Danielle Gottlieb

THE ROLE OF ORGAN LEVEL CONDITIONING ON THE PROMOTION OF ENGINEERED HEART VALVE TISSUE DEVELOPMENT IN-VITRO USING MESENCHYMAL STEM CELLS

We have previously shown that combined flexure and flow (CFF) augment engineered heart valve tissue formation using bone marrow-derived mesenchymal stem cells (MSC) seeded on polyglycolic acid (PGA)/poly-L-lactic acid (PLLA) blend nonwoven fibrous scaffolds (Engelmayr, et al., Biomaterials 2006; vol. 27 pp. 6083-95). In the present study, we sought to determine if these phenomena were reproducible at the organ level in a functional tri-leaflet valve. Tissue engineered valve constructs (TEVC) were fabricated using PGA/PLLA nonwoven fibrous scaffolds then seeded with MSCs. Tissue formation rates using both standard and augmented (using basic fibroblast growth factor [bFGF] and ascorbic acid-2-phosphate [AA2P]) media to enhance the overall production of collagen were evaluated, along with their relation to the local fluid flow fields. The resulting TEVCs were statically cultured for 3 weeks, followed by a 3 week dynamic culture period using our organ level bioreactor (Hildebrand et al., ABME, Vol. 32, pp. 1039-49, 2004) under approximated pulmonary artery conditions. Results indicated that supplemented media accelerated collagen formation (approximately 185% increase in collagen mass/MSC compared to standard media), as well as increasing collagen mass production from 3.90 to 4.43 pg/cell/week from 3 to 6 weeks. Using augmented media, dynamic conditioning increased collagen mass production rate from 7.23 to 13.65 pg/cell/week (88.8%) during the dynamic culture period, along with greater preservation of net DNA. Moreover, when compared to our previous CFF study, organ level conditioning increased the collagen production rate from 4.76 to 6.42 pg/cell/week (35%). Newly conducted CFD studies of the CFF specimen flow patterns suggested that oscillatory surface shear stresses were surprisingly similar to a tri-leaflet valve. Overall, we found that the use of simulated pulmonary artery conditions resulted in substantially larger collagen mass production levels and rates found in our earlier CFF study. Moreover, given the fact that the scaffolds underwent modest strains (approximately 7% max) during either CFF or physiological conditioning, the oscillatory surface shear stresses estimated in both studies may play a substantial role in eliciting MSC collagen production in the highly dynamic engineered heart valve fluid mechanical environment.

Regional Structural and Biomechanical Alterations of the Ovine Main Pulmonary Artery During Postnatal Growth

The engineering foundation for novel approaches for the repair of congenital defects that involve the main pulmonary artery (PA) must rest on an understanding of changes in the structure-function relationship that occur during postnatal maturation. In the present study, we quantified the postnatal growth patterns in structural and biomechanical behavior in the ovine PA in the juvenile and adult stages. The biaxial mechanical properties and collagen and elastin fiber architecture were studied in four regions of the PA wall, with the collagen recruitment of the medial region analyzed using a custom biaxial mechanical-multiphoton microscopy system. Circumferential residual strain was also quantified at the sinotubular junction and bifurcation locations, which delimit the PA. The PA wall demonstrated significant mechanical anisotropy, except in the posterior region where it was nearly isotropic. Overall, we observed only moderate changes in regional mechanical properties with growth. We did observe that the medial and lateral locations experience a moderate increase in anisotropy. There was an average of about 24% circumferential residual stain present at the luminal surface in the juvenile stage that decreased to 16% in the adult stage with a significant decrease at the bifurcation, implying that the PA wall remodels toward the bifurcation with growth. There were no measurable changes in collagen and elastin content of the tunica media with growth. On average, the collagen fiber recruited more rapidly with strain in the adult compared to the juvenile. Interestingly, the PA thickness remained constant with growth. When this fact is combined with the observed stable overall mechanical behavior and increase in vessel diameter with growth, a simple Laplace Law wall stress estimate suggests an increase in effective PA wall stress with postnatal maturation. This observation is contrary to the accepted theory of maintenance of homeostatic stress levels in the regulation of vascular function and suggests alternative mechanisms regulate postnatal somatic growth. Understanding the underlying mechanisms, incorporating important structural features during growth, will help to improve our understanding of congenital defects of the PA and lay the basis for functional duplication in their repair and replacement.

Estimated in Vivo Postnatal Surface Growth Patterns of the Ovine Main Pulmonary Artery and Ascending Aorta

Delineating the normal postnatal development of the pulmonary artery (PA) and ascending aorta (AA) can inform our understanding of congenital abnormalities, as well as pulmonary and systolic hypertension. We thus conducted the following study to delineate the PA and AA postnatal growth deformation characteristics in an ovine model. MR images were obtained from endoluminal surfaces of 11 animals whose ages ranged from 1.5 months/15.3 kg mass (very young) to 12 months/56.6 kg mass (adult). A bicubic Hermite finite element surface representation was developed for the each artery from each animal. Under the assumption that the relative locations of surface points were retained during growth, the individual animal surface fits were subsequently used to develop a method to estimate the time-evolving local effective surface growth (relative to the youngest measured animal) in the end-diastolic state. Results indicated that the spatial and temporal surface growth deformation patterns of both arteries, especially in the circumferential direction, were heterogeneous, leading to an increase in taper and increase in cross-sectional ellipticity of the PA. The longitudinal PA growth stretch of a large segment on the posterior wall reached 2.57 ± 0.078 (mean ± SD) at the adult stage. In contrast, the longitudinal growth of the AA was smaller and more uniform (1.80 ± 0.047). Interestingly, a region of the medial wall of both arteries where both arteries are in contact showed smaller circumferential growth stretches-specifically 1.12 ± 0.012 in the PA and 1.43 ± 0.071 in the AA at the adult stage. Overall, our results indicated that contact between the PA and AA resulted in increasing spatial heterogeneity in postnatal growth, with the PA demonstrating the greatest changes. Parametric studies using simplified geometric models of curved arteries during growth suggest that heterogeneous effective surface growth deformations must occur to account for the changes in measured arterial shapes during the postnatal growth period. This result suggests that these first results are a reasonable first-approximation to the actual effective growth patterns. Moreover, this study clearly underscores how functional growth of the PA and AA during postnatal maturation involves complex, local adaptations in tissue formation. Moreover, the present results will help to lay the basis for functional replacement by defining critical geometric metrics.

Successful Treatment of Mediastinitis in a Young Child by Omental Translocation Following Extracardiac Fontan Graft Placement

Mediastinitis after a midline sternotomy can become a serious complication, especially after implantation of prosthetic vascular grafts. We present a case of a three-year-old boy with hypoplastic left heart syndrome who developed mediastinitis following his third-stage palliation (Fontan operation). Rather than following the “traditional” surgical therapy of graft explantation, debridement, and replacement, we chose to preserve the graft and protect it by omental translocation. The relative merits of this therapeutic approach, which is rarely utilized and underappreciated in children, are outlined and discussed.

Mechanical Characterization of the Wall of a Tissue Engineered Pulmonary Valve Conduit: A Twenty Week In Vivo Study

Current clinical options for congential pulmonary valve disease are limited and associated with several complications, including lack of somatic growth in replacements. Often, surgical intervention also requires the reconstruction of the right ventricular outflow tract. Tissue engineered pulmonary valved conduits have received much attention as a potential therapy, offering prospective long-term functional improvements and accommodating somatic growth [1]. Though in vitro work has been performed, little is known concerning the physical properties and quality of the tissue produced in vivo, owing to small specimen sample sizes and a lack of detailed mechanical analyses. This work focuses on elucidating in vivo time-course changes in the mechanical quality of tissue engineered pulmonary valve conduit.Copyright

Effect of In Vivo Physical Interaction of the Ascending Aorta and Main Pulmonary Artery on Postnatal Surface Growth Patterns in Ovine

Congenital abnormalities of the main pulmonary artery (MPA) and ascending aorta (AA) often necessitate surgical repair or the use of a valved conduit replacement, requiring multiple re-interventions due to regurgitation or failure of the prosthetic conduit. In recent years there has been a growing interest in the development of a living autologous tissue graft that could address the critical need for growing substitutes in the repair of congenital cardiovascular defects [1]. Regardless of the particulars of the therapeutic approach, the detailed growth characteristics of the native artery is required to establish the baseline dimensional changes post-implantation. During normal embryogenesis the Truncus Arteriosus begins to split and form into the anterior pulmonary artery and the posterior aorta [2]. Due to their common embryologic origin from a single outflow tract, there are disease conditions that originate in one artery and eventually affect both arteries [3]. Therefore, the postnatal growth deformation of both the MPA and AA was computed to quantify the effects of the mechanical association of these two arteries.Copyright

Three-Dimensional High Resolution Scaffold Fiber Architecture and Morphology in Tissue Engineered Heart Valve Tissue

Engineered heart valve tissue (EHVT) has received much attention as a potential pediatric valve replacement therapy, offering prospective long-term functional improvements over current options. A significant gap in the literature exists, however, regarding estimating tissue mechanical properties from tissue-scaffold composites. Detailed three-dimensional structural information prior to implantation (in vitro) and after implantation in (in vivo) is needed for improved modeling of tissue properties. As such, a novel high-resolution imaging technique will be employed to obtain three-dimensional microstructural information. Analysis techniques will be used to fully quantify constituents of interest including scaffold, collagen, and cellular information and to develop appropriate two-dimensional sectioning sampling protocols. It is the intent of this work to guide modeling efforts to better elucidate EHVT tissue-specific mechanical properties.Copyright