Epidermal allografts

Ovine model of burn wounds grafted with ovine cadaver skin

Shortage in autograft to cover burn wounds involves a frequent use of cadaver skin (CS) as a temporary cover to prevent infection, dehydration and preparation of wounds for subsequent autografting. We aimed to establish an ovine model of burn wound healing using ovine CS (OCS). Quality and efficacy of fresh and frozen OCS overlaid on to excised 3rd degree flame burn wounds in sheep were evaluated in comparison to autograft. Histologically, autografted wounds maintained normal skin structure at different time points. Wounds overlaid with fresh OCS graft showed signs of rejection starting from day 7. At day 14, the epidermis was mostly rejected. The rejection was completed by day 20 with signs of immunoreaction and presence of many immune cells. Frozen OCS was rejected in the same pattern. Immediately prior to grafting, the thickness was comparable between freshly prepared and frozen OCS for 10 or 40 days. Significant reduction in viability was detected in OCS frozen for 40 days. Both fresh or frozen ovine OCS were rejected within 10 days that mimics CS rejection time in humans (∼8.4 days), suggesting that ovine model of burn wound grafted with OCS can successfully be used in burn wound research mimicking clinical scenario.

Assessment of organ culture for the conservation of human skin allografts

OBJECTIVE

Human skin allografts are used in the treatment of severe burns and their preservation is therefore critical for optimal clinical benefit. Current preservation methods, such as 4 degrees C storage or cryopreservation, cannot prevent the decrease of tissue viability. The aim of this study was to assess viability and function of skin allografts in a new skin organ culture model, allowing conservation parameters as close as possible to physiological conditions: 32 degrees C, air-liquid interface and physiological skin tension.

DESIGN

Twelve skin samples, harvested from 6 living surgical donors, were conserved 35 days in two conditions: conservation at 4 degrees C and organ culture. Viability and function of skin samples were investigated at Day 0, 7, 14, 21, 28 and 35 using cell culture methods (trypan blue exclusion, Colony Forming Efficiency and Growth Rate), histopathological and histoenzymological studies (Ki67 immunostaining).

RESULTS

In the two conditions, fibroblast and keratinocyte viability was progressively affected by storage, with a significant decrease observed after 35 days. No statistical difference could be observed between the two conditions. The two methods were also comparable regarding alterations of fibroblast and keratinocyte culture parameters, which were respectively significantly reduced at Day 7 and 21, compared to fresh skin. By contrast, histopathological and histoenzymological studies revealed a better preservation of skin architecture and proliferative potential at 4 degrees C, as compared to organ culture.

CONCLUSION

These results indicate that skin organ culture does not provide significant advantages for skin allograft preservation. However, its potential use as an experimental model to study skin physiology and wound healing should be further evaluated.

[Autologous cultured skin substitutes]

Progress in cell culture and biomaterial technologies has resulted in commercially available autologous and allogeneic skin substitutes that are composed of keratinocytes and/or fibroblats, in part combined with allogeneic (fibrin) or xenogeneic (collagen, hyaluronan) matrix substances. So far, clinical testing of tissue-engineered products focused on chronic wounds (vascular leg ulcers, diabetic foot ulcers); another major indication, however, is large acute skin defects (burns). During the last decade, partly-controlled clinical trials have been performed with several cultured skin substitutes, studying primarily vascular leg ulcers; a few of these products have been approved for defined indications by the regulatory authorities of various countries. To fulfill regulatory requirements and be eligible for reimbursement, safety as well as cost-effectiveness have to be documented for these novel therapies in contrast to established methods for clearly defined clinical settings; this, in combination with restricted health care resources, is actually hampering the clinical breakthrough of tissue engineering in the treatment of skin wounds, despite undiminished research activities.

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.

Successful treatment of chronic leg ulcers with epidermal equivalents generated from cultured autologous outer root sheath cells

The outer root sheath cells of hair follicles can substitute for interfollicular epidermal keratinocytes, as during healing of skin wounds when these cells migrate onto the denuded area and contribute to epidermal regeneration. Using improved culture techniques, we generated epidermal equivalents from cultured outer root sheath cells of patients suffering from recalcitrant chronic leg ulcers, primarily of vascular origin. In such epidermal equivalents, tissue organization as well as immunolocalization of epidermal differentiation products (keratin 10, involucrin, filaggrin) and integrins were indistinguishable from normal epidermis. As determined by the number of bromodeoxyuridine-incorporating cells, the basal layer contained a large compartment of proliferative cells irrespective of donor age. FACS analysis of the outer root sheath cells, used to prepare the epidermal equivalents, disclosed a fraction of small cells with enhanced expression of beta1-integrin, a potential stem cell marker. in contrast to acute wounds, a major definitive take of grafted cultured autologous keratinocytes has not been convincingly demonstrated in chronic wounds. In a pilot study, grafting of epidermal equivalents generated in vitro from autologous outer root sheath cells on 11 ulcers in five patients resulted in a definitive take rate of about 80%, with subsequent complete healing within 2 to 3 wk of five out of seven ulcers grafted with densely arranged cultures. This improvement in the treatment of chronic leg ulcers with cultured autologous keratinocytes probably depends on the large compartment of proliferative cells as well as on a well-developed horny layer which prevents disintegration of the grafts. Practical advantages of the new technique are its noninvasiveness, the lack of need for surgical facilities or anesthesia, and a short immobilization period after grafting.

Treatment of burns with skin substitutes

More than 2 million persons sustain thermal injuries in the United States annually (Monafo and Crabtree, 1985) and more than 10,000 burn victims die (Collini and Kealey, 1989). The principal factors affecting mortality are the total area burned and the area of third degree (full thickness) burns (Tompkins et al., 1985) with wound sepsis being the leading cause of mortality. Early aggressive excision and immediate covering of the wounds improve survival (Herndon and Parks, 1986). Various biological and synthetic substrates have been employed to replace the injured skin. Most of these provide a permeability barrier which substitutes for the epidermal function of the lost skin. An ideal skin replacement should also provide a substitute for dermis, which provides both support and stability for the epidermal replacement and prevents wound contraction. The dermal and epidermal replacement should be firmly integrated by a complete basement membrane zone (BMZ).