Defining the epithelial stem cell niche in skin

Many adult regenerative cells divide infrequently but have high proliferative capacity. We developed a strategy to fluorescently label slow-cycling cells in a cell type-specific fashion. We used this method to purify the label-retaining cells (LRCs) that mark the skin stem cell (SC) niche. We found that these cells rarely divide within their niche but change properties abruptly when stimulated to exit. We determined their transcriptional profile, which, when compared to progeny and other SCs, defines the niche. Many of the >100 messenger RNAs preferentially expressed in the niche encode surface receptors and secreted proteins, enabling LRCs to signal and respond to their environment.

The cornified envelope: a model of cell death in the skin

The epidermis functions as a barrier against the environment by means of several layers of terminally differentiated, dead keratinocytes - the cornified layer, which forms the endpoint of epidermal differentiation and death. The cornified envelope replaces the plasma membrane of differentiating keratinocytes and consists of keratins that are enclosed within an insoluble amalgam of proteins, which are crosslinked by transglutaminases and surrounded by a lipid envelope. New insights into the molecular mechanisms and the physiological endpoints of cornification are increasing our understanding of the pathological defects of this unique form of programmed cell death, which is associated with barrier malfunctions and ichthyosis.

Stem cells and their niches

A constellation of intrinsic and extrinsic cellular mechanisms regulates the balance of self-renewal and differentiation in all stem cells. Stem cells, their progeny, and elements of their microenvironment make up an anatomical structure that coordinates normal homeostatic production of functional mature cells. Here we discuss the stem cell niche concept, highlight recent progress, and identify important unanswered questions. We focus on three mammalian stem cell systems where large numbers of mature cells must be continuously produced throughout adult life: intestinal epithelium, epidermal structures, and bone marrow.

Stem cells in the hair follicle bulge contribute to wound repair but not to homeostasis of the epidermis

The discovery of long-lived epithelial stem cells in the bulge region of the hair follicle led to the hypothesis that epidermal renewal and epidermal repair after wounding both depend on these cells. To determine whether bulge cells are necessary for epidermal renewal, here we have ablated these cells by targeting them with a suicide gene encoding herpes simplex virus thymidine kinase (HSV-TK) using a Keratin 1-15 (Krt1-15) promoter. We show that ablation leads to complete loss of hair follicles but survival of the epidermis. Through fate-mapping experiments, we find that stem cells in the hair follicle bulge do not normally contribute cells to the epidermis which is organized into epidermal proliferative units, as previously predicted. After epidermal injury, however, cells from the bulge are recruited into the epidermis and migrate in a linear manner toward the center of the wound, ultimately forming a marked radial pattern. Notably, although the bulge-derived cells acquire an epidermal phenotype, most are eliminated from the epidermis over several weeks, indicating that bulge stem cells respond rapidly to epidermal wounding by generating short-lived 'transient amplifying' cells responsible for acute wound repair. Our findings have implications for both gene therapy and developing treatments for wounds because it will be necessary to consider epidermal and hair follicle stem cells as distinct populations.

THE PIG AS A MODEL FOR HUMAN WOUND HEALING

The medical literature describes numerous in vitro and in vivo wound-healing models. The selection of an animal model depends on a number of factors including availability, cost, ease of handling, investigator familiarity, and anatomical/functional similarity to humans. Small mammals are frequently used for wound healing studies, however, these mammals differ from humans in a number of anatomical and physiological ways. Anatomically and physiologically, pig skin is more similar to human skin. The many similarities between man and pig would lead one to believe that the pig should make an excellent animal model for human wound healing. The purpose of this paper is to review the existing literature for evidence of this supposition and determine how well the various models correlate to human wound healing. Studies of wound dressings, topical antimicrobials, and growth factors are examined. Over 180 articles were utilized for this comparative review. Our conclusion is that the porcine model is an excellent tool for the evaluation of therapeutic agents destined for use in human wounds.

Barrier function of the skin: "la raison d'être" of the epidermis

The primary function of the epidermis is to produce the protective, semi-permeable stratum corneum that permits terrestrial life. The barrier function of the stratum corneum is provided by patterned lipid lamellae localized to the extracellular spaces between corneocytes. Anucleate corneocytes contain keratin filaments bound to a peripheral cornified envelope composed of cross-linked proteins. The many layers of these specialized cells in the stratum corneum provide a tough and resilient framework for the intercellular lipid lamellae. The lamellae are derived from disk-like lipid membranes extruded from lamellar granules into the intercellular spaces of the upper granular layer. Lysosomal and other enzymes present in the extracellular compartment are responsible for the lipid remodeling required to generate the barrier lamellae as well as for the reactions that result in desquamation. Lamellar granules likely originate from the Golgi apparatus and are currently thought to be elements of the tubulo-vesicular trans-Golgi network. The regulation of barrier lipid synthesis has been studied in a variety of models, with induction of several enzymes demonstrated during fetal development and keratinocyte differentiation, but an understanding of this process at the molecular genetic level awaits further study. Certain genetic defects in lipid metabolism or in the protein components of the stratum corneum produce scaly or ichthyotic skin with abnormal barrier lipid structure and function. The inflammatory skin diseases psoriasis and atopic dermatitis also show decreased barrier function, but the underlying mechanisms remain under investigation. Topically applied "moisturizers" work by acting as humectants or by providing an artificial barrier to trans-epidermal water loss; current work has focused on developing a more physiologic mix of lipids for topical application to skin. Recent studies in genetically engineered mice have suggested an unexpected role for tight junctions in epidermal barrier function and further developments in this area are expected. Ultimately, more sophisticated understanding of epidermal barrier function will lead to more rational therapy of a host of skin conditions in which the barrier is impaired.

Klf4 is a transcription factor required for establishing the barrier function of the skin

Located at the interface between body and environment, the epidermis must protect the body against toxic agents and dehydration, and protect itself against physical and mechanical stresses. Acquired just before birth and at the last stage of epidermal differentiation, the skin's proteinaceous/lipid barrier creates a surface seal essential for protecting animals against microbial infections and dehydration. We show here that Kruppel-like factor 4 (Klf4, encoded by the gene Klf4), highly expressed in the differentiating layers of epidermis, is both vital to and selective for barrier acquisition. Klf4-/- mice die shortly after birth due to loss of skin barrier function, as measured by penetration of external dyes and rapid loss of body fluids. The defect was not corrected by grafting of Klf4-/- skin onto nude mice. Loss of the barrier occurs without morphological and biochemical alterations to the well-known structural features of epidermis that are essential for mechanical integrity. Instead, late-stage differentiation structures are selectively perturbed, including the cornified envelope, a likely scaffold for lipid organization. Using suppressive subtractive hybridization, we identified three transcripts encoding cornified envelope proteins with altered expression in the absence of Klf4. Sprr2a is one, and is the only epidermal gene whose promoter is known to possess a functional Klf4 binding site. Our studies provide new insights into transcriptional governance of barrier function, and pave the way for unravelling the molecular events that orchestrate this essential process.

Synthesis and characterization of a model extracellular matrix that induces partial regeneration of adult mammalian skin

Regeneration of the dermis does not occur spontaneously in the adult mammal. The epidermis is regenerated spontaneously provided there is a dermal substrate over which it can migrate. Certain highly porous, crosslinked collagen-glycosaminoglycan copolymers have induced partial morphogenesis of skin when seeded with dermal and epidermal cells and then grafted on standard, full-thickness skin wounds in the adult guinea pig. A mature epidermis and a nearly physiological dermis, which lacked hair follicles but was demonstrably different from scar, were regenerated over areas as large as 16 cm2. These chemical analogs of extracellular matrices were morphogenetically active provided that the average pore diameter ranged between 20 and 125 microns, the resistance to degradation by collagenase exceeded a critical limit, and the density of autologous dermal and epidermal cells inoculated therein was greater than 5 x 10(4) cells per cm2 of wound area. Unseeded copolymers with physical structures that were within these limits delayed the onset of wound contraction by about 10 days but did not eventually prevent it. Seeded copolymers not only delayed contraction but eventually arrested and reversed it while new skin was being regenerated. The data identify a model extracellular matrix that acts as if it were an insoluble growth factor with narrowly specified physiochemical structure, functioning as a transient basal lamina during morphogenesis of skin.

Analysis of the actin-myosin II system in fish epidermal keratocytes: mechanism of cell body translocation

While the protrusive event of cell locomotion is thought to be driven by actin polymerization, the mechanism of forward translocation of the cell body is unclear. To elucidate the mechanism of cell body translocation, we analyzed the supramolecular organization of the actin-myosin II system and the dynamics of myosin II in fish epidermal keratocytes. In lamellipodia, long actin filaments formed dense networks with numerous free ends in a brushlike manner near the leading edge. Shorter actin filaments often formed T junctions with longer filaments in the brushlike area, suggesting that new filaments could be nucleated at sides of preexisting filaments or linked to them immediately after nucleation. The polarity of actin filaments was almost uniform, with barbed ends forward throughout most of the lamellipodia but mixed in arc-shaped filament bundles at the lamellipodial/cell body boundary. Myosin II formed discrete clusters of bipolar minifilaments in lamellipodia that increased in size and density towards the cell body boundary and colocalized with actin in boundary bundles. Time-lapse observation demonstrated that myosin clusters appeared in the lamellipodia and remained stationary with respect to the substratum in locomoting cells, but they exhibited retrograde flow in cells tethered in epithelioid colonies. Consequently, both in locomoting and stationary cells, myosin clusters approached the cell body boundary, where they became compressed and aligned, resulting in the formation of boundary bundles. In locomoting cells, the compression was associated with forward displacement of myosin features. These data are not consistent with either sarcomeric or polarized transport mechanisms of cell body translocation. We propose that the forward translocation of the cell body and retrograde flow in the lamellipodia are both driven by contraction of an actin-myosin network in the lamellipodial/cell body transition zone.

Getting under the skin of epidermal morphogenesis

At the surface of the skin, the epidermis serves as the armour for the body. Scientists are now closer than ever to understanding how the epidermis accomplishes this extraordinary feat, and is able to survive and replenish itself under the harshest conditions that face any tissue. By combining genetic engineering with cell-biological studies and with human genome data analyses, skin biologists are discovering the mechanisms that underlie the development and differentiation of the epidermis and hair follicles of the skin. This explosion of knowledge paves the way for new discoveries into the genetic bases of human skin disorders and for developing new therapeutics.

Moisturization and skin barrier function

Over the past decade, great progress has been made toward elucidating the structure and function of the stratum corneum (SC), the outermost layer of the epidermis. SC cells (corneocytes) protect against desiccation and environmental challenge by regulating water flux and retention. Maintenance of an optimal level of hydration by the SC is largely dependent on several factors. First, intercellular lamellar lipids, organized predominantly in an orthorhombic gel phase, provide an effective barrier to the passage of water through the tissue. Secondly, the diffusion path length also retards water loss, since water must traverse the tortuous path created by the SC layers and corneocyte envelopes. Thirdly, and equally important, is natural moisturizing factor (NMF), a complex mixture of low-molecular-weight, water-soluble compounds first formed within the corneocytes by degradation of the histidine-rich protein known as filaggrin. Each maturation step leading to the formation of an effective moisture barrier--including corneocyte strengthening, lipid processing, and NMF generation--is influenced by the level of SC hydration. These processes, as well as the final step of corneodesmolysis that mediates exfoliation, are often disturbed upon environmental challenge, resulting in dry, flaky skin conditions. The present paper reviews our current understanding of the biology of the SC, particularly its homeostatic mechanisms of hydration.

Ultrathin conformal devices for precise and continuous thermal characterization of human skin

Precision thermometry of the skin can, together with other measurements, provide clinically relevant information about cardiovascular health, cognitive state, malignancy and many other important aspects of human physiology. Here, we introduce an ultrathin, compliant skin-like sensor/actuator technology that can pliably laminate onto the epidermis to provide continuous, accurate thermal characterizations that are unavailable with other methods. Examples include non-invasive spatial mapping of skin temperature with millikelvin precision, and simultaneous quantitative assessment of tissue thermal conductivity. Such devices can also be implemented in ways that reveal the time-dynamic influence of blood flow and perfusion on these properties. Experimental and theoretical studies establish the underlying principles of operation, and define engineering guidelines for device design. Evaluation of subtle variations in skin temperature associated with mental activity, physical stimulation and vasoconstriction/dilation along with accurate determination of skin hydration through measurements of thermal conductivity represent some important operational examples.

Stratum corneum defensive functions: an integrated view

Most epidermal functions can be considered as protective, or more specifically, as defensive in nature. Yet, the term "barrier function" is often used synonymously with only one such defensive function, though arguably its most important, i.e., permeability barrier homeostasis. Regardless of their relative importance, these protective cutaneous functions largely reside in the stratum corneum (SC). In this review, I first explore the ways in which the multiple defensive functions of the SC are linked and interrelated, either by their shared localization or by common biochemical processes; how they are co-regulated in response to specific stressors; and how alterations in one defensive function impact other protective functions. Then, the structural and biochemical basis for these defensive functions is reviewed, including metabolic responses and signaling mechanisms of barrier homeostasis. Finally, the clinical consequences and therapeutic implications of this integrated perspective are provided.

Induction of tenascin in healing wounds

The distribution of the extracellular matrix glycoprotein, tenascin, in normal skin and healing skin wounds in rats, has been investigated by immunohistochemistry. In normal skin, tenascin was sparsely distributed, predominantly in association with basement membranes. In wounds, there was a marked increase in the expression of tenascin at the wound edge in all levels of the skin. There was also particularly strong tenascin staining at the dermal-epidermal junction beneath migrating, proliferating epidermis. Tenascin was present throughout the matrix of the granulation tissue, which filled full-thickness wounds, but was not detectable in the scar after wound contraction was complete. The distribution of tenascin was spatially and temporally different from that of fibronectin, and tenascin appeared before laminin beneath migrating epidermis. Tenascin was not entirely codistributed with myofibroblasts, the contractile wound fibroblasts. In EM studies of wounds, tenascin was localized in the basal lamina at the dermal-epidermal junction, as well as in the extracellular matrix of the adjacent dermal stroma, where it was either distributed homogeneously or bound to the surface of collagen fibers. In cultured skin explants, in which epidermis migrated over the cut edge of the dermis, tenascin, but not fibronectin, appeared in the dermis underlying the migrating epithelium. This demonstrates that migrating, proliferating epidermis induces the production of tenascin. The results presented here suggest that tenascin is important in wound healing and is subject to quite different regulatory mechanisms than is fibronectin.

Essential involvement of IL-6 in the skin wound-healing process as evidenced by delayed wound healing in IL-6-deficient mice

To clarify interleukin (IL)-6 roles in wound healing, we prepared skin excisions in wild-type (WT) and IL-6-deficient BALB/c [knockout (KO)] mice. In WT mice, the wound area was reduced to 50% of original size at 6 days after injury. Microscopically, leukocyte infiltration was evident at wound sites. Furthermore, the re-epithelialization rate was approximately 80% at 6 days after injury with increases in angiogenesis and hydroxyproline contents. The gene expression of IL-1, chemokines, adhesion molecules, transforming growth factor-beta1, and vascular endothelial growth factor was enhanced at the wound sites. In contrast, the enhanced expression of these genes was significantly reduced in KO mice. Moreover, in KO mice, the reduction of wound area was delayed with attenuated leukocyte infiltration, re-epithelialization, angiogenesis, and collagen accumulation. Finally, the administration of a neutralizing anti-IL-6 monoclonal antibody significantly delayed wound closure in WT mice. These observations suggest that IL-6 has crucial roles in wound healing, probably by regulating leukocyte infiltration, angiogenesis, and collagen accumulation.

Structural and lipid biochemical correlates of the epidermal permeability barrier

As reviewed in this article, the stratum corneum must now be accorded the respect due to a structurally heterogeneous tissue possessing a selected array of enzymatic activity. The sequestration of lipids to intercellular domains and their organization into a unique multilamellar system have broad implications for permeability barrier function, water retention, desquamation, and percutaneous drug delivery. Yet, the functions and organization of specific lipid species in this membrane system are still unknown. Certain novel insights have resulted from comparative studies in avians and marine mammals. Further elucidation of the molecular architecture and interactions of lipid and nonlipid components of the stratum corneum intercellular domains will be a prerequisite for a comprehensive understanding of stratum corneum function.

The transformation suppressor Pdcd4 is a novel eukaryotic translation initiation factor 4A binding protein that inhibits translation

Pdcd4 is a novel transformation suppressor that inhibits tumor promoter-induced neoplastic transformation and the activation of AP-1-dependent transcription required for transformation. A yeast two-hybrid analysis revealed that Pdcd4 associates with the eukaryotic translation initiation factors eIF4AI and eIF4AII. Immunofluorescent confocal microscopy showed that Pdcd4 colocalizes with eIF4A in the cytoplasm. eIF4A is an ATP-dependent RNA helicase needed to unwind 5' mRNA secondary structure. Recombinant Pdcd4 specifically inhibited the helicase activity of eIF4A and eIF4F. In vivo translation assays showed that Pdcd4 inhibited cap-dependent but not internal ribosome entry site (IRES)-dependent translation. In contrast, Pdcd4(D418A), a mutant inactivated for binding to eIF4A, failed to inhibit cap-dependent or IRES-dependent translation or AP-1 transactivation. Recombinant Pdcd4 prevented eIF4A from binding to the C-terminal region of eIF4G (amino acids 1040 to 1560) but not to the middle region of eIF4G(amino acids 635 to 1039). In addition, both Pdcd4 and Pdcd4(D418A) bound to the middle region of eIF4G. The mechanism by which Pdcd4 inhibits translation thus appears to involve inhibition of eIF4A helicase, interference with eIF4A association-dissociation from eIF4G, and inhibition of eIF4A binding to the C-terminal domain of eIF4G. Pdcd4 binding to eIF4A is linked to its transformation-suppressing activity, as Pdcd4-eIF4A binding and consequent inhibition of translation are required for Pdcd4 transrepression of AP-1.

Epithelial stem cells: a folliculocentric view

Putative epithelial stem cells were identified in the hair follicle bulge as quiescent "label retaining cells". The study of these cells was hindered until the identification of bulge cell molecular markers, such as CD34 expression and K15 promoter activity. This allowed for the isolation and characterization of bulge cells from mouse follicles. Bulge cells possess stem cell characteristics, including multipotency, high proliferative potential, and their cardinal feature of quiescence. Lineage analysis demonstrated that all epithelial layers within the adult follicle and hair originated from bulge cells. Bulge cells only contribute to the epidermis during wound healing, but after isolation, when combined with neonatal dermal cells, they regenerate new hair follicles, epidermis, and sebaceous glands. Bulge cells maintain their stem cell characteristics after propagation in vitro, thus ultimately they may be useful for tissue engineering applications. Understanding the signals important for directing movement and differentiation of bulge cells into different lineages will be important for developing treatments based on stem cells as well as clarifying their role in skin disease.

Electroporation of mammalian skin: a mechanism to enhance transdermal drug delivery

Mammalian skin owes its remarkable barrier function to its outermost and dead layer, the stratum corneum. Transdermal transport through this region occurs predominantly through intercellular lipids, organized largely in bilayers. Electroporation is the creation of aqueous pores in lipid bilayers by the application of a short (microseconds to milliseconds) electric pulse. Our measurements suggest that electroporation occurs in the intercellular lipid bilayers of the stratum corneum by a mechanism involving transient structural changes. Flux increases up to 4 orders of magnitude were observed with human skin in vitro for three polar molecules having charges between -1 and -4 and molecular weights up to slightly more than 1000. Similar flux increases were observed in vivo with animal skin. These results may have significance for drug delivery and other medical applications.

Materials and optimized designs for human-machine interfaces via epidermal electronics

Thin, soft, and elastic electronics with physical properties well matched to the epidermis can be conformally and robustly integrated with the skin. Materials and optimized designs for such devices are presented for surface electromyography (sEMG). The findings enable sEMG from wide ranging areas of the body. The measurements have quality sufficient for advanced forms of human-machine interface.

Genetic analysis of Netrin genes in Drosophila: Netrins guide CNS commissural axons and peripheral motor axons

Two tandem Netrin genes in Drosophila are expressed at the midline of the developing CNS and in different subsets of neurons, muscles, and epidermal patches. In embryos carrying a small deficiency that deletes both genes, CNS axon commissures are partially missing or thinner. This phenotype is rescued by expressing either Netrin gene at the midline. Pan-neural expression of either gene causes disruption of commissural and longitudinal tracts, indicating that the pattern of Netrin expression is crucial and that Netrins function as instructive cues. The double mutant also shows defects in motor axon projections. Expression of either Netrin gene in all muscles also results in aberrant motor projections. Thus, Drosophila Netrins are required for the guidance of commissural axons at the midline, and of motor axons to their target muscles.

Multifunctional epidermal electronics printed directly onto the skin

Materials and designs are presented for electronics and sensors that can be conformally and robustly integrated onto the surface of the skin. A multifunctional device of this type can record various physiological signals relevant to health and wellness. This class of technology offers capabilities in biocompatible, non-invasive measurement that lie beyond those available with conventional, point-contact electrode interfaces to the skin.

Epidermal barrier formation and recovery in skin disorders

Skin is at the interface between the complex physiology of the body and the external, often hostile, environment, and the semipermeable epidermal barrier prevents both the escape of moisture and the entry of infectious or toxic substances. Newborns with rare congenital barrier defects underscore the skin's essential role in a terrestrial environment and demonstrate the compensatory responses evoked ex utero to reestablish a barrier. Common inflammatory skin disorders such as atopic dermatitis and psoriasis exhibit decreased barrier function, and recent studies suggest that the complex response of epidermal cells to barrier disruption may aggravate, maintain, or even initiate such conditions. Either aiding barrier reestablishment or dampening the epidermal stress response may improve the treatment of these disorders. This Review discusses the molecular regulation of the epidermal barrier as well as causes and potential treatments for defects of barrier formation and proposes that medical management of barrier disruption may positively affect the course of common skin disorders.

Alterations in NF-kappaB function in transgenic epithelial tissue demonstrate a growth inhibitory role for NF-kappaB

Stratified epithelium contains a mitotically active basal layer of cells that cease proliferating, then migrate outwards and undergo terminal differentiation. The control of this process, which is abnormal in cutaneous neoplasia and inflammation, is not well understood. In normal epidermis, NF-kappaB proteins were found to exist in the cytoplasm of basal cells and then to localize in the nuclei of suprabasal cells, suggesting a role for NF-kappaB in the switch from proliferation to growth arrest and differentiation. Functional blockade of NF-kappaB by expressing dominant-negative NF-kappaB inhibitory proteins in transgenic murine and human epidermis produced hyperplastic epithelium in vivo. Consistent with this, application of a pharmacologic inhibitor of NF-kappaB to intact skin induced epidermal hyperplasia. In contrast, overexpression of active p50 and p65 NF-kappaB subunits in transgenic epithelium produced hypoplasia and growth inhibition. These data suggest that spatially restricted NF-kappaB activation occurs in stratified epithelium and indicate that NF-kappaB activation in this tissue, in contrast to its role in other settings, is important for cellular growth inhibition.

Skin ageing and its treatment

The effects of chronic sun exposure on skin are readily apparent when skin not typically exposed to the sun and skin regularly exposed to the sun are compared. While the sun is not the only aetiological factor in the dynamic process of skin ageing, it is the primary exogenous cause among several internal and environmental elements. Thus, photo-ageing, the main focus of this article, is a subset of extrinsic skin ageing. The influence of the sun in extrinsic skin ageing, as well as its role in potentially altering the normal course of intrinsic (also known as natural or cellular) ageing, is discussed. Telomeres, the specialized structures found at the ends of chromosomes, are believed to be integral to cellular ageing as well as in the development of cancer. The ageing process, both intrinsic and extrinsic, is also believed to be influenced by the formation of free radicals, also known as reactive oxygen species. The loss of collagen is considered the characteristic histological finding in aged skin. Wrinkling and pigmentary changes are directly associated with photo-ageing and are considered its most salient cutaneous manifestations. Such photodamage represents the cutaneous signs of premature ageing. In addition, deleterious consequences of chronic sun exposure, specifically various forms of photo-induced skin cancer, are also linked to acute and chronic sun exposure. The only known strategies aimed at preventing photo-ageing include sun avoidance, using sunscreens to block or reduce skin exposure to UV radiation, using retinoids to inhibit collagenase synthesis and to promote collagen production, and using anti-oxidants, particularly in combination, to reduce and neutralize free radicals.