Tissue engineering: an evolving 21st-century science to provide biologic replacement for reconstruction and transplantation

Smart gel system of Linum usitatissimum mucilage as a vehicle of an ophthalmic drug

Smart gel systems involve the utilization of the rapid sol–gel phase transition of an ocular solution in the cul-de-sac of eye in response to physicochemical stimuli. The current investigative scheme has been designed to formulate and assess a smart in situ gelling system of a quinolone antibiotic (ofloxacin) in combination with biodegradable polymers: Linum usitatissimum mucilage (LUM) and sodium alginate. The medicated formulations were clear and isotonic in the pH range 5·57–6·23 and converted into gel in the presence of simulated lachrymal fluid. The formulations were scrutinized through content uniformity, gelling capacity and rheological parameters and in vitro diffusion studies. The formulations were characterized by ultraviolet–visible and Fourier transform infrared spectroscopy for unveiling of the chemical interactions and preliminary structural elucidation. Sterility and stability, antibacterial and eye safety tests using in vitro and in vivo models proved that the optimized formulation (F2) is stable, therapeutically efficacious and non-irritant and provides complete and sustained release of the drug over a 12 h period in a predetermined manner. The data obtained thus suggest that a novel in situ gel system can be successfully designed by using biodegradable polymers for sustained ofloxacin delivery, leading toward the goal of a viable alternate ophthalmic medicine.

Alternating air-medium exposure in rotating bioreactors optimizes cell metabolism in 3D novel tubular scaffold polyurethane foams

Background

In vitro dynamic culture conditions play a pivotal role in developing engineered tissue grafts, where the supply of oxygen and nutrients, and waste removal must be permitted within construct thickness. For tubular scaffolds, mass transfer is enhanced by introducing a convective flow through rotating bioreactors with positive effects on cell proliferation, scaffold colonization and extracellular matrix deposition. We characterized a novel polyurethane-based tubular scaffold and investigated the impact of 3 different culture configurations over cell behavior: dynamic (i) single-phase (medium) rotation and (ii) double-phase exposure (medium-air) rotation; static (iii) single-phase static culture as control.

Methods

A new mixture of polyol was tested to create polyurethane foams (PUFs) as 3D scaffold for tissue engineering. The structure obtained was morphologically and mechanically analyzed tested. Murine fibroblasts were externally seeded on the novel porous PUF scaffold, and cultured under different dynamic conditions. Viability assay, DNA quantification, SEM and histological analyses were performed at different time points.

Results

The PUF scaffold presented interesting mechanical properties and morphology adequate to promote cell adhesion, highlighting its potential for tissue engineering purposes. Results showed that constructs under dynamic conditions contain enhanced viability and cell number, exponentially increased for double-phase rotation; under this last configuration, cells uniformly covered both the external surface and the lumen.

Conclusions

The developed 3D structure combined with the alternated exposure to air and medium provided the optimal in vitro biochemical conditioning with adequate nutrient supply for cells. The results highlight a valuable combination of material and dynamic culture for tissue engineering applications.

Controlling collagen fiber microstructure in three-dimensional hydrogels using ultrasound

Type I collagen is the primary fibrillar component of the extracellular matrix, and functional properties of collagen arise from variations in fiber structure. This study investigated the ability of ultrasound to control collagen microstructure during hydrogel fabrication. Under appropriate conditions, ultrasound exposure of type I collagen during polymerization altered fiber microstructure. Scanning electron microscopy and second-harmonic generation microscopy revealed decreased collagen fiber diameters in response to ultrasound compared to sham-exposed samples. Results of mechanistic investigations were consistent with a thermal mechanism for the effects of ultrasound on collagen fiber structure. To control collagen microstructure site-specifically, a high frequency, 8.3-MHz, ultrasound beam was directed within the center of a large collagen sample producing dense networks of short, thin collagen fibrils within the central core of the gel and longer, thicker fibers outside the beam area. Fibroblasts seeded onto these gels migrated rapidly into small, circularly arranged aggregates only within the beam area, and clustered fibroblasts remodeled the central, ultrasound-exposed collagen fibrils into dense sheets. These investigations demonstrate the capability of ultrasound to spatially pattern various collagen microstructures within an engineered tissue noninvasively, thus enhancing the level of complexity of extracellular matrix microenvironments and cellular functions achievable within three-dimensional engineered tissues.

Biomaterial selection for tooth regeneration

Biomaterials are native or synthetic polymers that act as carriers for drug delivery or scaffolds for tissue regeneration. When implanted in vivo, biomaterials should be nontoxic and exert intended functions. For tooth regeneration, biomaterials have primarily served as a scaffold for (1) transplanted stem cells and/or (2) recruitment of endogenous stem cells. This article critically synthesizes our knowledge of biomaterial use in tooth regeneration, including the selection of native and/or synthetic polymers, three-dimensional scaffold fabrication, stem cell transplantation, and stem cell homing. A tooth is a complex biological organ. Tooth loss represents the most common organ failure. Tooth regeneration encompasses not only regrowth of an entire tooth as an organ, but also biological restoration of individual components of the tooth including enamel, dentin, cementum, or dental pulp. Regeneration of tooth root represents perhaps more near-term opportunities than the regeneration of the whole tooth. In the adult, a tooth owes its biological vitality, arguably more, to the root than the crown. Biomaterials are indispensible for the regeneration of tooth root, tooth crown, dental pulp, or an entire tooth.

Fabrication and characterization of prosurvival growth factor releasing, anisotropic scaffolds for enhanced mesenchymal stem cell survival/growth and orientation

Scaffolds that not only mimic the mechanical and structural properties of the target tissue but also support cell survival/growth are likely necessary for the development of mechanically functional cardiovascular tissues. To reach these goals, we have generated scaffolds that are elastic to approximate soft tissue mechanical properties, are nanofibrous to mimic fibrous nature of extracellular matrix (ECM), have aligned structure to guide cellular alignment, and are capable of releasing insulin-like growth factor (IGF-1) to administrate cellular growth and survival. We have developed a technique that can quickly fabricate (<3 h) such scaffolds by simultaneously electrospinning elastase-sensitive polyurethaneurea nanofibers, encapsulating IGF-1 into poly(lactide-co-glycolide) (PLGA) microspheres and assembling them into scaffolds. Scaffold morphology, mechanical properties, degradation with or without elastase, and bioactivity of the released IGF-1 were assessed. The scaffolds had degree of alignment approximately 70%. They were flexible and relatively strong, with tensile strengths of 3.4-11.1 MPa, elongations at break of 71-88%, and moduli of 2.3-7.9 MPa at the alignment direction. IGF-1 release profile and bioactivity were dependent on PLGA content and molecular weight and IGF-1 loading. The released IGF-1 remained bioactive for 4 weeks. The fabricated nanofibers were elastase-sensitive with weight remaining <59% after a 4-week degradation in the presence of elastase. Mesenchymal stem cells (MSCs) were seeded on the scaffolds and cultured either under normal culture conditions (21% O(2), 5% CO(2), and 20% fetal bovine serum (FBS)) or hypoxia/nutrient starvation conditions (5% O(2), 5% CO(2), and 1% FBS) to evaluate the effect of IGF-1 loading on cell growth and survival. Under normal culture conditions, MSCs were found to align on the scaffolds with a degree of alignment matching that of the scaffold. The IGF-1 loaded scaffolds enhanced MSC growth during a 7-day culture period, with higher IGF-1 content showing better stimulus effect. Under hypoxia/nutrient starvation conditions, the IGF-1 loaded scaffolds were found to significantly improve MSC survival.

A novel approach for reducing ischemic mitral regurgitation by injection of a polymer to reverse remodel and reposition displaced papillary muscles

BACKGROUND

Ischemic mitral regurgitation (MR) relates to displacement of the papillary muscles from ischemic ventricular distortion. We tested the hypothesis that repositioning of the papillary muscles can be achieved by injection of polyvinyl-alcohol (PVA) polymer, a biologically inert biomaterial that has been specially formulated to produce an encapsulated, stable, resilient gel once injected into the myocardium. The purpose is to materially support the infarcted myocardium while at the same time repositioning the papillary muscles that become apically tethered in MR.

METHODS AND RESULTS

Nine sheep underwent ligation of circumflex branches to produce acute ischemic MR. PVA polymer was then injected by echo guidance into the myocardium underlying the infarcted papillary muscle. Hemodynamic data, left ventricular ejection fraction, elastance, tau (relaxation constant), left ventricular stiffness coefficient, and 2-dimensional and 3-dimensional echocardiograms were obtained post-MR and post-PVA injection. One animal died after coronary ligation and 2 did not develop MR. In the remaining 6, moderate MR developed. With PVA injection, the MR decreased significantly from moderate to trace-mild (vena contracta: 5+/-0.4 mm versus 2+/-0.7 mm, post-MR versus post-PVA injection; P<0.0001). This was associated with a decrease in infarcted papillary muscle-to-mitral annulus tethering distance (27+/-4 to 24+/-4 mm, post-MR versus post-PVA, P<0.001). Importantly, PVA injection was not associated with significant decreases in left ventricular ejection fraction (43+/-6% versus 37+/-4%, post-MR versus post-PVA, P=nonsignificant), elastance (3.5+/-1.4 versus 2.9+/-1.3; post-MR versus post-PVA injection, P=nonsignificant). Measures of left ventricular diastolic function, tau (100+/-51 ms to 84+/-37 ms, post-MR versus post-PVA; P=nonsignificant), and left ventricular stiffness coefficient (0.18+/-0.12 versus 0.14+/-0.08, post-MR versus post-PVA; P=nonsignificant) did not increase post-PVA.

CONCLUSIONS

PVA polymer injection resulted in acute reverse remodeling of the ventricle with papillary muscle repositioning to decrease MR. This was not associated with an adverse effect on left ventricular systolic and diastolic function. This new approach to alter pathological anatomy after infarction may offer an alternative strategy for relieving ischemic MR by correcting the position of the affected papillary muscle, thus relieving apical tethering.

Intestinal regeneration by a novel surgical procedure

BACKGROUND

Treatment of short bowel syndrome is problematical. Small bowel tissue engineering has achieved modest results in animal studies. The aim of this study was to investigate intestinal regeneration in a novel surgical model.

METHODS

Roux-en-Y bypass procedures were performed on 40 Wistar rats weighing 250-350 g. Animals were killed at 1, 2, 3, 4, 8, 12 and 24 weeks after implantation with a 3-cm silicone tube. The spatio temporal relationship of intestinal regeneration was analysed using three-dimensional multislice computed tomography, and examination of sequential morphological changes on gross or histological findings and measurement of missing intestinal tissue (growth defects).

RESULTS

Progressive intestinal regeneration on a silicone tube was identifiable in 35 animals. Most adhesions were initially localized on the tube but spread to a distal site 4 weeks after implantation. Growth defects decreased with time, with a marked reduction in the first 4 weeks and a gradual reduction to week 24 after implantation. Luminal patency shown radiologically as well as sequential histological findings, such as mucosal lining, matrix remodelling and muscular regeneration, suggested that regeneration of intestinal tissue took place, not merely scar contraction.

CONCLUSION

Non-invasive as well as histomorphological assessment followed intestinal regeneration over time in this model, which provides scope for further studies.

Physicochemical degradation studies of calcium phosphate glass ceramic in the CaO-P2O5-MgO-TiO2 system

The aim of this work was to evaluate the in vitro degradation behaviour of a 45CaO-37P(2)O(5)-5MgO-13TiO(2) (mol.%) glass ceramic, under two different simulated physiological conditions: normal physiological pH 7.4, and pH 3.0, which was designed to simulate the acidic conditions produced by osteoclast cells. The in vitro testing was carried out at 37 degrees C for up to 42 days for the pH 7.4 solution and for up to 1 day for the pH 3.0 solution. The incorporation of TiO(2) into the glass structure leads to the precipitation of specific crystalline phases in the glass matrix, namely alpha- and beta-Ca(2)P(2)O(7), TiP(2)O(7) and CaTi(4)(PO(4))(6). The degradation testing at pH 3.0 showed a higher weight loss compared with degradation testing at pH 7.4; the weight loss under the acidic condition after 1 day (24 h) was about 10 times higher than the weight loss after 42 days of immersion at pH 7.4. The ionic release profile of Ca(2+), PO(4)(3-), Mg(2+) and Ti(4+) showed a continuous increase in concentration over all immersion times for both testing solutions. After 1 day of immersion at pH 3.0, the concentration levels of Mg(2+), Ca(2+), PO(4)(3-) were about six times higher than the levels achieved after 42 days of immersion at pH 7.4. The glass ceramic showed similar degradation to hydroxyapatite, and therefore has potential to be used in certain clinical applications where relatively slow resorption of the implant and replacement by bone is required, e.g. cranioplasty.

Left Ventricular Functional Recovery with Percutaneous, Transvascular Direct Myocardial Delivery of Bone Marrow-Derived Cells

BACKGROUND

The potential for cellular cardiomyoplasty to provide functional left ventricular recovery in the chronically injured heart remains unclear.

METHODS

Yorkshire swine (n = 10; 35-50 kg) had anterolateral myocardial infarction (MI) induced by coil embolization of the left anterior descending artery. Approximately 5 weeks post-MI, a composite, intravascular ultrasound-guided catheter system (TransAccess) was used to deliver an autologous, labeled, bone marrow-derived cell sub-population (approximately 3 x 10(8) cells) or saline control (approximately 50 injections/arm) through coronary veins directly into infarct and peri-infarct myocardium. Two months post-transplant, the animals had blinded endocardial and epicardial left ventricular electrical scar mapping and biventricular electrical stimulation. Coronary angiography and quantitative biplane ventriculography were performed at baseline, transplant, and sacrifice time-points.

RESULTS

Robust, viable, predominantly desmin-negative cell grafts were demonstrated post-mortem in all treatment animals. Baseline and pre-transplant global and regional wall motion was similar between groups. The cell treatment group demonstrated functional recovery with a left ventricular ejection fraction of 38.1% at the time of transplant increasing to 48.5% (p = 0.005) at sacrifice, whereas the control arm was unchanged (38.0% vs 34.3%, respectively; p = NS). The regional improvement corresponded with a reduction in percentage of hypokinetic (52.1%-42.9%, p = 0.002) and percentage of akinetic (24.8%-17.7%, p = 0.04) segments in the cell-treated animals. Epicardial scar area was not different (37 cm2 vs 23 cm2, p = 0.37) between groups.

CONCLUSIONS

Percutaneous, transvascular, direct intramyocardial bone marrow cell transplantation is safe and feasible in chronically infarcted tissue. In this pilot study, cell therapy improved overall left ventricular systolic function by recruiting previously hypokinetic or akinetic myocardial tissue.

New Insight into Progenitor/Stem Cells in Dental Pulp Using Col1a1-GFP Transgenes

In recent years there has been increasing progress in identifying stem cells from adult tissues and their potential application in tissue engineering. These advances provide a promising future for tooth replacement/regeneration. Essential for this approach is the identification of donor stem cells for various components of the teeth. Our studies show that pOBCol3.6GFPtpz and pOBCol2.3GFPemd transgenic animals provide a unique model to gain insight into stem cells in the dental pulp. Our in vivo studies of the developing teeth of these transgenic lines show both Col1a1-GFP transgenes are expressed in functional and fully differentiated odontoblasts. The patterns of expression of Col1a1-GFP transgenes during odontoblast differentiation correlates with the expression of DSPP. In the developing craniofacial bones both Col1a1-GFP transgenes are also expressed in osteoblasts and osteocytes of alveolar and calvarial bones. In the alveolar bones, the expression of Col1a1-GFP in osteocytes correlates with the expression of DMP1. Col1a1-3.6-GFP is expressed in the entire layer of the periosteum and in suture mesenchyme containing osteoprogenitor cells. On the other hand, Col1a1-2.3- GFP expression was limited to the osteoblastic layer of the periosteum and was not detected in the fibroblastic layer of the periosteum or in the suture mesenchyme. These observations indicate that Col1a1-3.6-GFP and Col1a1-2.3-GFP transgenes identify different subpopulations of cells during intramembranous ossification. By using the coronal portion of dental pulps isolated from postnatal transgenic mice our observations also provide direct evidence that the dental pulp contains progenitor/stem cells capable of giving rise to a new generation of odontoblast-like cells, as well as osteoblast-like cells.

Clinical applications of tissue engineered constructs

The reconstruction of soft tissue defects poses a challenge for plastic surgeons and tissue engineers. The construction of a biologically, functionally, and cosmetically successful replacement part will involve the combination of a composite that contains endoderm, mesoderm, and ectoderm. It will be active in immune surveillance and function. It must be durable to withstand the stress and strain encountered by the skin. Such a composite will involve the use of bone, cartilage, muscle, blood vessels, nerves, connective tissue, dermis, and epidermis. Fortunately, many of these tissues are among the best studied by tissue engineers. The future of this field will likely involve to some degree the co-mingling of current reconstructive modalities, including the techniques of prefabrication and pre-lamination, with more aggressive and successful tissue engineering technology and the rapidly developing science of stem cell biology. Tissues synthesized in vitro with better structure, color, and texture can be pre-laminated to a site that has already been prefabricated. Prefabrication of a bio-absorbable matrix can create a well perfused scaffold onto which larger subunits can be prelaminated. The future of this field of endeavor is exciting, and, with further research, experience, and interdisciplinary collaboration, bioengineered tissue constructs will become a reality.

Percutaneous transvenous cellular cardiomyoplasty. A novel nonsurgical approach for myocardial cell transplantation

OBJECTIVES

The study evaluated a nonsurgical means of intramyocardial cell introduction using the coronary venous system for direct myocardial access and cell delivery.

BACKGROUND

Direct myocardial cell repopulation has been proposed as a potential method to treat heart failure.

METHODS

We harvested bone marrow from Yorkshire swine (n = 6; 50 to 60 kg), selected culture-flask adherent cells, labeled them with the gene for green fluorescence protein, expanded them in culture, and resuspended them in a collagen hydrogel. Working through the coronary sinus, a specialized catheter system was easily delivered to the anterior interventricular coronary vein. The composite catheter system (TransAccess) incorporates a phased-array ultrasound tip for guidance and a sheathed, extendable nitinol needle for transvascular myocardial access. A microinfusion (IntraLume) catheter was advanced through the needle, deep into remote myocardium, and the autologous cell-hydrogel suspension was injected into normal heart. Animals were sacrificed at days 0 (n = 2), 14 (n = 1, + 1 control/collagen biogel only), and 28 (n = 2), and the hearts were excised and examined.

RESULTS

We gained widespread intramyocardial access to the anterior, lateral, septal, apical, and inferior walls from the anterior interventicular coronary vein. No death, cardiac tamponade, ventricular arrhythmia, or other procedural complications occurred. Gross inspection demonstrated no evidence of myocardial perforation, and biogel/black tissue dye was well localized to sites corresponding to fluoroscopic landmarks for delivery. Histologic analysis demonstrated needle and microcatheter tracts and accurate cell-biogel delivery.

CONCLUSIONS

Percutaneous intramyocardial access is safe and feasible by a transvenous approach through the coronary venous system. The swine offers an opportunity to refine approaches used for cellular cardiomyoplasty.

Tissue engineering

Tissue engineering will potentially change the practice of plastic surgery more than any other clinical specialty. It is an interdisciplinary field that promises new methods of tissue repair. There has been more than $3.5 billion invested in this field since 1990. Relevant areas of progress include advanced computing, biomaterials, cell technology, growth factor fabrication and delivery, and gene manipulation. Beneficial clinical techniques will emerge from continued investigation in each of these areas. Techniques that are developed must be scaled up to industry with products cleared by regulatory agencies and acceptable to clinicians and patients. A goal of tissue engineering is to change clinical practice, yielding improved patient outcomes and lower costs of care.