Guided tissue regeneration of vascular grafts in the peritoneal cavity

* A Rat Model for the In Vivo Assessment of Biological and Tissue-Engineered Valvular and Vascular Grafts

The demand for an improvement of the biocompatibility and durability of vascular and valvular implants requires translational animal models to study the in vivo fate of cardiovascular grafts. In the present article, a review on the development and application of a microsurgical rat model of infrarenal implantation of aortic grafts and aortic valved conduits is provided. By refinement of surgical techniques and inclusion of hemodynamic considerations, a functional model has been created, which provides a modular platform for the in vivo assessment of biological and tissue-engineered grafts. Through optional addition of procalcific diets, disease-inducing agents, and genetic modifications, complex multimorbidity scenarios mimicking the clinical reality in cardiovascular patients can be simulated. Applying this model, crucial aspects of the biocompatibility, biofunctionality and degeneration of vascular and valvular implants in dependency on graft preparation, and modification as well as systemic antidegenerative treatment of the recipient have been and will be addressed.

[Tissue engineering of heart valves]

With the introduction of heart valve prostheses cardiac valvular disease has become much more accessible to therapeutic options. However, currently available prostheses display significant limitations, such as limited long-term durability (biological prostheses) and a long-term necessity for anticoagulation therapy. Hence, alternative prosthesis types have been extensively explored in recent years particularly aiming at the development of vital and regenerative prostheses by means of tissue engineering. In the scientific field, different competing concepts have been introduced, including biological or synthetic scaffolds which can be further enhanced by cellular or extracellular components to promote further in vivo development of the prosthesis after implantation. Nowadays, decellularized donor heart valves are among the most advanced prosthesis types experiencing growing clinical attention and widespread use.

[Heart valve and myocardial tissue engineering]

Cardiac function, including the heart muscle and valves, can be severely altered by congenital and acquired heart diseases. Several graft materials are currently used to replace diseased cardiac tissue and valvular segments. Implantable grafts are either non-vital or can trigger an immune response which leads to graft calcification and degeneration. None of the existing grafts have the ability to remodel and grow in tandem with the physiological growth of a child and therefore require re-operation. Novel approaches such as tissue engineering have emerged as possible alternatives for cardiac reconstruction. The main concept of tissue engineering includes the use of biological and artificial scaffolds that form the shape of the organ structures for subsequent tissue replacement, which will provide absolute biocompatibility, no thrombogenicity, no teratogenicity, long-term durability and growth.Heart valve tissue engineering represents an important field especially in pediatric patients with valve pathologies. In order to create an autologous valve equivalent myofibroblasts and/or endothelial cells are seeded on specially designed scaffolds. Here we describe the different types of cell sources and different types of matrices currently used in heart valve tissue engineering. Valve manufacture is carried out in specially designed bioreactors providing physiological conditions. The number of clinical studies using tissue engineered valves is still limited; however, several promising results have already demonstrated their durability and ability to grow.Myocardial tissue engineering aims to repair, replace and regenerate damaged cardiac tissue using tissue constructs created ex vivo. Conceivable indications for clinical application of tissue engineered myocardial-implant substitutes include ischemic cardiomyopathies, as well as right ventricular outflow tract reconstruction in patients with congenital heart diseases. Therapeutic application of functional (contractile) tissue engineered heart muscle appears feasible once key issues such as identification of the suitable human cell source, large scale expansion and suitable scaffolds are solved. In addition, the present article discusses the importance of vascularization as an important prerequisite for successful bio-artificial myocardial tissue.Further experimental and clinical research on cardiovascular tissue engineering is felt to be of great importance for others as well as for us in order to create an ideal heart valve/myocardial substitute and help our patients with advanced cardiac pathologies.

Animal models for the assessment of novel vascular conduits

The development of an ideal small-diameter conduit for use in vascular bypass surgery has yet to be achieved. The ongoing innovation in biomaterial design generates novel conduits that require preclinical assessment in vivo, and a number of animal models have been used for this purpose. This article examines the rationale behind animal models used in the assessment of small-diameter vascular conduits encompassing the commonly used species: baboons, sheep, pigs, dogs, rabbits, and rodents. Studies on the comparative hematology for these species relative to humans are summarized, and the hydrodynamic values for common implant locations are also compared. The large- and small-animal models are then explored, highlighting the characteristics of each that determine their relative utility in the assessment of vascular conduits. Where possible, the performance of expanded polytetrafluoroethylene is given in each animal and in each location to allow direct comparisons between species. New challenges in animal modeling are outlined for the assessment of tissue-engineered graft designs. Finally, recommendations are given for the selection of animal models for the assessment of future vascular conduits.

A subpopulation of peritoneal macrophages form capillarylike lumens and branching patterns in vitro

Objective: We have previously shown that monocytes/macrophages (MC/Mph) influence neovascularization by extracellular matrix degradation, and by direct incorporation into growing microvessels. To date, neither the phenotype of these cells, nor the stages of their capillary-like conversion were sufficiently characterized. Methods: We isolated mouse peritoneal Mph from transgenic mice expressing fluorescent proteins either ubiquitously, or specifically in the myelocytic lineage. These Mph were embedded in Matrigel which contained fluorescent protease substrates, exposed to an MCP-1 chemotactic gradient, and then examined by confocal microscopy after various intervals. Results: Within 3 hrs after gel embedding, we detected TIMP-1 and MMP-12 dependent proteolysis of the matrix surrounding Mph, mostly in the direction of high concentrations of MCP-1. After 2 days, Mph developed intracellular vacuoles containing degradation product. At 5 days these vacuoles were enlarged and/or fused to generate trans-cellular lumens in approximately 10% of cells or more (depending on animal’s genetic background). At this stage, Mph became tubular, and occasionally organized in three-dimensional structures resembling branched microvessels. Conclusion: Isolated mouse peritoneal Mph penetrate Matrigel and form tunnels via a metalloprotease-driven proteolysis and phagocytosis. Following a morphological adjustment driven by occurrence, enlargement and/or fusion process of intracellular vacuoles, similar to that described in bona fide endothelium, a subpopulation of these cells end up by lining a capillary-like lumen in vitro. Thus we show that adult Mph, not only the more primitive ‘endothelial progenitors’, have functional properties until now considered defining of the endothelial phenotype.

Bioartificial tracheal grafts: can tissue engineering keep its promise?

A huge variety of graft materials and transplantation approaches have been applied for decades in order to generate a clinically applicable tracheal substitute; so far, without success. Today, tissue engineering, the creation of man-made functional biological organs or tissue replacements from biodegradable carrier structures and autologous cells, may represent an alternative to the shortage of suitable grafts for reconstructive airway surgery. Partial success has been obtained by numerous groups following different concepts and strategies. In this article, tissue engineering approaches towards the bioartificial airway prosthesis are discussed, focusing primarily on recent developments in the field.

Cardiovascular tissue engineering: state of the art

In patients requiring coronary or peripheral vascular bypass procedures, autogenous arterial or vein grafts remain as the conduit of choice even in the case of redo patients. It is in this class of redo patients that often natural tissue of suitable quality becomes unavailable; so that prosthetic material is then used. Prosthetic grafts are liable to fail due to graft occlusion caused by surface thrombogenicity and lack of elasticity. To prevent this, seeding of the graft lumen with endothelial cells has been undertaken and recent clinical studies have evidenced patency rates approaching reasonable vein grafts. Recent advances have also looked at developing a completely artificial biological graft engineered from the patient's cells with surface and viscoelastic properties similar to autogenous vessels. This review encompasses both endothelialisation of grafts and the construction of biological cardiovascular conduits.

Functional adaptation: the key to plasticity of cardiovascular "stem" cells?

There is increasing evidence that cells of disparate phenotypes displaying various degrees of proliferative capacity engraft and function heterotopically in adult organisms. Efforts were made to reconcile these findings with the embryologic notions of pluripotent stem or progenitor cell, although the nature of the 'stemness' remained elusive. This topic is particularly important for the cardiovascular system, in which cytotrophoblasts, certain tumor cells, monocytes/macrophages, peritoneal mesothelial cells, and others acquire endothelial properties and/or perform endothelial functions. Here we suggest that this pluripotency reflects a fundamental characteristic of cellular diversity, which is manifested as the adaptive response to a functional pressure exerted by the cell's biochemical and biophysical microenvironments that would drive their differentiation. In this model, differentiation is a dynamic, reversible, and open-ended process where the cells would maintain the flexibility to respond to changing environmental clues with a fine tuning of their structure, a property that was previously called cellular plasticity. Pluripotent adult stem cells that display this property in culture, and, perhaps upon in vivo administration, were described. Therefore, we also suggest that differentiation of stem cells is a form of cellular plasticity within the larger context of functional adaptation, whereas their stemness remains associated with self-renewal.

Tissue-engineered blood vessels: alternative to autologous grafts?

Although vascular bypass grafting remains the mainstay for revascularization for ischemic heart disease and peripheral vascular disease, many patients do not have healthy vessels suitable for harvest. Thus, prosthetic grafts made of synthetic polymers were developed, but their use is limited to high-flow/low-resistance conditions because of poor elasticity, low compliance, and thrombogenicity of their synthetic surfaces. To fill this need, several laboratories have produced in vivo or in vitro tissue-engineered blood vessels using molds or prosthetic or biodegradable scaffolds, but each artificial graft has significant problems. Recently, conduits have been grown in the peritoneal cavity of the same animals in which they will be grafted, ensuring no rejection, in the short time of 2 to 3 weeks. Remodeling occurs after grafting such that the tissue is almost indistinguishable from native vessels. This conduit is derived from cells of bone marrow origin, opening new possibilities in vascular modeling and remodeling.

The use of animal models in developing the discipline of cardiovascular tissue engineering: a review

Cardiovascular disease remains one of the major causes of death and disability in the Western world. Tissue engineering offers the prospect of being able to meet the demand for replacement of heart valves, vessels for coronary and lower limb bypass surgery and the generation of cardiac tissue for addition to the diseased heart. In order to test prospective tissue-engineered devices, these constructs must first be proven in animal models before receiving CE marking or FDA approval for a clinical trial. The choice of animal depends on the nature of the tissue-engineered construct being tested. Factors that need to be considered include technical requirements of implanting the construct, availability of the animal, cost and ethical considerations. In this paper, we review the history of animal studies in cardiovascular tissue engineering and the uses of animal tissue as sources for tissue engineering.

Mesothelial progenitor cells and their potential in tissue engineering

The mesothelium consists of a single layer of flattened mesothelial cells that lines serosal cavities and the majority of internal organs, playing important roles in maintaining normal serosal integrity and function. A mesothelial 'stem' cell has not been identified, but evidence from numerous studies suggests that a progenitor mesothelial cell exists. Although mesothelial cells are of a mesodermal origin, they express characteristics of both epithelial and mesenchymal phenotypes. In addition, following injury, new mesothelium regenerates via centripetal ingrowth of cells from the wound edge and from a free-floating population of cells present in the serosal fluid, the origin of which is currently unknown. Recent findings have shown that mesothelial cells can undergo an epithelial to mesenchymal transition, and transform into myofibroblasts and possibly smooth muscle cells, suggesting plasticity in nature. Further evidence for a mesothelial progenitor comes from tissue engineering applications where mesothelial cells seeded onto tubular constructs have been used to generate vascular replacements and grafts to bridge transected nerve fibres. These findings suggest that mesothelial cell progenitors are able to switch between different cell phenotypes depending on the local environment. However, only by performing detailed investigations involving selective cell isolation, clonal analysis together with cell labelling and tracking studies, will we begin to determine the true existence of a mesothelial stem cell.

Dynamics of proteins in Golgi membranes: comparisons between mammalian and plant cells highlighted by photobleaching techniques

In less than a decade the green fluorescent protein (GFP) has become one of the most popular tools for cell biologists for the study of dynamic processes in vivo. GFP has revolutionised the scientific approach for the study of vital organelles, such as the Golgi apparatus. As Golgi proteins can be tagged with GFP, in most cases without altering their targeting and function, it is a great substitute to conventional dyes used in the past to highlight this compartment. In this review, we cover the application of GFP and its spectral derivatives in the study of Golgi dynamics in mammalian and plant cells. In particular, we focus on the technique of selective photobleaching known as fluorescence recovery after photobleaching, which has successfully shed light on essential differences in the biology of the Golgi apparatus in mammalian and plant cells.