Vascular tissue engineering

Reconstructive surgery using autologous vessels is the conventional approach for substitution of diseased vessels or for generation of bypass to improve blood supply downstream of stenosed vessels. In some circumstances the use of autologous material is not possible due to concomitant diseases or previous use, and artificial grafts must be used. Unfortunately, these grafts cannot substitute small-caliber arterial vessels because of thrombotic complications. The objective of tissue engineering at the vascular level is then to generate biological substitutes of arterial conduits with functional characteristics of native vessels, combining cellular components with biodegradable scaffolds. These research projects started in several laboratories, in the late 1990s, and have expanded in different directions using a number of experimental approaches. The objective of this review is to give an overview of the results so far obtained in this area of research, and to discuss the problems related to these investigations, at the experimental and clinical level. The article provides an overview of different biodegradable scaffolds used, experimental techniques for vessels maturation in vitro under mechanical stimulation, and of differentiated as well as precursors of vascular cells, which opens new opportunities for further development of this form of cell transplantation. Finally, the current available results in clinical research will be discussed.     

Bladder tissue engineering

The bladder can lose the ability to store and empty effectively as a result of numerous conditions. When conservative methods to maximize patient safety and quality of life fail, surgical reconstruction of the bladder is usually considered. Augmentation cystoplasty can be performed with the use of the small bowel, large bowel, or less often, stomach. An alternative approach, tissue engineering, identifies the body's own potential for regeneration and supports this propensity with appropriate raw materials and growth factors so that the body's original structure and function may be restored. Tissue engineering can involve the use of a scaffold or matrix alone or of cell-seeded matrices. Harvesting cells and culturing them has become an important tool in tissue engineering. Multiple possibilities for sources of cells have been investigated, including stem cells and differentiated cells from organs other than the bladder; however, to date, autologous bladder cells remain the gold standard for culture and seeding.     

Skin tissue engineering

The coverage of extensive wounds with viable autologous keratinocytes remains the only option of treatment if autologous donor skin is not obtainable. There is evidence that proliferating keratinocytes, as suspended cells or as a single layer, are adequate for wound closure. Understanding keratinocyte-matrix interactions not only allows us to influence keratinocyte outgrowth, adhesion, and migration, but may also guide us to modify matrix molecules for enhancing keratinocyte take. Further approaches may include the generation of genetically manipulated keratinocytes, which allow the use of an off-the-shelf epidermal replacement. As surgeons, our goal is to help burn patients with the best quality of skin in the shortest time possible. As tissue engineers, we have not achieved the goal of a universal skin product. By continually reviewing the options and using them, we can at least use the proper material in the adequate situation. Because of the limited resources, the need for comparisons of clinical effectiveness and cost are ever more important. As anatomy and physiology of engineered skin substitutes improve, they will become more similar to native skin autografts. Improvement of skin substitutes will result from inclusion of additional cell types (eg, melanocytes) and from modifications of culture media and scaffolds. Skin-substitute materials may be able to stimulate regeneration rather than repair, and tissue-engineered skin may match the quality of split-skin autografts, our present gold standard.     

Hepatic tissue engineering

Fulminant hepatic failure (FHF) attributes to rising medical cost and accounts for many deaths each year in the United States. Currently, the only solution is organ transplantation. Due to increasing donor organ shortage, many in need of transplantation continue to remain on the waiting list. Liver Assist Devices (LADs) are being used to temporarily sustain liver function and bridge the period between FHF and transplantation. Hepatic Tissue Engineering is a step toward alleviating the need for donor organs; yet many challenges must be overcome including scaffold choice, cell source and immunological barriers. Bioreactors have aided in hepatocyte survival and have proven to sustain viable cells for several weeks. Achieving the necessary functions required for hepatic replacement is aided by the incorporation of growth factors and mitogens many that now can be bound to the polymer scaffold and released in a timely manner. Utilizing concepts such as MicroElectroMechanical systems (MEMs) technology, our laboratory is able to mimic the natural vasculature of the liver and sustain functional and viable hepatocytes. Expanding and improving upon this platform technology, advancements made will continue toward the development of a fully functioning and implantable liver.     

血管组织工程的进展

血管病变尤其是冠状动脉的闭塞性病变为人类的主要病死原因之一。其主要的治疗方法是血管搭桥手术,需要各种直径的血管移植物作为修补材料。临床已经应用的血管移植物包括自体血管、异体血管和合成材料管道。然而目前无论那种血管移植物都无法满足临床需要,临床需要一种新的血管替代物,这种管道应具有高度的组织相容性、可生     

Cardiac tissue engineering

Recent progress in implantations of differentiated cardiac and non-cardiac cells as well as adult stem cells into the heart suggests that the irreversible loss of viable cardiac myocytes that occurs during myocardial infarction can be at least partly substituted. We evaluated an alternative approach by reconstituting cardiac tissue grafts in vitro and implanting them as spontaneously and coherently contracting tissues. For this purpose we have optimized a method to generate ring-shaped three-dimensional engineered heart tissue (EHT) in vitro from neonatal rat cardiac myocytes. When subjected to isometric force measurements in organ baths, electrically stimulated EHTs exhibit a Frank-Starling behavior, a positive inotropic response to increases in extracellular calcium, a positive inotropic and lusitropic response to isoprenaline, and a negative inotropic response to the muscarinic agonist carbachol ('accentuated antagonism'). Twitch tension under maximal calcium amounts to 1-2 mN/ mm2. Importantly, passive (resting) tension is low, yielding a ratio of active/passive tension of approximately 1.5 under basal and 14 under maximal calcium. Morphologically, EHTs represent a highly interconnected three-dimensional network of cardiac myocytes resembling loose cardiac tissue with a high fraction of binucleated cardiac myocytes, strong eosin staining and elongated centrally located nuclei. Electron microscopy demonstrated well developed sarcomeric structures, T-tubules, SR vesicles, T-tubule-SR-junctions, all types of intercellular connective structures, and a basement membrane. Thus, EHTs comprise functional and morphological properties of intact, ventricular myocardium. First implantation experiments of EHTs in the peritoneum of Fischer 344 rats showed that EHTs survived for at least 14 days, maintained a network of differentiated cardiac myocytes, and were strongly vascularized. Thus, EHTs may serve as material for a novel tissue replacement approach.     

Biomechanics and tissue engineering

Development of artificial scaffold for musculo-skeletal applications, especially in load-bearing situations, requires the consideration of biomechanical aspects for its integrity and its function. However, the biomechanical loading could also be used to favour tissue formation through mechano-transduction phenomena. Design of scaffold could take advantages of this intrinsic mechanical loading.     

Biomaterials for tissue engineering

Biomaterials play a critical role in the engineering of new functional genitourinary tissues for the replacement of lost or malfunctioning tissues. They provide a temporary scaffolding to guide new tissue growth and organization and may provide bioactive signals (e.g., cell-adhesion peptides and growth factors) required for the retention of tissue-specific gene expression. A variety of biomaterials, which can be classified into three types – naturally derived materials (e.g., collagen and alginate), acellular tissue matrices (e.g., bladder submucosa and small-intestinal submucosa), and synthetic polymers [e.g., polyglycolic acid, polylactic acid, and poly(lactic-co-glycolic acid)] – have proved to be useful in the reconstruction of a number of genitourinary tissues in animal models. Some of these materials are currently being used clinically for genitourinary applications. Ultimately, the development or selection of appropriate biomaterials may allow the engineering of multiple types of functional genitourinary tissues.     

Heart valve tissue engineering

Valvular heart disease is a significant cause of morbidity and mortality world-wide. Classical replacement surgery involves the implantation of mechanical valves or biological valves (xeno- or homografts). Tissue engineering of heart valves represents a new experimental concept to improve current modes of therapy in valvular heart surgery. Various approaches have been developed differing either in the choice of scaffold (synthetic biodegradable polymers, decellularised xeno- or homografts) or cell source for the production of living tissue (vascular derived cells, bone marrow cells or progenitor cells from the peripheral blood). The use of autologous bone marrow cells in combination with synthetic biodegradable scaffolds bears advantages over other tissue engineering approaches: it is safe, it leads to complete autologous prostheses and the cells are more easily obtained in the clinical routine. Even though we demonstrated the feasibility to construct living functional tissue engineered heart valves from human bone marrow cells, so far their general potential to differentiate into non-hematopoietic cell lineages is not fully exploited for tissue engineering applications.     

体内组织工程技术—小肠黏膜下层的应用

组织工程技术分为两类 :一类称为体外组织工程技术 ;另一类为体内组织工程技术。1　体外组织工程技术体外组织工程技术先从宿主组织中取得宿主细胞 ,在体外进行培养 ,再植入到某种可降解生物材料中 ,这样组成复合植入物 ,一同植回宿主体内 ,使其继续再生过程〔1～ 3〕。体外组织工程技术目前存在许多困难。首先 ,体外组织工程所取得的宿主细胞在传代过程中的分化、变异目前尚没有办法进行很好的控制 ,因此 ,宿主细胞在体外达到有效浓度就极为困难 ,更不用说由宿主细胞行使有效的代谢功能。这样 ,植回宿主体内的仍然是生物可降解材料和少量宿…     

Nanofiber-based scaffolds for tissue engineering

For successful tissue engineering, it is essential to have as many biomimetic scaffolds as possible. With increasing interest in nanotechnology, development of nanofibers (n-fibers) by using the technique of electrospinning is having a new momentum. Among important potential applications of n-fiber-based scaffolds for tissue engineering represent an important advancing front. Nanoscaffolds (n-scaffolds) mimic natural extracellular matrix (ECM) and its nanoscale fibrous structure. With electrospinning, it is possible to develop submicron fibers from biodegradable polymers and these can also be used for developing multifunctional drug-releasing and bioactive scaffolds. Developed n-scaffolds are tested for their cytocompatibility using various cell models. In addition, they were seeded with cells for engineering tissue constructs. There is a large area ahead for further applications and development of these scaffolds. For instance, multifunctional scaffolds that can be used as controlled delivery system do have a potential and have yet to be investigated for improved engineering of various tissues. So far, there are only very few in vivo studies on n-scaffolds, but in the future many are expected to emerge. With the convergence of the fields of nanotechnology, drug release, and tissue engineering, new solutions could be found for the current limitations of tissue engineering. In this paper, nanoscaffolds developed by using electrospinning, used polymers so far, cytocompatibility and applications in tissue engineering are reviewed.     

Bladder tissue engineering through nanotechnology

The field of tissue engineering has developed in phases: initially researchers searched for "inert" biomaterials to act solely as replacement structures in the body. Then, they explored biodegradable scaffolds-both naturally derived and synthetic-for the temporary support of growing tissues. Now, a third phase of tissue engineering has developed, through the subcategory of "regenerative medicine." This renewed focus toward control over tissue morphology and cell phenotype requires proportional advances in scaffold design. Discoveries in nanotechnology have driven both our understanding of cell-substrate interactions, and our ability to influence them. By operating at the size regime of proteins themselves, nanotechnology gives us the opportunity to directly speak the language of cells, through reliable, repeatable creation of nanoscale features. Understanding the synthesis of nanoscale materials, via "top-down" and "bottom-up" strategies, allows researchers to assess the capabilities and limits inherent in both techniques. Urology research as a whole, and bladder regeneration in particular, are well-positioned to benefit from such advances, since our present technology has yet to reach the end goal of functional bladder restoration. In this article, we discuss the current applications of nanoscale materials to bladder tissue engineering, and encourage researchers to explore these interdisciplinary technologies now, or risk playing catch-up in the future.     

Biologic gels in tissue engineering

There are many different types of scaffold materials now available for tissue engineering applications. Hydrogels form one group of materials that have been used in a wide variety of applications. These hydrogels can be formed using natural materials, synthetic materials, or some combination of the two. There are advantages and disadvantages to using each type of material, and detailed investigations into the effects on various aspects of cell behavior of chemical and physical properties of the materials are needed to make an informed decision as to which material is best suited for a given application. By combining appropriate scaffold materials, such as hydrogels, with cells and proper signaling for those cells, more commercial tissue engineering products will become available for general use.     

脂肪组织工程研究进展

成功构建组织工程化脂肪的关键在于：①种子细胞的选定和获取;②具有良好生物降解性和组织相容性的三维支架材料;③种子细胞增殖和分化的微环境。本文就这三方面的研究进展综述如下：     

组织工程和肾脏

组织工程的研究已取得了许多重大成果，有的已经用于临床。但组织工程在肾脏领域的研究才开始起步．本文就目前组织工程在肾脏领域研究的现状及前景作一综述。