Skin Replacement in Burn Wounds

Genomic Reprogramming and Skin-Like Maturation of Engineered Human Skin Substitutes

BACKGROUND

Cultured skin substitutes (CSS) have been evaluated in clinical trials as an adjunctive treatment for large full-thickness burn wounds. Prepared with autologous fibroblasts, keratinocytes, and biopolymers, CSS can provide permanent wound closure upon engraftment to excised burns.

THE PROBLEM

CSS containing only two cell types are limited in anatomy and physiology compared with normal uninjured skin. Identifying deficiencies in CSS can instruct further tissue engineering advances.

BASIC/CLINICAL SCIENCE ADVANCES

Expression profiling of CSS during in vitro maturation and after transplantation in vivo with Affymetrix GeneChip^® Arrays was used to characterize pathways that are abnormal or deficient in CSS compared with normal human skin. Examination of the large data set generated from microarray expression analysis revealed similarities between healed CSS and normal skin, particularly in expression of genes involved in epidermal differentiation and barrier function. However, deficiencies in several pathways were also noted, such as the genetic pathways regulating development of adnexal structures, including hair follicles.

CLINICAL CARE RELEVANCE

A deeper understanding of the cellular and molecular events guiding morphogenesis of engineered skin can lead to improvements that will increase clinical efficacy.

CONCLUSION

The results of GeneChip analysis highlighted the processes that act to regulate tissue development in vitro and adaptation to the wound environment and healing in vivo. This knowledge can be used to inform modifications to the model that will facilitate incorporation of additional cell types for increased homology with native human skin and improved functional outcome for burn patients treated with engineered skin grafts.

A rapid fabricated living dermal equivalent for skin tissue engineering: an in vivo evaluation in an acute wound model

The encapsulation of both cells and a surgical mesh in a polymerizing collagen hydrogel followed by mechanical compression, after polymerization, results in the rapid formation of a living dermal equivalent (LDE) with physical properties suitable for in vivo application. It was found in the current study that the LDE supported the attachment, growth, and differentiation of keratinocytes, allowing for the formation of living skin equivalents (LSEs) with a monolayer epidermis (LSE-M) and a stratified epidermis (LSE-S). The utility of the LDE for the fabrication of living wound dressings was further evaluated by testing the safety and efficacy of the LSE-M and LSE-S in a lapine model of an acute full-thickness skin defect. It was found that the LSE-S significantly stimulated blood vessel formation and accelerated epidermal wound closure compared with controls. The LSE-M showed similar trends but these were not significant. These findings indicate the clinical usefulness of the LDE in the treatment of acute and possibly chronic wounds, such as venous and diabetic ulcerations. The 1-h fabrication time of the LDE is a significant reduction compared with that of dermal components of current FDA-approved dressings, such as Dermagraft, Apligraf, and OrCel, which require days to weeks of in vitro culture. It is therefore proposed that the presented method could reduce the high cost associated with the production of living, tissue-engineered dressings.

Comparison of healing parameters in porcine full-thickness wounds transplanted with skin micrografts, split-thickness skin grafts, and cultured keratinocytes

BACKGROUND

Transplantation of skin micrografts (MGs), split-thickness skin grafts (STSGs), or cultured autologous keratinocytes (CKs) enhances the healing of large full-thickness wounds. This study compares these methods in a porcine wound model, investigating the utility of micrograft transplantation in skin restoration.

STUDY DESIGN

Full-thickness wounds were created on Yorkshire pigs and assigned to one of the following treatment groups: MGs, STSGs, CKs, wet nontransplanted, or dry nontransplanted. Dry wounds were covered with gauze and the other groups' wounds were enclosed in a polyurethane chamber containing saline. Biopsies were collected 6, 12, and 18 days after wounding. Quantitative and qualitative wound healing parameters including macroscopic scar appearance, wound contraction, neoepidermal maturation, rete ridge formation, granulation tissue thickness and width, and scar tissue formation were studied.

RESULTS

Transplanted wounds scored lower on the Vancouver Scar Scale compared with nontransplanted wounds, indicating a better healing outcome. All transplanted wounds exhibited significantly lower contraction compared with nontransplanted wounds. Wounds transplanted with either MGs, STSGs, or CKs showed a significant increase in re-epithelialization compared with nontransplanted wounds. Wounds transplanted with MGs or STSGs exhibited improved epidermal healing compared with nongrafted wounds. Furthermore, transplantation with STSGs or MGs led to less scar tissue formation compared with the nontransplanted wounds. No significant impact on scar formation was observed after transplantation of CKs.

CONCLUSIONS

Qualitative and quantitative measurements collected from full-thickness porcine wounds show that transplantation of MGs improve wound healing parameters and is comparable to treatment with STSGs.

Acellular dermal matrix in the management of the burn patient

Although the principles of burn management are still primarily focused on survival, as advances are realized in resuscitation, nutrition, and wound management, the functional and aesthetic outcomes following burn injury have become increasingly important. Acellular dermal matrix materials, which allow surgeons to minimize skin graft donor site morbidity in the process of repairing injured areas, play a role in addressing these important issues. Many favorable reports have been published, but they are generally characterized by small sample sizes, limited objective testing, and retrospective analysis. There does appear to be some evidence for ADM application in patient populations in whom donor site availability (those with massive burns) or morbidity (children, the elderly) is a concern, but more studies are needed. In this article, the authors discuss the current applications for ADM in burn management, review the existing literature, and present opportunities for future research.

Cell therapy of burns

Severe burns remain a life-threatening local and general inflammatory condition often with serious sequelae, despite remarkable progress in their treatment over the past three decades. Cultured epidermal autografts, the first and still most up-to-date cell therapy for burns, plays a key role in that progress, but drawbacks to this need to be reduced by using cultured dermal-epidermal substitutes. This review focuses on what could be, in our view, the next major breakthrough in cell therapy of burns - use of mesenchymal stromal cells (MSCs). After summarizing current knowledge, including our own clinical experience with MSCs in the pioneering field of cell therapy of radiation-induced burns, we discuss the strong rationale supporting potential interest in MSCs in treatment of thermal burns, including limited but promising pre-clinical and clinical data in wound healing and acute inflammatory conditions other than burns. Practical options for future therapeutic applications of MSCs for burns treatment, are finally considered.

Hyaluronan: From Biomimetic to Industrial Business Strategy

Hyaluronan (hyaluronic acid) is a naturally occurring polysaccharide of a linear repeating disaccharide unit consisting of β-(1→4)-linked D-glucopyranuronic acid and β-(1→3)-linked 2-acetamido-2-deoxy-D-glucopyranose, which is present in extracellular matrices, the synovial fluid of joints, and scaffolding that comprises cartilage.

In its mechanism of synthesis, its size, and its physico-chemical properties, hyaluronan is unique amongst other glycosaminoglycans. The network-forming, viscoelastic and its charge characteristics are important to many biochemical properties of living tissues. It is an important pericellular and cell surface constituent; its interaction with other macromolecules such as proteins, participates in regulating cell behavior during numerous morphogenic, restorative, and pathological processes in the body. The knowledge of HA in diseases such as various forms of cancers, arthritis and osteoporosis has led to new impetus in research and development in the preparation of biomaterials for surgical implants and drug conjugates for targeted delivery.

A concise and focused review on hyaluronan is timely. This review will cover the following important aspects of hyaluronan: (i) biological functions and synthesis in nature; (ii) current industrial production and potential biosynthetic processes of hyaluronan; (iii) chemical modifications of hyaluronan leading to products of commercial significance; and (iv) and the global market position and manufacturers of hyaluronan.

Dermal substitute-assisted healing: enhancing stem cell therapy with novel biomaterial design

The use of dermal substitutes is increasingly widespread but the outcomes of substitute-assisted healing remain functionally deficient. Presently, the most successful scaffolds are acellular polymer matrices, prepared through lyophilization and phase separation techniques, designed to mimic the dermal extracellular matrix. The application of scaffolds containing viable cells has proven to be problematic due to short shelf-life, high cost and death of transplanted cells as a result of immune rejection and apoptosis. Recent advances in biomaterial science have made new techniques available capable of increasing scaffold complexity, allowing the creation of 3D microenvironments that actively control cell behaviour. Importantly, it may be possible through these sophisticated novel techniques, including bio-printing and electrospinning, to accurately direct stem cell behaviour. This complex proposal involves the incorporation of cell-matrix, cell-cell, mechanical cues and soluble factors delivered in a spatially and temporally pertinent manner. This requires accurate modelling of three-dimensional stem cell interactions within niche environments to identify key signalling molecules and mechanisms. The application of stem cells within substitutes containing such environments may result in greatly improved transplanted cell viability. Ultimately this may increase cellular organization and complexity of skin substitutes. This review discusses progress made in improving the efficacy of cellular dermal substitutes for the treatment of cutaneous defects and the potential of evolving new technology to improve current results.