Animal models for the assessment of novel vascular conduits

Submillimeter Diameter Poly(Vinyl Alcohol) Vascular Graft Patency in Rabbit Model

Microvascular surgery is becoming a prevalent surgical practice. Replantation, hand reconstruction, orthopedic, and free tissue transfer procedures all rely on microvascular surgery for the repair of venous and arterial defects at the millimeter and submillimeter levels. Often, a vascular graft is required for the procedure as a means to bridge the gap between native arteries. While autologous vessels are desired for their bioactivity and non-thrombogenicity, the tedious harvest process, lack of availability, and caliber or mechanical mismatch contribute to graft failure. Thus, there is a need for an off-the-shelf artificial vascular graft that has low thrombogenic properties and mechanical properties matching those of submillimeter vessels. Poly(vinyl alcohol) hydrogel (PVA) has excellent prospects as a vascular graft due to its bioinertness, low thrombogenicity, high water content, and tunable mechanical properties. Here, we fabricated PVA grafts with submillimeter diameter and mechanical properties that closely approximated those of the rabbit femoral artery. In vitro platelet adhesion and microparticle release assay verified the low thrombogenicity of PVA. A stringent proof-of-concept in vivo test was performed by implanting PVA grafts in rabbit femoral artery with multilevel arterial occlusion. Laser Doppler measurements indicated the improved perfusion of the distal limb after implantation with PVA grafts. Moreover, ultrasound Doppler and angiography verified that the submillimeter diameter PVA vascular grafts remained patent for 2 weeks without the aid of anticoagulant or antithrombotics. Endothelial cells were observed in the luminal surface of one patent PVA graft. The advantageous non-thrombogenic and tunable mechanical properties of PVA that are retained even in the submillimeter diameter dimensions support the application of this biomaterial for vascular replacement in microvascular surgery.

In Vivo Functional Evaluation of Tissue-Engineered Vascular Grafts Fabricated Using Human Adipose-Derived Stem Cells from High Cardiovascular Risk Populations

Many preclinical evaluations of autologous small-diameter tissue-engineered vascular grafts (TEVGs) utilize cells from healthy humans or animals. However, these models hold minimal relevance for clinical translation, as the main targeted demographic is patients at high cardiovascular risk such as individuals with diabetes mellitus or the elderly. Stem cells such as adipose-derived mesenchymal stem cells (AD-MSCs) represent a clinically ideal cell type for TEVGs, as these can be easily and plentifully harvested and offer regenerative potential. To understand whether AD-MSCs sourced from diabetic and elderly donors are as effective as those from young nondiabetics (i.e., healthy) in the context of TEVG therapy, we implanted TEVGs constructed with human AD-MSCs from each donor type as an aortic interposition graft in a rat model. The key failure mechanism observed was thrombosis, and this was most prevalent in grafts using cells from diabetic patients. The remainder of the TEVGs was able to generate robust vascular-like tissue consisting of smooth muscle cells, endothelial cells, collagen, and elastin. We further investigated a potential mechanism for the thrombotic failure of AD-MSCs from diabetic donors; we found that these cells have a diminished potential to promote fibrinolysis compared to those from healthy donors. Together, this study served as proof of concept for the development of a TEVG based on human AD-MSCs, illustrated the importance of testing cells from realistic patient populations, and highlighted one possible mechanistic explanation as to the observed thrombotic failure of our diabetic AD-MSC-based TEVGs.

Establishment of a rat and guinea pig aortic interposition graft model reveals model-specific patterns of intimal hyperplasia

OBJECTIVE

Although the aortic interposition bypass model has been widely used to evaluate biomaterials for bypass grafting, there is no comprehensive description of the procedure or of the distribution of intimal hyperplasia that results. The objectives of this study were to (1) review and summarize approaches of aortic interposition grafting in animal models, (2) determine the pertinent anatomy for this procedure, (3) validate this model in the rat and guinea pig, and (4) compare the distribution of intimal hyperplasia that develops in each species.

METHODS

A literature search was performed in PubMed from 1980 to the present to analyze the use of anesthesia, anticoagulation, antiplatelet agents, graft material, suture, and anastomotic techniques. Using 10-week-old male Sprague-Dawley rats and Hartley guinea pigs, we established pertinent aortic anatomy, developed comparable models, and assessed complications for each model. At 30 days, the graft and associated aorta were explanted, intimal formation was assessed morphometrically, and cellularity was assessed via nuclear counting.

RESULTS

We reviewed 30 articles and summarized the pertinent procedural findings. Upon establishing both animal models, key anatomic differences between the species that affect this model were noted. Guinea pigs have a much larger cecum, increased retroperitoneal fat, and lack the iliolumbar vessels compared with the rat. Surgical outcomes for the rat model included a 53% technical success rate and a 32% technical error rate. Surgical outcomes for the guinea pig model included a 69% technical success rate and a 31% technical error rate. These two species demonstrated unique distribution of intimal hyperplasia at 30 days. Intimal hyperplasia in the rat model was greatest at two areas, the proximal graft (5400 μm²; P < .001) and distal graft (2800 μm²; P < .04), whereas the guinea pig model developed similar intimal hyperplasia throughout the graft (4500-5100 μm²; P < .01).

CONCLUSIONS

In this report, we summarize the literature on the aortic interposition graft model, present a detailed description of the anatomy and aortic interposition graft procedure in the rat and guinea pig, and describe a unique distribution of intimal formation that results in both species. This information will be helpful when designing studies to evaluate novel graft materials in the future.

Rapid endothelialization and thin luminal layers in vascular grafts using silk fibroin

The histological effects of silk fibroin in vascular grafts were clarified comprehensively on a large-animal model.

Lymphatic Vascular Regeneration: The Next Step in Tissue Engineering

The lymphatic system plays a crucial role in interstitial fluid drainage, lipid absorption, and immunological defense. Lymphatic dysfunction results in lymphedema, fluid accumulation, and swelling of soft tissues, as well as a potentially impaired immune response. Lymphedema significantly reduces quality of life of patients on a physical, mental, social, and economic basis. Current therapeutic approaches in treatment of lymphatic disease are limited. Over the last decades, great progress has been made in the development of therapeutic strategies to enhance vascular regeneration. These solutions to treat vascular disease may also be applicable in the treatment of lymphatic diseases. Comparison of the organogenic process and biological organization of the vascular and lymphatic systems and studies in the regulatory mechanisms involved in lymphangiogenesis and angiogenesis show many common features. In this study, we address the similarities between both transport systems, and focus in depth on the biology of lymphatic development. Based on the current advances in vascular regeneration, we propose different strategies for lymphatic tissue engineering that may be used for treatment of primary and secondary lymphedema.

Case Report: Apixaban-Associated Gluteal Artery Extravasation Reversed With PCC3 Without FFP

Apixaban, an oral factor Xa inhibitor, has no commercially available assay to measure its activity and no specific antidote. To date, recommendations for managing bleeding associated with apixaban are based on studies with animal models and healthy volunteers (who do not have identified thrombogenic risk factors) and expert opinion. No clinical experience has been published in the literature. Ideally, apixaban would be reversed sufficiently to stop a perilous bleed without producing more thrombogenic risk than the patients' underlying risk factors. Three-factor prothrombin complex concentrate (PCC3) is the least thrombogenic among the suggested reversal agents. Fresh frozen plasma (FFP) is sometimes recommended to add to PCC3, but it adds considerable volume. We describe successful management of an active left gluteal arterial extravasation due to trauma and associated apixaban, in a patient with aortic stenosis and atrial fibrillation, by administration of PCC3 alone, without the added volume of FFP.

The Tissue-Engineered Vascular Graft-Past, Present, and Future

Cardiovascular disease is the leading cause of death worldwide, with this trend predicted to continue for the foreseeable future. Common disorders are associated with the stenosis or occlusion of blood vessels. The preferred treatment for the long-term revascularization of occluded vessels is surgery utilizing vascular grafts, such as coronary artery bypass grafting and peripheral artery bypass grafting. Currently, autologous vessels such as the saphenous vein and internal thoracic artery represent the gold standard grafts for small-diameter vessels (<6 mm), outperforming synthetic alternatives. However, these vessels are of limited availability, require invasive harvest, and are often unsuitable for use. To address this, the development of a tissue-engineered vascular graft (TEVG) has been rigorously pursued. This article reviews the current state of the art of TEVGs. The various approaches being explored to generate TEVGs are described, including scaffold-based methods (using synthetic and natural polymers), the use of decellularized natural matrices, and tissue self-assembly processes, with the results of various in vivo studies, including clinical trials, highlighted. A discussion of the key areas for further investigation, including graft cell source, mechanical properties, hemodynamics, integration, and assessment in animal models, is then presented.

Surgical technique for the implantation of tissue engineered vascular grafts and subsequent in vivo monitoring

The development of Tissue Engineered Vessels (TEVs) is advanced by the ability to routinely and effectively implant TEVs (4-5 mm in diameter) into a large animal model. A step by-step protocol for inter-positional placement of the TEV and real-time digital assessment of the TEV and native carotid arteries is described here. In vivo monitoring is made possible by the implantation of flow probes, catheters and ultrasonic crystals (capable of recording dynamic diameter changes of implanted TEVs and native carotid arteries) at the time of surgery. Once implanted, researchers can calculate arterial blood flow patterns, invasive blood pressure and artery diameter yielding parameters such as pulse wave velocity, augmentation index, pulse pressures and compliance. Data acquisition is accomplished using a single computer program for analysis throughout the duration of the experiment. Such invaluable data provides insight into TEV matrix remodeling, its resemblance to native/sham controls and overall TEV performance in vivo.

In vivo evaluation of biomimetic fluorosurfactant polymer-coated expanded polytetrafluoroethylene vascular grafts in a porcine carotid artery bypass model

OBJECTIVE

The objective of this study was to evaluate the potential for biomimetic self-assembling fluorosurfactant polymer (FSP) coatings incorporating heptamaltose (M7-FSP) to block nonspecific protein adsorption, the cell adhesive RGD peptide (RGD-FSP), or the endothelial cell-selective CRRETAWAC peptide (cRRE-FSP) to improve patency and endothelialization in small-diameter expanded polytetrafluoroethylene (ePTFE) vascular graft implants.

METHODS

ePTFE vascular grafts (4 mm in diameter, 5 cm in length) were coated with M7-FSP, RGD-FSP, or cRRE-FSP by dissolving FSPs in distilled water and flowing solution through the graft lumen for 24 hours. Coatings were confirmed by receding water contact angle measurements on the lumen surface. RGD-FSP and cRRE-FSP grafts were presodded in vitro with porcine pulmonary artery endothelial cells (PPAECs) using a custom-designed flow system. PPAEC coverage on the lumen surface was visualized with epifluorescent microscopy and quantified. Grafts were implanted as carotid artery interposition bypass grafts in seven pigs for 33 ± 2 days (ePTFE, n = 3; M7-FSP, n = 4; RGD-FSP, n = 3; cRRE-FSP, n = 4). Patency was confirmed immediately after implantation with duplex color flow ultrasound and at explantation with contrast-enhanced angiography. Grafts were sectioned for histology and stained: Movat pentachrome stain to outline vascular layers, immunofluorescent staining to identify endothelial cells (anti-von Willebrand factor antibody), and immunohistochemical staining to identify smooth muscle cells (anti-smooth muscle α-actin antibody). Neointima to lumen area ratio was determined to evaluate neointimal hyperplasia.

RESULTS

Receding water contact angle measurements on graft luminal surfaces were significantly lower (P < .05) on FSP-coated ePTFE surfaces (M7-FSP, 40 ± 16 degrees; RGD-FSP, 25 ± 10 degrees; cRRE-FSP, 33 ± 16 degrees) compared with uncoated ePTFE (126 ± 2 degrees), confirming presence of the FSP layer. In vitro sodding of PPAECs on RGD-FSP and cRRE-FSP grafts resulted in a confluent monolayer of PPAECs on the luminal surface, with a similar cell population on RGD-FSP (1200 ± 187 cells/mm(2)) and cRRE-FSP (1134 ± 153 cells/mm(2)) grafts. All grafts were patent immediately after implantation, and one of three uncoated, two of three RGD-FSP, two of four M7-FSP, and two of four cRRE-FSP grafts remained patent after 1 month. PPAEC coverage of the lumen surface was seen in all patent grafts. RGD-FSP grafts had a slightly higher neointima to lumen area ratio (0.53 ± 0.06) compared with uncoated (0.29 ± 0.15), M7-FSP (0.20 ± 0.15), or cRRE-FSP (0.17 ± 0.09) grafts.

CONCLUSIONS

Biomimetic FSP-coated ePTFE grafts can be used successfully in vivo and have potential to support endothelialization. Grafts modified with the M7-FSP and cRRE-FSP showed lower intimal hyperplasia compared with RGD-FSP grafts.

Experimental study of sutureless vascular anastomosis with use of glued prosthesis in rabbits

OBJECTIVE

The objective of this study is to explore the feasibility and efficacy of a new technique for sutureless vascular anastomosis, using glued prosthesis, as a sole anastomosis fixation method in rabbits.

METHODS

Ten rabbits were randomly selected to conduct the experiment. Five rabbits underwent direct anastomosis of infrarenal abdominal aorta, with glued prosthesis. In five other rabbits, reconstruction was done by sutured anastomosis. All animals were immediately examined by echo-Doppler for patency of anastomosis. The burst pressure of the glued anastomosis was measured and compared with that of a sutured artery. The animals were euthanized, and tissue samples were taken for histological examination immediately after the experiment.

RESULTS

Compared to conventional anastomoses, sutureless vascular anastomoses required shorter time of creation and significantly reduced blood loss (P<5%). There was no significant difference on the average blood flow through the anastomosis between two groups at the end of surgery. All anastomoses with glued prosthesis, examined by echo-Doppler, were patent at the anastomotic site, except one, which was stenosed immediately after surgery. In the control group, except one with stenosis, all conventional anastomoses were patent. Mean burst pressure at the anastomotic site for sutureless anastomoses was lower than in control group. Macroscopically, the BioGlue did not demonstrate any adhesion to the surrounding tissue as it was covered by the vascular prosthesis. Histological examination showed low-grade inflammatory reaction in glued anastomoses versus no inflammatory reaction at the sutured anastomoses.

CONCLUSION

This technique may provide a feasible and successful alternative in vascular surgery. However, further long-term studies are necessary to elucidate the break pressure and degree of inflammation at the anastomotic site.

Arterial grafts exhibiting unprecedented cellular infiltration and remodeling in vivo: the role of cells in the vascular wall

OBJECTIVE

To engineer and implant vascular grafts in the arterial circulation of a pre-clinical animal model and assess the role of donor medial cells in graft remodeling and function.

APPROACH AND RESULTS

Vascular grafts were engineered using Small Intestinal Submucosa (SIS)-fibrin hybrid scaffold and implanted interpositionally into the arterial circulation of an ovine model. We sought to demonstrate implantability of SIS-Fibrin based grafts; examine the remodeling; and determine whether the presence of vascular cells in the medial wall was necessary for cellular infiltration from the host and successful remodeling of the implants. We observed no occlusions or anastomotic complications in 18 animals that received these grafts. Notably, the grafts exhibited unprecedented levels of host cell infiltration that was not limited to the anastomotic sites but occurred through the lumen as well as the extramural side, leading to uniform cell distribution. Incoming cells remodeled the extracellular matrix and matured into functional smooth muscle cells as evidenced by expression of myogenic markers and development of vascular reactivity. Interestingly, tracking the donor cells revealed that their presence was beneficial but not necessary for successful grafting. Indeed, the proliferation rate and number of donor cells decreased over time as the vascular wall was dominated by host cells leading to significant remodeling and development of contractile function.

CONCLUSIONS

These results demonstrate that SIS-Fibrin grafts can be successfully implanted into the arterial circulation of a clinically relevant animal model, improve our understanding of vascular graft remodeling and raise the possibility of engineering mural cell-free arterial grafts.

Intraoperative cerebral blood flow imaging of rodents

Intraoperative monitoring of cerebral blood flow (CBF) is of interest to neuroscience researchers, which offers the assessment of hemodynamic responses throughout the process of neurosurgery and provides an early biomarker for surgical guidance. However, intraoperative CBF imaging has been challenging due to animal's motion and position change during the surgery. In this paper, we presented a design of an operation bench integrated with laser speckle contrast imager which enables monitoring of the CBF intraoperatively. With a specially designed stereotaxic frame and imager, we were able to monitor the CBF changes in both hemispheres during the rodent surgery. The rotatable design of the operation plate and implementation of online image registration allow the technician to move the animal without disturbing the CBF imaging during surgery. The performance of the system was tested by middle cerebral artery occlusion model of rats.

Animal Models for Studying Pathophysiology of Hemodialysis Access

Despite extensive efforts, most approaches to reduce arteriovenous (AV) access-related complications did not results in substantial improvement of AV access patency thus far. Part of this disappointing progress relates to incomplete understanding of the underlying pathophysiology of hemodialysis access failure. In order to unravel the pathophysiology of hemodialysis access failure, animal models that closely mimic human pathology are of utmost importance. Indeed, it is impossible to study the extremely complex response of the AV access at a molecular and cellular level in great detail in dialysis patients. Over the past decades, numerous animal models have been developed in an attempt to unravel the vascular pathology of AV access failure and to design new therapeutic strategies aimed to improve durability of these vascular conduits. While large animals such as pigs are suitable for intervention studies, murine models have the greatest potential to gain more insight in the molecular mechanisms underlying AV access failure due to the availability of transgenic mice. In the present review, we describe several existing models of AV access failure and discuss the advantages and limitations of these models.

Implantation of completely biological engineered grafts following decellularization into the sheep femoral artery

The performance of completely biological, decellularized engineered allografts in a sheep model was evaluated to establish clinical potential of these unique arterial allografts. The 4-mm-diameter, 2-3-cm-long grafts were fabricated from fibrin gel remodeled into an aligned tissue tube in vitro by ovine dermal fibroblasts. Decellularization and subsequent storage had little effect on graft properties, with burst pressure exceeding 4000 mmHg and the same compliance as the ovine femoral artery. Grafts were implanted interpositionally in the femoral artery of six sheep (n=9), with contralateral sham controls (n=3). At 8 weeks (n=5) and 24 weeks (n=4), all grafts were patent and showed no evidence of dilatation or mineralization. Mid-graft lumen diameter was unchanged. Extensive recellularization occurred, with most cells expressing αSMA. Endothelialization was complete by 24 weeks with elastin deposition evident. These completely biological grafts possessed circumferential alignment/mechanical anisotropy characteristic of native arteries and were cultured only 5 weeks prior to decellularization and storage as "off-the-shelf" grafts.

Biomechanics and biocompatibility of the perfect conduit-can we build one?

No currently available conduit meets the criteria for an ideal coronary artery bypass graft. The perfect conduit would combine the availability and complication-free harvest of a synthetic vessel with the long-term patency performance of the internal mammary artery. However, current polymer conduits suffer from inelastic mechanical properties and especially poor surface biocompatibility, resulting in early loss of patency as a coronary graft. Approaches to manufacture an improved conduit using new polymers or polymer surfaces, acellular matrices, or cellular constructs have to date failed to achieve a commercially successful alternative. Elastin, by mimicking the native extracellular environment as well as providing elasticity, provides the 'missing link' in vascular conduit design and brings new hope for realization of the perfect conduit.

Fabrication of small-diameter vascular scaffolds by heparin-bonded P(LLA-CL) composite nanofibers to improve graft patency

The poor patency rate following small-diameter vascular grafting remains a major hurdle for the widespread clinical application of artificial blood vessels to date. Our previous studies found that electrospun poly(L-lactide-co-epsilon-caprolactone) (P[LLA-CL]) nanofibers facilitated the attachment and growth of endothelial cells (EC), and heparin incorporated into P(LLA-CL) nanofibers was able to release in a controlled manner. Hence, we hypothesized that heparin-bonded P(LLA-CL) vascular scaffolds with autologous EC pre-endothelialization could significantly promote the graft patency rate. To construct a small-diameter vascular scaffold, the inner layer was fabricated by heparin-bonded P(LLA-CL) nanofibers through coaxial electrospinning, while the outer layer was woven by pure P(LLA-CL) nanofibers. Except dynamic compliance (5.4 1.7 versus 12.8 2.4 × 10⁻⁴/mmHg, P < 0.05), maximal tensile strength, burst pressure, and suture retention of the composite, scaffolds were comparable to those of canine femoral arteries. In vitro studies indicated that the scaffolds can continuously release heparin for at least 12 weeks and obtain desirable endothelialization through dynamic incubation, which was confirmed by EC viability and proliferation assay and scanning electronic microscopy. Furthermore, in vivo studies demonstrated that pre-endothelialization by autologous ECs provided a better effect on graft patency rate in comparison with heparin loading, and the united application of pre-endothelialization and heparin loading markedly promoted the 24 weeks patency rate of P(LLA-CL) scaffolds (88.9% versus 12.5% in the control group, P < 0.05) in the canine femoral artery replacement model. These results suggest that heparin-bonded P(LLA-CL) scaffolds have similar biomechanical properties to those of native arteries and possess a multiporous and biocompatible surface to achieve satisfactory endothelialization in vitro. Heparin-bonded P(LLA-CL) scaffolds with autologous EC pre-endothelialization have the potential to be substitutes for natural small-diameter vessels in planned vascular bypass surgery.

The use of antithrombotic therapies in reducing synthetic small-diameter vascular graft thrombosis

BACKGROUND

Thrombosis of synthetic small-diameter bypass grafts remains a major problem. The aim of this article is to review the antithrombotic strategies that have been used in an attempt to reduce graft thrombogenicity.

METHODS

A PubMed/MEDLINE search was performed using the search terms "vascular graft thrombosis," "small-diameter graft thrombosis," "synthetic graft thrombosis" combined with "antithrombotic," "antiplatelet," "anticoagulant," "Dacron," "PTFE," and "polyurethane."

RESULTS

The majority of studies on antithrombotic therapies have used either in vitro models or in vivo animal experiments. Many of the therapies used in these settings do show antithrombotic efficacy against synthetic graft materials. There is however, a distinct lack of human in vivo studies to further delineate the performance and limitations of therapies displaying good antithrombotic characteristics.

CONCLUSION

Very few antithrombotic therapies have translated into clinical use. More human in vivo studies are required to assess the efficacy and safety of such therapies.

Allogeneic human tissue-engineered blood vessel

BACKGROUND

Arterial bypass graft implantation remains the primary therapy for patients with advanced cardiovascular disease; however, there is no available synthetic small-diameter vascular graft.

METHODS

Tissue-engineered vessels were grown from human smooth muscle cells that were seeded on a biodegradable scaffold using a biomimetic perfusion system. The human tissue-engineered vessels (hTEV) were decellularized by a two-step process using a combination of detergents and hypertonic solutions. The mechanical characteristics were assessed by suture retention strength and burst pressure. The decellularized hTEV were implanted as aortic interpositional grafts in nude rats to evaluate in vivo performance as an arterial graft over a 6-week period.

RESULTS

The human tissue-engineered structure formed a vessel composed of smooth muscle cells and the extracellular matrix proteins, including collagen. After decellularization, the collagen matrix remained intact while the cellular components were removed. The mechanical strength of the hTEV after decellularization was similar to human vein in vitro, with a burst pressure of 1,567 ± 384 mm Hg (n = 3) versus 1,680 ± 307 mm Hg for human saphenous vein. The hTEVs had a high patency rate (four of five grafts) without evidence of rupture or aneurysm over a 6-week period as an aortic interpositional graft in a nude rat model. Histologic analysis showed a thin neointima with a confluent endothelium and a subendothelial layer of smooth muscle cells on the explanted tissue-engineered vessels. Transmission electron microscopy on the explanted tissue demonstrated elastin formation in the neointima and intact residual collagen fibers from the tissue-engineered vessel.

CONCLUSIONS

The hTEV had a high patency rate and remained mechanically stable as an aortic interpositional graft in a nude rat. The vessel supported the growth of a neointima with endothelial cells and smooth muscle cells. The host remodeling suggested the engineered matrix had a positive effect to create a regenerated vascular graft.