Tissue-engineered skin preserving the potential of epithelial cells to differentiate into hair after grafting

Early-stage bilayer tissue-engineered skin substitute formed by adult skin progenitor cells produces an improved skin structure in vivo

BACKGROUND

In recent years, significant progress has been made in developing highly complex tissue-engineered skin substitutes (TESSs) for wound healing. However, the lack of skin appendages, such as hair follicles and sweat glands, and the time required, are two major limitations that hinder its broad application in the clinic. Therefore, it is necessary to develop a competent TESS in a short time to meet the needs for clinical applications.

METHODS

Adult scalp dermal progenitor cells and epidermal stem cells together with type I collagen as a scaffold material were used to reconstitute bilayer TESSs in vitro. TESSs at 4 different culture times (5, 9, 14, and 21 days) were collected and then grafted onto full-thickness wounds created in the dorsal skin of athymic nude/nude mice. The skin specimens formed from grafted TESSs were collected 4 and 8 weeks later and then evaluated for their structure, cell organization, differentiation status, vascularization, and formation of appendages by histological analysis, immunohistochemistry, and immunofluorescent staining.

RESULTS

Early-stage bilayer TESSs after transplantation had a better efficiency of grafting. A normal structure of stratified epidermis containing multiple differentiated layers of keratinocytes was formed in all grafts from both early-stage and late-stage TESSs, but higher levels of the proliferation marker Ki-67 and the epidermal progenitor marker p63 were found in the epidermis formed from early-stage TESSs. Interestingly, the transplantation of early-stage TESSs produced a thicker dermis that contained more vimentin- and CD31-positive cells, and importantly, hair follicle formation was only observed in the skin grafted from early-stage TESSs. Finally, early-stage TESSs expressed high levels of p63 but had low expression levels of genes involved in the activation of the apoptotic pathway compared to the late-stage TESSs in vitro.

CONCLUSIONS

Early-stage bilayer TESSs reconstituted from skin progenitor cells contained more competent cells with less activation of the apoptotic pathway and produced a better skin structure, including hair follicles associated with sebaceous glands, after transplantation, which should potentially provide better wound healing when applied in the clinic in the future.

Regeneration of skin appendages and nerves: current status and further challenges

Tissue-engineered skin (TES), as an analogue of native skin, is promising for wound repair and regeneration. However, a major drawback of TES products is a lack of skin appendages and nerves to enhance skin healing, structural integrity and skin vitality. Skin appendages and nerves are important constituents for fully functional skin. To date, many studies have yielded remarkable results in the field of skin appendages reconstruction and nerve regeneration. However, patients often complain about a loss of skin sensation and even cutaneous chronic pain. Restoration of pain, temperature, and touch perceptions should now be a major challenge to solve in order to improve patients’ quality of life. Current strategies to create skin appendages and sensory nerve regeneration are mainly based on different types of seeding cells, scaffold materials, bioactive factors and involved signaling pathways. This article provides a comprehensive overview of different strategies for, and advances in, skin appendages and sensory nerve regeneration, which is an important issue in the field of tissue engineering and regenerative medicine.

Therapeutic effects of platelet derived growth factor overexpressed-mesenchymal stromal cells and sheets in canine skin wound healing model

Adipose-derived mesenchymal stromal cells (Ad-MSCs) have excellent potential for skin wound repair. Moreover, platelet-derived growth factor (PDGF) has strong wound healing properties. The purpose of the present study was to compare the healing effects of PDGF-overexpressing canine allogeneic Ad-MSCs (PDGF-MSCs) and their cell sheets (PDGF-CSs) as compared to unexpressed Ad-MSCs (U-MSCs) and their cell sheets (UCSs) in a cutaneous wound healing model induced upon dogs. In in vitro study, the expression of immunomodulatory and growth factors was assessed by qRT-PCR. In in vivo study, cells and sheets were transplanted into a square-shaped full-thickness (1.5×1.5 cm) skin defect model created in 12 dogs. After 5 and 10 days, wounds were harvested and evaluated macroscopically and histopathologically. The qRT-PCR results showed that the PDGF-B gene was significantly upregulated (p<0.05) in PDGF-CS and PDGF-MSCs groups. Upon gross analysis of the wound, all stromal cells and their sheet groups showed accelerated (p<0.05) cutaneous wound healing compared to the negative control groups. As compared to U-MSCs and UCSs, the PDGF-MSCs showed significant epithelization on days 5 and 10 of healing, whereas PDGF-CSs showed improved epithelization only on day 10. In the granulation tissue analysis, PDGF-CSs and UCSs promoted more formation (p<0.05) of upper granulation tissue, collagen, and activated fibroblasts than PDGF-MSCs, and U-MSCs. Especially, the PDGF-CSs presented the highest formation and maturation of granulation tissue among all groups. All considered, PDGF overexpressed stromal cells or cells sheets can improve cutaneous wound healing in a canine model.

Immune tolerance of tissue-engineered skin produced with allogeneic or xenogeneic fibroblasts and syngeneic keratinocytes grafted on mice

Organs are needed for the long-term replacement of diseased or wounded tissues. Various technologies based on cells seeded in synthetic or biomaterial scaffolds, or scaffold-free methods have been developed in order to produce substitutes that mimic native organs and tissues. For cell-based approaches, the use of living allogeneic fibroblasts could potentially lead to the production of "off-the-shelf" bioengineered organs/tissues. However, questions remain regarding the outcome of allogeneic grafts in terms of persistence of allogeneic cells, tolerance and the host immune reaction against the tissue after implantation. To evaluate graft tolerance of engineered-tissues containing non-autologous fibroblasts, tissue-engineered skin substitutes (TESs) produced with syngeneic, allogeneic or xenogeneic fibroblasts associated with syngeneic, allogeneic or xenogeneic epithelial cells were grafted in mice as primary and secondary grafts. The immune response was evaluated by histological analysis and immunodetection of M2 macrophages, CD4- and CD8-positive T cells, 15, 19, 35 and 56 days after grafting. Tissue-engineered skin composed of non-autologous epithelial cells were rejected. In contrast, TESs composed of non-autologous fibroblasts underlying syngeneic epithelial cells were still present 56 days after grafting. This work shows that TES composed of non-autologous fibroblasts and autologous epithelial cells are not rejected after grafting. STATEMENT OF SIGNIFICANCE: We found that tissue-engineered skin substitutes produced by a scaffold-free cell-based approach from allogeneic fibroblasts and autologous epithelial cells are not rejected after grafting and allow for the permanent coverage of a full-thickness skin wounds. In the field of tissue engineering, these findings open the possibility of selecting a human fibroblastic or stromal cell population based on its biological properties and adequate biosafety, banking it, in order to produce "ready-to-use" bioengineered organs/tissues that could be grafted to any patient without eliciting immune reaction after grafting. Our results can be generalized to any organs produced from fibroblasts. Thus, it is a great step with multiple applications in tissue engineering and transplantation.

Human Reconstructed Skin in a Mouse Model

Currently, no ideal in vivo skin model, to exactly mimic the native human skin, has been utilized for laboratory and clinical application. Here, we describe a method to in vivo reconstitute a human skin model, so-called hRSK, by using culture-expanded skin cells. We grafted a mixture of dissociated human epidermal and dermal cells onto an excision wound on the back of immunodeficient mouse to generate the hRSK, and the hRSK, containing epidermis, dermis, and subcutis and also appendages such as hair follicles, histologically mirrors in situ human skin.

Recombinant Spider Silk Functionalized Silkworm Silk Matrices as Potential Bioactive Wound Dressings and Skin Grafts

Silk is considered to be a potential biomaterial for a wide number of biomedical applications. Silk fibroin (SF) can be retrieved in sufficient quantities from the cocoons produced by silkworms. While it is easy to formulate into scaffolds with favorable mechanical properties, the natural SF does not contain bioactive functions. Spider silk proteins, on the contrary, can be produced in fusion with bioactive protein domains, but the recombinant procedures are expensive, and large-scale production is challenging. We combine the two types of silk to fabricate affordable, functional tissue-engineered constructs for wound-healing applications. Nanofibrous mats and microporous scaffolds made of natural silkworm SF are used as a bulk material that are top-coated with the recombinant spider silk protein (4RepCT) in fusion with a cell-binding motif, antimicrobial peptides, and a growth factor. For this, the inherent silk properties are utilized to form interactions between the two silk types by self-assembly. The intended function, that is, improved cell adhesion, antimicrobial activity, and growth factor stimulation, could be demonstrated for the obtained functionalized silk mats. As a skin prototype, SF scaffolds coated with functionalized silk are cocultured with multiple cell types to demonstrate formation of a bilayered tissue construct with a keratinized epidermal layer under in vitro conditions. The encouraging results support this strategy of fabrication of an affordable bioactive SF-spider silk-based biomaterial for wound dressings and skin substitutes.

Full-thickness skin with mature hair follicles generated from tissue culture expanded human cells

The goal of regenerative medicine is to reconstruct fully functional organs from tissue culture expanded human cells. In this study, we report a method for human reconstructed skin (hRSK) when starting with human cells. We implanted tissue culture expanded human epidermal and dermal cells into an excision wound on the back of immunodeficient mice. Pigmented skin covered the wound 4 weeks after implantation. Hair shafts were visible at 12 weeks and prominent at 14 weeks. Histologically, the hRSK comprises an intact epidermis and dermis with mature hair follicles, sebaceous glands and most notably, and unique to this system, subcutis. Morphogenesis, differentiation, and maturation of the hRSK mirror the human fetal process. Human antigen markers demonstrate that the constituent cells are of human origin for at least 6 months. The degree of new skin formation is most complete when using tissue culture expanded cells from fetal skin, but it also occurs with expanded newborn and adult cells; however, no appendages formed when we grafted both adult dermal and epidermal cells. The hRSK system promises to be valuable as a laboratory model for studying biological, pathological, and pharmaceutical problems of human skin.

Wnt1a maintains characteristics of dermal papilla cells that induce mouse hair regeneration in a 3D preculture system

Hair follicle morphogenesis and regeneration depend on intensive but well-orchestrated interactions between epithelial and mesenchymal components. Therefore, an alternative strategy to reproduce the process of epithelial-mesenchymal interaction in vitro could use a 3D system containing appropriate cell populations. The 3D air-liquid culture system for reproducibly generating hair follicles from dissociated epithelial and dermal papilla (DP) cells combined with a collagen-chitosan scaffold is described in this study. Wnt-CM was prepared from the supernatant of Wnt1a-expressing bone marrow mesenchymal stem cells (BM-MSCs) that maintain the hair-inducing gene expression of DP cells. The collagen-chitosan scaffold cells (CCS cells) were constructed using a two-step method by inoculating the Wnt-CM-treated DP cells and epidermal (EP) cells into the CCS. The cells in the air-liquid culture formed dermal condensates and a proliferative cell layer in vitro. The CCS cells were able to induce hair regeneration in nude mice. The results demonstrate that Wnt-CM can maintain the hair induction ability of DP cells in expansion cultures, and this approach can be used for large-scale preparation of CCS cells in vitro to treat hair loss. Copyright © 2015 John Wiley & Sons, Ltd.

Dermal papilla cells improve the wound healing process and generate hair bud-like structures in grafted skin substitutes using hair follicle stem cells

Tissue-engineered skin represents a useful strategy for the treatment of deep skin injuries and might contribute to the understanding of skin regeneration. The use of dermal papilla cells (DPCs) as a dermal component in a permanent composite skin with human hair follicle stem cells (HFSCs) was evaluated by studying the tissue-engineered skin architecture, stem cell persistence, hair regeneration, and graft-take in nude mice. A porcine acellular dermal matrix was seeded with HFSCs alone and with HFSCs plus human DPCs or dermal fibroblasts (DFs). In vitro, the presence of DPCs induced a more regular and multilayered stratified epidermis with more basal p63-positive cells and invaginations. The DPC-containing constructs more accurately mimicked the skin architecture by properly stratifying the differentiating HFSCs and developing a well-ordered epithelia that contributed to more closely recapitulate an artificial human skin. This acellular dermal matrix previously repopulated in vitro with HFSCs and DFs or DPCs as the dermal component was grafted in nude mice. The presence of DPCs in the composite substitute not only favored early neovascularization, good assimilation and remodeling after grafting but also contributed to the neovascular network maturation, which might reduce the inflammation process, resulting in a better healing process, with less scarring and wound contraction. Interestingly, only DPC-containing constructs showed embryonic hair bud-like structures with cells of human origin, presence of precursor epithelial cells, and expression of a hair differentiation marker. Although preliminary, these findings have demonstrated the importance of the presence of DPCs for proper skin repair.

Concise review: tissue-engineered skin and nerve regeneration in burn treatment

Burns not only destroy the barrier function of the skin but also alter the perceptions of pain, temperature, and touch. Different strategies have been developed over the years to cover deep and extensive burns with the ultimate goal of regenerating the barrier function of the epidermis while recovering an acceptable aesthetic aspect. However, patients often complain about a loss of skin sensation and even cutaneous chronic pain. Cutaneous nerve regeneration can occur from the nerve endings of the wound bed, but it is often compromised by scar formation or anarchic wound healing. Restoration of pain, temperature, and touch perceptions should now be a major challenge to solve in order to improve patients' quality of life. In addition, the cutaneous nerve network has been recently highlighted to play an important role in epidermal homeostasis and may be essential at least in the early phase of wound healing through the induction of neurogenic inflammation. Although the nerve regeneration process was studied largely in the context of nerve transections, very few studies have been aimed at developing strategies to improve it in the context of cutaneous wound healing. In this concise review, we provide a description of the characteristics of and current treatments for extensive burns, including tissue-engineered skin approaches to improve cutaneous nerve regeneration, and describe prospective uses for autologous skin-derived adult stem cells to enhance recovery of the skin's sense of touch.

Human epithelial stem cells persist within tissue-engineered skin produced by the self-assembly approach

To adequately and permanently restore organ function after grafting, human tissue-engineered skin substitutes (TESs) must ultimately contain and preserve functional epithelial stem cells (SCs). It is therefore essential that a maximum of SCs be preserved during each in vitro step leading to the production of TESs such as the culture process and the elaboration of a skin cell bank by cryopreservation. To investigate the presence and functionality of epithelial SCs within the human TESs made by the self-assembly approach, slow-cycling cells were identified using 5'-bromo-2'-deoxyuridine (BrdU) in the three-dimensional construct. A subset of basal epithelial cells retained the BrdU label and was positive for the SC-associated marker keratin 19 within TESs after a chase of 21 days in culture post-BrdU labeling. Moreover, keratinocytes harvested from TESs gave rise to SC-like colonies in secondary monolayer subcultures, indicating that SCs were preserved within TESs. To evaluate the effect of cryopreservation with dimethyl sulfoxide and storage in liquid nitrogen on SCs, human epithelial cells were extracted from skin samples, amplified in culture, and used to produce TESs, before cryopreservation as well as after thawing. We found that the proportion and the growth potential of epithelial SCs in monolayer culture and in TESs remained constant before and after cryopreservation. Further, the functionality of these substitutes was demonstrated by successfully grafting human TESs on athymic mice for 6 months. We conclude that human epithelial skin SCs are adequately preserved upon human tissue reconstruction. Thus, these TESs produced by the self-assembly approach are suitable for clinical applications.

Bioengineered skin substitutes: key elements and novel design for biomedical applications

Significant progress has been made in the development of in vitro-engineered skin substitutes that mimic human skin, either to be used for the replacement of lost skin or for the establishment of in vitro skin research models. However, at the present time, there are no models of bioengineered skin that completely replicate the nature of uninjured skin. Obviously, there is still much room for improvement of the components of bioengineered skin and their interplay. This review summarises the important new discoveries in key elements of engineering of tissue-engineered skin including cell sources, biomaterials and growth factors, etc. Furthermore, basic and clinical applications for engineered skin substitutes in cell therapy, tissue engineering, and biomedical research continue to drive design improvements premised on these structure and function-based engineering paradigms.

Human fibroblast-derived ECM as a scaffold for vascular tissue engineering

The self-assembly approach is based on the capability of mesenchymal cells to secrete and organize their own extracellular matrix (ECM). This tissue engineering method allows for the fabrication of autologous living tissues, such as tissue-engineered blood vessels (TEBV) and skin. However, the secretion of ECM by smooth muscle cells (SMCs), required to produce the vascular media, may represent a long process in vitro. The aim of this work was to reduce the time required to produce a tissue-engineered vascular media (TEVM) and extend the production of TEVM with SMCs from all patients without compromising its mechanical and functional properties. Therefore, we developed a decellularized matrix scaffold (dMS) produced from dermal fibroblasts (DF) or saphenous vein fibroblasts (SVF), in which SMCs were seeded to produce a TEVM. Mechanical and contractile properties of these TEVM (referred to as nTEVM) were compared to standard self-assembled TEVM (sTEVM). This approach reduced the production time from 6 to 4 weeks. Moreover, nTEVM were more resistant to tensile load than sTEVM and their vascular reactivity was also improved. This new fabrication technique allows for the production of a vascular media using SMCs isolated from any patient, regardless of their capacity to synthesize ECM. Moreover, these scaffolds can be stored to be available when needed, in order to accelerate the production of the vascular substitute using autologous vascular cells.

Using stem cells in skin regeneration: possibilities and reality

Tissue-engineered skin has a long history of clinical applications, yet current treatments are not capable of completely regenerating normal, uninjured skin. Nonetheless, the field has experienced a tremendous development in the past 10 years, encountering the summit of tissue engineering (TE) and the arising of stem cell research. Since then, unique features of these cells such as self-renewal capacity, multi-lineage differentiation potential, and wound healing properties have been highlighted. However, a realistic perspective of their outcome in skin regenerative medicine applications is still absent. This review intends to discuss the directions that adult and embryonic stem cells (ESCs) can take, strengthening the skin regeneration field. Distinctively, a critical overview of stem cells' differentiation potential onto skin main lineages, along with a highlight of their participation in wound healing mechanisms, is herein provided. We aim to compile and review significant work to allow a better understanding of the best skin TE approaches, enabling the embodiment of the materialization of a new era in skin regeneration to come, with a conscious overview of the current limitations.