture of human scalp skin

In order to better understand the molecular mechanisms of human hair growth control and to test hair growth-modulatory drugs, appropriate in vitro models are required. Here, we report the long-term growth, shaft elongation, and spontaneous regression of human hair follicles in histoculture of intact scalp skin. Human scalp skin with abundant hair follicles in various stages of the hair growth cycle was grown for up to 40 days in a gelatin sponge-supported histoculture system at the air/liquid interface. Isolated follicles placed in the gelatin-sponge matrix also supported hair shaft elongation, with the hair follicle cells remaining proliferative and viable for very long periods. Hair shaft elongation occurred mainly during the first 10 days of histoculture of both intact skin and isolated follicles. However, hair follicles were viable and follicle keratinocytes continued to incorporate [3H]thymidine for up to several weeks after shaft elongation had ceased as shown by fluorescent-dye double staining, measured by confocal laser scanning microscopy, and by histological autoradiography of [3H]thymidine incorporation, respectively. Hair follicles could continue their cycle in histoculture; for example, apparent spontaneous catagen induction was observed both histologically and by the actual regression of the hair follicle. In addition, vellus follicles were shown to be viable at day 40 after initiation of culture. In the histocultured human scalp we demonstrated the association of mast cells with anagen follicles and macrophages with catagen follicles, suggesting a role of these cells in the hair cycle. This histoculture technique should serve as a powerful tool for future hair research in the human system as well as a screening assay for compounds that can perturb the hair cycle.

Distribution of proteoglycans during the hair growth cycle in human skin

The involvement of proteoglycans in hair growth has been recognized through the observation of increased hair growth in diseases such as the mucopolysaccharidoses and pre-tibial myxedema, which involve an increase in skin proteoglycan content. In an attempt to understand this, we have examined the distribution of chondroitin 6 sulphate (C6S), unsulphated chondroitin (COS), dermatan sulphate (DS), and heparan sulphate proteoglycans (HSPG) in frozen tissue sections of normal scalp by immunostaining. Results show that during anagen, the thick connective tissue sheath around the follicle strains strongly for C6S, COS, and DS. COS is uniquely associated with this region and is not found beneath the epidermis or infundibular epithelium. HSPG is, however, localized in the basement membrane zone adjacent to the outer root sheath. In addition, all of these proteoglycans are localized in the dermal papilla. In mid-catagen, we observed significant loss of C6S and COS staining from both the dermal papilla and the connective tissue sheath, but no decrease in staining for HSPG. In late catagen, very little staining of C6S and COS was observed. In early anagen, we observed that C6S was again present in the connective tissue sheath and dermal papilla; however, COS staining appeared to be weaker and less closely associated with the follicle. HSPG staining was observed in early anagen in a pattern very similar to that found for other basement membrane components. Results for DS were not obtained for catagen or early anagen. These results provide further evidence that hair growth is associated with the presence of chondroitin proteoglycans in the follicle environment and that the cessation of growth is associated with their removal. Further studies are underway to characterize the relationship between hair growth and proteoglycans.

Growth factors specifically alter hair follicle cell proliferation and collagenolytic activity alone or in combination

A three-dimensional culture model for isolated murine pelage hair follicles in a type I collagen gel has been utilized to study the effects of selected growth factors on follicle cell proliferation and release of collagenolytic factors. Cultured follicle organoids differentially express cytokeratins 6 and 14 in a pattern suggesting they contain cells of the outer root sheath, inner root sheath and follicle matrix. Using incorporation of [3H]thymidine as a measure of proliferation, follicle organoids show a peak of DNA synthesis between day 1 and 5 of culture, depending on plating density, and then have a low rate of DNA synthesis. Thymidine incorporation is stimulated by transforming growth factor-alpha (TGF-alpha) in a dose-dependent response. Only peripheral cells presumably of the outer root sheath, incorporate thymidine in basal or stimulated conditions. TGF-beta 1 and TGF-beta 2 inhibit constitutive cell proliferation and oppose growth stimulation by TGF-alpha. Hair follicles lyse the collagen gel matrix when exposed to certain cytokines. Epidermal growth factor (EGF) and TGF-alpha stimulate gel lysis, but TGF-beta 1, TGF-beta 2 and cholera toxin do not. Other skin-derived cells, such as interfollicular epidermal cells, dermal fibroblasts, or combinations thereof, do not lyse gels in this culture model even when exposed to growth factors. Combinations of EGF or TGF-alpha with TGF-beta 1 or TGF-beta 2 are synergistic for collagenase release. These cytokines stimulate release of multiple species of matrix metalloproteinases, but the 92-kDa and 72 kDa type IV procollagenases and their activated derivatives predominate on zymograms. In cytokine-stimulated follicles, both peripheral and centrally located cells in the organoids express the 72-kDa type IV collagenase and a similar immunostaining pattern is present in developing follicles in vivo. Thus growth factors appear to work in concert for certain hair follicle responses and in opposition for others. These combined actions may play a role in different phases of hair follicle development that require cell replication and invasion into the deeper dermis.

Dynamic ultrastructural changes of the connective tissue sheath of human hair follicles during hair cycle

Ultrastructural changes of the connective tissue sheath (CTS), including the hyaline membrane, of human hair follicles during the hair cycle, were studied in normal scalp skin specimens. In early anagen, the CTS was composed of a thin basal lamina and surrounding collagen tissue. The collagen tissue gradually thickened during the development of the hair and hair follicle. In mature anagen hair follicles, the collagen tissue was separated into three layers. The inner collagen layer, just outside the basal lamina, was thin and composed of collagen fibres running longitudinally parallel to the hair axis. The middle collagen layer was very thick with its collagen fibres running transversely against the hair axis and surrounding the inner hair tissue. Many fibroblasts were present among the collagen fibres in the middle layer, whereas the inner layer contained almost none. In the outer collagen layer, collagen fibres ran in various directions parallel to the outer surface of the outer root sheath cells. In late anagen, the basal lamina became very thick. In catagen, the basal lamine and the inner collagen layer became corrugated and showed oedematous change and degeneration. Surrounding fibroblasts showed active production of new collagen fibres, which seemed to fill the spaces left by the retraction of the hair follicle and hyaline membrane. These ultrastructural changes of the CTS show that there may be dynamic metabolic changes of the connective tissue around human hair follicles during the hair cycle.

Phagocytosis of collagen by mouse decidual cells

Collagen fibrils were present within membrane-bound vacuoles in the cytoplasm of mouse decidual cells on the 7th day of pregnancy. The space between the vacuole membranes and the fibrils was narrow and frequently filled with a granular electron-dense material. The loss of banding of the collagen fibrils, their association with lysosomelike bodies, and the demonstration of acid phosphatase activity in the vacuoles indicate that the fibrils were internalized by the decidual cells and were being digested. It is suggested that phagocytosis of collagen is a mechanism of remodeling of the mouse decidua.

Bilateral idiopathic inflammation of the optic nerve sheaths. Light and electron microscopic findings

Idiopathic perioptic neuritis is a term used to describe noninfectious inflammatory disorders of the optic nerve sheaths, the causes of which are unknown. In the following report, a 68-year-old woman with bilateral visual loss was found to have chronic inflammation with necrobiotic granulomas of her optic nerve sheaths. The patient, who had no systemic condition known to be associated with necrobiotic granuloma, lost vision from infarction of the optic nerve parenchyma and from compression due to thickened meninges. Although there are similarities between the inflammatory reaction in this case to the necrobiotic dermatoses, the pathogenesis of this condition remains obscure.

The effect of coumarin (5,6 benzo-alpha-pyrone) on elicited members of the mononuclear system in dogs with chronic secondary lymphedema

The effect of coumarin is investigated on the morphological features of members of the mononuclear phagocytic system (MPS) attaching to subcutaneously (s.c.) implanted coverslips in "normal" and chronically lymphedematous tissues. In untreated "normal" tissues there was no attachment suggesting that the presence of lymphedema elsewhere influences MPS activity. Coumarin restored attachment proportions to those found in normal tissues of dogs with no lymphedema. In lymphedema, coumarin significantly increased the proportion of macrophages which were round with less than ten pseudopods (new recruits) and the proportion with distinct pseudopods combined with more than ten vacuoles (active elicited members of the MPS). Coumarin resulted in a significant decline in the proportion of macrophages which were round and with more than ten apparently lipid-containing vacuoles, suggesting a facilitation of their removal to other sites.

Microscopical studies of necrotising scleritis. II. Collagen degradation in the scleral stroma

Electron microscopy of tissue excised during surgery in eight cases of advanced anterior necrotising scleritis showed degradation of collagen in the scleral stroma by both intracellular and extracellular mechanisms. In the first of these mechanisms cells resembling active fibroblasts and macrophages were observed in the process of phagocytosis of collagen fibrils into vacuoles associated with dense cytoplasmic granules. In the second mechanism collagen fibrils in large areas of the scleral stroma appeared swollen and unravelled, or completely solubilised, without close association with stromal cells. Both activation and degeneration of stromal fibrocytes were evident in zones of extracellular fibril degradation. These changes appeared to take place prior to invasion of the stroma by cells of the granuloma.

Electron-microscopical study of the operculum in anuran tadpole after extirpation of the right forelimb during metamorphosis

The process of histolysis and fenestration of the skin of the prospective opercular perforation region of Rana japonica after extirpation of the right forelimb was observed during metamorphosis by transmission and scanning electron microscopy. Epidermal cells of the belly of the tadpole, including the operculum, are extremely similar in their ultrastructure. Epidermal cells of the prospective opercular perforation region during metamorphosis become thin and vacuolated especially around the nucleus perhaps by autolysis, associated with lysosomal activity. The histolysis and formation of the perforation of the operculum occurs in the complete absence of forelimb. Macrophages containing phagosomes and lymphocytes or other blood cells are almost always found in the intercellular epidermis. Necrotic epidermal cells progressively separate by cleft formation and slough off without cornification.

Intracellular collagen fibres in regenerating tendon

Intracytoplasmic collagen fibres contained in elongated membrane-bound structures were observed in fibroblasts of regenerating tendon in the later stages of the repair process. The observations appear to support the hypothesis that collagen fibres of the intercellular matrix may be phagocytosed by the cells and degraded in the intracellular environment. Phagocytosis of extracellular collagen fibres probably represents an additional mechanism of collagen resorption during the later stages of remodelling of the regenerating tendon.

Collagen phagocytosis in multicentric reticulohistiocytosis

A large number of dermal cells engaged in collagen phagocytosis were observed in a case of multicentric reticulohistiocytosis. These cells apparently start to engulf collagen by deep invaginations of the plasma membrane. Fusion of phagosomes containing the engulfed collagen with vesicles probably originating from the Golgi apparatus would give rise to secondary lysosomes. The process of collagen degradation leads to the appearance of fine fibrils and myelin bodies. The presence in the dermis of unusually thick collagen fibers might contribute to this process which appears to occur only under particular physiological and pathological conditions.

Morphological changes in the proximal area of the rat's hair follicle during early catagen. An electron-microscopic study

This electron-microscopic study of the catagen phase shows that the first alteration of regression of the follicle is localized in the papilla, where the cells withdraw their offshoots and break the contact with the basal lamina. Both at the level of the papilla and of the bulb structures appear that increase the cell cohesion. Under the influence of the outer root sheath an upward migration occurs. This is followed by plication and thickening of the basal lamina. The alterations in the connective tissue sheath occur in a further stage. The first signs of autolysis occur in the center of the epithelial column. At the end of the catagen stage macrophages take care of the clearing-up.

Excess plasma proteins as a cause of chronic inflammation and lymphodema: quantitative electron microscopy

The subcutaneous tissue of rats was injected with plasma. PVP, or saline. In spite of precautions to avoid immunological reactions and the release of mediators, it was found that the plasma-injected animals had many of the signs of chronic inflammation-even at 64 days. These were considerably reduced by coumarin, a benzo-pyrone, which is known to enhance the lysis of proteins by macrophages. The numbers of these cells were greatly increased in the injection site-especially with the administration of coumarin. PVP also moderately increased macrophage numbers, but did not cause the increase in fibroblast numbers or collagen formation associated with plasma-injections. It is concluded that altered proteins in the tissues are a cause of chronic inflammation and that, in particular, chronic lymphoedema is a form of chronic inflammation.

Crohn's disease: transmission electron microscopic studies. III. Target tissues. Proliferation of and injury to smooth muscle and the autonomic nervous system

Transmission electron microscopy was done on surgical specimens from 12 patients with Crohn's disease and three control subjects. Nonulcerated involved areas of ileum as well as proximal, grossly uninvolved resection margins were chosen for study. Specimens for transmission electron microscopy were prepared by a variety of techniques and 112 blocks were examined by electron microscopy. The study was concentrated on two target tissues of the gut: the autonomic nervous system and the smooth muscle. Proliferative and injurious changes were found in each. Proliferation, myofibroblastic transformation, hypercontraction, and necrosis characterized the smooth muscle changes seen in Crohn's disease of the ileum. Autonomic nervous system changes included proliferation of axons containing dense core granules (catecholamines) and axonal necrosis. The possible pathogenetic significance of these changes is discussed here and in the accompanying article beginning on page 606 of this issue.

The effect of rat serum on the morphology of rat hair follicles in tissue culture

Rat hair follicles obtained from the skin of 12-day-old animals were cultured in the presence of 20% fetal calf serum and compared with those cultured in the presence of 20% rat serum obtained from animals aged 12-14, 18-20 days; and 1 year. Follicles cultured in the presence of fetal calf or 12-14-day rat serum showed no morphological change after 2 days of culture. Follicles cultured in the presence of 18-20 day serum or adult serum for 2 days showed greatly altered morphology, the dermal papilla degenerated, keratinization of hair cells occurred, and an enlarged outer sheath developed. The cells in this sheath contained many filaments which were periodic acid-Schiff positive. These changes are compared with those occurring during the catagen phase of the hair cycle in vivo.

Intracellular collagen in recurrent ameloblastic fibroma

Electron microscopic examination of tissue from a twice recurrent ameloblastic fibroma revealed the presence of intracellular collagen fibres in fibroblasts active in protein synthesis. The intracellular fibres were morphologically identical to collagen fibres located extracellularly. The literature on intracellular collagen in biological systems and pathological states has been reviewed, and attention is focussed on collagen phagocytosis and degradation by fibroblasts which are currently considered to represent the basis of connective tissue remodelling and turnover.

Response of periodontal ligament cells to orthodontic force: ultrastructural identification of proliferating fibroblasts

The morphologic response of periodontal ligament (PDL) cells in an area of tension created by orthodontic force has been assessed by transmission electron microscopy. Young adult male rats were sacrificed at 24, 48, 72, 96 and 120 hours following orthodontic stimulation. The earliest detectable response was the appearance of increased numbers of mitotic cells in the PDL at 24 hours post-stimulation. The most significant ultrastructural feature of these cells was the presence of intracellular vesicles containing collagen microfibrils. These vesicles were identical to profiles present in interphase PDL fibroblasts involved in collagen phagocytosis associated with turnover of the ligament. Between 48 and 120 hours the alveolar bone surface in the region examined was characterized by the presence of newly generated osteoblasts and active bone formation. Intracellular collagen was never observed in osteoblasts. These observations suggest that at least a portio- of the population of PDL cells which proliferate in response to orthodontic force represent functional ligament fibroblasts.