Vascularization of the Arteriovenous Loop in a Rat Isolation Chamber Model—Quantification of Hypoxia and Evaluation of Its Effects

Hypoxia as a stimulus for tissue formation: The concept of organogenesis in microsurgically vascularized tissue engineering constructs

Axial vascularization of tissue constructs is essential to maintain an adequate blood supply for a stable regeneration of a clinically relevant tissue size. The versatility of the arterio-venous loop (AVL) has been previously shown in various small and large animal models as well as in clinical reports for bone regeneration. We have previously demonstrated the capability of the AVL to induce axial vascularization and to support the nourishment of tissue constructs in small animal models after applying high doses of ionizing radiation comparable to those applied for adjuvant radiotherapy after head and neck cancer. We hypothesize that this robust ability to induce regeneration after irradiation could be related to a state of hypoxia inside the constructs that triggers the HIF1 (hypoxia induced factor 1) - SDF1 (stromal derived factor 1) axis leading to chemotaxis of progenitor cells and induction of tissue regeneration and vascularization. We analyzed the expression of HIF1 and SDF1 via immunofluorescence in axially vascularized bone tissue engineering constructs in Lewis rats 2 and 5 weeks after local irradiation with 9Gy or 15Gy. We also analyzed the expression of various genes for osteogenic differentiation (collagen 1, RUNX, alkaline phosphatase and osteonectin) via real time PCR analysis. The expression of HIF1 and SDF1 was enhanced two weeks after irradiation with 15Gy in comparison to non-irradiated constructs. The expression of osteogenic markers was enhanced at the 5-weeks time point with significant results regarding collagen, alkaline phosphatase and osteonectin. These results indicate that the hypoxia within the AVL constructs together with an enhanced SDF1 expression probably play a role in promoting tissue differentiation. The process of tissue generation triggered by hypoxia in the vicinity of a definite vascular axis with enhanced tissue differentiation over time resembles hereby the well-known concept of organogenesis in fetal life.

Vascular persistence following precision micropuncture

OBJECTIVE

The success of engineered tissues continues to be limited by time to vascularization and perfusion. Recently, we described a simple microsurgical approach, termed micropuncture (MP), which could be used to rapidly vascularize an adjacently placed scaffold from the recipient macrovasculature. Here we studied the long-term persistence of the MP-induced microvasculature.

METHODS

Segmental 60 μm diameter MPs were created in the recipient rat femoral artery and vein followed by coverage with a simple Type 1 collagen scaffold. The recipient vasculature and scaffold were then wrapped en bloc with a silicone sheet to isolate intrinsic vascularization. Scaffolds were harvested at 28 days post-implantation for detailed analysis, including using a novel artificial intelligence (AI) approach.

RESULTS

MP scaffolds demonstrated a sustained increase of vascular density compared to internal non-MP control scaffolds (p < 0.05) secondary to increases in both vessel diameters (p < 0.05) and branch counts (p < 0.05). MP scaffolds also demonstrated statistically significant increases in red blood cell (RBC) perfused lumens.

CONCLUSIONS

This study further highlights that the intrinsic MP-induced vasculature continues to persist long-term. Its combination of rapid and stable angiogenesis represents a novel surgical platform for engineered scaffold and graft perfusion.

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.

Prefabrication-a Vascularized Skin Flap Using an Arteriovenous LoopPrefabricated Flap With Arteriovenous Loop: An Experimental Study in Minipigs

BACKGROUND

Arteriovenous loops have a high potency to induce angiogenesis and are promising to solve the problem of scarce implanted pedicle sources and insufficient neovascularization in flap prefabrication. But there is a lack of large animal experiments to support their clinical application. Therefore, we aimed to explore the feasibility of prefabricating large flaps based on arteriovenous loops in pigs.

METHODS

Five minipigs were used. In the experimental group, a 10-cm-long ear vein graft was microanastomosed with the saphenous artery and vein to form an arteriovenous loop and implanted under the medial thigh flap. A month later, a 10×10 cm prefabricated flap pedicled with the arteriovenous loop was elevated and sutured in situ. In the control group, a 10×10 cm flap with no vascular pedicle was elevated completely and sutured in situ in the same position. The patency of the arteriovenous loop was evaluated by angiography 30 days after implantation, and the viability of flaps was assessed by macroscopic analysis 10 days after elevation. Three animals received arteriovenous loop flaps unilaterally and no-pedicle flaps unilaterally. Two animals received arteriovenous loop flaps bilaterally.

RESULTS

In the experimental group, no thrombi were exhibited in any arteriovenous loop. All 7 prefabricated flaps survived uneventfully. In the control group, 3 flaps were completely necrotic.

CONCLUSION

The arteriovenous loops with long interpositional venous grafts can be used as vascular pedicles to prefabricated large area and well-vascularized flaps. This approach can greatly expand the application of flap prefabrication.

Systemically injected bone marrow mononuclear cells specifically home to axially vascularized tissue engineering constructs

Inducing axial vascularisation of tissue engineering constructs is a well-established method to support tissue growth in large 3-dimensional tissues. Progenitor cell chemotaxis towards axially vascularized tissues has not been well characterized. In a prospective randomized controlled study including 32 male syngeneic Lewis rats we investigated the capability of the axially vascularized constructs to attract systemically injected bone marrow mononuclear cells (BMMNCs). The underlying mechanism for cell homing was investigated focusing on the role of hypoxia and the SDF1-CXCR4-7 axis. Sixteen animals were used as donors for BMMNCs. The other animals were subjected to implantation of a tissue engineering construct in the subcutaneous groin region. These constructs were axially vascularized either via an arteriovenous loop (AVL, n = 6) or via uninterrupted flow-through vessels (non-AVL, n = 10). BMMNCs were labelled with quantum dots (Qdot® 655) and injected 12 days after surgery either via intra-arterial or intravenous routes. 2 days after cell injection, the animals were sacrificed and examined using fluorescence microscopy. The Qdot® 655 signals were detected exclusively in the liver, spleen, AVL constructs and to a minimal extent in the non-AVL constructs. A significant difference could be detected between the number of labelled cells in the AVL and non-AVL constructs with more cells detected in the AVL constructs specially in central zones (p <0.0001). The immunohistological analysis showed a significant increase in the absolute expression of HIF-1 in the AVL group in comparison to the non-AVL group. The PCR analysis confirmed a 1.4-fold increase in HIF-1 expression in AVL constructs. Although PCR analysis showed an enhanced expression of CXCR4 and CXCR7 in AVL constructs, no significant differences in SDF1 expression were detected via immunohistological or PCR analysis. At the examined time point, the AVL constructs can attract BMMNCs in a mechanism probably related to the hypoxia associated with a robust tissue formation.

Hypoxia in Bone and Oxygen Releasing Biomaterials in Fracture Treatments Using Mesenchymal Stem Cell Therapy: A Review

Bone fractures have a high degree of severity. This is usually a result of the physical trauma of diseases that affect bone tissues, such as osteoporosis. Due to its highly vascular nature, the bone is in a constant state of remodeling. Although those of younger ages possess bones with high regenerative potential, the impact of a disrupted vasculature can severely affect the recovery process and cause osteonecrosis. This is commonly seen in the neck of femur, scaphoid, and talus bone. In recent years, mesenchymal stem cell (MSC) therapy has been used to aid in the regeneration of afflicted bone. However, the cut-off in blood supply due to bone fractures can lead to hypoxia-induced changes in engrafted MSCs. Researchers have designed several oxygen-generating biomaterials and yielded varying degrees of success in enhancing tissue salvage and preserving cellular metabolism under ischemia. These can be utilized to further improve stem cell therapy for bone repair. In this review, we touch on the pathophysiology of these bone fractures and review the application of oxygen-generating biomaterials to further enhance MSC-mediated repair of fractures in the three aforementioned parts of the bone.

[Research progress of in vivo bioreactor for bone tissue engineering]

Objective

To review the research progress of in vivo bioreactor (IVB) for bone tissue engineering in order to provide reference for its future research direction.

Methods

The literature related to IVB used in bone tissue engineering in recent years was reviewed, and the principles of IVB construction, tissue types, sites, and methods of IVB construction, as well as the advantages of IVB used in bone tissue engineering were summarized.

Results

IVB takes advantage of the body's ability to regenerate itself, using the body as a bioreactor to regenerate new tissues or organs at injured sites or at ectopic sites that can support the regeneration of new tissues. IVB can be constructed by tissue flap (subcutaneous pocket, muscle flap/pocket, fascia flap, periosteum flap, omentum flap/abdominal cavity) and axial vascular pedicle (axial vascular bundle, arteriovenous loop) alone or jointly. IVB is used to prefabricate vascularized tissue engineered bone that matched the shape and size of the defect. The prefabricated vascularized tissue engineered bone can be used as bone graft, pedicled bone flap, or free bone flap to repair bone defect. IVB solves the problem of insufficient vascularization in traditional bone tissue engineering to a certain extent.

Conclusion

IVB is a promising method for vascularized tissue engineered bone prefabrication and subsequent bone defect reconstruction, with unique advantages in the repair of large complex bone defects. However, the complexity of IVB construction and surgical complications hinder the clinical application of IVB. Researchers should aim to develop a simple, safe, and efficient IVB.

Induction of scaffold angiogenesis by recipient vasculature precision micropuncture

The success of engineered tissues continues to be limited by time to vascularization and perfusion. Here, we studied the effects of precision injury to a recipient macrovasculature in promoting neovessel formation in an adjacently placed scaffold. Segmental 60 μm diameter micropunctures (MP) were created in the recipient rat femoral artery and vein followed by coverage with a simple collagen scaffold. Scaffolds were harvested at 24, 48, 72, and 96 h post-implantation for detailed analysis. Those placed on top of an MP segment showed an earlier and more robust cellular infiltration, including both endothelial cells (CD31) and macrophages (F4/80), compared to internal non-micropunctured control limbs (p < 0.05). At the 96-hour timepoint, MP scaffolds demonstrated an increase in physiologic perfusion (p < 0.003) and a 2.5-fold increase in capillary network formation (p < 0.001). These were attributed to an overall upsurge in small vessel quantity. Furthermore, MP positioned scaffolds demonstrated significant increases in many modulators of angiogenesis, including VEGFR2 and Tie-2 despite a decrease in HIF-1α at all timepoints. This study highlights a novel microsurgical approach that can be used to rapidly vascularize or inosculate contiguously placed scaffolds and grafts. Thereby, offering an easily translatable route towards the creation of thicker and more clinically relevant engineered tissues.

Collagen and Endothelial Cell Coculture Improves β-Cell Functionality and Rescues Pancreatic Extracellular Matrix

The use of biomaterials and biomaterial functionalization is a promising approach to support pancreatic islet viability posttransplantation in an effort to reduce insulin dependence for patients afflicted with diabetes mellitus type 1. Extracellular matrix (ECM) proteins are known to impact numerous reparative functions in the body. Assessing how endogenously expressed pancreatic ECM proteins are affected by posttransplant-like hypoxic conditions may provide significant insights toward the development of tissue-engineered therapeutic strategies to positively influence β-cell survival, proliferation, and functionality. Here, we investigated the expression of three relevant groups of pancreatic ECM proteins in human native tissue, including basement membrane (BM) proteins (collagen type 4 [COL4], laminins [LAM]), proteoglycans (decorin [DCN], nidogen-1 [NID1]), and fibril-forming proteins (fibronectin [FN], collagen type 1 [COL1]). In an in vitro hypoxia model, we identified that ECM proteins were differently affected by hypoxic conditions, contributing to an overall loss of β-cell functionality. The use of a COL1 hydrogel as carrier material demonstrated a protective effect on β-cells mitigating the effect of hypoxia on proteoglycans as well as fibril-forming protein expression, supporting β-cell functionality in hypoxia. We further showed that providing endothelial cells (ECs) into the COL1 hydrogel improves β-cell response as well as the expression of relevant BM proteins. Our data show that β-cells benefit from a microenvironment composed of structure-providing COL1 with the incorporation of ECs to withstand the harsh conditions of hypoxia. Such hydrogels support β-cell survival and can serve as an initial source of ECM proteins to allow cell engraftment while preserving cell functionality posttransplantation. Impact statement Expression analysis identifies hypoxia-induced pathological changes in extracellular matrix (ECM) homeostasis as potential targets to support β-cell transplants by encapsulation in biomaterials for the treatment of diabetes mellitus. A collagen-1 hydrogel is shown to attenuate the effect of hypoxia on β-cells and their ECM expression. The functionalization of the hydrogel with endothelial cells increases the β-cell response to glucose and rescues essential basement membrane proteins.

The Microvascular Peroneal Artery Perforator Flap as a "Lifeboat" for Pedicled Flaps

Pedicled perforator flaps have expanded reconstructive options in extremity reconstruction. Despite preoperative mapping, intraoperative findings may require microvascular tissue transfer when no adequate perforators can be found. The free peroneal artery perforator flap may serve as a reliable back-up plan in small defects.

Keratinocyte Monolayers on Hyaluronic Acid Membranes as "Upside-Down" Grafts Reconstitute Full-Thickness Wounds

BACKGROUND Skin replacement by means of cultured epithelial keratinocytes is a well-accepted method. However, several clinical drawbacks of sheet autografts (CEA - cultured epithelial autografts) have stimulated various efforts to optimize cell culture and cell delivery. Recent developments include use of cell monolayers instead of a fully differentiated epithelium, as well as use of various biomaterials to grow and transport the cultured cells. To optimize the transfer of human keratinocytes directly to the recipient wound bed, we used an "upside-down" technique, delivering cultured cells directly to the wound with the carrier material on top. MATERIAL AND METHODS Subconfluent second-passage human keratinocyte monolayers on esterified hyaluronic acid membranes (KHAMC - Keratinocyte-Hyaluronic-Acid-Membrane-Composites) were transplanted either as upside-down grafts or as upside-up grafts onto standardized full-thickness wounds in athymic nude mice versus controls with the cell-free membrane alone. RESULTS In the upside-down group, 14 days after grafting, a multi-layered, differentiating epidermis was found, whereas the wounds in the upside-up group and in the control group were not completely closed up to day 21. Persistence of human keratinocytes was shown in the upside-down group only, from day 7 until day 35 after grafting. CONCLUSIONS This study confirms that upside-down grafting of subconfluent monolayers of serum-free cultured human keratinocytes on esterified hyaluronic acid membranes is a suitable means to transfer actively proliferative keratinocytes, and reduces wound contraction. Compared to standard grafting protocols of cultured epithelium, such as CEA sheet grafts, it is easier to apply, does not need enzymatic detachment of cells from the culture dish, and limits the number of production steps required.