Matthew P. Brennan

Tissue-engineered vascular grafts transform into mature blood vessels via an inflammation-mediated process of vascular remodeling

Biodegradable scaffolds seeded with bone marrow mononuclear cells (BMCs) are the earliest tissue-engineered vascular grafts (TEVGs) to be used clinically. These TEVGs transform into living blood vessels in vivo, with an endothelial cell (EC) lining invested by smooth muscle cells (SMCs); however, the process by which this occurs is unclear. To test if the seeded BMCs differentiate into the mature vascular cells of the neovessel, we implanted an immunodeficient mouse recipient with human BMC (hBMC)-seeded scaffolds. As in humans, TEVGs implanted in a mouse host as venous interposition grafts gradually transformed into living blood vessels over a 6-month time course. Seeded hBMCs, however, were no longer detectable within a few days of implantation. Instead, scaffolds were initially repopulated by mouse monocytes and subsequently repopulated by mouse SMCs and ECs. Seeded BMCs secreted significant amounts of monocyte chemoattractant protein-1 and increased early monocyte recruitment. These findings suggest TEVGs transform into functional neovessels via an inflammatory process of vascular remodeling.

Small-diameter biodegradable scaffolds for functional vascular tissue engineering in the mouse model.

The development of neotissue in tissue engineered vascular grafts remains poorly understood. Advances in mouse genetic models have been highly informative in the study of vascular biology, but have been inaccessible to vascular tissue engineers due to technical limitations on the use of mouse recipients. To this end, we have developed a method for constructing sub-1mm internal diameter (ID) biodegradable scaffolds utilizing a dual cylinder chamber molding system and a hybrid polyester sealant scaled for use in a mouse model. Scaffolds constructed from either polyglycolic acid or poly-l-lactic acid nonwoven felts demonstrated sufficient porosity, biomechanical profile, and biocompatibility to function as vascular grafts. The scaffolds implanted as either inferior vena cava or aortic interposition grafts in SCID/bg mice demonstrated excellent patency without evidence of thromboembolic complications or aneurysm formation. A foreign body immune response was observed with marked macrophage infiltration and giant cell formation by post-operative week 3. Organized vascular neotissue, consisting of endothelialization, medial generation, and collagen deposition, was evident within the internal lumen of the scaffolds by post-operative week 6. These results present the ability to create sub-1mm ID biodegradable tubular scaffolds that are functional as vascular grafts, and provide an experimental approach for the study of vascular tissue engineering using mouse models.

Tissue engineered vascular grafts demonstrate evidence of growth and development when implanted in a juvenile animal model

Introduction:The development of a living, autologous vascular graft with the ability to grow holds great promise for advancing the field of pediatric cardiothoracic surgery. Objective:To evaluate the growth potential of a tissue-engineered vascular graft (TEVG) in a juvenile animal model. Methods:Polyglycolic acid nonwoven mesh tubes (3-cm length, 1.3-cm id; Concordia Fibers) coated with a 10% copolymer solution of 50:50 l-lactide and &egr;-caprolactone were statically seeded with 1 × 106 cells/cm2 autologous bone marrow derived mononuclear cells. Eight TEVGs (7 seeded, 1 unseeded control) were implanted as inferior vena cava (IVC) interposition grafts in juvenile lambs. Subjects underwent bimonthly magnetic resonance angiography (Siemens 1.5 T) with vascular image analysis (www.BioimageSuite.org). One of 7-seeded grafts was explanted after 1 month and all others were explanted 6 months after implantation. Neotissue was characterized using qualitative histologic and immunohistochemical staining and quantitative biochemical analysis. Results:All grafts explanted at 6 months were patent and increased in volume as measured by difference in pixel summation in magnetic resonance angiography at 1 month and 6 months. The volume of seeded TEVGs at explant averaged 126.9% ± 9.9% of their volume at 1 month. Magnetic resonance imaging demonstrated no evidence of aneurysmal dilation. TEVG resembled the native IVC histologically and had comparable collagen (157.9 ± 26.4 &mgr;g/mg), elastin (186.9 ± 16.7 &mgr;g/mg), and glycosaminoglycan (9.7 ± 0.8 &mgr;g/mg) contents. Immunohistochemical staining and Western blot analysis showed that Ephrin-B4, a determinant of normal venous development, was acquired in the seeded grafts 6 months after implantation. Conclusions:TEVGs demonstrate evidence of growth and venous development when implanted in the IVC of a juvenile lamb model.

Self-assembly of pH-responsive fluorinated dendrimer-based particulates for drug delivery and noninvasive imaging

Dendrimers are nanoscale macromolecules with well-defined branching chemical structures. Control over the architecture and function of these structures has enabled many advances in materials science and biomedical applications. Though dendrimers are directly synthesized by iteration of simple repetitive steps, generation of the larger, more complex structures required for many biomedical applications by covalent synthetic methods has been challenging. Here we demonstrate a spontaneous self-assembly of poly(amidoamine) dendrimers into complex nanoscopic and microscopic particulates following partial fluorination of the constituent dendrimer subunits. These dense particulates exhibit a stimulus-induced response to low external pH that causes their disassembly over time, enabling controlled release of encapsulated agents. In addition, we show that these assemblies offer a sufficiently high density of fluorine spins to enable detection of their site-specific accumulation in vivo by (19)F magnetic resonance imaging ((19)F MRI). Fluorinated dendrimer-based particulates present new features and capabilities important for a wide variety of emerging biomedical applications.

A non-parametric vessel detection method for complex vascular structures

Modern medical imaging techniques enable the acquisition of in vivo high resolution images of the vascular system. Most common methods for the detection of vessels in these images, such as multiscale Hessian-based operators and matched filters, rely on the assumption that at each voxel there is a single cylinder. Such an assumption is clearly violated at the multitude of branching points that are easily observed in all, but the most focused vascular image studies. In this paper, we propose a novel method for detecting vessels in medical images that relaxes this single cylinder assumption. We directly exploit local neighborhood intensities and extract characteristics of the local intensity profile (in a spherical polar coordinate system) which we term as the polar neighborhood intensity profile. We present a new method to capture the common properties shared by polar neighborhood intensity profiles for all the types of vascular points belonging to the vascular system. The new method enables us to detect vessels even near complex extreme points, including branching points. Our method demonstrates improved performance over standard methods on both 2D synthetic images and 3D animal and clinical vascular images, particularly close to vessel branching regions.

Tissue-engineered arterial grafts: long-term results after implantation in a small animal model

BACKGROUND Use of prosthetic vascular grafts in pediatric vascular surgical applications is limited because of risk of infection, poor durability, potential for thromboembolic complications, and lack of growth potential. Construction of an autologous neovessel using tissue engineering technology offers the potential to create an improved vascular conduit for use in pediatric vascular applications. METHODS Tissue-engineered vascular grafts were assembled from biodegradable tubular scaffolds fabricated from poly-L-lactic acid mesh coated with epsilon-caprolactone and L-lactide copolymer. Thirteen scaffolds were seeded with human aortic endothelial and smooth muscle cells and implanted as infrarenal aortic interposition grafts in SCID/bg mice. Grafts were analyzed at time-points ranging from 4 days to 1 year after implantation. RESULTS All grafts remained patent without evidence of thromboembolic complications, graft stenosis, or graft rupture as documented by serial ultrasound and computed tomographic angiogram, and confirmed histologically. All grafts demonstrated extensive remodeling leading to the development of well-circumscribed neovessels with an endothelial inner lining, neomedia containing smooth muscle cells and elastin, and a collagen-rich extracellular matrix. CONCLUSIONS The development of second-generation tissue-engineered vascular grafts shows marked improvement over previous grafts and confirms feasibility of using tissue engineering technology to create an improved arterial conduit for use in pediatric vascular surgical applications.

Functional Small-Diameter Human Tissue–Engineered Arterial Grafts in an Immunodeficient Mouse Model: Preliminary Findings

HYPOTHESIS The immunodeficient (severe combined immunodeficiency beige [SCID/bg]) mouse model provides a useful model for investigating vascular neotissue formation in human tissue-engineered arterial conduits (TEAC). DESIGN Human aortic smooth muscle cells and endothelial cells were statically seeded on porous biodegradable polymeric scaffolds for vascular tissue engineering. These 2-cell tissue-engineered vascular conduits were implanted into immunodeficient female mice as aortic interposition grafts. Grafts were evaluated over a 30-week course to investigate their patency and structure. SETTING In vivo animal study. PATIENTS Thirteen female C.B-17 SCID/bg mice. INTERVENTION The TEACs implanted as infrarenal abdominal aortic interposition grafts. MAIN OUTCOME MEASURES Selective microcomputed tomography with intra-arterial contrast revealed graft patency and structure. Histological and immunohistochemical evaluations revealed cellularity and extracellular matrix composition. Species-specific immunohistochemical evaluation determined the source of cells within TEACs. RESULTS All TEACs were patent without evidence of thrombosis or rupture over the 30-week course. Histological and immunohistochemical evaluation revealed a von Willebrand factor-positive luminal monolayer surrounded by concentric collagen-rich layers of alpha-smooth muscle actin-positive cells. CONCLUSIONS The SCID/bg mouse is a useful model for investigating vascular neotissue formation in human TEACs. We see evidence that these grafts remain patent while developing into vascular neotissue histologically similar to native aorta. This chimeric animal model also enables determination of seeded cell retention, providing insight into cellular mechanisms underlying neotissue formation.

Accuracy and Reproducibility of Absolute Quantification of Myocardial Focal Tracer Uptake from Molecularly Targeted SPECT/CT: A Canine Validation

Accurate and reproducible SPECT quantification of myocardial molecular processes remains a challenge because of the complication of heterogeneous background and extracardiac activity adjacent to the heart, which causes errors in the estimation of myocardial focal tracer uptake. Our aim in this study was to introduce a heuristic method for the correction of extracardiac activity into SPECT quantification and validate the modified quantification method for accuracy and reproducibility using a canine model. Methods: Dual-isotope–targeted 99mTc and 201Tl perfusion SPECT images were acquired using a hybrid SPECT/CT camera in 6 dogs at 2 wk after myocardial infarction. Images were reconstructed with and without CT-based attenuation correction, and the reconstructed SPECT images were filtered and quantified simultaneously with incorporation of extracardiac radioactivity correction, gaussian fitting, and total-count sampling. Absolute myocardial focal tracer uptake was quantified from SPECT images using 3 different normal limits (maximum entropy [ME], mean-squared-error minimization [MSEM], and global minimum [GM]). SPECT-quantified percentage injected dose (%ID) was calculated and compared with the well-counted radioactivity measured from the postmortem myocardial tissue. SPECT quantitative processing was performed by 2 different individuals with extensive experience in cardiac image processing, to assess reproducibility of the quantitative analysis. Results: Correlations between SPECT-quantified and well-counted %IDs using 3 different normal limits were excellent (ME: r = 0.82, y = 0.932x − 0.0102; MSEM: r = 0.73, y = 1.1413x − 0.0052; and GM: r = 0.7, y = 1.2147x − 0.0002). SPECT quantification using ME normal limits resulted in an underestimation of %ID, as compared with well-counted %ID. Myocardial focal tracer uptake quantified from SPECT images without CT-based attenuation correction was significantly lower than that with the attenuation correction. The %IDs quantified from attenuation-corrected SPECT images using MSEM and GM normal limits were not significantly different from well-counted %IDs. Reproducibility of the SPECT quantitative analysis was excellent (ME: r = 0.98, y = 0.9221x + 0.0001; MSEM: r = 0.97, y = 0.9357x + 0.0004; and GM: r = 0.96, y = 0.9026x + 0.001). Conclusion: Our SPECT/CT quantification algorithm for the assessment of regional radioactivity may allow for accurate and reproducible serial noninvasive evaluation of molecularly targeted tracers in the myocardium.

Generic Iterative Reconstruction of Multi-pinhole SPECT

Single photon emission computerized tomographic (SPECT) images often suffer from low resolution and low count density. To improve spatial resolution of SPECT it is possible to use a pinhole collimator; however, this further reduces the system sensitivity. A potential solution to this problem is to use coded apertures, which offers increased sensitivity by using multiple pinholes, at the cost of increased image reconstruction time. A generic reconstruction algorithm has been developed which allows for arbitrary acquisition geometry via affine transforms (translation and rotation). The reconstruction process uses a (Siddon) ray projector, the expectation maximization (EM) algorithm and a 1 to n pinhole position matrix. Iteration times scale as a function of the number of pinholes in the collimator. Resolution recovery has also been incorporated into the reconstruction algorithm. The algorithm developed allows for the investigation of optimal imaging settings for small animal imaging. Simulated acquisitions of an ex-vivo rat heart with 1, 5 and 8 pinholes, over 360 degree acquisition, showing that multi-pinhole imaging can be successfully applied to rat cardiac imaging. Further refinement of the acquisition parameters, such as image overlap, collimator pinhole configuration and geometrical imaging configuration, will predict the theoretical settings for quantitative cardiac multi-pinhole SPECT imaging.