Expression of Insulin-Like Growth Factor I by Cultured Skin Substitutes Does Not Replace the Physiologic Requirement for Insulin In Vitro

Exogenous Keratinocyte Growth Factor Is Not Required for Pigmentation of Skin Substitutes with Three Isogeneic Cell Types

Engineered skin substitutes (ESS) containing human fibroblasts (hF) and human keratinocytes (hK) provide significant medical benefits for treatment of acute and chronic skin wounds, including, but not limited to, burns, burn scars, congenital skin lesions, and cutaneous ulcers. However, anatomic deficiencies, such as lack of pigment, can contribute to long-term morbidity, including hypopigmentation and reduced solar protection. To address the deficiency of hypopigmentation, ESS were populated sequentially with cultured hF, human melanocytes (hM), and hK to generate ESS with pigment (ESS-P). Constructs were incubated in media containing 0.0, 1.5, or 5.0 ng/mL keratinocyte growth factor (KGF), which promotes survival and differentiation of hM in ESS-P, and had media changed at 24 or 48 h intervals. ESS-P were evaluated in vitro for surface hydration, surface color, and distribution of hM. Proliferation was assessed by measuring incorporation of 5-bromo-2'-deoxyuridine into replicating DNA in basal epidermal cells. ESS-P from test conditions were grafted to immunodeficient mice, and were assessed over 12 weeks for pigmented area, pigment density, and distribution of hM in healed human grafts. The in vitro data showed differences among test groups, including increase in hydration of the epidermal surface with higher KGF, increase of surface pigmentation with 24 h media changes, increase of hM density with higher KGF and 24 h media changes, and time-dependent decrease of proliferation. At 12 weeks after grafting, differences among groups were found for pigment density, but not for distribution of hM or percentage of pigmented area. These differences demonstrate that a higher concentration of KGF (5 ng/mL) in the maturation medium of ESS-P and more frequent media changes (24 h interval) promote higher viability and hM differentiation of ESS-P before grafting, but are not required for full pigmentation (pigmented area, pigment density, hM distribution) of grafted wounds. Based on these results, reductions of the concentration of KGF (i.e., 1.5 ng/mL) in the maturation medium, and of the frequency of medium changes (48 h intervals) would be expected to support survival, continued replication, and restoration of skin color by hM in therapeutic transplantation of ESS-P. Impact Statement Restoration of skin color after traumatic injury affects personal identity and provides protection from exposure to solar radiation. Keratinocyte growth factor (KGF) and nutrient supply are known to regulate survival of melanocytes before transplantation in engineered skin substitutes with pigment (ESS-P). This report demonstrates that exogenous KGF is not required to restore skin color and that replacement of the nutrient medium at lower frequency (48 versus 24 h) does not inhibit development of skin color after melanocyte transplantation. These results offer new alternatives to conserve resources in fabrication of ESS-P and to maintain efficacy for restoration of skin color.

Quantitative Assay for Quality Assurance of Human Cells for Clinical Transplantation

Transplantation of human cells after isolation and culture has become an important alternative for treatment of acute or chronic skin wounds. To increase the efficacy and reduce cost for transplantation of skin cells, more efficient and accurate techniques for evaluation of cell proliferation are needed. Hemocytometer counts provide a valid assessment of cell proliferation and viability, but they are very labor intensive and require removal of the cells from their substrate. In this study, hemocytometer counts were compared with a fluorometric assay (n = 21 per condition) that uses the commercially available reagent alamarBlue™, which is reduced to a fluorescent substrate by cellular dehydrogenases. Human epidermal keratinocytes were inoculated at 200, 600, 2000, and 6000 cells/cm2 incubated for 6 days in modified MCDB 153 medium. Alamar Blue™ was incubated with cells for 2 h at 37°C, and fluorescence was measured with a microplate reader at 590 nm. Hemocytometer counts (×10-4) from the respective cell inoculation densities were 0.30 ± 0.04, 1.07 ± 0.10, 6.37 ± 0.62, and 16.99 ± 0.96. Fluorescence values (×10–3) for the respective inoculation densities were 0.14 ± 0.01, 0.34 ± 0.02, 1.20 ± 0.09, and 1.79 ± 0.12. Regression analysis showed a statistical significant (p < 0.0001) correlation (r2 = 0.87) between cell counts and optical density from the alamarBlue™ assay. These data demonstrate that alamarBlue™ provides a valid substitute for cell counts to assess cell proliferation before clinical transplantation of engineered skin. AlamarBlue™ also allows repeated, nondamaging assessment of living cells over time. These advantages are expected to increase the validity and reliability of quality assurance standards for transplanted skin cells, and to increase the efficacy of healing of cutaneous wounds.

Controlled-rate freezing to regulate the structure of collagen-glycosaminoglycan scaffolds in engineered skin substitutes

Controlled-rate freezing (CRF) of biopolymer scaffolds may increase reproducibility of microstructure compared with analog processes. Freezing of collagen-glycosaminoglycan (CG) scaffolds by CRF with liquid nitrogen at chamber cooling rates of -80, -40, -20, or -10°C/min, was compared with submersion in 95% ethanol at -55°C. Cooling rates of -80 or -40°C/min generated scaffolds with pore areas and pore fractions that were comparable to scaffolds frozen in ethanol. Test and control scaffolds were populated with human dermal fibroblasts and epidermal keratinocytes to generate engineered skin substitutes (ESS) and evaluated for surface hydration and mitochondrial metabolism. ESS with scaffolds frozen by CRF at -80 or -40°C/min were comparable with, or better than, ESS with control scaffolds (p < 0.05). These results demonstrate that fabrication of CG scaffolds by CRF offers advantages of digital programming, as well as greater reproducibility, safety, and simplicity than submersion in chilled ethanol without compromise of biological properties required for biomedical applications.

Immunotherapy of radioresistant mammary tumors with early metastasis using molecular chaperone vaccines combined with ionizing radiation

In the current study, exposure of mammary tumor cells derived from mice transgenic for the polyomavirus middle T oncogene to ionizing radiation resulted in the generation of a tumor cell population that preferentially expressed cancer stem cell markers. In addition, these cells were more resistant to subsequent radiation treatments and appeared to acquire an enhanced capacity for dissemination to the lungs of mice. Therefore, we tested an immunotherapy approach to the treatment of local and disseminated mammary tumor cells in a murine model using a recently developed molecular chaperone-based vaccine that specifically targets the radioresistant subpopulation of tumor cells. Heat shock protein 70-peptide complexes (Hsp70.PC-F) were extracted from fusions of dendritic cells and radiation-enriched tumor cells, and the resulting chaperone vaccines were used to treat mice with pre-existing lung metastases. Immunization of mice with the Hsp70.PC-F vaccine resulted in a T cell-mediated immune response, including a significant increase in CD4 and CD8 T cell proliferation and the induction of effector T cells capable of targeting radioresistant tumor cells. Importantly, the growth of primary tumors was inhibited, and the number of tumor cells metastasizing to lung was reduced significantly by combining chaperone vaccine with radiotherapy. These results indicate that Hsp70.PC-F vaccine can induce specific immunity to radioresistant populations of mammary tumor cells and, thus, can complement radiotherapy, leading to synergistic killing.

Assessment of replication rates of human keratinocytes in engineered skin substitutes grafted to athymic mice

Stable closure of skin wounds with engineered skin substitutes (ESS) requires indefinite mitotic capacity to generate the epidermis. To evaluate whether keratinocytes in ESS exhibit the stem cell phenotype of label retention, ESS (n = 6-9/group) were pulsed with 5-bromo-2'-deoxyuridine (BrdU) in vitro, and after grafting to athymic mice (n = 3-6/group). Pulse and immediate chase in vitro labeled virtually all basal keratinocytes at day 8, with label uptake decreasing until day 22. Label retention in serial chase decreased more rapidly from day 8 to day 22, with a reorganization of BrdU-positive cells into clusters. Similarly, serial chase of labeled basal keratinocytes in vivo decreased sharply from day 20 to day 48 after grafting. Label uptake was assessed by immediate chases of basal keratinocytes, and decreased gradually to day 126, while total labeled cells remained relatively unchanged. These results demonstrate differential rates of label uptake and retention in basal keratinocytes of ESS in vitro and in vivo, and a proliferative phenotype with potential for long-term replication in the absence of hair follicles. Regulation of a proliferative phenotype in keratinocytes of ESS may improve the biological homology of tissue-engineered skin to natural skin, and contribute to more rapid and stable wound healing.

A denatured collagen microfiber scaffold seeded with human fibroblasts and keratinocytes for skin grafting

Biomaterial scaffolds are categorized into artificial or natural polymers, or combinations of the two. Artificial polymers often undergo serum protein adsorption, elicit foreign body and encapsulation immune responses post-implantation. Large pore bovine electrospun collagen I was therefore screened as a candidate for human keratinocyte and fibroblast cell scaffolds. Human HaCaT keratinocyte and dermal fibroblasts were seeded on electrospun denatured collagen I microfiber (DCM) scaffolds and after 72 h Livedead(®) assays performed to determine adhesive cell, survival and scaffold penetration. Both keratinocytes and fibroblasts attached to and survived on DCM scaffolds, however only fibroblasts migrated over and into this biomaterial. HaCaT keratinocytes remained largely stationary on the scaffold surface in discrete islands of monolayered cells. For this reason, normal human epidermal keratinocyte (NHEK) scaffold interactions were assessed using scanning and transmission electron microscopy (EM) that demonstrated DCM scaffolds comprised networks of interlocking and protruding collagen fibers with a mean diameter of 2-5 μm, with a mean inter-fiber pore size of 6.7 μm (range 3-10 μm) and scaffold thickness 50-70 μm. After 72 h the keratinocytes and fibroblasts on DCM scaffolds had attached, flattened and spread over the entire scaffold with assembly of lamellapodia and focal adhesion (FA)-like junctions. Using transmission EM, NHEKs and HaCaT keratinocytes assembled desmosomes, lamellapodia and FA junctions, however, neither hemidesmosomes nor basal lamina were present. In long term (21 day) co-culture fibroblasts migrated throughout the scaffold and primary keratinocytes (and to a lesser extend HaCaTs) stratified on the scaffold surface forming a human skin equivalent (HSE). In vivo testing of these HSEs on immunocompetent (BalbC) and immunodeficient (SCID) excisionally wounded model mice demonstrated scaffold wound biocompatibility and ability to deliver human cells after scaffold biodegradation.

Induction of cytotoxic T lymphocytes against ovarian cancer-initiating cells

The majority of patients with stage III/IV ovarian carcinoma that respond initially to standard therapies ultimately undergo relapse due to the survival of small populations of cells with tumor-initiating potential. These ovarian cancer (OVCA)-initiating cells (OCIC) are sometimes called cancer stem cells (CSC) because they express stem cell markers, and can survive conventional therapies such as chemotherapy, which usually target rapidly replicating tumor cells, and give rise to recurrent tumors that are more chemo-resistant and more aggressive. Thus, it would be desirable to develop a therapy that could selectively target OCIC and be used to complement the conventional therapies. In this study, we isolated a subset of OVCA cells with a CD44(+) phenotype in samples from patients with OVCA that possess CSC properties including the formation of spheroids in culture, self-renewal and the ability to be engrafted in immune-compromised mice. We next explored the use of immunotherapy using fusions of dendritic cells and OCIC to specifically target the OCIC subpopulations. Fusion cells (FCs) prepared in this way activated T cells to express elevated levels of IFN-γ with enhanced killing of CD44(+) OVCA cells. We envision a combined approach where conventional therapies such as chemotherapy kill the bulk of tumor cells, whereas OCIC-reactive cytotoxic T lymphocytes target the resistant OCIC fraction. A combined therapy such as this may represent a promising approach for the treatment of OVCA.

The effectiveness of basic fibroblast growth factor in fibrin-based cultured skin substitute in vivo

Cultured skin substitute (CSS), comprised keratinocytes and fibroblasts in a biopolymer matrix, is useful for adjunctive burn therapy. However, the vascularization of CSS is much slower than split-thickness autografts, because it lacks a vascular plexus. This study evaluated the influence of basic fibroblast growth factor (bFGF) on fibrin-based CSS grafting in vivo. Fibrin-based CSS treated with 0, 0.26, 1.3, 6.5, 13, or 130 microg/cm bFGF was transplanted into athymic mice, and macroscopic and histologic examinations of the graft were performed on day 21 posttransplantation. Engrafted CSS of the 0.26 to 6.5 microg/cm bFGF treatment groups were similar to the untreated control. However, the engrafted area was significantly suppressed in the 13 microg/cm bFGF treatment group, and the 130 microg/cm bFGF treatment group was not engrafted. Neovascularization of CSS was significantly increased in the 1.3 microg/cm bFGF treatment group compared with the control (P < .05). The number of human fibroblastic cells in CSS that were positive for vimentin increased significantly in the 0.26 and 1.3 microg/cm bFGF treatment groups (P < .01). CSS treated with 0.26 to 6.5 microg/cm bFGF showed normal epidermis with keratinizing stratified squamous epithelium, whereas the thickness of the epidermis and proliferation of keratinocytes in the basal layer was decreased. These results demonstrated that bFGF treatment (1.3 microg/cm) in fibrin-based CSS may enhance angiogenesis and fibroblast proliferation after transplantation.

Improvement of Epidermal Barrier Properties in Cultured Skin Substitutes after Grafting onto Athymic Mice

Barrier function in cultured skin substitutes (CSS) prepared from human cell sources was measured by noninvasive (surface hydration, transepidermal water loss) and invasive methods (water permeation, niacinamide flux) before and after grafting onto athymic mice. In vitro measurements were made on days 7 and 14. Although three of the four measures of barrier function improved markedly from day 7 to 14, the values obtained were still far from those obtained with native human skin controls. Additional CSS were grafted onto athymic mice on day 14, and skin was harvested 2 and 6 weeks after grafting. Grafting brought about a substantial decrease in all measurements by 2 weeks and almost complete normalization of barrier function after 6 weeks. The most sensitive measure of this recovery was niacinamide permeability, which decreased from (280 +/- 40) x 10(-4) cm/h in vitro to (17 +/- 30) x 10(-4) cm/h 2 weeks after grafting and (5 +/- 2) x 10(-4) cm/h 6 weeks after grafting, versus control values of (2 +/- 2) x 10(-4) cm/h in human cadaver skin and (0.6 +/- 0.4) x 10(-4) cm/h in human epidermal membrane prepared from freshly excised breast skin. These results demonstrate the reformation of epidermal barrier function after transplantation and provide insights for the development of a functional epidermal barrier in CSS in vitro.

Increased expression of integrins and decreased apoptosis correlate with increased melanocyte retention in cultured skin substitutes

Losses of human melanocytes (HM) in transplantation of cultured skin substitutes (CSS) may result from poor cellular attachments. To test this hypothesis, HM integrin expression was measured in four culture media: (a) melanocyte growth medium (MGM), an HM proliferation medium; (b) UCMC 160, a CSS maturation medium; (c) mMGM, modified MGM with 1.8 mM calcium; and (d) modified UCMC 160 with HM supplements (mUCMC 160). HM grew well in all media except UCMC 160. Increased expression of beta1, beta4, alpha3beta1 and alpha5 integrins on HM cultured in MGM and mMGM versus UCMC 160 was found by flow cytometry. Annexin V-allophycocyanin (APC) labeled HM in apoptosis and increased significantly in UCMC 160 (31.1%) compared with MGM (11.9%) or mMGM (13.9%). CSS were incubated in UCMC 160, mMGM or mUCMC 160 media, and grafted to athymic mice. In the mMGM group, grafts were darker as measured with a chromameter through 6 weeks and the average number of basal HM per field was greater at 12 weeks post-grafting. Increased graft loss was observed in the mMGM group which corresponded with the poor epidermal morphology in vitro. Although HM retention improved in vivo using mMGM to culture the CSS, the stability of the epidermis decreased. These results indicate that expression of integrins on HM in vitro correlates with HM retention in CSS and short-term survival after transplantation, but that long-term survival depends also on stable epithelium.

Preparation of Cultured Skin for Transplantation Using Insulin-like Growth Factor I in Conjunction with Insulin-like Growth Factor Binding Protein 5, Epidermal Growth Factor, and Vitronectin

BACKGROUND

Cultured skin for transplantation is routinely prepared by growing patient keratinocytes in the presence of semidefined sources of growth factors including serum and feeder cells, but these materials require substantial risk remediation and can contribute to transplant rejection.

METHODS

We have therefore investigated the potential of a novel combination of recombinant and purified growth factors to replace serum and feeder cells in cultures of human keratinocytes suitable for clinical application. Our technique was investigated with respect to culture establishment, serial propagation, colony-forming efficiency, immunocytochemistry, epidermal reconstruction, and suitability to support transplantation by aerosolization.

RESULTS

We demonstrate that insulin-like growth factor (IGF)-I--used in conjunction with epidermal growth factor (EGF), insulin-like growth factor binding protein (IGFBP)-5 and vitronectin--supports growth in the absence of serum. Moreover, a threefold greater number of cells are generated within 7 days compared to those grown under current best practice conditions using serum (P<0.05). The resulting test cultures are suitable for epidermal reconstruction and support the option for delivery in the form of an aerosolized cell suspension. Serial propagation, with the view to producing confluent sheets for extensive injuries, was achieved but with less consistency and this result correlated with a significant decline in colony-forming efficiency compared to controls.

CONCLUSIONS

IGF-I used in conjunction with IGFBP-5, EGF, and vitronectin provides a superior alternative to serum for the rapid expansion and transplantation of cultured keratinocytes within the first week of treatment. Nevertheless, further optimization is required with respect to elimination of feeder cells and serial expansion of cultures for treatment of extensive injuries.

Microarray analysis of gene expression in cultured skin substitutes compared with native human skin

Cultured skin substitutes (CSS), prepared using keratinocytes, fibroblasts, and biopolymers, can facilitate closure of massive burn wounds by increasing the availability of autologous tissue for grafting. But because they contain only two cell types, skin substitutes cannot replace all of the functions of native human skin. To better understand the physiological and molecular differences between CSS and native skin, we undertook a comprehensive analysis of gene expression in native skin, cultured keratinocytes, cultured fibroblasts, and skin substitutes using Affymetrix gene chip microarrays. Hierarchical tree clustering identified six major clusters of coordinately regulated genes, using a list of 1030 genes that were the most differentially expressed between groups. These clusters correspond to biomarker pools representing expression signatures for native skin, fibroblasts, keratinocytes, and cultured skin. The expression analysis revealed that entire clusters of genes were either up- or downregulated upon combination of fibroblasts and keratinocytes in cultured skin grafts. Further, several categories of genes were overexpressed in CSS compared with native skin, including genes associated with hyperproliferative skin or activated keratinocytes. The observed pattern of expression indicates that CSS in vitro, which display a well-differentiated epidermal layer, exhibit a hyperproliferative phenotype similar to wounded native skin.

Human mesenchymal stem cells successfully improve skin-substitute wound healing

BACKGROUND

Large or deteriorated skin defects are sometimes life threatening. There is increasing evidence that adult stem cells are useful for tissue regeneration. Human mesenchymal stem cells (hMSCs) are self-renewing and are potent in differentiating into multiple cells and tissues.

OBJECTIVES

To investigate the effects of hMSCs in cutaneous wound healing.

METHODS

Wound healing was studied in an hMSC-populated porcine skin substitute, using a nude rat model to minimize immune reactions. Full-thickness skin and soft tissue defects of 1.5 x 1.5 cm in size, including the panniculus carnosus, were excised and covered with hMSCs and basic fibroblast growth factor (bFGF)-soaked skin substitutes and an evaluation was made of wound size, histology and protein expression at 3, 7 and 42 days after injury.

RESULTS

The wound size was significantly smaller in the hMSC-treated groups (P < 0.01) and any dose of bFGF (1, 10, 100 microg) enhanced the healing (P < 0.01). The re-epithelialization markers integrin alpha3 and skin-derived antileucoproteinase were remarkably increased with the presence of bFGF in a dose-dependent manner, while the mesenchymal cell surface markers CD29 and CD44 were downregulated in a time-dependent manner. Human pancytokeratin, which does not cross-react with rat antigens, was observed by Western blotting at 38 kDa and 42 kDa from the hMSC-treated tissues on day 7. The expression levels were elevated by 10 microg bFGF (P < 0.01). The immunohistochemical expression of human pancytokeratin was only observed in the hMSC-treated groups.

CONCLUSIONS

These data suggest that hMSCs together with bFGF in a skin defect model accelerate cutaneous wound healing as the hMSCs transdifferentiate into the epithelium.

Expression of human beta-defensins HBD-1, HBD-2, and HBD-3 in cultured keratinocytes and skin substitutes

Defensins are effector molecules of the innate host defense system with antimicrobial activity against a variety of pathogens, including microorganisms commonly found in burn units. beta-Defensins are variably expressed in the epithelia of skin and other organs. Cultured skin substitutes (CSS) grafted to burn wounds lack a vascular plexus and are therefore more susceptible to microbial contamination than split thickness skin autograft. To investigate whether beta-defensins can contribute to host defense in CSS, we examined expression of human beta-defensins HBD-1, HBD-2, and HBD-3 in cultured keratinocytes and CSS from uninjured donors and burn patients. HBD-1 was expressed in all keratinocyte strains analyzed. HBD-2 expression in keratinocyte monolayers was highly variable but did not correlate with burn injury. HBD-3 was expressed at variable levels in all but one keratinocyte strain. CSS were prepared from two donors that lacked expression of HBD-2 in keratinocyte monolayers. All three genes were readily detected in CSS from both donors, suggesting up-regulation of HBD-2 and HBD-3. In sections of CSS, HBD-1, HBD-2, and HBD-3 proteins were localized to distinct epidermal regions. We conclude that beta-defensins can potentially contribute to innate immunity in CSS, but their levels may be too low to prevent contamination after grafting.

Vascular endothelial growth factor overexpression increases vascularization by murine but not human endothelial cells in cultured skin substitutes grafted to athymic mice

Cultured skin substitutes (CSS) consisting of fibroblasts, keratinocytes, and biopolymers are an adjunctive treatment for large burns. Because CSS lack a vascular plexus, they vascularize more slowly than split-thickness autografts. Previously, CSS were prepared with dermal microvascular endothelial cells (ECs), which formed vascular analogs at a low frequency but did not contribute to increased vascularization after grafting. The present study addressed whether keratinocytes genetically modified to overexpress vascular endothelial growth factor (VEGF), an endothelial cell mitogen, could improve the persistence and organization of ECs in CSS. CSS were prepared with control or VEGF-modified keratinocytes, with (CSS + ECs) or without added ECs, and were grafted to full-thickness wounds in athymic mice. Elevated VEGF expression was detected in VEGF-modified CSS and CSS + ECs compared with controls, but no significant difference in EC density in vitro was observed. After grafting, VEGF-modified CSS and CSS + ECs showed enhanced vascularization, and organization of human ECs into multicellular structures in CSS + ECs was observed. However, VEGF overexpression did not significantly enhance the proliferation of human ECs, suggesting that other factors may be required. Improved persistence and organization of human ECs in vitro will likely be required for their participation in vascularization of CSS + ECs after grafting.

Epidermal homeostasis: the role of the growth hormone and insulin-like growth factor systems

GH and IGF-I and -II were first identified by their endocrine activity. Specifically, IGF-I was found to mediate the linear growth-promoting actions of GH. It is now evident that these two growth factor systems also exert widespread activity throughout the body and that their actions are not always interconnected. The literature highlights the importance of the GH and IGF systems in normal skin homeostasis, including dermal/epidermal cross-talk. GH activity, sometimes mediated via IGF-I, is primarily evident in the dermis, particularly affecting collagen synthesis. In contrast, IGF action is an important feature of the dermal and epidermal compartments, predominantly enhancing cell proliferation, survival, and migration. The locally expressed IGF binding proteins play significant and complex roles, primarily via modulation of IGF actions. Disturbances in GH and IGF signaling pathways are implicated in the pathophysiology of several skin perturbations, particularly those exhibiting epidermal hyperplasia (e.g., psoriasis, carcinomas). Additionally, many studies emphasize the potential use of both growth factors in the treatment of skin wounds; for example, burn patients. This overview concerns the role and mechanisms of action of the GH and IGF systems in skin and maintenance of epidermal integrity in both health and disease.

Plasticity of hair follicle dermal cells in wound healing and induction

The capacity of adult hair follicle dermal cells to participate in new follicle induction and regeneration, and to elicit responses from diverse epithelial partners, demonstrates a level of developmental promiscuity and influence far exceeding that of interfollicular fibroblasts. We have recently suggested that adult follicle dermal cells have extensive stem or progenitor cell activities, including an important role in skin dermal wound healing. Given that up to now tissue engineered skin equivalents have several deficiencies, including the absence of hair follicles, we investigated the capacity of follicle dermal cells to be incorporated into skin wounds; to form hair follicles in wound environments; and to create a hair follicle-derived skin equivalent. In our study, we implanted rat follicle dermal cells labelled with a vital dye into ear and body skin wounds. We found that they were incorporated into the new dermis in a manner similar to skin fibroblasts, but that lower follicle dermal sheath also assimilated into hair follicles. Using different combinations of follicle dermal cells and outer root sheath epithelial cells in punch biopsy wounds, we showed that new hair follicles were formed only with the inclusion of intact dermal papillae. Finally by combining follicle dermal sheath and outer root sheath cells in organotypic chambers, we created a skin equivalent with characteristic dermal and epidermal architecture and a normal basement membrane - the first skin to be produced entirely from hair follicle cells. These data support the hypothesis that follicle dermal cells may be important in wound healing and demonstrate their potential usefulness in human skin equivalents and skin substitutes. While we have made progress towards producing skin equivalents that contain follicles, we suggest that the failure of cultured dermal papilla cells to induce follicle formation in wounds illustrates the complex role the follicle dermis may play in skin. We believe that it demonstrates a genuine dichotomy of activity for follicle cells within skin.

Human dermal microvascular endothelial cells form vascular analogs in cultured skin substitutes after grafting to athymic mice

Cultured skin substitutes (CSS) consisting of autologous fibroblasts and keratinocytes combined with biopolymers are an adjunctive treatment for large excised burns. CSS containing two cell types are limited by anatomical deficiencies, including lack of a vascular plexus, leading to slower vascularization after grafting than split-thickness autograft. To address this limitation, CSS were prepared containing human keratinocytes, fibroblasts, and dermal microvascular endothelial cells (HDMEC) isolated from a single skin sample. After 16 days in culture, control CSS and CSS containing HDMEC (CSS+EC) were grafted to full-thickness wounds in athymic mice. In CSS+EC in vitro, HDMEC persisted in the dermal substitutes and formed multicellular aggregates. One wk after grafting, HDMEC in CSS+EC organized into multicellular structures, some containing lumens. By 4 wk after grafting, HDMEC were found in linear and circular organizations resembling vascular analogs associated with basement membrane deposition. In some cases, colocalization of HDMEC with mouse perivascular cells was observed. The results demonstrate HDMEC transplantation in a clinically relevant cultured skin model, persistence of HDMEC after grafting, and HDMEC organization into vascular analogs in vitro and in vivo. All cells were derived from the same donor tissue, indicating the feasibility of preparing CSS containing autologous HDMEC for grafting to patients.

Vitamin C regulates keratinocyte viability, epidermal barrier, and basement membrane in vitro, and reduces wound contraction after grafting of cultured skin substitutes

Cultured skin substitutes have become useful as adjunctive treatments for excised, full-thickness burns, but no skin substitutes have the anatomy and physiology of native skin. Hypothetically, deficiencies of structure and function may result, in part, from nutritional deficiencies in culture media. To address this hypothesis, vitamin C was titrated at 0.0, 0.01, 0.1, and 1.0 mM in a cultured skin substitute model on filter inserts. Cultured skin substitute inserts were evaluated at 2 and 5 wk for viability by incorporation of 5-bromo-2'-deoxyuridine (BrdU) and by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) conversion. Subsequently, cultured skin substitute grafts consisting of cultured human keratinocytes and fibroblasts attached to collagen-glycosaminoglycan substrates were incubated for 5 wk in media containing 0.0 mM or 0.1 mM vitamin C, and then grafted to athymic mice. Cultured skin substitutes (n = 3 per group) were evaluated in vitro at 2 wk of incubation for collagen IV, collagen VII, and laminin 5, and through 5 wk for epidermal barrier by surface electrical capacitance. Cultured skin substitutes were grafted to full-thickness wounds in athymic mice (n = 8 per group), evaluated for surface electrical capacitance through 6 wk, and scored for percentage original wound area through 8 wk and for HLA-ABC-positive wounds at 8 wk after grafting. The data show that incubation of cultured skin substitutes in medium containing vitamin C results in greater viability (higher BrdU and MTT), more complete basement membrane development at 2 wk, and better epidermal barrier (lower surface electrical capacitance) at 5 wk in vitro. After grafting, cultured skin substitutes with vitamin C developed functional epidermal barrier earlier, had less wound contraction, and had more HLA-positive wounds at 8 wk than without vitamin C. These results suggest that incubation of cultured skin substitutes in medium containing vitamin C extends cellular viability, promotes formation of epidermal barrier in vitro, and promotes engraftment. Improved anatomy and physiology of cultured skin substitutes that result from nutritional factors in culture media may be expected to improve efficacy in treatment of full-thickness skin wounds.

Overexpression of vascular endothelial growth factor accelerates early vascularization and improves healing of genetically modified cultured skin substitutes

Cultured skin substitutes (CSS) lack a vascular plexus, leading to slower vascularization after grafting than split-thickness skin autograft. CSS containing keratinocytes genetically modified to overexpress vascular endothelial growth factor (VEGF) were previously shown to exhibit enhanced vascularization up to 2 weeks after grafting to athymic mice. The present study examines whether enhanced vascularization compared with controls persists after stable engraftment is achieved and analyzes VEGF expression, wound contraction, and engraftment. Control and VEGF-modified (VEGF+) CSS were grafted onto full-thickness wounds in athymic mice. VEGF expression was detected in VEGF+ CSS 14 weeks after grafting. Graft contraction was significantly lower in VEGF+ CSS compared with controls, suggesting more stable engraftment and better tissue development. Positive HLA-ABC staining, indicating persistence of human cells, was seen in 86.7% (13/15) of grafted VEGF+ CSS, compared with 58.3% (7/12) of controls. Differences in dermal vascularization between control and VEGF+ grafts were significant 1 week after surgery, but not at later times. However, the distribution of vessels was different, with more vessels in the upper dermis of VEGF+ grafts. These results suggest that VEGF overexpression in genetically modified CSS acts to accelerate early graft vascularization and can contribute to improved healing of full-thickness skin wounds.

Tissue-engineered skin substitutes

The last two years have seen new tissue-engineered skin substitutes come onto the market and begin to resolve the various roles to which each is best suited. It is becoming evident that some of the very expensive cell-based products have cost-benefit advantage despite their high price and are valuable within the restricted applications for which they are intended. The use of skin substitutes for testing purposes has extended from epidermal keratinocytes to other integumentary epithelia and into preparations containing multiple cell types in which reactions resulting from paracrine interactions can be examined. Challenges remain in the application of gene therapy techniques to skin substitutes, both the control of transgene expression and in the selection of suitable genes to transfect. A coming challenge is the production of tissue-engineered products without the use of animal products other than human cells. A challenge that may be diminishing is the importance of acute rejection of allogeneic tissue-engineered skin substitutes.