A novel concept for scaffold-free vessel tissue engineering: self-assembly of microtissue building blocks.

Current scientific attempts to generate in vitro tissue-engineered living blood vessels (TEBVs) show substantial limitations, thereby preventing routine clinical use. In the present report, we describe a novel biotechnology concept to create living small diameter TEBV based exclusively on microtissue self-assembly (living cellular re-aggregates). A novel bioreactor was designed to assemble microtissues in a vascular shape and apply pulsatile flow and circumferential mechanical stimulation.

Tissue engineering of heart valves: advances and current challenges.

It is estimated that the number of patients requiring heart valve replacement will triple over the next five decades. None of the current replacement valves can fully restore native valve function because they lack growth and remodeling capabilities. Heart valve tissue engineering is a promising technology to overcome these limitations.

Hypoxia induces near-native mechanical properties in engineered heart valve tissue.

Background: Previous attempts in heart valve tissue engineering (TE) failed to produce autologous valve replacements with native-like mechanical behavior to allow for systemic pressure applications. Because hypoxia and insulin are known to promote protein synthesis by adaptive cellular responses, a physiologically relevant oxygen tension and insulin supplements were applied to the growing heart valve tissues to enhance their mechanical properties.

Methods And Results: Scaffolds of rapid-degrading polyglycolic acid meshes coated with poly-4-hydroxybutyrate were seeded with human saphenous vein myofibroblasts.

Living heart valve and small-diameter artery substitutes--an emerging field for intellectual property development.

Cardiovascular diseases, such as heart valve dysfunction and coronary artery stenosis, are next to cancer the leading cause of death in the US. Treatments involve replacement of the heart valve or bypassing the obstructed coronary artery with a small-diameter vascular graft. The major limitation of currently used replacements is their inability to grow, adapt and repair in the patient.

Straining mode-dependent collagen remodeling in engineered cardiovascular tissue.

Similar to native cardiovascular tissues, the mechanical properties of engineered cardiovascular constructs depend on the composition and quality of the extracellular matrix, which is a net result of matrix remodeling processes within the tissue. To improve tissue remodeling, and hence tissue mechanical properties, various mechanical conditioning protocols, such as strain-based or flow-based conditioning, have been applied to engineered cardiovascular constructs with promising results. We hypothesize that tissue remodeling is dependent on the mode of straining.

Intermittent straining accelerates the development of tissue properties in engineered heart valve tissue.

Tissue-engineered heart valves lack sufficient amounts of functionally organized structures and consequently do not meet in vivo mechanical demands. To optimize tissue architecture and hence improve mechanical properties, various in vitro mechanical conditioning protocols have been proposed, of which intermittent straining is most promising in terms of tissue properties. We hypothesize that this is due to an improved collagen matrix synthesis, maturation, and organization, triggered by periodic straining of cells.

Stress related collagen ultrastructure in human aortic valves--implications for tissue engineering.

Understanding the response of tissue structures to mechanical stress is crucial for optimization of mechanical conditioning protocols in the field of heart valve tissue engineering. In heart valve tissue, it is unclear to what extent mechanical loading affects the collagen fibril morphology. To determine if local stress affects the collagen fibril morphology, in terms of fibril diameter, its distribution, and the fibril density, this was investigated in adult native human aortic valve leaflets.

Tailoring fiber diameter in electrospun poly(epsilon-caprolactone) scaffolds for optimal cellular infiltration in cardiovascular tissue engineering.

Despite the attractive features of nanofibrous scaffolds for cell attachment in tissue-engineering (TE) applications, impeded cell ingrowth has been reported in electrospun scaffolds. Previous findings have shown that the scaffold can function as a sieve, keeping cells on the scaffold surface, and that cell migration into the scaffold does not occur in time. Because fiber diameter is directly related to the pore size of an electrospun scaffold, the objective of this study was to systematically evaluate how cell delivery can be optimized by tailoring the fiber diameter of electrospun poly(epsilon-caprolactone) (PCL) scaffolds.

Transendothelial migration drives dissociation of plateletmonocyte complexes.

Monocytes and platelets are both crucially involved in atherogenesis. Importantly, activated platelets bound to circulating monocytes increase adhesion of the monocytes and thus mediate colocalization of both cell types at the vessel wall. We examined the fate of the platelets upon migration of these potentially pro-atherogenic platelet-monocyte complexes (PMC) across activated endothelium.

In vitro heart valve tissue engineering.

Heart valve replacement represents the most common surgical therapy for end-stage valvular heart diseases. A major drawback all contemporary heart valve replacements have in common is the lack of growth, repair, and remodeling capabilities. To overcome these limitations, the emerging field of tissue engineering is focusing on the in vitro generation of functional, living heart valve replacements.

Prenatally fabricated autologous human living heart valves based on amniotic fluid derived progenitor cells as single cell source.

Background: A novel concept providing prenatally tissue engineered human autologous heart valves based on routinely obtained fetal amniotic fluid progenitors as single cell source is introduced.

Methods And Results: Fetal human amniotic progenitors were isolated from routinely sampled amniotic fluid and sorted using CD133 magnetic beads. After expansion and differentiation, cell phenotypes of CD133- and CD133+ cells were analyzed by immunohistochemistry and flowcytometry.

Effect of biomimetic conditions on mechanical and structural integrity of PGA/P4HB and electrospun PCL scaffolds.

The selection of an appropriate scaffold represents one major key to success in tissue engineering. In cardiovascular applications, where a load-bearing structure is required, scaffolds need to demonstrate sufficient mechanical properties and importantly, reliable retention of these properties during the developmental phase of the tissue engineered construct. The effect of in vitro culture conditions, time and mechanical loading on the retention of mechanical properties of two scaffold types was investigated.

The role of collagen cross-links in biomechanical behavior of human aortic heart valve leaflets--relevance for tissue engineering.

A major challenge in tissue engineering of functional heart valves is to determine and mimic the dominant tissue structures that regulate heart valve function and in vivo survival. In native heart valves, the anisotropic matrix architecture assures sustained and adequate functioning under high-pressure conditions. Collagen, being the main load-bearing matrix component, contributes significantly to the biomechanical strength of the tissue.

Engineering of biologically active living heart valve leaflets using human umbilical cord-derived progenitor cells.

This study demonstrates the engineering of biologically active heart valve leaflets using prenatally available human umbilical cord-derived progenitor cells as the only cell source. Wharton's Jelly-derived cells and umbilical cord blood-derived endothelial progenitor cells were subsequently seeded on biodegradable scaffolds and cultured in a biomimetic system under biochemical or mechanical stimulation or both. Depending on the stimulation, leaflets showed mature layered tissue formation with functional endothelia and extracellular matrix production comparable with that of native tissues.

Functional growth in tissue-engineered living, vascular grafts: follow-up at 100 weeks in a large animal model.

Background: Living autologous vascular grafts with the capacity for regeneration and growth may overcome the limitations of contemporary artificial prostheses. Particularly in congenital cardiovascular surgery, there is an unmet medical need for growing replacement materials. Here we investigate growth capacity of tissue-engineered living pulmonary arteries in a growing lamb model.

Autologous human tissue-engineered heart valves: prospects for systemic application.

Background: Tissue engineering represents a promising approach for the development of living heart valve replacements. In vivo animal studies of tissue-engineered autologous heart valves have focused on pulmonary valve replacements, leaving the challenge to tissue engineer heart valves suitable for systemic application using human cells.

Methods And Results: Tissue-engineered human heart valves were analyzed up to 4 weeks and conditioning using bioreactors was compared with static culturing.

Living autologous heart valves engineered from human prenatally harvested progenitors.

Background: Heart valve tissue engineering is a promising strategy to overcome the lack of autologous growing replacements, particularly for the repair of congenital malformations. Here, we present a novel concept using human prenatal progenitor cells as new and exclusive cell source to generate autologous implants ready for use at birth.

Methods And Results: Human fetal mesenchymal progenitors were isolated from routinely sampled prenatal chorionic villus specimens and expanded in vitro.

Modeling the mechanics of tissue-engineered human heart valve leaflets.

Mathematical models can provide valuable information to assess and evaluate the mechanical behavior of tissue-engineered constructs. In this study, a structurally based model is applied to describe and analyze the mechanics of tissue-engineered human heart valve leaflets. The results from two orthogonal uniaxial tensile tests are used to determine the model parameters of the constructs after two, three and four weeks of culturing.

Platelet binding to monocytes increases the adhesive properties of monocytes by up-regulating the expression and functionality of β and β integrins.

Human monocytes adhere to activated platelets, resulting in the formation of platelet-monocyte complexes (PMC). Complex formation depends on the interaction between platelet-displayed P-selectin and the specific ligand for P-selectin on leukocytes, P-selectin glycoprotein ligand-1 (PSGL-1). We have recently shown that monocytes within PMC have increased adhesive capacity to the activated endothelium.

Platelet binding to monocytes increases the adhesive properties of monocytes by up-regulating the expression and functionality of beta1 and beta2 integrins.

Human monocytes adhere to activated platelets, resulting in the formation of platelet-monocyte complexes (PMC). Complex formation depends on the interaction between platelet-displayed P-selectin and the specific ligand for P-selectin on leukocytes, P-selectin glycoprotein ligand-1 (PSGL-1). We have recently shown that monocytes within PMC have increased adhesive capacity to the activated endothelium.

Tissue engineering of human heart valve leaflets: a novel bioreactor for a strain-based conditioning approach.

Current mechanical conditioning approaches for heart valve tissue engineering concentrate on mimicking the opening and closing behavior of the leaflets, either or not in combination with tissue straining. This study describes a novel approach by mimicking only the diastolic phase of the cardiac cycle, resulting in tissue straining. A novel, yet simplified, bioreactor system was developed for this purpose by applying a dynamic pressure difference over a closed tissue engineered valve, thereby inducing dynamic strains within the leaflets.

Living patches engineered from human umbilical cord derived fibroblasts and endothelial progenitor cells.

Objective: A major shortcoming in contemporary congenital heart surgery is the lack of viable replacement materials with the capacity of growth and regeneration. Here we focused on living autologous patches engineered from human umbilical cord derived fibroblasts and endothelial progenitor cells (EPCs) as a ready-to-use cell source for paediatric cardiovascular tissue engineering.

Methods: EPCs were isolated from 20 ml fresh umbilical cord blood by density gradient centrifugation and myofibroblasts were harvested from umbilical cord tissue.

Fibrin as a cell carrier in cardiovascular tissue engineering applications.

In cardiovascular tissue engineering approaches, efficient seeding methods are essential. To achieve this and to save time, cells can be encapsulated in gels. Combining the advantages of a gel as a cell carrier with the advantages of a fiber-based scaffold, providing structural integrity to the developing tissue, might offer several advantages.