Dean et al.: Translation of vascular tissue engineering to pediatric CHD 235 block of neotissue” [28]. Rather, these stud- ies suggest a regenerative medicine para- digm, in which seeded cells merely promote the host’s natural healing process. In line with this paradigm are recent studies show- ing that male-derived vessel segments im- planted in female wild type mice previously given green fluorescent protein bone mar- row transplants give rise to neovessel tissue originating from migration of the adjacent vessel segment rather than from the seeded cells or bone marrow [47]. This finding was further substantiated by studies investigating the use of serial magnetic resonance imaging (MRI) for non- invasive monitoring of implanted vascular grafts in a mouse model. The technique was based upon the uptake of ultra-small super- paramagnetic iron oxide (USPIO) particles by monocyte derivatives which were then seeded onto a biodegradable scaffold. MRI of the grafts confirmed the presence of the seeded cells immediately after implantation, but subsequent imaging revealed a rapid re- turn to control levels within 2 hours. The re- turn to baseline correlated with the loss of USPIO-labeled cells from the scaffolds as verified with Prussian blue staining for the presence of iron oxide-labeled cells. These data support the theory of rapid loss of seeded cells from the TEVGs post-implan- tation and argues against their role as direct cellular precursors in the process of neoves- sel formation [48]. Extracellular Matrix In addition to the cellular composition of the neovessel, the characteristics of the extracellular matrix (ECM) play a critical role in the determination of the vessel’s bio- mechanical properties and overall structural integrity. Deviations from the normal phys- iological processes of ECM deposition and remodeling can lead to alterations in the bio- mechanical properties of the graft, resulting in complications such as aneurismal dilata- tion and stenosis. The primary components of the ECM include collagen, elastin, and GAG [39]. Zinc-dependent enzymes re- ferred to as matrix metalloproteinases (MMP) also have been shown to be inti- mately involved in the process of ECM degradation and remodeling [49]. Using a C57BL/6 murine model, the quantitative and qualitative characteristics of these ECM components were assessed in implanted TEVGs over a 4-week period. Results indi- cated a trend of early deposition of Type I and Type III collagen in the neovessel, peak- ing at 2 weeks. Gene expression levels of both Type I and Type III collagen were sig- nificantly higher in the TEVG versus native IVC at that time point, but there was no sta- tistical difference by 4 weeks. Type IV col- lagen was expressed at a lower level in the graft when compared to native IVC at early time points but approximated that of vena cava by 4 weeks. Similarly, analysis of the total collagen content of the neovessel re- vealed no difference between the implanted graft and normal vena cava by the 4-week time point [50]. Additional experiments revealed that gene expression levels of elastin increased within the TEVGs over the course of the ex- periment, and overall production of GAG was also significantly greater when com- pared to earlier time points. Gene expression studies on the metalloproteinases MMP-9 and MMP-2 revealed high early expression of MMP-9 followed by a gradual decline, while MMP-2 expression increased steadily over time [50]. These patterns are consistent with previously described models of vascu- lar injury [51,52]. The presumed roles of MMP-9 and MMP-2 include induction of an early inflammatory foreign body reaction coupled with ECM degradation and remod- eling, thereby allowing the vascular graft to adapt to the specific biomechanical envi- ronment in which it is implanted. These in- vestigations provide some of the first qualitative and quantitative data clarifying the sequence of ECM deposition and re- modeling in TEVGs. These processes play a critical role in neovessel formation and may be implicated in many of the complica- tions arising from deficiencies in the struc- tural integrity of the implanted conduit. Further work is needed in order to more fully characterize these mechanisms so that an improved vascular graft can be designed.