Keratinocytes synthesize basal-lamina proteins in culture

Characterization of Pseudogymnoascus destructans conidial adherence to extracellular matrix: Association with fungal secreted proteases and identification of candidate extracellular matrix binding proteins

Pseudogymnoascus destructans is the etiological agent of white-nose syndrome (WNS), a fungal skin infection of hibernating bats. Pathophysiology of the disease involves disruption of bat metabolism and hibernation patterns, which subsequently causes premature emergence and mortality. However, information on the mechanism(s) and virulence factors of P. destructans infection is minimally known. Typically, fungal adherence to host cells and extracellular matrix (ECM) is the critical first step of the infection. It allows pathogenic fungi to establish colonization and provides an entry for invasion in host tissues. In this study, we characterized P. destructans conidial adherence to laminin and fibronectin. We found that P. destructans conidia adhered to laminin and fibronectin in a dose-dependent, time-dependent and saturable manner. We also observed changes in the gene expression of secreted proteases, in response to ECM exposure. However, the interaction between fungal conidia and ECM was not specific, nor was it facilitated by enzymatic activity of secreted proteases. We therefore further investigated other P. destructans proteins that recognized ECM and found glyceraldehyde-3-phosphate dehydrogenase and elongation factor 1-alpha among the candidate proteins. Our results demonstrate that P. destructans may use conidial surface proteins to recognize laminin and fibronectin and facilitate conidial adhesion to ECM. In addition, other non-specific interactions may contribute to the conidial adherence to ECM. However, the ECM binding protein candidates identified in this study highlight additional potential fungal virulence factors worth investigating in the P. destructans mechanism of infection in future studies.

Human skin equivalents: Impaired barrier function in relation to the lipid and protein properties of the stratum corneum

To advance drug development representative reliable skin models are indispensable. Animal skin as test model for human skin delivery is restricted as their properties greatly differ from human skin. In vitro 3D-human skin equivalents (HSEs) are valuable tools as they recapitulate important aspects of the human skin. However, HSEs still lack the full barrier functionality as observed in native human skin, resulting in suboptimal screening outcome. In this review we provide an overview of established in-house and commercially available HSEs and discuss in more detail to what extent their skin barrier biology is mimicked in vitro focusing on the lipid properties and cornified envelope. Further, we will illustrate how underlying factors, such as culture medium improvements and environmental factors affect the barrier lipids. Lastly, potential improvements in skin barrier function will be proposed aiming at a new generation of HSEs that may replace animal skin delivery studies fully.

The Human Tissue-Engineered Cornea (hTEC): Recent Progress

Each day, about 2000 U.S. workers have a job-related eye injury requiring medical treatment. Corneal diseases are the fifth cause of blindness worldwide. Most of these diseases can be cured using one form or another of corneal transplantation, which is the most successful transplantation in humans. In 2012, it was estimated that 12.7 million people were waiting for a corneal transplantation worldwide. Unfortunately, only 1 in 70 patients received a corneal graft that same year. In order to provide alternatives to the shortage of graftable corneas, considerable progress has been achieved in the development of living corneal substitutes produced by tissue engineering and designed to mimic their in vivo counterpart in terms of cell phenotype and tissue architecture. Most of these substitutes use synthetic biomaterials combined with immortalized cells, which makes them dissimilar from the native cornea. However, studies have emerged that describe the production of tridimensional (3D) tissue-engineered corneas using untransformed human corneal epithelial cells grown on a totally natural stroma synthesized by living corneal fibroblasts, that also show appropriate histology and expression of both extracellular matrix (ECM) components and integrins. This review highlights contributions from laboratories working on the production of human tissue-engineered corneas (hTECs) as future substitutes for grafting purposes. It overviews alternative models to the grafting of cadaveric corneas where cell organization is provided by the substrate, and then focuses on their 3D counterparts that are closer to the native human corneal architecture because of their tissue development and cell arrangement properties. These completely biological hTECs are therefore very promising as models that may help understand many aspects of the molecular and cellular mechanistic response of the cornea toward different types of diseases or wounds, as well as assist in the development of novel drugs that might be promising for therapeutic purposes.

Next-Generation Biomaterials for Culture and Manipulation of Stem Cells

Stem cell fate decisions are informed by physical and chemical cues presented within and by the extracellular matrix. Despite the generally attributed importance of extracellular cues in governing self-renewal, differentiation, and collective behavior, knowledge gaps persist with regard to the individual, synergistic, and competing effects that specific physiochemical signals have on cell function. To better understand basic stem cell biology, as well as to expand opportunities in regenerative medicine and tissue engineering, a growing suite of customizable biomaterials has been developed. These next-generation cell culture materials offer user-defined biochemical and biomechanical properties, increasingly in a manner that can be controlled in time and 3D space. This review highlights recent innovations in this regard, focusing on advances to culture and maintain stemness, direct fate, and to detect stem cell function using biomaterial-based strategies.

Microfluidics-based skin irritation test using in vitro 3D angiogenesis platform

A global ban on animal experiments has been proposed. Hence, it is imperative to develop alternative models. Artificial skin models should reflect the responses of subcutaneous blood vessels and the immune system to elucidate disease and identify cosmetics' base materials. Notably, in vivo skin-irritation cascades involve disruption of the epidermal barrier and the release of proinflammatory mediators in response to chemical stimuli. Such proinflammatory factors promote angiogenesis and blood vessel permeability, as observed in irritant contact dermatitis. As an alternative to animal models, we propose a novel skin-irritation model based on a three-dimensional in vitro angiogenesis platform, in which irritated keratinocytes biochemically stimulate vascular endothelial growth factors. Our microfluidic platform hosts interactions between keratinocytes and dermal fibroblasts, which promote angiogenic sprouting. We use sodium lauryl sulfate (SLS) and steartrimonium chloride (SC) as chemical irritants. The irritative effects of SLS and SC are of particular interest due to the ubiquity of both SLS and SC in cosmetics. SLS was observed to significantly affect angiogenic performance, with increasing sprout length. Further promotion of vessel sprouting and lumen formation was observed with 10, 20, and 60 μM of SC, despite its classification as nonirritating and use in supposedly safe formulations. This platform provides an alternative to animal testing as a basis for testing cosmetics and pharmaceutical substances, in addition to serving as a disease model for irritant contact dermatitis.

Heparanase Inhibitors Facilitate the Assembly of the Basement Membrane in Artificial Skin

Recent research suggests that the basement membrane at the dermal-epidermal junction of the skin plays an important role in maintaining a healthy epidermis and dermis, and repeated damage to the skin can destabilize the skin and accelerate the aging process. Skin-equivalent models are suitable for studying the reconstruction of the basement membrane and its contribution to epidermal homeostasis because they lack the basement membrane and show abnormal expression of epidermal differentiation markers. By using these models, it has been shown that reconstruction of the basement membrane is enhanced not only by supplying basement membrane components, but also by inhibiting proteinases such as urokinase and matrix metalloproteinase. Although matrix metalloproteinase inhibitors assist in the reconstruction of the basement membrane structure, their action is not sufficient to promote its functional recovery. However, heparanase inhibitors stabilize the heparan sulfate chains of perlecan (a heparan sulfate proteoglycan) and promote the regulation of heparan sulfate binding growth factors in the basement membrane. Heparan sulfate promotes effective protein-protein interactions, thereby facilitating the assembly of type VII collagen anchoring fibrils and elastin-associated microfibrils. Using both matrix metalloproteinase inhibitors and heparanase inhibitors, the basement membrane in a skin-equivalent model comes close to recapitulating the structure and function of an in vivo basement membrane. Therefore, by using an appropriate dermis model and suitable protease inhibitors, it may be possible to produce skin-equivalent models that are more similar to natural skin

Expression of the High-Affinity Laminin Receptor (67 kDa) in Normal Human Skin and Appendages

The interaction of cells with extracellular matrix components plays a significant role in the regulation of cell biology. Laminin is a large glycoprotein involved in fundamental interactions between cells and the basement membrane. Several cell surface receptors are responsible for cell-matrix interactions. The 67 kDa high affinity laminin receptor, 67LR, is involved in the adhesion of normal cells to the laminin network and is also associated with the metastatic phenotype of some tumoral cells. We have investigated the expression of laminin and of the 67LR in normal human skin using immunoperoxidase staining. Twenty samples of skin were analyzed. Antibody against laminin reacted in a continuous linear band at the dermal-epidermal junction, as well as basement membranes of hair follicles, sebaceous and eccrine sweat glands, and dermal blood vessels. The epidermis and the follicular epithelium were negative for laminin. The 67LR seemed not to be expressed on the basal surface of basal keratinocytes. The major expression of this receptor may be detected in the upper half of the spinous layer and in the granular layer. The cells of the outer root sheath in hair follicle showed the same immunohistochemical pattern described for epidermis. In sebaceous glands and in eccrine sweat glands the secreting epithelium was positive. Endothelial cells of dermal blood vessels were routinely positive for 67LR. We observed that the expression of the 67LR in normal human skin is mostly located in epidermal areas in which the keratinizing process was particularly advanced.

Making more matrix: enhancing the deposition of dermal-epidermal junction components in vitro and accelerating organotypic skin culture development, using macromolecular crowding

Skin is one of the most accessible tissues for experimental biomedical sciences, and cultured skin cells represent one of the longest-running clinical applications of stem cell therapy. However, culture-generated skin mimetic multicellular structures are still limited in their application by the time taken to develop these constructs in vitro and by their incomplete differentiation. The development of a functional dermal-epidermal junction (DEJ) is one of the most sought after aspects of cultured skin, and one of the hardest to recreate in vitro. At the DEJ, dermal fibroblasts and epidermal keratinocytes interact to form an interlinked basement membrane of extracellular matrix (ECM), which forms as a concerted action of both keratinocytes and fibroblasts. Successful formation of this basement membrane is essential for take and stability of cultured skin autografts. We studied interactive matrix production by monocultures and cocultures of primary human keratinocytes and fibroblasts in an attempt to improve the efficiency of basement membrane production in culture using mixed macromolecular crowding (mMMC); resulting ECM were enriched with the deposition of collagens I, IV, fibronectin, and laminin 332 (laminin 5) and also in collagen VII, the anchoring fibril component. Our in vitro data point to fibroblasts, rather than keratinocytes, as the major cellular contributors of the DEJ. Not only did we find more collagen VII production and deposition by fibroblasts in comparison to keratinocytes, but also observed that decellularized fibroblast ECM stimulated the production and deposition of collagen VII by keratinocytes, over and above that of keratinocyte monocultures. In confrontation cultures, keratinocytes and fibroblasts showed spontaneous segregation and demarcation of cell boundaries by DEJ protein deposition. Finally, mMMC was used in a classical organotypic coculture protocol with keratinocytes seeded over fibroblast-containing collagen gels. Applied during the submerged phase, mMMC was sufficient to accelerate the emergence of collagen VII along the de novo DEJ, together with stronger transglutaminase activity in the neoepidermis. Our findings corroborate the role of fibroblasts as important players in producing collagen VII and inducing collagen VII deposition in the DEJ, and that macromolecular crowding leads to organotypic epidermal differentiation in tissue culture in a significantly condensed time frame.

Semaphorin 7a promotes spreading and dendricity in human melanocytes through beta1-integrins

Described as secreted and membrane-bound proteins important for neural pathfinding, the class of proteins called Semaphorins are expressed in multiple tissue types and are involved in diverse biologic processes. In this study, we describe the function of Semaphorin 7a, a membrane-bound Semaphorin known to stimulate neurite outgrowth, on human melanocytes. We show that Semaphorin 7a is expressed by human keratinocytes and fibroblasts in vitro and in vivo and that melanocytes express Plexin C1, a receptor for Semaphorin 7a. Upregulation of Semaphorin 7a was observed in fibroblasts treated with UV irradiation, a potent stimulus for melanocyte dendricity. Because of the importance of melanocyte dendrites in cutaneous photoprotection, we performed functional studies examining the effect of Semaphorin 7a in melanocyte dendrite formation. We also examined the contribution of beta1-integrin and Plexin C1 receptor signaling in mediating effects of Semaphorin 7a in melanocytes. We show that Semaphorin 7a induces significant melanocyte spreading and dendricity in human melanocytes. Furthermore, we show that beta1-integrins and Plexin C1 receptors are ligands for Semaphorin 7a, and that signaling by these receptors has opposing effects on Semaphorin 7a-induced dendrite formation.

Engineering skin to study human disease--tissue models for cancer biology and wound repair

Recent advances in the engineering of three-dimensional tissues known as skin equivalents, that have morphologic and phenotypic properties of human skin, have provided new ways to study human disease processes. This chapter will supply an overview of two such applications--investigations of the incipient development of squamous cell cancer, and studies that have characterized the response of human epithelium during wound repair. Using these novel tools to study cancer biology, it has been shown that cell-cell interactions inherent in three-dimensional tissue architecture can suppress early cancer progression by inducing a state of intraepithelial dormancy. This dormant state can be overcome and cancer progression enabled by altering tissue organization in response to tumor promoters or UV irradiation or by modifying the interaction of tumor cells with extracellular matrix proteins or their adjacent epithelia. By adapting skin equivalent models of human skin to study wound reepithelialization, it has been shown that several key responses, including cell proliferation, migration, differentiation, growth-factor responsiveness and protease expression, will mimic the response seen in human skin. In this light, these engineered models of human skin provide powerful new tools for studying disease processes in these tissues as they occur in humans.

Organotypic cocultures as skin equivalents: A complex and sophisticated in vitro system

To assess the role of genes required for skin organogenesis, tissue regeneration and homeostasis, we have established in vitro skin equivalents composed of primary cells or cell lines, respectively. In these organotypic cocultures keratinocytes generate a normal epidermis irrespective of the species and tissue origin of fibroblasts. The combination of cells derived from mouse and human tissues facilitates the identification of the origin of compounds involved in epidermal tissue reconstitution and thus the precise analysis of growth regulatory mechanisms.

Analysis of microenvironmental factors contributing to basement membrane assembly and normalized epidermal phenotype

To understand further the role of the dynamic interplay between keratinocytes and stromal components in the regulation of the growth, differentiation, morphogenesis, and basement membrane assembly of human stratified squamous epithelium, we have generated novel, three-dimensional organotypic cultures in which skin keratinocytes were grown in the absence or presence of pre-existing basement membrane components and/or dermal fibroblasts. We found that keratinocytes cultured in the presence of pre-existing basement membrane components and dermal fibroblasts for 9 d showed rapid assembly of basement membrane, as seen by a nearly complete lamina densa, hemidesmosomes, and the polarized, linear distribution of laminin 5 and a6 integrin subunit. Basement membrane assembly was somewhat delayed in the absence of dermal fibroblasts, but did occur at discrete nucleation sites when pre-existing basement membrane components were present. No basement membrane developed in the absence of pre-existing basement membrane components, even in the presence of dermal fibroblasts. Bromodeoxyuridine incorporation studies showed that early keratinocyte growth was independent of mesenchymal support, but by 14 d, both fibroblasts and assembled basement membrane were required to sustain growth. Normalization of keratinocyte differentiation was independent of both dermal fibroblasts and structured basement membrane. These results indicated that epithelial and mesenchymal components play a coordinated role in the generation of structured basement membrane and in the regulation of normalized epithelial growth and tissue architecture in an in vitro model of human skin.

Basement membrane formation during wound healing is dependent on epidermal transplants

The purpose of the study was to compare directly the effect of healing and the formation of the basement membrane during wound healing from two autologous primary keratinocyte cultures in the liquid environment in full-thickness wounds in pigs. Wounds were either transplanted with cultured epidermal autografts (n = 26) or autologous keratinocyte suspensions (n = 24) or treated with saline alone (n = 40) and covered with a chamber. All wounds transplanted with cultured epidermal autografts and keratinocyte cell suspensions had positive "take" after transplantation. Healing times were significantly shorter for wounds treated with either cultured epidermal autografts or keratinocyte suspensions (p = 0.0001) compared with saline-treated wounds but were not different from each other (p = 0.1835). There were no differences in cytokeratin and laminin expression; however, staining with monoclonal antibody against collagen type VII showed a lower signal for cultured epidermal autografts only on days 8 and 16 compared with keratinocyte suspensions. Electron microscope evaluation showed a higher incidence of anchoring fibrils and a more mature dermal-epidermal junction in wounds treated with keratinocyte cell suspensions at day 8. These findings may be due to the single, noncontact-inhibited cells and the early formation of an in vivo neodermis to the wet wound environment. These data suggest that wounds transplanted with autologous keratinocyte suspensions in a wet environment may be an alternative method in the treatment of wounds.

Characterization of a new tissue-engineered human skin equivalent with hair

We designed a new tissue-engineered skin equivalent in which complete pilosebaceous units were integrated. This model was produced exclusively from human fibroblasts and keratinocytes and did not contain any synthetic material. Fibroblasts were cultured for 35 d with ascorbic acid and formed a thick fibrous sheet in the culture dish. The dermal equivalent was composed of stacked fibroblast sheets and exhibited some ultrastructural organization found in normal connective tissues. Keratinocytes seeded on this tissue formed a stratified and cornified epidermis and expressed typical markers of differentiation (keratin 10, filaggrin, and transglutaminase). After 4 wk of culture, a continuous and ultrastructurally organized basement membrane was observed and associated with the expression of laminin and collagen IV and VII. Complete pilosebaceous units were obtained by thermolysin digestion and inserted in this skin equivalent in order to assess the role of the transfollicular route in percutaneous absorption. The presence of hair follicles abolished the lag-time observed during hydrocortisone diffusion and increased significantly its rate of penetration in comparison to the control (skin equivalent with sham hair insertion). Therefore, this new hairy human skin equivalent model allowed an experimental design in which the only variable was the presence of pilosebaceous units and provided new data confirming the importance of hair follicles in percutaneous absorption.

Tissue engineering of skin

The skin plays a crucial role in protecting the integrity of the body's internal milieu. The loss of this largest organ is incompatible with sustained life. In reconstructive surgery or burn management, substitution of the skin is often necessary. In addition to traditional approaches such as split- or full-thickness skin grafts, tissue flaps and free-tissue transfers, skin bioengineering in vitro or in vivo has been developing over the past decades. It applies the principles and methods of both engineering and life sciences toward the development of substitutes to restore and maintain skin structure and function. Currently, these methods are valuable alternatives or complements to other techniques in reconstructive surgery. This review article deals with the evolution and current approaches to the development of in vitro and in vivo epidermis and dermis.

The co-culture of dermal fibroblasts with human epidermal keratinocytes induces increased prostaglandin E2 production and cyclooxygenase 2 activity in fibroblasts

During wound healing, cell-cell interactions between epidermal keratinocytes and dermal fibroblasts contribute to the organization of epidermis, in which prostaglandin E2 (PGE2) is considered to be involved in proliferation and differentiation of keratinocytes. In the current study, we investigated the regulation of PGE2 biosynthesis in co-culture of human epidermal keratinocytes and human dermal fibroblasts. The production of PGE2 was synergistically enhanced in the co-culture at cell ratios of keratinocytes to fibroblasts between 1/8 and 4, whereas the production of PGE2 was negligible in individual monolayer cultures of keratinocytes or fibroblasts. To address the mechanism of PGE2 production induced by the co-culture of keratinocytes and fibroblasts, we asked whether either cell-derived soluble factor(s) or direct cell-cell contact was required to augment the production of PGE2. Neither the fibroblast-conditioned medium nor membrane fractions influenced the production of PGE2 in keratinocytes. Keratinocyte-conditioned medium greatly enhanced the production of PGE2 in fibroblasts, however, whereas the effect of keratinocyte-membrane fractions was weaker. The main soluble fraction in the keratinocyte-conditioned medium contained a precursor of interleukin-1 alpha (proIL-1alpha) by western blot analysis, and PGE2 production was inhibited by anti-IL-1alpha antibody, but not by anti-IL-1beta or by anti-tumor necrosis factor-alpha antibody. The enhanced production of PGE2 in fibroblasts upon culturing with keratinocytes was due to the induction of COX-2 mRNA mediated by proIL-1alpha released from keratinocytes. These results suggest that cell-cell interactions of keratinocytes and fibroblasts augment the production of PGE2 by a mechanism in which the activity of COX-2 in fibroblasts is increased by the keratinocyte-derived proIL-1alpha in a paracrine manner.

Keratinocytes contract human dermal extracellular matrix and reduce soluble fibronectin production by fibroblasts in a skin composite model

Composites of human de-epidermised acellular dermis and normal adult human keratinocytes and fibroblasts were examined for the ability of cells to contract these composites. Image analysis of the outline of the composites showed that, in this model, keratinocytes alone or in the presence of fibroblasts caused highly significant contraction (of the order of 25% by day 12). There was no significant contraction of the dermis with fibroblasts alone or in the absence of cells. The presence or absence of basement membrane antigens did not influence the effect of keratinocytes on dermal contraction. Analysis of the conditioned media from these composites showed that the greatest fibronectin production was seen with fibroblasts alone in the presence of basement membrane. Keratinocytes alone produced little fibronectin irrespective of the presence of the basement membrane. If keratinocytes were present with fibroblasts, however, then fibronectin production was significantly reduced both in the presence and absence of the basement membrane, indicating that keratinocytes modify dermal fibroblast extracellular matrix production. This study shows that while keratinocytes and fibroblasts are clearly influencing each other's activity in this human skin composite model, under the circumstances we describe it is the keratinocyte and not the fibroblast which causes contraction of the human de-epidermised acellular dermis.

Disruption of microfilaments alters laminin synthesis but not laminin trafficking in NHEK in vitro

Laminin synthesis and deposition are concomitant with the development of a basal lamina between the human epidermis and the underlying dermis. One of the challenges in tissue engineering of human epidermal models is to develop substrates and conditions that encourage the development of a basement membrane. The purpose of this study was to determine if actin filaments and/or microtubules are involved in the synthesis/secretion of laminin by normal human epidermal keratinocytes (NHEK) in vitro. NHEK synthesize and secrete laminin subunits B1, B2, and M but little, if any, of laminin subunit A. Data indicate that disruption of microfilaments by the destabilizing agent, cytochalasin D, had no apparent effect on the relative synthesis rates of most cytosolic proteins as revealed by one-dimensional sodium dodecyl sulfate (SDS) gel electrophoresis. This drug, however, increased laminin B2 synthesis several fold over untreated controls. This enhanced synthetic rate was independent of the type of collagen matrix on which the NHEK were grown. Similar increases in synthesis of the M and B1 laminin chains were not observed. To determine if this increase in synthesis lead to increases in laminin B2 secretion, laminin B2 was immunoprecipitated from both the apical and basal domains of NHEK cells grown on microporous membranes. While more laminin B1, B2, and M were secreted basally than apically, an observation consistent with laminin's role in basal lamina formation, cytochalasin D had no apparent effect on either basal or apical laminin B2 secretion. Experiments with the microtubule destabilizer, nocodazole, showed no similar effects on laminin synthesis and/or secretion. We conclude that (a) disruption of the actin network in NHEK selectively increases the synthesis of laminin B2, (b) the secretion of laminin B2 from NHEK cells is not governed by either the microfilamentous cytoskeleton or the amount of laminin synthesized by NHEK, and (c) disruption of the microtubular network does not alter laminin synthesis or secretion.

Early basement membrane formation following the grafting of cultured epidermal sheets detached with thermolysin or Dispase

The basement membrane zone is important for graft adhesion and stability. The aim of the present study was to visualize the regeneration of the basement membrane and determine the sequential appearance of its constituents in the early postgrafting period of cultured human epidermal sheets. A keratinocyte single cell suspension, devoid of dermal fibroblast contamination, was obtained from human skin by a two-step tissue digestion method with thermolysin and trypsin. After culturing, epidermal sheets were generated, detached enzymatically by incubating with thermolysin (for 20-30 min) or Dispase (for 45-60 min), and deposited on a muscular graft bed of athymic mice. Immunohistochemistry and ultrastructural analyses were performed on biopsies harvested 2, 4 and 21 days postgrafting. Bullous pemphigoid antigens and laminin were detected at the dermo-epidermal junction, showing an almost continuous line 2 days postgrafting. Type IV collagen was generally absent at this time, but it was detected 4 days postgrafting. Type VII collagen was labelled as a discontinuous line of increasing intensity from 2 to 21 days postgrafting. Ultrastructural analysis revealed hemidesmosomes and a discontinuous lamina densa 2 days postgrafting, and a complete basement membrane with a continuous lamina densa, hemidesmosomes and anchoring fibrils 21 days postgrafting. The sequence of appearance of major basement membrane components was similar for cultured sheets detached with thermolysin or Dispase. However, it differed from that of other wound healing models. Results are discussed in terms of the variable keratinocyte migration requirement between various wound healing models.

Expression of laminin, type IV procollagen and 230 kDa bullous pemphigoid antigen genes by keratinocytes and fibroblasts in culture: application of the polymerase chain reaction for detection of small amounts of messenger RNA

In order to clarify whether keratinocytes and/or fibroblasts express genes encoding basement membrane zone macromolecules, we examined laminin, type IV collagen and 230 kDa bullous pemphigoid antigen (BPAG1) gene expression in keratinocytes and fibroblasts in culture. Northern transfer analysis revealed the presence of specific mRNA transcripts for alpha 1(IV) and alpha 2(IV) chains of type IV collagen as well as B1 and B2 chains of laminin in both fibroblast and keratinocyte RNA. Laminin A mRNA, however, was detected in fibroblasts but not in keratinocytes. In contrast, BPAG1 mRNA was detected in keratinocytes but not in fibroblasts using the same RNA preparations. A polymerase chain reaction (PCR) using laminin A and BPAG1-specific primers produced amplified DNAs with the predicted sizes in reverse-transcripted cDNA derived from keratinocyte and fibroblast RNA, respectively. These results provide evidence that normal human skin keratinocytes and fibroblasts express genes encoding laminin A, B1, B2, alpha 1(IV), and BPAG1 at a steady-state level. Moreover, the PCR for detecting small amounts of mRNA suggested that both keratinocytes and fibroblasts can be utilized for the analysis of DNA mutations in inherited skin diseases affecting the basement membrane zone, such as epidermolysis bullosa.

The human keratinocyte two-dimensional gel protein database: update 1993

The master two-dimensional gel database of human keratinocytes currently lists 3038 cellular proteins (2127 isoelectric focusing, IEF; and 911 nonequilibrium pH gradient electrophoresis, NEPHGE) many of which correspond to post-translational modifications. 763 proteins have been identified (protein name, organelle components, etc.) and they are listed both in alphabetical order and with increasing SSP number, together with their M(r), pI, cellular localization and credit to the investigator(s) that aided in the identification. Furthermore we have listed 176 proteins that have been microsequenced so far and that are recorded in this database. We also include synthetic images depicting some interesting sets of proteins identified so far; these include components of hnRNP's, proteasomes or prosomes, ribosomes, as well as assorted organelle markers, GTP-binding proteins, calcium binding proteins, stress proteins, autoantigens, differentiation markers and psoriasis upregulated proteins. The aim of the comprehensive database is to gather, through a systematic study of keratinocytes, qualitative and quantitative information on proteins and their genes that may allow us to identify abnormal patterns of gene expression and ultimately to pinpoint signaling pathways and components affected in various skin diseases, cancer included.

Production of basement membrane components by a reconstructed epidermis cultured in the absence of serum and dermal factors

A fully differentiated epithelium displaying features of human epidermis was obtained in vitro by culturing second-passage normal human keratinocytes for 14 days in defined medium and on an inert polycarbonate filter substratum at the air-liquid interface. Vertical sections stained for histology and indirect immunofluorescence studies showed that the 'basal' cells synthesize and secrete all major markers of hemidesmosomes and the lamina lucida. Components of the lamina densa are also expressed. Collagen VII is synthesized, but not secreted. Ultrastructural studies showed the presence of hemidesmosomes with major dense plaques and anchoring filaments, and a basement membrane-like structure was clearly identified. These results show that epidermal cells are able to produce hemidesmosomes and to secrete the major components of the dermo-epidermal junction in the absence of serum and dermal factors, suggesting that basement membrane synthesis and hemidesmosome assembly are not dependent on the presence of dermis.

A Human Epidermal Model that can be Used in an Automated Multiple Endpoint Assay

Normal human epidermal keratinocytes were differentiated on Millipore Millicell CM microporous membranes overlaid with an acellular cross-linked collagen gel. The extracellular calcium concentration was elevated, and stratification of the monolayer to a 20-cell thick multilayer ensued within 5–7 days. Transmission electron microscopy of this stratified, human epidermal model (HEM) revealed typical ultrastructural markers of differentiation, including numerous desmosomes, keratinohyalin granules, keratin filaments, lamellar-type bodies, and squamous-shaped cells in the apical region, but not the basal region, of this HEM. Preparations lifted to the air–liquid interface had electron-dense apical cells suggestive of cornification. An electron-dense lamina appeared on the basal surface in both submerged and air–liquid interface cultures, suggesting that this structure can be produced without the help of underlying dermal cells and is independent of the air–liquid interface conditions. The uniform thickness of the epidermis, the absence of dermal fibroblasts, the lack of contraction of the epidermis, and the thinness of the underlying collagen gel, made this HEM a suitable candidate for use in an automated, fluorescence multiple endpoint assay. Fluorescent probes monitoring lysosomal integrity, epidermal permeability and plasma membrane integrity were monitored using the CytoFluor 2300 fluorescent plate reader. Our data suggest the feasibility of using multilayered human tissues for automated multiple endpoint analysis.

Altered distribution of 1-2B7B antigen in basal cell carcinoma, squamous cell carcinoma and Bowen's disease

Skin lesions of basal cell carcinoma (BCC), squamous cell carcinoma (SCC) and Bowen's disease were immunohistochemically examined using the 1-2B7B monoclonal antibody, which recognizes a 120 kDa polypeptide component found in hemidesmosomes of normal human epidermis and hemidesmosome-like adhesion junction of vascular endothelial cells, to disclose altered characteristics of the interface between the tumor cell aggregate and stromal tissues of the epidermal neoplasms. In BCC, 1-2B7B antigen was rarely expressed at the tumor cell aggregate-stromal tissue interface, where poorly developed hemidesmosometonofibril complexes and a normal-looking lamina densa were detectable. In SCC and Bowen's disease, 1-2B7B antigen was expressed not only along the interface of the tumor nest and stromal tissue, but also in the intercellular space of the desmosomes and other adhesion junction structures that lack associating tonofibrils. In the invading front of SCC, 1-2B7B antigen had partly disappeared from the tumor cell aggregate-stromal tissue interface, where neither hemidesmosomes nor lamina densa were noted. The altered distribution of this hemidesmosomal component in the epidermal neoplasms seems to reflect aberrant interaction of neoplasmic cells and surrounding stromal tissue.

A comprehensive two-dimensional gel protein database of noncultured unfractionated normal human epidermal keratinocytes: towards an integrated approach to the study of cell proliferation, differentiation and skin diseases

A two-dimensional (2-D) gel database of cellular proteins from noncultured, unfractionated normal human epidermal keratinocytes has been established. A total of 2651 [35S]methionine-labeled cellular proteins (1868 isoelectric focusing, 783 nonequilibrium pH gradient electrophoresis) were resolved and recorded using computer-aided 2-D gel electrophoresis. The protein numbers in this database differ from those reported in an earlier version due to changes in the scanning hardware (Celis et al., Electrophoresis 1990, 11, 242-254). Annotation categories reported include: "protein name" (listing 207 known proteins in alphabetical order), "basal cell markers", "differentiation markers", "proteins highly up-regulated in psoriatic skin", "microsequenced proteins" and "human autoantigens". For reference, we have also included 2-D gel (isoelectric focusing) patterns of cultured normal and psoriatic keratinocytes, melanocytes, fibroblasts, dermal microvascular endothelial cells, peripheral blood mononuclear cells and sweat duct cells. The keratinocyte 2-D gel protein database will be updated yearly in the November issue of Electrophoresis.

Cytokines alter mRNA steady state levels for basement membrane proteins in human skin fibroblasts

Keratinocytes and fibroblasts synthesize basement membrane proteins and even contribute to the formation of basement membrane structures following injury or tissue damage. Under these conditions many cellular functions are regulated by mediators e.g. transforming growth factor-beta, tumor necrosis factor alpha, interferon-gamma or interleukin-1 alpha. We therefore describe here their influence on synthesis of basement membrane proteins in human skin fibroblasts. A comparative analysis of mRNA steady levels coding for BM-40, nidogen, laminin B1 and B2 chains and collagen IV in fibroblasts, in primary human keratinocytes and a epidermal cell line grown in monolayer culture demonstrated that the highest amounts were present in human fibroblasts. Interferon-gamma reduces all mRNA steady state levels dose dependently in comparison to the control, while transforming growth factor-beta simultaneously induces BM-40, alpha 1 and alpha 2 (IV) collagen mRNAs. TGF-beta, however, has no effect on nidogen and laminin mRNA levels. Interleukin-1 alpha and tumor necrosis factor alpha do not affect the mRNA levels of most basement membrane proteins. However, the alpha 1 (IV) collagen mRNA is upregulated by both cytokines to 300%. These data demonstrate a specific control of the expression of several basement membrane proteins by cytokines and indicate that fibroblasts could contribute to basement membrane formation during wound healing and tissue repair.

Healing of full-thickness cutaneous wounds in the pig. I. Immunohistochemical study of epidermo-dermal junction regeneration

In order to determine the kinetics of epidermo-dermal junction (EDJ) regeneration during would healing, we studied the regeneration of five EDJ components during reepidermization. Cutaneous wounds (50-mm length, 2-mm width, and 5-mm depth) were produced on the flank area of two pigs and left unsutured. Daily biopsies from day 1 to day 20 were studied by light microscopy on paraffin-embedded sections and by indirect immunofluorescence on cryostat sections using human sera to bullous pemphigoid antigen (BPA) with specificity previously confirmed by indirect immuno-electron microscopy, rabbit antisera to type IV collagen (Coll IV) and to fibronectin, and the monoclonal antibodies (MoAb) 4C 12-8 to laminin and NP-76 to type VII collagen (Coll VII). Histologically, reepidermization started from day 1 and progressed unidirectionally and exclusively from the wound edges. Up to day 9, the distal tips of the neo-epidermal tongues generally extended between the crust and the granulation tissue (GT). They fused on day 10, restoring epidermal continuity. For each EDJ component, the date of appearance (emergence), the spreading under the neo-epidermis tongue (expression), and the morphologic aspect of the labeling were studied. BPA and Coll IV were detected from day 1 to day 20 and found to be expressed all along the neo-EDJ. Fibronectin and laminin were detected from day 1, were present in the proximal and median zones of the neo-EDJ before day 7, up to the distal tip from day 7 to day 9 and were all along the neo-EDJ from day 10 to day 20. Coll VII was only detected from day 3. It was present in the proximal zone on day 3 and day 4, in the proximal and median zones on day 5 and day 6, than all along the neo-EDJ from day 7 to day 20. From day 10, all the labeling characteristics of the five components were found to be similar in the neo-EDJ and in the normal EDJ. With regard to the neo-epidermis progression, we found a synchronism of emergence and expression for BPA and Coll IV, a synchronism of emergence but a delay of expression for fibronectin and laminin and lastly, a delay of emergence and expression for Coll VII. We concluded that BPA and Coll IV could constitute the framework on which the neo-EDJ is progressively built by adjunction of the other components, restitution being obtained just after epidermal continuity is restored.

In vitro and post-transplantation differentiation of human keratinocytes grown on the human type IV collagen film of a bilayered dermal substitute

Using human type IV and type I + III collagens and a new, nontoxic cross-linking procedure, we have developed a cell-free bilayered human dermal substitute for organotypic culture and transplantation of human skin keratinocytes. We have studied the formation of the basement membrane, and the differentiation of keratinocytes grown on the type IV collagen layer of this dermal substitute, in vitro and after grafting onto nude mice. These studies demonstrated the formation of essential constituents of the basement membrane in culture: hemidesmosomes and deposition of extracellular matrix on the top of the type IV collagen were observed as early as 6 days after plating of human keratinocytes. Although the keratinocytes formed a well-organized multilayered epithelium, they exhibited limited differentiation when grown submerged in liquid medium. However, the multilayered sheet obtained after 14 days in submerged culture was composed of a regular basal cell layer, several nucleated suprabasal cell layers containing granular cells, and several dense, anucleated cell layers. The grafting experiments have shown a good biocompatibility of the dermal substitute. It is repopulated by fibroblasts, newly synthesized collagen, vessels, and a few mononuclear cells. At Day 14 after grafting, the type IV collagen layer was still present and very dense, and the basement membrane appeared as in culture, with numerous well-structured hemidesmosomes and deposition of extracellular matrix resembling lamina densa. At Day 55 after transplantation, even if the epidermal graft did not exhibit all the characteristics of the normal epidermis in vivo, it was very close to it. At this stage, the basement membrane was complete, with structures clearly indicative of anchoring fibrils. This new dermal substitute offers many advantages. It is stable and easy to handle. Its production is standardized. The oxidation induced by periodic acid led to a nontoxic cross-linked matrix. This dermal substitute is the first one entirely composed of human collagens. The type I + III collagen underlayer is reorganized when grafted. It supports a type IV collagen top layer which offers an excellent substrate for keratinocytes, favors their anchorage, and favors the formation of the basement membrane in vitro. This dermal substitute could be useful for wound coverage or as an in vitro model for toxicological and pharmacological studies.

Effect of decidua angiogenic factors on experimental dermis allografts

Meshed human dermis allografts containing human uterine angiogenic factor were implanted over full thickness surgical skin wounds in rats. Enhanced angiogenesis of the wound bed, three times more than that observed in control grafts, was found in experimental grafts at 3 days postimplantation. This was accompanied by a greater amount of granulation tissue formation, enhanced granulation tissue penetration into the dermal grafts and accelerated incorporation of the grafts. Angiogenesis of the wound bed regressed to control levels at 7 days postimplantation. The growth and penetration of the granulation tissue and the accelerated incorporation of the dermis grafts, however, continued ahead in the experimental grafts while necrosis appeared in the control grafts. The relevance of these results for acceleration of wound healing and improvement of the wound bed for subsequent application of cultured epidermal grafts is discussed. This method may be extended to the treatment of ulcers and burns.

Differential expression of type IV procollagen and laminin genes by fetal vs adult skin fibroblasts in culture: determination of subunit mRNA steady-state levels

Basement membrane zone gene expression by fibroblast cultures, established from individuals varying in age from 14 fetal weeks to 61 years, was examined by molecular hybridizations with human sequence specific cDNAs corresponding to type IV procollagen and laminin subunit polypeptides. Northern transfer analysis with poly(A)+RNA revealed the presence of specific mRNA transcripts for alpha 1(IV) and alpha 2(IV) chains of type IV procollagen as well as B1 and B2 chains of laminin. Laminin A chain mRNAs were not detected using the same RNA preparations. Quantitative estimates of the steady-state levels of type IV procollagen and laminin mRNAs indicated that they were of relatively low abundance, as compared with mRNA to type I procollagen. The expression of alpha 1(IV) and alpha 2(IV) collagen genes was high in fetal fibroblasts but was reduced to low, yet detectable, levels in cultures established from 3-d to 61-year-old individuals. In contrast, the laminin B1 and B2 chain mRNA levels showed little age-associated variation within the cultures examined. These results provide evidence for differential regulation of the expression of different basement membrane zone macromolecules during chronologic aging.

Bullous pemphigoid and cicatricial pemphigoid: immunoblotting detection of involved autoantigens

Bullous pemphigoid (BP) and cicatricial pemphigoid (CP) are subepidermal bullous autoimmune diseases which have distinct clinical features but identical immunological status. In order to determine whether these diseases could be dissociated on the basis of qualitative differences in serum antibodies to basement membrane zone (BMZ) antigens, the reactivity of sera from 7 CP and 29 BP patients with proteins extracted from normal human epidermal sheets (containing most of the lamina lucida components) was analysed using immunoblotting and compared to that of 10 normal sera. 20 out of the 29 BP sera contained antibodies recognizing one or several protein(s) of 240, 200, 180 and 165 kD molecular weight (MW). Antibodies in 4 out 7 CP sera specifically reacted with one or two polypeptides of 240 and 120 kD MW. These data confirm the heterogeneity of BP antigens and show the presence in CP of a novel 120 kD MW polypeptide which is found only in CP but not in BP. Taken together these findings demonstrate that in BP and CP, autoantibodies are directed to both common and specific BMZ antigens, their physiopathological significance need to be understood.

Reconstitution of structure and cell function in human skin grafts derived from cryopreserved allogeneic dermis and autologous cultured keratinocytes

Grafts of allogeneic dermis plus autologous epidermal cell cultures were used to replace extensively burned skin. Cryopreserved split-thickness cadaveric skin was grafted onto debrided burn wound, and autologous keratinocytes were cultured from uninjured donor sites. Several weeks later, allograft epidermis was abraded and replaced with the keratinocyte cultures. The final grafts were thus composites of autologous cultured epidermis and allogeneic dermis. In a case with 28 months follow-up, reconstitution of the dermal-epidermal (BMZ.1) and microvascular (BMZ.2) basement membrane zones was studied immunohistochemically and ultrastructurally. Immediately before grafting, thawed cryopreserved skin reacted with antibodies against laminin and type IV collagen in normal patterns. Twenty-nine days after grafting, BMZ.1 reacted weakly with both antibodies, and anticollagen type IV reactivity was absent from BMZ.2. Antilaminin reactivity of BMZ.2, however, was moderately intense, consistent with recent neovascularization. On day 29, the allograft epidermis was replaced with autologous keratinocyte cultures. Twenty-five days later (54 d after allografting), staining of both BMZs was intense with both antibodies. Ultrastructurally, at day 76 (47 d after culture placement) BMZ.1 revealed only small hemidesmosomes, few incipient anchoring fibrils, and a discontinuous lamina densa. BMZ.2, however, was fully reconstituted. By 124 d, both BMZs appeared normal. Observations in the dermis at 76 d included the presence of lymphocytes, organellar debris, and hyperactive collagen fibrillogenesis, all indicative of dermal remodelling. The microvasculature was well differentiated, but no elastic fibers or nerves were found. In the epidermis, melanocytes and evidence of melanosome transfer were seen at 5, 47, and 95 d after grafting of keratinocyte cultures. We conclude that the composite procedure reconstitutes skin with excellent textural and histologic qualities.

Growth of stratified squamous epithelium on reconstituted extracellular matrices: long-term culture

Oral keratinocytes grown at an air-liquid interface on stabilized matrices of collagen or a basement membrane exhibit a pattern of tissue organization more similar to the parent tissue than the same cells cultured conventionally. An orderly sequence of cell migration and differentiation is maintained, and the full complement of terminally differentiated cells is retained on the surface of the culture for up to 65 days following subculture. The pattern of histodifferentiation of cultured stratified squamous epithelium differs according to the matrix upon which it is grown. Pliant, fine meshed gels of type III collagen are corrugated by the cultured keratinocytes with adjustments occurring in the various suprabasal cell strata that result in the retention of a flat stratum corneum. Such pliant gels can be stabilized by pouring a supporting underlayer of coarse type I guinea pig collagen. Keratinocytes grown directly on the irregular surface of guinea pig type I collagen migrate into spaces between collagen fibrillar bundles and aberrantly keratinize 20-30 days following subculture. Keratinocytes grown on a basement membrane do not aberrantly keratinize, suggesting that contact with a basement membrane may suppress signals for keratinocyte differentiation. Keratinocytes also form hemidesmosomes opposite a basement membrane but not opposite collagen fibrils. The keratin pattern of oral keratinocytes cultured in different configurations does not change; a finding that indicates that a greater degree of tissue organization does not automatically result in the synthesis of keratins more characteristic of upper cell strata or cornified cells in the native tissue.

Basement membrane formation by malignant mouse keratinocyte cell lines in organotypic culture and transplants: correlation with degree of morphologic differentiation

Six malignant C3H mouse epidermal cell lines (HEL-30, HEL-37, HELP I, HELP IV, HD II, H3L), with different capacities for epidermal differentiation, were analyzed for their organized growth behavior and basement membrane (BM) formation in organotypical cultures in vitro and after transplantation into syngeneic mice. Expression and deposition of five BM components (type IV collagen, laminin, bullous pemphigoid antigen, fibronectin, heparan sulfate proteoglycan) were determined on frozen sections by indirect immunofluorescence. Additionally, synthesis and secretion of BM components by the line HEL-30 (in submersed cultures) were identified by metabolic labeling and immunoprecipitation. Morphologic differentiation features and formation of a structured BM were studied by electron microscopy. All cell lines were tumorigenic and invasive but nevertheless able to synthesize BM constituents in vitro and in vivo, although pronounced variations in the expression and the polarity and continuity of deposition were found. Irrespective of the amount of BM components synthesized, none of the cell lines formed a structured BM in organotypical cultures in vitro. After transplantation the production of BM components was improved and the newly formed epithelia were separated from the mesenchyme by a structured BM. The formation of BM occurred whether the epithelial cells were in immediate contact with the mesenchyme or separated by a 1 to 2 mm thick native collagen gel. Deposition of BM constituents and formation of BM structures occurred both at the superficial epithelium and around invading cell cords. The studies clearly demonstrated that malignant epidermal cells maintain their capacity to synthesize BM components. The extent of production and the polarity of deposition of the constituents and the quality of BM formation were usually higher in well differentiated cell lines and obviously correlated well with their preserved degree of differentiation. Comparable to normal keratinocytes, formation of structured BM requires interaction with living mesenchyme but occurs independently of direct epidermal-mesenchymal contact.

The growth and structure of human oral keratinocytes in culture

Human keratinocytes derived from explants of cheek (buccal) mucosa grow vigorously in culture and can be subcultivated twice. The structure of the oral keratinocytes in vitro is the same in primary cultures and subcultures. The cells stratify, are characterized by well-developed tonofibrillar-desmosomal complexes, and rarely exhibit signs of terminal differentiation. Unique features of the culture system that favor keratinocyte growth are: incubation at 34 degrees C, inclusion of 0.5% dimethyl sulfoxide in the culture medium, and initiating subcultures as 5.0 mm colonies containing 100,000/20 microliter of medium. One primary culture can yield 6 first-passage subcultures, which subsequently achieve confluence in 10-12 days. Such cultures are a useful source of human keratinocytes that stratify but generally do not undergo terminal differentiation.

The role of epithelial cell differentiation in the expression of herpes simplex virus type 1 in normal human oral mucosa in culture

We have examined by immunofluorescent antibody staining technique the expression of herpes simplex virus type 1 (HSV-1) in organ cultures of the normal human oral mucosa. The expression of HSV-1 antigen was found selectively in the epithelial cell layers in relatively undifferentiated states such as basal layer and lower prickle cell layer in addition to the basement membrane. When the epithelial cells dissociated from the oral mucosa were infected with HSV-1 and association of the HSV-1 expression with the cellular differentiation was examined, the epithelial cells containing laminin in an undifferentiated state were permissive for the expression of HSV-1 antigen whereas terminally differentiated epithelial cells with the cornified envelope did not express HSV-1 antigen. These findings indicate that the expression of HSV-1 antigen is restricted in the mucosal epithelial cells in a differentiated state, although the possibility that the cornified envelope might protect the cells from infection is not excluded.

Expression of anionic sites at the dermoepibolic junction

The emergence of anionic sites during basement membrane zone formation was studied using migrating epidermis in organ culture as a model system. Ultrastructural investigations using a strongly cationized probe revealed that the heparitinase-sensitive, anionic sites were formed synchronously with the newly built basal lamina after 7 days in culture.

Immunohistochemical studies of basal cell carcinomas transplanted into nude mice

Xenografting into nude mice forms a system for analysis of human tissues under experimental conditions. In this study, normal skin samples and basal cell carcinomas were investigated, prior to and after transplantation, using immunofluorescence methods with antibodies against keratins, laminin, and collagen type IV. Three groups of transplants were studied: intact tissue samples, human epithelium (either normal or neoplastic) recombined with normal human dermis and, human epithelium recombined with normal mouse dermis. Transplants recovered after 3 weeks showed the following characteristics. The xenograft system was satisfactory in terms of host survival and rate of successful tissue recovery except for recombinants between human epithelium and mouse dermis. Intact and recombined samples of normal skin retained their preexisting patterns of architecture, cytodifferentiation, and basement membrane staining. Solid nonfibrosing basal cell carcinomas showed altered architecture and differentiation of both the epithelium and the basement membrane zone after transplantation: the solid tumor pattern changed towards spreading of tumor cells, a more squamous differentiation pattern was apparent and was confirmed by reactivity with antibodies against large keratins. Discontinuities of the basement membrane zone were detected with antibodies against laminin and collagen type IV. These changes were seen in both intact and recombined tumor transplants.

Ultrastructural morphometry of normal human dermal-epidermal junction. The influence of age, sex, and body region on laminar and nonlaminar components

To obtain baseline data for future studies on such processes as wound healing, carcinogenesis, and blistering, a morphometric analysis of the dermal-epidermal junction was undertaken on normal skin from 3 or 4 standard sites on the arm and leg of 12 subjects aged 20-60 years. Lamina densa was thinner in females than in males (p less than 0.01) but no sex difference was apparent for lamina lucida. Both laminar elements were thinner beneath melanocytes than beneath keratinocytes. Sex, age, and body region had no apparent influence on numbers of hemidesmosomes or basal cell plasmalemmal vesicles, nor was there a significant variation of these structures among individuals. Numbers of dermal microfibril bundles diminished with age (p less than 0.01). Anchoring fibril counts varied widely both among individuals (p less than 0.025) and within the same subject; there were fewer in the upper arm compared with different parts of the leg (p less than 0.005). These results emphasize the importance of appropriate controls in studies of physiologic and pathologic conditions involving the dermal-epidermal junction.