Cultured keratinocyte sheets enhance spontaneous re-epithelialization in a dermal explant model of partial-thickness wound healing

Role of neuropeptides, neurotrophins, and neurohormones in skin wound healing

Due to the close interactions between the skin and peripheral nervous system, there is increasing evidence that the cutaneous innervation is an important modulator of the normal wound healing process. The communication between sensory neurons and skin cells involves a variety of molecules (neuropeptides, neurohormones, and neurotrophins) and their specific receptors expressed by both neuronal and nonneuronal skin cells. It is well established that neurotransmitters and nerve growth factors released in skin have immunoregulatory roles and can exert mitogenic actions; they could also influence the functions of the different skin cell types during the wound healing process.

Migration of the epidermal over the dermal component (epiboly) in a bilayered bioengineered skin construct

A bilayered bioengineered living skin construct (LSC) consisting of viable human neonatal keratinocytes over a collagenous dermis seeded with dermal fibroblasts has been used extensively in difficult-to heal human wounds. Its biological properties include production of several mediators, cytokines, and growth factors and the ability to heal itself upon injury. In this study, we investigated the process of keratinocyte migration in LSC. At baseline, 6-mm punch biopsies of the construct were placed in serum-free medium (AIM-V) or Dulbecco's modified Eagle medium. At varying time points, the LSC samples were processed and analyzed using histology and immunohistochemistry. By 72 h, in a time-dependent manner, the overlying epidermis had migrated over and enveloped the entire underlying dermis, a process known as epiboly. Increasing concentrations of neutralizing antibodies to epidermal growth factor or interleukin-1 alpha down-regulated the extent of epiboly, as measured using computerized planimetry, but antibodies to transforming growth factor-beta 1 did not affect it. The consistent expression of laminin V, alpha3beta1 integrin, and vitronectin (epibolin) and its integrin receptor (alphavbeta5) characterized the tongue of migrating epidermis. Increasing concentrations of antibodies to vitronectin blocked the process of epiboly, as did antibodies to the alphavbeta5 integrin receptor, which mediates vitronectin-driven keratinocyte locomotion. This process of epiboly provides novel mechanisms of action for bioengineered skin constructs.

In vitro cultured autologous pre-confluent oral keratinocytes for experimental prefabrication of oral mucosa

The reconstruction of large defects after head and neck cancer resection often requires composite tissue transfer to replace a combination of bone, muscle and mucosa. Thus, tissue engineering techniques may be useful for oral mucosal reconstructive surgery to prefabricate mucosal tissue on the muscle flap in vivo, instead of using conventional skin-bearing composite flaps. The aim of this study was to investigate whether autogenous pre-confluent oral keratinocytes (PCOK) cultured in vitro can create mucosal coverage on muscle in vivo, in a single grafting procedure. In 30 Wistar rats, with a small piece of oral mucosa (2 mm x 5 mm), oral keratinocytes were isolated and then seeded on a hydrophilic PTFE membrane (n = 50) in serum-free culture condition. After 48 h, the membrane, together with the PCOK, was transplanted onto the gracilis muscle to fabricate a mucosal flap in vivo. The wound bed was closed primarily until the time of examination. Biopsies were carried out 1, 2, 3, and 4 weeks, respectively, after transplantation and were evaluated immunohistochemically (AE1/AE3 anti-pancytokeratin, cytokeratin 5/6, collagen IV, laminin, lectin-specific labeling of N-acetylglucosamine oligomeres of endothelial cells) with relation to the following criteria: (1) graft acceptance; (2) inflammatory signs; (3) structural changes and keratinocyte lining; (4) expression of basement membrane components; and (5) vascularization. Ninety-one percent of the grafts showed uniform epithelial layers. The mean number of reconstructed epithelial cell layers was 1.7, 2.0, 1.85 and 2.7 at 1, 2, 3 and 4 weeks, respectively after transplantation (P = 0.342). Collagen IV, laminin and lectin-specific capillaries developed between the neoepithelium and the underlying muscular layer. Only two specimens showed signs of infection 2 weeks after transplantation. In conclusion, this experiment demonstrated that PCOK grafts on muscle in vivo can achieve uniform multi-layered oral epithelial coverage in a short period of time. This technique may be a useful alternative tool for oropharyngeal reconstructive surgery and is also worth considering for further clinical studies.

Thrombospondin-1 promotes fibroblast-mediated collagen gel contraction caused by activation of latent transforming growth factor beta-1

BACKGROUND

Grafting of cultured epithelium has become a useful technique for the treatment of epithelial defects, since grafted epithelial cells secrete factors promoting wound healing. We identified one such factor produced by cultured oral epithelial cells as thrombospondin-1 (TSP-1). Recently, TSP-1 was reported to act as an activator of transforming growth factor-beta1 (TGF-beta1).

OBJECTIVE

The role of TSP-1 in wound healing and its mechanism were investigated in vitro and in vivo.

METHODS

The cultured oral epithelial cell-conditioned medium was harvested and applied to Heparin-Sepharose affinity chromatography. Proteins were analyzed by N-Terminal sequencer. TSP-1 and the other factors were applied to fibroblasts-mediated collagen gel contraction assay. The amount of TGF-beta1 (latent TGF-beta1 (LTGF) and active TGF-beta1) in collagen gels was quantified by ELISA and Western blotting analysis. Collagen sponges were soaked with TSP-1 and implanted subcutaneously into rats.

RESULTS

A 38 kDa protein secreted from cultured oral epithelial cells was found to be human TSP-1. TSP-1 promoted collagen gel contraction activity, and anti-human TSP-1 and TGF-beta1 antibody inhibited the activity. The diameters of the gels treated with LTGF and TSP-1 were reduced to a greater extent than those of gels treated with either factor alone. Although there were no significant differences in the amounts of total TGF-beta1, which include LTGF, the amount of 25 kDa TGF-beta1 was 3.30-fold greater in TSP-1-treated samples than controls. In vivo, 7 days after implantation, increased numbers of fibroblasts were observed in the sponges treated with TSP-1.

CONCLUSION

These findings suggested that TSP-1 causes collagen gel contraction by activation of LTGF. TSP-1 is expected to be especially suitable for regulating wound healing.

Evaluation of dermal-epidermal skin equivalents ('composite-skin') of human keratinocytes in a collagen-glycosaminoglycan matrix(Integra artificial skin)

Integra artificial skin (Integra LifeSciences Corp., Plainsboro, NJ, USA) is a dermal template consisting of bovine collagen, chondroitin-6-sulphate and a silastic membrane manufactured as Integra. This product has gained widespread use in the clinical treatment of third degree burn wounds and full thickness skin defects of different aetiologies. The product was designed to significantly reduce the time needed to achieve final wound closure in the treatment of major burn wounds, to optimise the sparse autologous donor skin resources and to improve the durable mechanical quality of the skin substitute. The clinical procedure requires two stages. The first step creates a self neodermis, the second creates a self epidermis on the neodermis. However, it is desirable to cover major burn wounds early in a single step by a skin substitute consisting of a dermal equivalent seeded in vitro with autologous keratinocytes ('composite-skin') out of which a full thickness skin develops in vivo.The goal of this experimental study was to develop a method to integrate human keratinocytes in homogeneous distribution and depth into Integra Artificial Skin. The seeded cell-matrix composites were grafted onto athymic mice in order to evaluate their potential to reconstitute a human epidermis in vivo. We were able to demonstrate that the inoculated human keratinocytes reproducibly displayed a homogeneous pattern of distribution, adherence, proliferation and confluence. The cell-matrix composites grafted in this model exhibited good wound adherence, complete healing, minor wound contraction and had the potential to reconstitute an elastic, functional and durable human skin. Histologically we were able to show that the inoculated human keratinocytes in vivo colonised the matrix in a histomorphologically characteristic epidermal pattern (keratomorula, keratinocyte bubbling) and developed a persisting, stratified, keratinising epidermis which immunohistologically proved to be of human origin. These experimental results demonstrate the establishment of an effective cell cultivation process which may be suitable for scale-up production of the epidermal component as large-scale composite-skin grafts. When seeded into Integratrade mark and grafted onto the nude mouse a replacement skin with normal functioning dermal-epidermal components was developed. These results encourage the design of a clinical trial to assess the function of this composite graft in man.

Involvement of keratinocyte activation phase in cutaneous graft healing: comparison of full-thickness and split-thickness skin grafts

BACKGROUND

Little is known about keratinocytic activation in the graft take and healing process.

OBJECTIVE

To investigate the clinical and molecular differences between pure epidermal sheet graft (PESG), split-thickness skin graft (STSG), and full-thickness skin graft (FTSG).

METHODS

Three different thickness skin grafts (PESG, STSG, and FTSG) were performed onto three kinds of porcine wounds: shallow, deep, and full. Graft take, contraction, and Ki-67 and beta1 integrin expression in epidermis were studied.

RESULTS

All grafts took well. As expected, full wounds covered by PESG and STSG contracted more than those covered by FTSG, whereas shallow wounds covered by FTSG contracted more than those covered by STSG. No difference in contracture was observed among deep wounds covered by PESG, STSG, and FTSG. Up-regulation of Ki-67 and beta1 integrin expression was greater in PESG and STSG, compared with little expression in FTSG.

CONCLUSION

The keratinocytic activation phase may occur both in the STSG and PESG healing process, as well as serum imbibition, inosculatory, and revascularization phases.

Secretory leukocyte protease inhibitor is a novel inhibitor of fibroblast-mediated collagen gel contraction

Cultured epithelial cells, including those from the oral epithelium, have been successfully applied in the promotion of scarless wound healing. Factors released from the epithelial cells are thought to contribute significantly to the beneficial effects. In the conditioned medium of human oral epithelial cells, we found a factor that inhibited fibroblast-mediated collagen gel contraction, an in vitro model of wound healing and scar formation. Biochemical analysis identified the factor to be human secretory leukocyte protease inhibitor (SLPI). Fibroblasts transfected with SLPI cDNA showed reduced gel-contracting activity. SLPI purified from the conditioned medium inhibited gel contraction in a dose-dependent manner, and anti-SLPI antibody counteracted this activity. Upon SLPI treatment, human skin fibroblasts in collagen gel became shorter in length and were inhibited in pseudopodia extension. Furthermore, after SLPI treatment, alpha(1)-integrin immunoreactivity decreased, and cyclic AMP levels increased. Excessive gel contraction was observed when fibroblasts treated with TGF-beta1 and fibroblasts from hypertrophic and from keloid scar tissue were cultured in collagen gel. SLPI was also effective in inhibiting gel contraction in the above three models of scar formation. These results suggest that SLPI may be useful in promoting scarless wound healing.

Cultured human sole-derived keratinocyte grafts re-express site-specific differentiation after transplantation

Cultured epithelial autografts (CEA) derived from sole skin were transplanted to full-thickness wounds excised to muscle fascia over a variety of diverse body sites in 12 pediatric patients treated for acute burns or giant congenital nevi. The skin regenerated from the grafts was biopsied from 7 days to 6 years after grafting. The resultant epidermal phenotype was analyzed histologically and by immunohistochemical localization of keratin 9 (K9) as objective evidence of sole-type site-specific differentiation. Expression of K9 was also verified by one-dimensional gel electrophoresis of epidermal cytoskeletal extracts and K9 immunoblot analysis. Grafts prepared from epidermis of axilla; groin or foreskin and transplanted to wounds of comparable depth in an identical manner in the same patients served as controls of postgrafting differentiation. Biopsies of sole skin from amputation specimens from patients of comparable age served as normal positive controls, and biopsies of nonsole skin from patients of comparable age served as normal negative controls. As early as 2 weeks postgrafting, the histologic appearance of sole-derived CEA differed substantively from that of axilla- or groin-derived CEA controls and displayed a phenotype characteristic of sole skin with a thick compact stratum corneum, a thick stratum granulosum, and a distinct stratum lucidum. In sole-derived grafts rete ridges regenerated within 2 months postgrafting, whereas nonsole-derived grafts required 4-6 months for rete ridge regeneration. Once acquired, the sole skin phenotype was maintained long-term by all sole-derived CEA. In vitro, sole-derived keratinocytes synthesized little, if any, K9. However, within 7 days after grafting, K9 synthesis by multiple suprabasal keratinocytes was seen within the epidermis regenerated from sole-derived CEA. Protein of K9 appeared progressively more diffuse throughout the suprabasal layers, attaining a confluent pattern of expression comparable to normal controls of sole skin by 6 to 12 months postgrafting, and the confluent pattern of suprabasal K9 synthesis was maintained long-term. The results demonstrate that site-specific differentiation is an intrinsic property of postnatal human keratinocytes and can be expressed and maintained in a permissive environment in the absence of dermal tissue.

Characterization of epidermal wound healing in a human skin organ culture model: acceleration by transplanted keratinocytes

Few data are available on early regeneration of human epidermis in vivo. We have established a supravital skin organ culture model for epidermal wound healing by setting a central defect (3 mm diameter) in freshly excised skin specimens and culturing under air exposure. Re-epithelialization was followed for up to 7 d by histology and immunohistologic analysis of various markers for differentiation and proliferation. In 12 of 19 cases (63%; 5% fetal calf serum) or six of 21 cases (29%; 2% fetal calf serum), the wounds were re-epithelialized spontaneously after 7 d. After transplantation to the wounds of 1-2 x 10(6) dissociated allogenic cultured epidermal or about 1 x 10(6) autologous outer root sheath keratinocytes, 18 of 21 cases (86%; 5% fetal calf serum) or 17 of 21 cases (81%; 2% fetal calf serum) were healed within the same period. Histologically, early neoepithelium (3 d) was disordered after keratinocyte transplantation, whereas later (7 d) it had gained a more ordered stratification, exhibiting a thin discontinuous granular and a compact horny layer. At this stage, not only hyperproliferative (CK 6) but also, abundantly, maturation-associated cytokeratins (CK 1, CK 10) were detected immunohistochemically. Analyses of regenerated epidermis after transplantation of (i) keratinocytes labeled in vitro with BrdU and (ii) heterosexual keratinocytes by immunohistochemistry and fluorescence in situ hybridization for the Y chromosome, respectively, clearly showed that external keratinocytes are physically integrated into the regenerated epidermis and extendedly contribute to its formation. The data presented here demonstrate improvement and acceleration of epidermal re-epithelialization by transplantation of keratinocytes.

Vascularized collagen-glycosaminoglycan matrix provides a dermal substrate and improves take of cultured epithelial autografts

Cultured epithelial autografts are an important adjunct in treating severely burned patients, greatly expanding the epidermis using a small donor site. Problems with cultured epithelial autografts include the time delay to culture cells to confluence and variable take on full-thickness wounds. Dermal allografts have been used as a substrate to improve the take of cultured epithelial autografts. This study examined the effect of a vascularized collagen-glycosaminoglycan matrix as a substrate for cultured epithelial autografts. The matrix was grafted onto 12 full-thickness wounds in Yorkshire pigs and allowed to vascularize for 10 days. The cultured epithelial autografts were applied over the vascularized collagen-glycosaminoglycan matrix (n = 12) or onto freshly excised full-thickness wounds (n = 10). Gross and histologic observations were made over a 3-week period. Gross observations at 7 days indicated cultured epithelial autografts to have nearly complete confluence when applied to wounds treated by collagen-glycosaminoglycan, whereas cultured epithelial autografts applied to freshly excised wounds did not take. Gross determination of epithelial confluence was verified by histologic analysis of randomly selected wounds. Histologic epithelial confluence of cultured epithelial autografts on collagen-glycosaminoglycan (98 +/- 4 percent) was significantly greater than that on full-thickness wounds (4 +/- 10 percent). Electron microscopy of the cultured epithelial autografts/collagen-glycosaminoglycan construct demonstrated anchoring fibrils at the dermal-epidermal junction at day 7. The neoepidermis of wounds treated by cultured epithelial autografts/collagen-glycosaminoglycan was hyperplastic at day 7 but developed a normal maturation sequence by 21 days. Results from this study suggest that vascularized collagen-glycosaminoglycan matrices produce a favorable substrate for cultured epithelial autografts and may improve cultured epithelial autografts take in burn patients.

Effect of keratinocyte seeding of collagen-glycosaminoglycan membranes on the regeneration of skin in a porcine model

A collagen-glycosaminoglycan matrix, impregnated with autologous keratinocytes, was applied as island grafts onto full-thickness porcine wounds to determine whether complete epidermal coverage could be achieved in a single grafting procedure. Twenty-four grafts with seeding densities ranging from 0 to 3,000,000 cells/cm2 were used to determine the kinetics of epidermal coverage. The time sequence of epidermal formation was then studied between days 14 and 28 using four additional grafts, each seeded with a density of 500,000 cells/cm2. Autologous keratinocytes proliferated as the collagen-glycosaminoglycan matrix was vascularized to form a confluent epidermis by 2 weeks in matrices seeded with at least 100,000 cells/cm2. The epidermal thickness and the number of keratinocyte cysts observed in the neodermis at 2 weeks increased linearly with the logarithm of the seeding density. Sequential analysis of neoepidermis showed the nascent epidermis to be hyperplastic, parakeratotic, and focally lacking in granular layer differentiation at 2 weeks. After 2 weeks, it underwent normal maturation and differentiation. Irrespective of seeding density at 2 weeks the collagen-glycosaminoglycan matrix was well vascularized, contained a dense cellular infiltrate, and was almost completely degraded. These studies demonstrate that seeded keratinocytes proliferate and differentiate to form a confluent epidermis by 2 weeks in matrices seeded with at least 100,000 cells/cm2.

Retroviral marking identifies grafted autologous keratinocytes in porcine wounds receiving cultured epithelium

Cultured epithelial autografts are often applied to wounds with a capacity for regeneration from dermal appendages. It is unclear in these circumstances whether the cultured autografts act merely as a biologic dressing or whether they become incorporated into the new epithelium. We have used retroviral gene transfer techniques to identify autologous keratinocytes in an established porcine model of cultured epidermal (CE) grafting. Porcine keratinocytes were transduced with an MFG-lacZ nls vector produced by the amphotropic packaging line GP+EnvAm12. Transduction rates of 15.1%, in the absence of selection, were achieved by a single passage on gamma-irradiated retroviral producers as a feeder layer. Full-thickness wounds were created on Large White pigs and isolated from the surrounding skin by a polytetrafluoroethylene chamber. Wounds were grafted initially with autologous de-epidermized dermis (DED), followed 7 d later by sheets of retrovirally marked or unmarked CE autografts. Two weeks after grafting, the mean area of epithelium was 48.4% in wounds that received CE grafts and 32.3% in wounds that were left as DED alone. The epithelium on DED represents regeneration from dermal appendages. The contribution made by the autograft cells to the new epidermis was demonstrated unequivocally, however, by lacZ-positive areas visible macroscopically on the surface of the excised wound. In cryostat sections through the lacZ-positive areas, retrovirally marked cells were present at both superficial and basal positions in the new epithelium.

Characterization of a new in vitro model for studies of reepithelialization in human partial thickness wounds

Reepithelialization of artificial partial thickness wounds made in biopsies of human skin was determined after 3, 5, or 7 d of incubation, submerged or elevated to the air-liquid interface. The biopsies were reepithelialized within 5-7 d, with a more complete epidermal healing in wounds exposed to air. Both types of wounds showed similar time-course in deposition of basement membrane components, as detected by immunofluorescence labeling. Laminin and collagen type VII were deposited underneath the migrating tips, whereas collagen type IV was detected after reepithelialization. Markers of terminal differentiation showed a pattern close to normal in the air-liquid incubated wounds after reepithelialization. Involucrin was detected in the suprabasal regions of the migrating epidermis and thereafter in the upper half of neo-epidermis in the air-liquid incubated wound. Filaggrin could not be detected in the submerged wounds at any time during healing, whereas wounds exposed to air showed a well-differentiated epidermis by Day 7. Tritiated thymidine-incorporation indicated proliferation of epidermal and dermal cells during reepithelialization and a maintained viability, as shown by cultivation of endothelial- and fibroblast-like cells obtained from the dermis 7 d after wounding. Reepithelialization in this human in vitro model is supported by a matrix close to normal with the possibility of extracellular influences and cell-cell interactions and, in addition, the technique is simple and reproducible. Therefore, we suggest this model for studies of regeneration in culture and as a complement to in vivo studies on epidermal healing.

Differential epithelial outgrowth of plucked and microdissected human hair follicles in explant culture

In the present study we prepared explant cultures of plucked total hair follicles and of fragments microdissected from the following regions: B1 (bulb region), B2 (intermediate region), B3-1 (lower central outer root sheath, ORS), B3-2 (upper central ORS) and B4 (area of fracture). The growth capacities, the start of epithelial outgrowth, the stages of differentiation and apoptosis were studied immunohistochemically in early and late explant cultures using a battery of antibodies against cytokeratins, growth factor receptors and cell adhesion molecules and proliferation markers. Whole plucked hair follicles showed epithelial outgrowths exclusively in the upper central ORS (B3-2) starting early, mostly by day 3. In microdissected fragments, in contrast, outgrowths were more widespread, mostly in B3-2 and B3-1, and started early, but were also of late onset in some cases of B2 and B4. Epithelial outgrowths exhibited a basal layer of small cuboidal cells in a low stage of differentiation and one to two suprabasal layers of large prickle-like cells expressing late differentiation markers. The former expressed the receptor of nerve growth factor (NGF) heterogeneously whereas epidermal growth factor (EGF) receptor was not detectable. This is similar to ORS cells of this area in vivo. The proliferative activity of the outgrowths was always restricted to peripheral cells. Thus no essential differences in differentiation of outgrowing cells were detected. These results suggest that keratinocytes with the highest growth capacities in plucked human hair follicles are localized in the lower central ORS (corresponding to B3-2) and some with a lower capacity in the upper central ORS (corresponding to B3-1) as established after microdissection. This is in agreement with the bulge activation theory. NGF may also play a role in hair growth.

Randomized trial comparing cryopreserved cultured epidermal allografts with hydrocolloid dressings in healing chronic venous ulcers

BACKGROUND

Cultured epidermal allografts have been successfully used to treat a variety of wounds. Their postulated mechanism of action is through release of cytokines that stimulate epithelialization. On the basis of previous experience we expected ulcers treated with cryopreserved cultured allografts (CCAs) to be healed by 6 weeks. Hydrocolloid dressings (HCDs) have also been reported to be effective in the treatment of venous ulcers.

OBJECTIVE

Our purpose was to compare the effectiveness of CCAs with HCDs in healing chronic venous ulcers.

METHODS

Forty-three patients with 47 ulcers were enrolled in a randomized controlled trial. Ulcers not healed by 6 weeks were changed to the other treatment.

RESULTS

No difference in the number of healed ulcers between the two groups was observed at 6 weeks. Healing rate, percent reduction of initial ulcer size, and radial progression toward wound closure were significantly greater for CCAs than for HCDs. Pain relief was not significantly different.

CONCLUSION

CCAs achieve more rapid healing and greater reduction in ulcer size than HCDs.

Upper human hair follicle contains a subpopulation of keratinocytes with superior in vitro proliferative potential

We and others have shown previously that corneal keratinocyte stem cells can proliferate in vitro better than their progeny cells. In this paper, we applied this approach to the identification of hair follicular stem cells. When human scalp hair follicles were placed in explant culture, the bulge area yielded best outgrowths. In another experiment, we isolated different subpopulations of human follicular keratinocytes by micro-dissection, dispersed them by trypsin/EDTA into single cells, and grew them in the presence of 3T3 feeder cells. The keratinocytes were then subcultured under identical conditions to compare their in vitro life span. Our results indicate that the life span of keratinocytes of the upper follicle (containing mainly the isthmus area) > sebaceous gland > lower follicle (between the bulge and bulb) > bulb (containing the matrix cells). The cultured upper follicular keratinocytes tend to be small and relatively uniform in size. The poor in vitro growth of matrix cells may reflect their non-stem cell nature and/or special growth requirement(s) satisfied in vivo by the neighboring dermal papilla cells. Unexpectedly, we found that the upper follicular keratinocytes grow even better than epidermal keratinocytes. The existence of a subpopulation of keratinocytes with an in vitro growth potential superior than other known keratinocytes of the skin supports the hypothesis that follicular stem cells reside in the upper follicle. Our data also raise the possibility that putative follicular stem cells are involved not only in forming the follicle, but also in the long-term maintenance of the epidermis. Finally, we discuss the possibility that keratinocyte stem cells, as defined by their in vivo slow-cycling nature, are absent in culture.

Role of growth factors in cutaneous wound healing: a review

The well-orchestrated, complex series of events resulting in the repair of cutaneous wounds are, at least in part, regulated by polypeptide growth factors. This review provides a detailed overview of the known functions, interactions, and mechanisms of action of growth factors in the context of the overall repair process in cutaneous wounds. An overview of the cellular and molecular events involved in soft tissue repair is initially presented, followed by a review of widely studied growth factors and a discussion of commonly utilized preclinical animal models. The article concludes with a summary of the preliminary results from human clinical trials evaluating the effects of growth factors in the healing of chronic skin ulcers. Throughout, the interactions among the growth factors in the wound-healing process are emphasized.

Delivery of growth factor to wounds using a genetically engineered biological bandage

Increasing the rate of wound healing of acute wounds and promoting the closure of chronic ulcers is an important goal in wound therapy. Growth factors have been shown to facilitate this process; however, the systems described for growth factor delivery are not ideal. In the present report we demonstrate the feasibility of a new method of delivering growth factors to the wound site using a genetically engineered biological bandage. The bandage consists of keratinocytes (SCC-13 cells) that are engineered by gene transfer to produce high levels of bovine growth hormone (bGH). bGH was selected for these studies because it can be easily distinguished from rat and human growth hormone in wound fluids and culture medium. The bGH-producing cells are contained and maintained in serum-free medium inside an envelope composed of a low protein binding, 0.2 micron pore size, polysulfone membrane. The genetically engineered cells cannot escape from the bandage, but the bGH is freely released into the surrounding culture medium. When placed onto a full-thickness, surgically generated wound on rats, the cells within the bandage continue to produce and release bGH into the wound for at least 3 days. This system is a safe and reliable way of providing real-time delivery of any desired biomolecule into the wound site.

Lateral growth and terminal differentiation during repeated epidermal regeneration in vitro

By incubating multilayered primary cultures of human epidermal keratinocytes in a low calcium medium, the suprabasal layers can be stripped off leaving a basal cell monolayer. When this monolayer is refed normal calcium medium a reproducible series of cell kinetic, morphological and biochemical changes take place resulting in the regeneration of a multilayered tissue. The stripping procedure seems to induce the selective proliferation of a cohort of basal cells that is committed to vertical migration and rapid terminal differentiation. In contrast, when the basal cells are allowed to regenerate in the presence of the strong mitogen, cholera toxin, lateral growth and continued proliferation are favoured at the expense of the capacity of the cells to differentiate. Repeated stripping of the same cultures disclosed a considerable heterogeneity in the capacity of the basal cells to regenerate the suprabasal layers. The number of times the basal cells could restore the suprabasal layers after repeated stripping varied from four to nine times. A negative correlation between donor age and regenerative capacity was observed. The experiments with repeated stripping of the same cultures also showed that the capacity to proliferate and to restore the multilayering was fully retained for at least four cycles of stripping-regeneration, whereas the capacity to terminally differentiate was rapidly lost. It is suggested that the present system of regenerating epidermal tissue cultures may serve as an experimental model for the study of epidermal tissue homeostasis and cellular aging.

Hyaluronan and wound healing: a new perspective

Hyaluronan has long been associated with the remodelling extracellular matrix. Such remodelling occurs in development, growth and wound healing. This role has been thought to be related to the physical structure and chemical composition of the pure glycosaminoglycan chain. We question this proposition and present evidence which suggests that proteins associated with hyaluronan may be more critical determinants of tissue remodelling.

Epidermis generated in vitro: practical considerations and applications

The technology for culture of epidermis is one of the most advanced to date for generation of a tissue in vitro. Cultured epidermis is already used for a number of applications ranging from use as a permanent skin replacement to use as an organotypic culture model for toxicity testing and basic research. While simple epidermal sheets have been grafted successfully, more advanced models for skin replacement consisting of both dermal and epidermal components are in development and being tested in a number of laboratories. One of the most advanced in vitro models is the living skin equivalent, an organotypic model consisting of a collagen lattice contracted and nourished by dermal fibroblasts overlaid with a fully formed epidermis.