Skin in Complex Culture: The Transition from “Culture” Phenotype to Organotypic Phenotype

Biological and physical factors influencing the successful engraftment of a cultured human skin substitute

Skin tissue may be engineered in a variety of ways. Our cultured skin substitute (Graftskin, living skin equivalent or G-LSE), Apligraftrade mark, is an organotypic culture of skin, containing both a "dermis" and "epidermis." The epidermis is an important functional component of skin, responsible for biologic wound closure. The epidermis possesses a stratum corneum which develops with time in culture. The stratum corneum provides barrier function properties and gives the LSE improved strength and handling characteristics. Clinical experience indicated that the stratum corneum might play an important role in improving the clinical utility of the LSE. Handling and physical characteristics improved with time in culture. We examined the LSE at different stages of epidermal maturation for barrier function and ability to persist as a graft. LSE grafted onto athymic mice before significant development of barrier function did not withstand bandage removal at 7 days postgraft. LSE grafted after barrier function had been established in vitro were able to withstand bandage removal at day 7. Corneum lipid composition and structure are critical components for barrier function. Media modifications were used in an attempt to improve the fatty acid composition of the stratum corneum. The barrier developed more rapidly and was improved in a serum-free, lipid-supplemented condition. Lipid lamellar structure was improved with 10% of the stratum corneum exhibiting broad-narrow-broad lipid lamellar arrangements similar to human skin. Fatty acid metabolism was not appreciably altered. Barrier function in vitro was 4- to 10-fold more permeable than human skin. Epidermal differentiation does not compromise engraftment or the wound healing ability of the epidermis. The stratum corneum provides features beneficial for engraftment and clinical use. (c) 1996 John Wiley & Sons, Inc.

A pilot study to evaluate a tissue-engineered bilayered cell therapy as an alternative to tissue from the palate

BACKGROUND

This study evaluated the safety and effectiveness of a tissue-engineered skin product composed of viable neonatal keratinocytes and fibroblasts and compared it to a free gingival graft (FGG) in a procedure to enhance keratinized tissue (KT) and wound healing around teeth that do not require root coverage.

METHODS

Twenty-five subjects were enrolled who had at least two non-adjacent teeth in contralateral quadrants exhibiting an insufficient zone of attached gingiva requiring soft tissue grafting where root coverage was not desired. One tooth was randomized to receive an FGG, and the other was randomized to receive bilayered cell therapy (BCT). The amount of KT was measured at baseline and 3 and 6 months, and the texture and color of the grafted tissue were compared to the surrounding tissue at months 1, 3, and 6. A questionnaire was used to determine subject preference at 6 months. Biopsies and persistence studies were performed on a subset of the subjects.

RESULTS

The FGG generated statistically significantly (P <0.001) more KT than the test device (BCT) (4.5 +/- 0.80 mm versus 2.4 +/- 1.02 mm); no significant difference in recession or clinical attachment level was detected between treatment groups (P = 0.212 and P = 0.448, respectively); and no significant differences were detected at any time point for bleeding on probing (BOP), resistance to muscle pull, or inflammation. The BCT group had significantly better color and texture match with surrounding tissue (P <0.001), and subject preference was significantly greater for the BCT group (P = 0.041). No device-related adverse events or safety issues occurred during the course of the study.

CONCLUSIONS

The tissue-engineered graft BCT was safe and capable of generating de novo KT without the morbidity and potential clinical difficulties associated with donor-site surgery. The amount of KT generated with FGG was greater than generated with BCT; however, 24 of 25 test sites demonstrated an increase in KT at 6 months, with more than three-quarters of the sites yielding > or =2 mm bands of KT.

Kinetic Analysis on the Skin Disposition of Cytotoxicity as an Index of Skin Irritation Produced by Cetylpyridinium Chloride: Comparison of In Vitro Data using a Three-Dimensional Cultured Human Skin Model with In Vivo Results in Hairless Mice

PURPOSE

The aim of this study was to kinetically and dynamically analyze in vitro cytotoxicity as an index of skin irritation by use of a three-dimensional cultured human skin model and to compare the in vitro assay data with data from living animals.

METHODS

A cationic surfactant, cetylpyridinium chloride (CPC), was selected as a model irritant. Living skin equivalent-high (LSE-high) and hairless mice were used for the in vitro and in vivo tests, respectively. Skin irritation dermatodynamics was evaluated by calorimetric thiazoyl blue (MTT) conversion assay both for in vitro and in vivo tests, whereas dermatokinetics of CPC in LSE-high and mouse skin were evaluated using HPLC.

RESULTS

The time course of cell viability in the skin after application of CPC to intact skin was distinctly different from that of stratum-corneum-stripped skin in both LSE-high and hairless mice. Biphasic behavior characterized by two first-order rates with an inflection time point was observed in intact skin, whereas cell viability monoexponentially decreased immediately after CPC application in stripped skin. The time courses of cell viability in the skin and dermatodynamics were closely related to that of dermatokinetics of CPC.

CONCLUSION

The present study demonstrates that the in vitro cytotoxic profile was similar to the in vivo cytotoxicity test and that dermatodynamics was related to dermatokinetics of CPC.

The engineering of tissues using progenitor cells

The "engineering" of a tissue implies that it can be constructed by assembling the necessary components. However, tissues are formed through an evolving, interactive process, not through a collection of parts. This chapter focuses on the biology of the progenitor cell, the native precursor to new tissue, and its role in neogenesis, or the de novo generation of functional tissue. We present a working hypothesis for the generation of parenchymal cell populations and use this hypothesis as a basis for analysis of three parenchymal populations, epidermal cells, hepatocytes of the liver, and pancreatic islets, with a view toward what impact this information will have on the development of cell therapies. By comparing developmental processes, response to injury and disease, and behavior in vitro, we conclude that the adult progenitor cell retains the potential for substantial growth and organ neogenesis and that its biological properties make it the cell of first choice for the engineering of tissues.

A novel approach for the detection of proteolytically activated transglutaminase 1 in epidermis using cleavage site-directed antibodies

It has been suggested that transglutaminase 1 is proteolytically activated upon the terminal differentiation of the keratinocyte, but the mechanisms are not well understood. We have established two mouse hybridoma cell lines producing monoclonal antibodies that specifically detect proteolytically cleaved transglutaminase 1. One detects the amino-terminus of the fragment produced by cleavage between Arginine 93 and Glycine 94, and the other detects the amino-terminus of the fragment produced by cleavage between Arginine 573 and Glycine 574. Using these two antibodies, immunohistochemical analyses of the epidermis revealed that the cleavages of the transglutaminase 1 protein occur early in the terminal differentiation of keratinocytes in the basal layer of the epidermis, that the cleavage between Arginine 573 and Glycine 574 (producing the 574G fragment) precedes the cleavage between Arginine 93 and Glycine 94 (producing the 94G fragment), that the 94G fragment is localized to the plasma membrane of keratinocytes and has cross-linking activity, whereas the 574G fragment is dispersed in the cytosol and does not have detectable levels of activity on in situ transglutaminase assay, and that 1-alpha-25-dihydroxycholecalciferol or all-trans retinoic acid treatment and ultraviolet B exposure disturb the localization of the transglutaminase 1 fragments with changes in the morphology of differentiating keratinocytes. All these results demonstrate that the antibodies generated in this work are useful to dissect the mechanism by which transglutaminase 1 is activated, and would provide us with novel insights into the biogenesis of the epidermis.

Bilayered bioengineered skin substitute (Apligraf): a review of its use in the treatment of venous leg ulcers and diabetic foot ulcers

The bilayered bioengineered skin substitute (BBSS) [Apligraf] is used for the treatment of venous leg ulcers and diabetic foot ulcers. It has an epidermal layer formed from human keratinocytes and a dermal layer composed of human fibroblasts in a bovine type I collagen matrix. BBSS does not contain any antigen-presenting cells such as Langerhans cells, dermal dendritic cells, endothelial cells or leucocytes. In clinical trials, there was no evidence of clinical rejection and immunological tests indicated no humoral or cellular response to the keratinocytes or fibroblasts of BBSS. Further clinical trials are required to identify the exact mechanism of action of BBSS in chronic wounds. BBSS plus compression therapy was well tolerated and was superior in efficacy to compression therapy alone in a multicentre, randomised trial in patients with venous leg ulcers. At 6 months' follow-up, complete wound healing occurred in 63 versus 49% of patients and the median time to wound closure was 61 versus 181 days. In a subgroup of patients with hard-to-heal ulcers (>1 year's duration), wound healing was achieved in significantly more patients (47 vs 19%) and the median time to wound healing was significantly shorter (181 days vs not attained). In a multicentre, randomised trial, BBSS was well tolerated and effective in patients with full-thickness neuropathic diabetic foot ulcers. Ulcer healing occurred in significantly more patients (56 vs 38%) and the median time to wound healing was shorter (65 vs 90 days) with BBSS than with saline-moistened gauze at 12 weeks' follow-up. Patients in both groups also received standard diabetic foot care. The cost effectiveness of BBSS in patients with chronic ulcers has yet to be examined in well designed, prospective clinical trials. However, according to a modelled analysis incorporating data from a multicentre randomised trial, BBSS was cost effective in patients with hard-to-heal venous leg ulcers. The average annual medical cost of managing patients with ulcers of >1 year's duration was estimated to be 20,041 US dollars per patient treated with BBSS plus compression therapy and 27,493 US dollars per patient treated with compression therapy alone (1996 costs).

CONCLUSIONS

Clinical trials have shown that BBSS in conjunction with standard compression therapy was effective and well tolerated in patients with venous leg ulcers, especially patients with ulcers of >6 months' duration or that extended to the subcutaneous tissue. In addition, BBSS in conjunction with standard diabetic foot care was effective and well tolerated in patients with full-thickness neuropathic diabetic foot ulcers. BBSS represents a useful adjuvant to standard ulcer therapy in patients with venous leg ulcers or full-thickness neuropathic diabetic foot ulcers that do not respond to conventional ulcer therapy.

The use of cells in reparative medicine

Cells are the functional elements of reparative medicine and tissue engineering. The use of living cells as a therapy presents several challenges. These include identification of a suitable source, development of adequate methods, and proof of safety and efficacy. We are now well aware that stem or pluripotent cells offer an exciting potential source for a host of functional cell types. Their true potential will only be realized through continued effort to increase basic scientific understanding at all levels, the development of adequate methods to achieve a functional phenotype, and attention to safety issues associated with adequate control of cell localization, proliferation, and differentiation. There is also new understanding regarding the immunology of parenchymal cells and new promising approaches to immune modulation, which will open the door to broader therapies using allogeneic cell sources without prohibitive immune suppression. Control of cell growth and phenotypic expression does not end in the culture vessel, but goes beyond to the patient. A living therapy is not static but dynamic, as is the host response. The cells or tissue construct in most cases will not behave as a whole-organ transplant. It is therefore important that we understand a cell or tissue therapy's ability to react and interact within the host since clinical effectiveness has proven to be one of the most difficult milestones to achieve. A living cell therapy offers great potential to alter the human condition, encompassing alteration of the current biological state of a targeted tissue or organ, augmentation of depleted or lost function, or absolute functional tissue replacement. The extent to which we are able to achieve effective cell therapies will depend on assimilating a rapidly developing base of scientific knowledge with the practical considerations of design, delivery, and host response.

Cell proliferation and differentiation in a model of human skin equivalent

Recent advances in culturing technology has permitted the production of organotypic models that may be referred to as human skin equivalents (HSE). We have studied histochemical, ultrastructural, and kinetic aspects of an HSE composed by an epidermal equivalent and a dermal equivalent separated by a basement membrane. Only keratinocytes and fibroblasts were present in the epidermal and dermal equivalents, respectively; cells of other lineages were lacking. Keratinocyte stratification and differentiation seemed similar to natural skin. Evidence is shown that such an HSE may also release growth factors such as vascular endothelial growth factor that are believed to play a role in skin grafting. The distribution of cycling cells as well as the values of the growth fraction are comparable to those observed in natural skin. Although the absence of several cells populations that reside in natural skin is a remarkable feature of this HSE, the high levels of tissue organization and cell differentiation lead us to believe that such an HSE may be considered a candidate substitute of human skin in biological, pharmacologic, and clinical applications.

Physiological and cell biological aspects of perfusion culture technique employed to generate differentiated tissues for long term biomaterial testing and tissue engineering

Optimal results in biomaterial testing and tissue engineering under in vitro conditions can only be expected when the tissue generated resembles the original tissue as closely as possible. However, most of the presently used stagnant cell culture models do not produce the necessary degree of cellular differentiation, since important morphological, physiological, and biochemical characteristics disappear, while atypical features arise. To reach a high degree of cellular differentiation and to optimize the cellular environment, an advanced culture technology allowing the regulation of differentiation on different cellular levels was developed. By the use of tissue carriers, a variety of biomaterials or individually selected scaffolds could be tested for optimal tissue development. The tissue carriers are to be placed in perfusion culture containers, which are constantly supplied with fresh medium to avoid an accumulation of harmful metabolic products. The perfusion of medium creates a constant microenvironment with serum-containing or serum-free media. By this technique, tissues could be used for biomaterial or scaffold testing either in a proliferative or in a postmitotic phase, as is observed during natural development. The present paper summarizes technical developments, physiological parameters, cell biological reactions, and theoretical considerations for an optimal tissue development in the field of perfusion culture.

Using skin replacement products to treat burns and wounds

Much progress has been made toward the development of artificial skin replacement products. Continued research promises to bring more products to the marketplace, and each new product seems to develop a niche in the field of skin replacement. However, although each skin replacement product has unique properties and advantages, nothing works as well as a patient's own skin. Clinicians can only hope for an off-the-shelf skin replacement product that can be applied to a wound and yield a permanent, dependable dermis and epidermal skin replacement for all patients.

Human melanoma progression in skin reconstructs : biological significance of bFGF

Human skin reconstructs are three-dimensional in vitro models consisting of epidermal keratinocytes plated onto fibroblast-contracted collagen gels. Cells in skin reconstructs more closely recapitulate the in situ phenotype than do cells in monolayer culture. Normal melanocytes in skin reconstructs remained singly distributed at the basement membrane which separated the epidermis from the dermis. Cell lines derived from biologically early primary melanomas of the radial growth phase proliferated in the epidermis and the basement membrane was left intact. Growth and migration of the radial growth phase melanoma cells in the dermal reconstruct and tumorigenicity in vivo were only observed when cells were transduced with the basic fibroblast growth factor gene, a major autocrine growth stimulator for melanomas. Primary melanoma cell lines representing the more advanced stage vertical growth phase invaded the dermis in reconstructs and only an irregular basement membrane was formed. Metastatic melanoma cells rapidly proliferated and aggressively invaded deep into the dermis, with each cell line showing typical invasion and growth characteristics. Our results demonstrate that the growth patterns of melanoma cells in skin reconstructs closely correspond to those in situ and that basic fibroblast growth factor is critical for progression.

Constructing an in vitro cornea from cultures of the three specific corneal cell types

This paper presents a reliable method for establishing pure cultures of the three types of corneal cells. This is believed to be the first time, corneal cells have been cultured from fetal pig corneas. Cell growth studies were performed in different media. Subcultures of the three corneal cell types were passaged until the 30th generation without their showing signs of senescence. For engineering an in vitro cornea, corneal epithelial cells were cultured over corneal stromal cells in an artificial biomatrix of collagen with an underlying layer of corneal endothelial cells. The morphology, histology, and differentiation of the in vitro cornea were investigated to determine the degree of comparability to the cornea in vivo. The in vitro construct displayed signs of transition to an organotypic phenotype of which the most prominent was the formation of two basement membranes.

Effects of immunoregulatory cytokines on the immunogenic potential of the cellular components of a bilayered living skin equivalent

The purpose of this study was to determine if the immunocompatibility of an allogeneic living skin equivalent (LSE) would be affected by cytokines that would be potentially present at the wound site. Specifically, the ability of interleukin-1alpha (IL-1a), interleukin-6 (IL-6), or interleukin-12 (IL-12) to induce an allogeneic T cell response to "nonprofessional" antigen presenting cells (APC) was investigated in this series of experiments. Since cytokine concentrations at the wound site can vary greatly, recombinant IL-1a, IL-6, and IL-12 were used over a wide range of concentrations. These cytokines were either added directly to a mixed lymphocyte reaction (MLR) culture system or used to pretreat APC prior to use in the MLR culture. The addition of IL-12, IL-1alpha, or IL-6 into an MLR was examined as a possible means of providing the necessary costimulatory signal for functionally deficient APC, such as human keratinocytes (HK) and dermal fibroblasts (HF). While the results show that IL-1a and IL-12 can significantly augment a primary allogeneic response against appropriately equipped antigen presenting cells, the same was not true for HK or HF. Further experiments showed that pretreatment of HK, HF, or human umbilical vein endothelial cells (HUVEC) with Interferon-gamma (IFNgamma) and either IL-12, IL1alpha, or IL-6 had no significant affect on their ability to present alloantigen to immune-reactive T lymphocytes over IFNgamma-treatment alone. The data suggest that exposure of HK or HF to IL-1alpha, IL-6, or IL-12 in combination with IFNgamma does not provide the additional signal(s) required by these cells to effectively present alloantigen to unprimed T cells. The data suggests that exposure to these immunoregulatory cytokines in the wound bed would be unlikely to affect the immunocompatibility of the LSE.

Epidermal organization and differentiation of HaCaT keratinocytes in organotypic coculture with human dermal fibroblasts

The immortal human keratinocyte line HaCaT is frequently used as a paradigm for skin keratinocytes in vitro because of its highly preserved differentiation capacity. HaCaT cells form a nearly regular epidermal architecture when transplanted onto subcutaneous tissue of athymic mice. In order to analyze further their differentiation capacity in vitro, HaCaT cells were studied in organotypic cocultures on top of collagen gels containing human dermal fibroblasts. Within 1 wk HaCaT cells formed a still dysplastic epithelium, the thickness of which correlated with the number of fibroblasts in the collagen gel. With further culture time of up to 3 wk a remarkably well structured and differentiated squamous epithelium developed. After 1 wk, keratins 10 and 16, involucrin, and transglutaminase I were expressed in suprabasal layers, whereas filaggrin, keratin 2e, and loricrin appeared after 2-3 wk. Within this time, a nearly complete basement membrane had formed including hemidesmosomes and anchoring fibrils. Epithelial cell proliferation became restricted to the basal layer after 2 and 3 wk. Using the TdT-mediated dUTP nick end labeling assay, fragmentation of DNA was detectable in nuclei of the parakeratotic stratum corneum. Ultrastructurally, many features of keratinization accumulated after 2 and 3 wk, though an orthokeratotic keratinization was not achieved, in contrast to HaCaT transplants. This differentiation deficiency - as compared with normal keratinocytes -- might be due to a lack of paracrine factors important for keratinocyte differentiation or to a reduced sensitivity of these cells. Nevertheless, this high degree of differentiation under organotypic conditions qualifies this cell line as an appropriate model for elucidation of the molecular mechanisms regulating keratinocyte growth and differentiation and for use in pharmacotoxicology.

Tissue-engineered human skin substitutes developed from collagen-populated hydrated gels: clinical and fundamental applications

The field of tissue engineering has opened several avenues in biomedical sciences, through ongoing progress. Skin substitutes are currently optimised for clinical as well as fundamental applications. The paper reviews the development of collagen-populated hydrated gels for their eventual use as a therapeutic option for the treatment of burn patients or chronic wounds: tools for pharmacological and toxicological studies, and cutaneous models for in vitro studies. These skin substitutes are produced by culturing keratinocytes on a matured dermal equivalent composed of fibroblasts included in a collagen gel. New biotechnological approaches have been developed to prevent contraction (anchoring devices) and promote epithelial cell differentiation. The impact of dermo-epidermal interactions on the differentiation and organisation of bio-engineered skin tissues has been demonstrated with human skin cells. Human skin substitutes have been adapted for percutaneous absorption studies and toxicity assessment. The evolution of these human skin substitutes has been monitored in vivo in preclinical studies showing promising results. These substitutes could also serve as in vitro models for better understanding of the immunological response and healing mechanism in human skin. Thus, such human skin substitutes present various advantages and are leading to the development of other bio-engineered tissues, such as blood vessels, ligaments and bronchi.

Tissue engineering and the development of Apligraf, a human skin equivalent

In recent years, skin grafting has evolved from the initial autograft and allograft preparations to biosynthetic and tissue-engineered living skin replacements. This review details the pioneering work of numerous investigators that led to the following precursors of tissue-engineered skin replacement: cultured autologous keratinocyte grafts, cultured allogeneic keratinocyte grafts, autologous/allogeneic composites, acellular collagen matrices, and cellular matrices. It also discusses the rationale for the development of the newer products and describes the technical advances leading to the development of Apligraf, a tissue-engineered human skin product.

Fibroblasts can modulate the phenotype of malignant epithelial cells in vitro

An organotypic, tridimensional cell culture, also called a raft system, was used to study the influence of fibroblasts on epithelial carcinogenesis in a cell line derived from laryngeal squamous cell carcinoma and harboring a mutated p53. Differences between the effects of normal fibroblasts and those of tumor-derived fibroblasts were compared by means of fibroblasts taken from the normal skin and from the tumor of a cancer patient and cultivated with epithelial carcinoma cells in an organotypic culture. To study cell contact-mediated changes, the fibroblasts were either simply embedded in collagen matrix or additionally brought into direct contact with epithelial cells. Control epithelial cells were cultivated without any fibroblasts in an organotypic model. A protein panel [p53, p21, PCNA, bcl-2, Ki67, total cytokeratin (CK), CK 8, CK 10, CK 17, CK 18, CK 19, vimentin] involved in cell cycling and epithelial differentiation was assessed immunocytochemically in all organotypic cultures with fibroblasts, in tumor cells cultivated as a monolayer, and in the original tumor sample. The most dysplastic phenotype was obtained when tumor-derived fibroblasts were used in direct contact with epithelial cells, whereas the most benign phenotype was seen when skin fibroblasts had no contact with them. The intensive staining seen for p53 can be explained by p53 mutations also reflecting the weak expression of p21 and abundant expression of PCNA. The intensive Ki67 staining seen in all sections paralleled that of PCNA and marked active cellular proliferation. The CK staining pattern seen in cultured epithelia toward embryonic CKs, CK 8 and CK 18, suggested a simple epithelial phenotype. CK 19 was found only in the epithelium where no direct contacts had occurred. Vimentin expression increased when the raft epithelium was shifting toward a more benign phenotype. The results stress the importance of the origin of fibroblasts as well as the role of direct cellular contacts in modifying the epithelial phenotype even when the epithelial cells are malignant.

Comparison of an in vitro skin model to normal human skin for dermatological research

EpiDerm, an in vitro human skin equivalent (HSE), was compared to normal human breast skin (NHS) to morphologically and biochemically assess its feasibility for dermatological research. Intralot and interlot variability was studied in day 0, 1, 2, and 3 in vitro cultures and in day 0, 3, 5, and 7 NHS. For NHS, light microscopy (LM) at day 0 showed stratified epidermis which exhibited an increase in vacuoles and dark basal cells as storage increased to 3, 5, and 7 days. Transmission electron microscopy (TEM) revealed typical organelles in the epidermis and a convoluted basement membrane at day 0. With increased storage, vacuoles and paranuclear clefts became numerous, necrosis increased, tonofilaments became less organized, and overall cellular integrity decreased. Biochemical data showed consistent MTT and glucose utilization (GU) through day 5, while lactate production decreased to 75% by day 3. By LM, day 0 HSE consisted of a thick, compact, stratum corneum that sent projections between the stratum granulosum cells. By TEM, the configuration organization, differentiation, distribution, and frequency of the organelles differed slightly from NHS. In addition, the basement membrane of the HSE was not completely differentiated, and the dermis was thin and acellular. Although day 1 and 2 cultures showed little change, day 3 exhibited an overall degeneration. Biochemical analysis showed GU and the lactate production decreased through day 3. In conclusion, the EpiDerm HSE, although exhibiting slight differences, was morphologically and biochemically similar to normal human epidermis and may be a valuable model in assessing the toxicology, metabolism, or pharmacology of nonvesicating compounds.

A limited role for retinoic acid and retinoic acid receptors RAR alpha and RAR beta in regulating keratin 19 expression and keratinization in oral and epidermal keratinocytes

Different types of stratified squamous epithelia-for example, the "orthokeratinized" epidermis, the "parakeratinized" gingiva, and the "nonkeratinized" oral lining mucosal epithelia-are formed by intrinsically distinct keratinocyte subtypes. These subtypes exhibit characteristic patterns of keratin protein expression in vivo and in culture. Keratin 19 is an informative subtype-specific marker because the basal cells of only nonkeratinizing epithelia express K19 in vivo and in culture. Epidermal keratinocytes normally do not express K19, but can be induced to do so in culture by retinoic acid (RA). Keratinocyte subtypes express the retinoic acid receptor (RAR) beta at levels roughly correlated with their level of K19 expression in culture and their potential for forming a nonkeratinized epithelium in vivo. We tested the hypothesis that the level of RAR beta expressed by a keratinocyte determines its K19 expression and its form of suprabasal differentiation. Normal human epidermal and gingival keratinocytes stably overexpressing either RAR beta or RAR alpha were generated by defective retroviral transduction. Overexpression of either receptor enhanced the RA inducibility of K19 in conventional culture, in that the proportion of the transductants becoming K19+ in response to RA was markedly increased compared with controls. The pattern of differentiation of the epithelium formed in organotypic culture, assessed by basal K19 and suprabasal K1, K4, and filaggrin expression, however, was unaltered by RAR overexpression. Thus, the susceptibility of keratinocytes to regulation of K19 expression by retinoids is conditional, and levels of neither RAR beta nor RAR alpha are limiting to the intrinsic mechanism that specifies alternate differentiation pathways for stratified squamous epithelia.

Cultured skin as a 'smart material' for healing wounds: experience in venous ulcers

The healing of chronic wounds is a difficult and varied problem. The engineering of a cultured skin tissue offers an adaptive therapy for chronic wounds. Our hypothesis has been that living tissue can act as a 'smart material' to heal wounds. We have examined the healing characteristics of a bilayered cultured skin equivalent (Graftskin) in a controlled study and present clinical data from interim analyses for 233 patients over 6 months of treatment. All venous ulcer patients will be followed for up to 1 year. We report on three basic scenarios of healing: (i) promotion of healing by secondary intention, (ii) persistent biological wound closure with stimulation of underlying healing, and (iii) healing by frank graft take of the cultured material with remodelling of the tissue over time. Our results indicate that the cultured skin equivalent is responsive to individual wound conditions and thus acts as a 'smart material' in the chronic wound.

The matrix form of collagen and basal microporosity influence basal lamina deposition and laminin synthesis/secretion by stratified human keratinocytes in vitro

The ability of the collagen matrix form to support the formation of a basal lamina by cultured normal human epidermal keratinocytes (NHEK) was determined using transmission electron microscopy. The collagen matrix forms tested in this study were a) a dry type I collagen film and b) a type I collagen gel. NHEK were grown for 14 days on the following five different substrates: plain plastic culture dishes without the addition of collagen (PP); plain plastic culture dishes overlaid with a dry, aldehyde-crosslinked type I collagen film (DCF-P); plain plastic culture dishes overlaid with an aldehyde-crosslinked type I collagen gel (GEL-P); Millipore Millicell CM microporous membranes overlaid with a dry, aldehyde-crosslinked type I collagen film (DCF-CM); and Millipore Millicell CM microporous membranes overlaid with an aldehyde-crosslinked type I collagen gel (GEL-CM). NHEK maintained for 2 wk on PP and DCF-P were unable to secrete a basal lamina. NHEK grown for 2 wk on the GEL-P and GEL-CM substrates, however, secreted a contiguous basal lamina at the GEL-NHEK interface. To determine if the appearance of this basal lamina correlated with laminin synthesis, laminin was immunoprecipitated from cellular extracts, as well as media from the apical and basal chambers. NHEK grown on the GEL-P substrate synthesized more laminin than did NHEK grown on the other four alternative substrates. In addition, NHEK grown on GEL-CM were able to direct more laminin to the basal compartment than NHEK grown on DCF-CM substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of a serum-free epidermal culture model to show deleterious effects of epidermal growth factor on morphogenesis and differentiation

The presence of serum has limited the utility of many culture models for the study of cytokine effects because its complexity and variability can confound the interpretation of data. In the present study, a serum-free skin co-culture model was used to investigate the effect of exogenous epidermal growth factor (EGF) on epidermal proliferation and differentiation. Human keratinocytes cultured on collagen rafts at the air-liquid interface produced a well-differentiated epithelium that resembled normal epidermis. Keratin filaments, membrane-coating granules, and keratohyalin granules were all observed. Epidermal differentiation markers keratin K1/K10, involucrin, and transglutaminase were localized in most of the suprabasal layers, whereas profilaggrin/filaggrin was confined to the granular layers and stratum corneum. In the continual presence of 10-20 ng/mL EGF, the epidermis was less organized, thinner, and less proliferative. EGF also depressed several indicators of differentiation: The number of keratohyalin granules and membrane-coating granules was greatly decreased; antigen expression of profilaggrin/filaggrin appeared diminished by immunocytochemical staining; frequent nuclear retention was noted in the relatively thickened stratum corneum-like layers. As detected by immunohistochemical staining, the expression of EGF receptor in the epidermis was reduced by exogenous EGF. These data illustrate that EGF cannot be considered a simple mitogen. Our findings also underscore the importance of using sophisticated culture models to assess complex cytokine effects that may be dependent on the architecture of a differentiating epidermis.

The organotypic culture of human skin keratinocytes and fibroblasts to achieve form and function

We describe an organotypic model of human skin comprised of a stratified layer of human epidermal keratinocytes and dermal fibroblasts within a contracted collagen lattice. Feasible and reproducible production of the skin construct has required the use of traditional as well as specialized culture techniques. The configuration of the construct has been engineered to maintain polarity and permit extended culture at the air-liquid interface. Morphological, biochemical and kinetic parameters were assessed and functional assays were performed to determine the degree of similarity to human skin. Light and ultrastructural morphology of the epidermis closely resembled human skin. The immunocytochemical localization of a number of differentiation markers and extracellular matrix proteins was also similar to human skin. Kinetic data showed a transition of the epidermal layer to a more in vivo-like growth rate during the development of the construct at the air-liquid interface. The barrier properties of the construct also increased with time reaching a permeability to water of less than 2%-h after approximately 2 weeks at the air-liquid interface which is still on average 30-fold more water-permeable than normal human skin. The construct is currently used for in vitro research and testing and is also being tested in clinical applications.