Engineered Prevascularization for Oral Tissue Grafting: A Systematic Review

Multiscale Imaging to Monitor Functional SHED-Supported Engineered Vessels

Regeneration of orofacial tissues is hampered by the lack of adequate vascular supply. Implantation of in vitro engineered, prevascularized constructs has emerged as a strategy to allow the rapid vascularization of the entire graft. Given the angiogenic properties of dental pulp stem cells, we hereby established a preclinical model of prevascularized constructs loaded with stem cells from human exfoliating deciduous teeth (SHED) in a 3-dimensional-printed material and provided a functional analysis of their in vivo angiogenesis, vascular perfusion, and permeability. Three different cell-loaded collagen hydrogels (SHED-human umbilical vein endothelial cell [HUVEC], HUVEC with SHED-conditioned medium, and SHED alone) were cast in polylactic acid (PLA) grids and ectopically implanted in athymic mice. At day 10, in vivo positron emission tomography (PETscan) revealed a significantly increased uptake of radiotracer targeting activated endothelial cells in the SHED-HUVEC group compared to the other groups. At day 30, ex vivo micro-computed tomography imaging confirmed that SHED-HUVEC constructs had a significantly increased vascular volume compared to the other ones. Injection of species-specific lectins analyzed by 2-photon microscopy demonstrated blood perfusion of the engineered human vessels in both prevascularized groups. However, in vivo quantification showed increased vessel density in the SHED-HUVEC group. In addition, coinjection of fluorescent lectin and dextran revealed that prevascularization with SHED prevented vascular leakage, demonstrating the active role of SHED in the maturation of human-engineered microvascular networks. This preclinical study introduces a novel PLA prevascularized and implantable construct, along with an array of imaging techniques, to validate the ability of SHED to promote functional human-engineered vessels, further highlighting the interest of SHED for orofacial tissue engineering. Furthermore, this study validates the use of PETscan for the early detection of in vivo angiogenesis, which may be applied in the clinic to monitor the performance of prevascularized grafts.

Cell Sheet Technology: An Emerging Approach for Tendon and Ligament Tissue Engineering

Tendon and ligament injuries account for a substantial proportion of disorders in the musculoskeletal system. While non-operative and operative treatment strategies have advanced, the restoration of native tendon and ligament structures after injury is still challenging due to its innate limited regenerative ability. Cell sheet technology is an innovative tool for tissue fabrication and cell transplantation in regenerative medicine. In this review, we first summarize different harvesting procedures and advantages of cell sheet technology, which preserves intact cell-to-cell connections and extracellular matrix. We then describe the recent progress of cell sheet technology from preclinical studies, focusing on the application of stem cell-derived sheets in treating tendon and ligament injuries, as well as highlighting its effects on mitigating inflammation and promoting tendon/graft-bone interface healing. Finally, we discuss several prerequisites for future clinical translation including the selection of appropriate cell source, optimization of preparation process, establishment of suitable animal model, and the fabrication of vascularized complex tissue. We believe this review could potentially provoke new ideas and drive the development of more functional biomimetic tissues using cell sheet technology to meet the needs of clinical patients.

Recent trends and perspectives in reconstruction and regeneration of intra/extra-oral wounds using tissue-engineered oral mucosa equivalents

Many conditions, including cancer, trauma, and congenital anomalies, can damage the oral mucosa. Multiple cultures of oral mucosal cells have been used for biocompatibility tests and oral biology studies. In recent decades, the clinical translation of tissue-engineered products has progressed significantly in developing tangible therapies and inspiring advancements in medical science. However, the reconstruction of an intraoral mucosa defect remains a significant challenge. Despite the drawbacks of donor-site morbidity and limited tissue supply, the use of autologous oral mucosa remains the gold standard for oral mucosa reconstruction and repair. Tissue engineering offers a promising solution for repairing and reconstructing oral mucosa tissues. Cell- and scaffold-based tissue engineering approaches have been employed to treat various soft tissue defects, suggesting the potential clinical use of tissue-engineered oral mucosa (TEOMs). In this review, we first cover the recent trends in the reconstruction and regeneration of extra-/intra-oral wounds using TEOMs. Next, we describe the current status and challenges of TEOMs. Finally, future strategic approaches and potential technologies to support the advancement of TEOMs for clinical use are discussed.

Recent advancement in vascularized tissue-engineered bone based on materials design and modification

Bone is one of the most vascular network-rich tissues in the body and the vascular system is essential for the development, homeostasis, and regeneration of bone. When segmental irreversible damage occurs to the bone, restoring its vascular system by means other than autogenous bone grafts with vascular pedicles is a therapeutic challenge. By pre-generating the vascular network of the scaffold in vivo or in vitro, the pre-vascularization technique enables an abundant blood supply in the scaffold after implantation. However, pre-vascularization techniques are time-consuming, and in vivo pre-vascularization techniques can be damaging to the body. Critical bone deficiencies may be filled quickly with immediate implantation of a supporting bone tissue engineered scaffold. However, bone tissue engineered scaffolds generally lack vascularization, which requires modification of the scaffold to aid in enhancing internal vascularization. In this review, we summarize the relationship between the vascular system and osteogenesis and use it as a basis to further discuss surgical and cytotechnology-based pre-vascularization strategies and to describe the preparation of vascularized bone tissue engineered scaffolds that can be implanted immediately. We anticipate that this study will serve as inspiration for future vascularized bone tissue engineered scaffold construction and will aid in the achievement of clinical vascularized bone.

Type H blood vessels in coupling angiogenesis-osteogenesis and its application in bone tissue engineering

One specific capillary subtype, termed type H vessel, has been found with unique functional characteristics in coupling angiogenesis with osteogenesis. Researchers have fabricated a variety of tissue engineering scaffolds to enhance bone healing and regeneration through the accumulation of type H vessels. However, only a limited number of reviews discussed the tissue engineering strategies for type H vessel regulation. The object of this review is to summary the current utilizes of bone tissue engineering to regulate type H vessels through various signal pathways including Notch, PDGF-BB, Slit3, HIF-1α, and VEGF signaling. Moreover, we give an insightful overview of recent research progress about the morphological, spatial and age-dependent characteristics of type H blood vessels. Their unique role in tying angiogenesis and osteogenesis together via blood flow, cellular microenvironment, immune system and nervous system are also summarized. This review article would provide an insight into the combination of tissue engineering scaffolds with type H vessels and identify future perspectives for vasculized tissue engineering research.

Transforming one organ into another to overcome challenges in tissue engineering

Tissue engineering (TE) is promising for the regeneration of failed organs. However, immune rejection, shortage of seed cells, and unintegrated blood vessels restrict the development and clinical application of TE. The last factor is the most challenging and intractable. Harnessing the mature blood vessel network in existing dispensable organs could be a powerful approach to effectively overcome the obstacles. After being remodeled to harbor an immunosuppressive and proregenerative niche, these potential target organs can be transformed into other organs with specific physiological functions, compensating the latter's failed native functions. Organ transformation, such as a hepatized spleen, represents an effective and encouraging TE strategy. In this review, we discuss the current development and obstacles of TE and its feasibility and superiority in organ transformation.

Use of Human Gingival Fibroblasts for Pre-Vascularization Strategies in Oral Tissue Engineering

BACKGROUND

Cocultures of human gingival fibrobasts (hGF) and endothelial cells could enhance regeneration and repair models as well as improve vascularization limitations in tissue engineering. The aim of this study was to assess if hGF could support formation of stable vessel-like networks.

METHODS

Explant primary hGF were isolated from gum surgical wastes collected from healthy patients with no history of periodontitis. Human umbilical vein endothelial cells (HUVEC) were two-dimensional (2D) and three-dimensional (3D) cocultured in vitro with hGF at a cell ratio of 1:1 and medium of 1:1 of their respective media during at least 31 days. Vessel quantification of HUVEC networks was performed. In order to investigate the pericyte-like properties of hGF, the expression of perivascular markers α-SMA, NG2, CD146 and PDGFR-β was studied using immunocytochemistry and flow cytometry on 2D cultures.

RESULTS

hGF were able to support a long-lasting HUVEC network at least 31 days, even in the absence of a bioreactor with flow. As observed, HUVEC started to communicate with each other from day 7, constructing a network. Their interconnection increased significantly between day 2 and day 21 and lasted beyond the 31 days of observation. Moreover, we tried to explain the stability of the networks obtained and showed that a small population of hGF in close vicinity of HUVEC networks expressed perivascular markers.

CONCLUSION

These findings highlight a new interesting property concerning hGF, accentuating their relevance in tissue engineering and periodontal regeneration. These promising results need to be confirmed using more 3D applications and in vivo testing.

The Structure and Function of Next-Generation Gingival Graft Substitutes-A Perspective on Multilayer Electrospun Constructs with Consideration of Vascularization

There is a shortage of suitable tissue-engineered solutions for gingival recession, a soft tissue defect of the oral cavity. Autologous tissue grafts lead to an increase in morbidity due to complications at the donor site. Although material substitutes are available on the market, their development is early, and work to produce more functional material substitutes is underway. The latter materials along with newly conceived tissue-engineered substitutes must maintain volumetric form over time and have advantageous mechanical and biological characteristics facilitating the regeneration of functional gingival tissue. This review conveys a comprehensive and timely perspective to provide insight towards future work in the field, by linking the structure (specifically multilayered systems) and function of electrospun material-based approaches for gingival tissue engineering and regeneration. Electrospun material composites are reviewed alongside existing commercial material substitutes', looking at current advantages and disadvantages. The importance of implementing physiologically relevant degradation profiles and mechanical properties into the design of material substitutes is presented and discussed. Further, given that the broader tissue engineering field has moved towards the use of pre-seeded scaffolds, a review of promising cell options, for generating tissue-engineered autologous gingival grafts from electrospun scaffolds is presented and their potential utility and limitations are discussed.

Extracorporeal Shock Wave Therapy Improves In Vitro Formation of Multilayered Epithelium of Oral Mucosa Equivalents

Oral mucosa is used in various surgical fields as a graft for the reconstruction of tissue defects. Tissue engineering of oral mucosa equivalents using autologous cells represents a suitable less burdensome alternative. The survival of the multilayered epithelium is essential for the functionality of the tissues in vivo. To ensure its functionality after transplantation, mucosa equivalents in vitro were subjected to extracorporeal shock wave therapy (ESWT) to determine whether this treatment stimulated the formation and differentiation of the epithelium. Mucosa equivalents treated with ESWT were examined for cellular metabolic activity using AlamarBlue^TM assay. The formation of vascular structures, basement membrane, and multilayered epithelium were examined using confocal fluorescence microscopy and immunohistochemistry. The potential ingrowth in vivo was simulated using the chorioallantoic membrane model (CAM assay) in ovo. ESWT on culture day 19 of oral mucosa equivalents resulted in slightly increased cellular metabolic activity. The in vitro development of basement membrane and multilayer epithelium was stimulated by ESWT. Additionally, in the CAM assay, ESWT led to a more pronounced multilayered epithelium. Thus, ESWT stimulated the formation of a more distinct and differentiated multilayered epithelium of oral mucosa equivalents in vitro and might increase the chance of efficient ingrowth, survival, and functionality of tissue equivalents in vivo.

Human Periodontal Ligament Stem Cell and Umbilical Vein Endothelial Cell Co-Culture to Prevascularize Scaffolds for Angiogenic and Osteogenic Tissue Engineering

(1) Background: Vascularization remains a critical challenge in bone tissue engineering. The objective of this study was to prevascularize calcium phosphate cement (CPC) scaffold by co-culturing human periodontal ligament stem cells (hPDLSCs) and human umbilical vein endothelial cells (hUVECs) for the first time; (2) Methods: hPDLSCs and/or hUVECs were seeded on CPC scaffolds. Three groups were tested: (i) hUVEC group (hUVECs on CPC); (ii) hPDLSC group (hPDLSCs on CPC); (iii) co-culture group (hPDLSCs + hUVECs on CPC). Osteogenic differentiation, bone mineral synthesis, and microcapillary-like structures were evaluated; (3) Results: Angiogenic gene expressions of co-culture group were 6–9 fold those of monoculture. vWF expression of co-culture group was 3 times lower than hUVEC-monoculture group. Osteogenic expressions of co-culture group were 2–3 folds those of the hPDLSC-monoculture group. ALP activity and bone mineral synthesis of co-culture were much higher than hPDLSC-monoculture group. Co-culture group formed capillary-like structures at 14–21 days. Vessel length and junction numbers increased with time; (4) Conclusions: The hUVECs + hPDLSCs co-culture on CPC scaffold achieved excellent osteogenic and angiogenic capability in vitro for the first time, generating prevascularized networks. The hPDLSCs + hUVECs co-culture had much better osteogenesis and angiogenesis than monoculture. CPC scaffolds prevacularized via hPDLSCs + hUVECs are promising for dental, craniofacial, and orthopedic applications.

Osteogenically differentiated mesenchymal stem cells promote the apoptosis of human umbilical vein endothelial cells in vitro

The absence of blood vessels in tissue engineered bone often leads to necrosis of internal cells after implantation, ultimately affecting the process of bone repair. Herein, mesenchymal stem cells (MSCs) and human umbilical vein endothelial cells (HUVECs) were cocultured to induce osteogenesis and angiogenesis. Based on the findings, the number of HUVECs in the coculture system increased in the growth medium group, but decreased in the osteogenic induction medium (OIM) group. Considering that the paracrine effects of MSCs had changed, we tested the genes expression of osteogenically differentiated MSCs. The expression of osteogenic genes in MSCs increased during osteogenesis. Further, the expression levels of pigment epithelial-derived factor (PEDF) gene and protein, an antivascular factor, were also increased. To verify whether MSCs promote HUVECs apoptosis via PEDF, PEDF was silenced via siRNA. The conditioned medium of differentiated MSCs with PEDF silencing significantly improved the proliferation and apoptosis of HUVECs. Based on further experiments, PEDF mediated the apoptosis and proliferation of HUVECs through p53, BAX/BCL-2, FAS, and c-Caspase-3. However, when PEDF was silenced with siRNA, the osteogenic potential of MSCs was affected. The results of this study provide a theoretical basis for the construction of prevascularized bone tissues in vitro.

Characteristics of Large Animal Models for Current Cell-Based Oral Tissue Regeneration

The recent advances in the field of cell-based therapeutics open promising perspectives for oral tissue regeneration. The development of large animal models, which overcome the limits of the rodent models and allow to emulate clinical situations, is crucial for the validation of regenerative strategies to move toward clinical application. Currently, porcine, canine, and ovine models are mainly developed for oral regeneration and their specific characteristics have an impact on the outcomes of the studies. Thus, this systematic review investigates the application of porcine, canine, and ovine models in present cell-based oral regeneration, according to the species characteristics and the targeted tissue to regenerate. A customized search of PubMed, EMBASE, Scopus, and Web of Science databases from January 2015 to March 2020 was conducted. Relevant articles about cell-based oral tissues engineering in porcine, canine, and ovine models were evaluated. Among the evaluated articles, 58 relevant studies about cell-based oral regeneration in porcine, canine, and ovine models matched the eligibility criteria and were selected for full analysis. Porcine models, the most similar species with humans, were mostly used for bone and periodontium regeneration; tooth regeneration was reported only in pig, except for one study in dog. Canine models were the most transversal models, successfully involved for all oral tissue regeneration and notably in implantology. However, differences with humans and ethical concerns affect the use of these models. Ovine models, alternative to porcine and canine ones, were mainly used for bone and, scarcely, periodontium regeneration. The anatomy and physiology of these animals restrain their involvement. If consistency was found in defect specificities and cell trends among different species animal models of bone, dentin-pulp complex, or tooth regeneration, variability appeared in periodontium. Regeneration assessment methods were more elaborate in porcines and canines than in ovines. Risk of bias was low for selection, attrition and reporting, but unclear for performance and detection. Overall, if none of the large animal models can be considered an ideal one, they are of deemed importance for oral cell-based tissue engineering and researchers should consider their relevance to establish favorable conditions for a given preclinical cell-based therapeutics.

Acceleration of ligamentization and osseointegration processes after anterior cruciate ligament reconstruction with autologous tissue-engineered polyethylene terephthalate graft

Background

Despite the advantages of excellent mechanical properties for rapid return to sports and early rehabilitation after anterior cruciate ligament (ACL) reconstruction with polyethylene terephthalate (PET) artificial ligament, the graft failure rate during long-term follow-up is relatively high due to poor graft-host incorporation. The purpose of the present study was to investigate the effect of autologous tissue-engineered PET (ATE-PET) grafts on osseointegration and ligamentization after ACL reconstruction.

Methods

Forty-eight New Zealand white rabbits were randomly divided into PET group (n=24) and ATE-PET group (n=24). In the ATE-PET group, the rabbits initially underwent subcutaneous implantation of the PET ligament. Two weeks later, unilateral ipsilateral ACL reconstruction was performed using an ATE-PET graft. In the PET group, the rabbits underwent ACL reconstruction using PET grafts as controls. Macroscopic observation, micro-computed tomography, histological and immunofluorescent staining, and biomechanical tests were conducted to evaluate the effects at 4 and 12 weeks postoperatively.

Results

The ATE-PET graft was highly pre-vascularized with myofibroblast aggregation after two weeks of subcutaneous implantation. With regard to the intraosseous part of the graft, the ATE-PET group had significantly higher bone mineral density and bone volume/total volume ratio at 12 weeks. Histologically, the width of the interface between the graft and bone was smaller. Regarding the intra-articular part, thicker tissue coverage with a glossy appearance was observed in the ATE-PET group at 12 weeks on macroscopic observation. Histological staining also showed more collagen fibers grew in the grafts with fewer inflammatory reactions of the ATE-PET group at both 4 and 12 weeks. Immunofluorescently, both α-SMA-positive vessels and α-SMA-positive myofibroblasts were found to be significantly greater around the graft in the ATE-PET group at 4 weeks and markedly declined at 12 weeks. Moreover, the ATE-PET group presented significantly greater failure load and stiffness than the PET group at 12 weeks (53.7±5.4 vs. 42.5±4.5 N, P<0.01; 12.9±3.0 vs. 9.8±1.3 N/mm, P=0.04).

Conclusions

The ATE-PET artificial ligament with pre-vascularization and myofibroblast aggregation could effectively accelerate intra-articular graft ligamentization and intraosseous graft osseointegration, thus enhancing the biomechanical properties after ACL reconstruction in a rabbit model.