Technology update Advances in tissue-engineered skin substitutes


widely used biological substitutes worldwide are porcine skin, cadaveric skin and amnion. Depending on their composition, both


synthetic and biological skin substitutes can further be divided into dermal, epidermal or dermo-epidermal replacements. Sustainability is an additional factor that decides the temporary or permanent nature of skin substitutes. Temporary skin substitutes provide transient physiologic wound closure by protecting the wound from trauma, providing a barrier to bacteria and pathogens, and maintaining a moist wound environment until repair of the damaged tissue is complete[6]


.


Conversely, permanent skin substitutes are designed to provide permanent wound closure, replace the lost skin components (epidermis, dermis or both), and integrate with the recipient tissue [Box 3; Table 1].


TISSUE ENGINEERING OF THE SKIN To culture skin in the laboratory, a skin biopsy from the patient or a donor is obtained to isolate the different cell types [Figure 2]. After trimming all excess fat, it is surface-sterilised in alcohol and placed in an appropriate culture medium. After approximately 24 hours, the epidermis is separated from the dermis and the two layers are then enzymatically treated to digest the bonds that bind the different cell types together (eg the extracellular matrix in the dermis is digested using collagenase to isolate the fibroblasts). Likewise, the epidermal keratinocytes are isolated from the epidermal layer. Each cell type is allowed to proliferate in


its appropriate culture medium. The cultured cell types are then used either in isolation (eg epidermal substitutes such as Epicel® [Genzyme], Myskin™ [Altrika] and ReCell® [Avita Medical]) or in a collagen construct or scaffold (eg in dermal substitutes such as TransCyte® [Advanced BioHealing] and Dermagraft® [Advanced BioHealing]). Certain dermo-epidermal substitutes can be


constructed with keratinocytes, melanocytes and fibroblasts in a collagen hydrogel. Vascular endothelial cells can also be added into the hydrogel to help form capillaries[16-19]


. Acellular


dermal scaffold (eg Alloderm® [LifeCell], Integra® [Integra LifeSciences Corporation], Biobrane® [Smith & Nephew], Matriderm® [Eurosurgical]) can also be engineered to provide coverage of a deep wound or burn. However, an autograft in the form of a STSG is


Poor integration and multiple procedures In deep burns or wounds the lack of dermis and epidermis requires a product that can replace these two layers. Although


needed for epithelial cover if a dermal (cellular or acellular) product is used in isolation (without epithelial layer or keratinocytes).


MONITORING Although a reliable indicator or marker to monitor the effectiveness of an engineered skin substitute would be helpful, to date, they remain largely experimental. Cytokeratin 19 (CK19) is a protein that is expressed in basal keratinocytes and is a marker for epidermal homeostasis and an indicator of young keratinocytes or stem cells[20,21]


.


CK19 expression indicates a thriving and functional epidermis that signals a potentially successful transplantation. Other keratinocyte stem cell markers include integrin α6 chain (high expression) and the protein CD71 (low expression)[22,23]


. The accuracy of these


markers has to be corroborated with a reliable bioassay, in this case the formation of a normal-looking stratified epidermis approximately 12 weeks after transplantation. No epidermal marker is currently in clinical use — likewise there is no established dermal marker at present.


CONCERNS Although TESSs have many uses and have established an indispensable role in the management of a variety of wounds — particularly burns — there are a number of concerns that remain to be addressed, which currently preclude their widespread use in routine clinical practice.


Lack of level 1 evidence Randomised controlled trials are lacking for many TESSs, and the current evidence is based on non-randomised prospective trials, retrospective reviews, small case series (institutional or personal), and isolated case reports. No trials have evaluated the effectiveness of two comparable ‘like-to-like’ products (eg, dermal versus dermal skin substitute). Currently large, multicentre, double-blind randomised trials are underway for some products[24]


.


References


21. Pontiggia L, Biedermann T, Meuli M, et al. Markers to evaluate the quality and self-renewing potential of engineered human skin substitutes in vitro and after transplantation. J Invest Dermatol 2009; 129: 480–90.


22. Li A, Simmons PJ, Kaur P. Identification and isolation of candidate human keratinocyte stem cells based on cell surface phenotype. Proc Natl Acad Sci USA 1998; 95: 3902–7.


23. Webb A, Li A, Kaur P. Location and phenotype of human adult keratinocyte stem cells of the skin. Differentiation 2004; 72: 387–95.


24. Enoch S, Grey JE, Harding KG. Recent advances and emerging treatments. Br Med J 2006; 332: 962–5.


25. Haynes B. The history of burn care. In: Bosivich J, ed. The Art and Science of Burn Care. 1987; Aspen, Rockville, Md, USA: 3.


26.MacNeil S. Progress and opportunities for tissue engineered skin. Nature 2007; 445: 874–80.


27. Böttcher-Haberzeth S, Biedermann T, Reichmann E. Tissue engineering of skin. Burns 2010; 36(4): 450–60.


28. Abdel-Malek ZA. Endocrine factors as effectors of integumental pigmentation. Dermatol Clin 1988; 6: 175–183.


29. Nordlund JJ, Abdel-Malek ZA, Boissy RE, et al. Pigment cell biology: an historical review. J Invest Dermatol 1989; 92: 53S–60S.


30. Boyce ST, Kagan RJ, Meyer NA, et al. The 1999 Clinical Research Award. Cultured skin substitutes combined with Integra to replace native skin autograft and allograft for closure of full-thickness burns. J Burn Care Rehabil 1999; 20: 453–461.


31. Supp AP, Wickett RR; Swope VB, et al. Incubation of cultured skin substitutes in reduced humidity promotes cornification in vitro and stable engraftment in athymic mice. Wound Repair Regen 1999; 7: 226–37.


32. Swope VB, Supp AP, Cornelius JR, et al. Regulation of pigmentation in cultured skin substitutes by


www.woundsinternational.com


30


Technology and product reviews


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61