Determining the fate of seeded cells in venous tissue‐engineered vascular grafts using serial MRI

Abstract

A major limitation of tissue engineering research is the lack of noninvasive monitoring techniques for observations of dynamic changes in single tissue-engineered constructs. We use cellular magnetic resonance imaging (MRI) to track the fate of cells seeded onto functional tissue-engineered vascular grafts (TEVGs) through serial imaging. After in vitro optimization, murine macrophages were labeled with ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles and seeded onto scaffolds that were surgically implanted as inferior vena cava interposition grafts in SCID/bg mice. Serial MRI showed the transverse relaxation times (T(2)) were significantly lower immediately following implantation of USPIO-labeled scaffolds (T(2) = 44 ± 6.8 vs. 71 ± 10.2 ms) but increased rapidly at 2 h to values identical to control implants seeded with unlabeled macrophages (T(2) = 63 ± 12 vs. 63 ± 14 ms). This strongly indicates the rapid loss of seeded cells from the scaffolds, a finding verified using Prussian blue staining for iron containing macrophages on explanted TEVGs. Our results support a novel paradigm where seeded cells are rapidly lost from implanted scaffolds instead of developing into cells of the neovessel, as traditionally thought. Our findings confirm and validate this paradigm shift while demonstrating the first successful application of noninvasive MRI for serial study of cellular-level processes in tissue engineering.

Bioengineered blood vessels

“…Monitoring the maturation of TEBV may be achieved through destructive and nondestructive (noninvasive) manners. Methods that require the 'sacrifice' of the samples include histology and molecular analyses, whereas noninvasive methods, such as MRI [78,79], ultrasound, X-ray CT imaging and optical imaging, allow real-time and repetitive measurements of a single sample [78,80]. Ultrasound has a relatively low resolution (sub-millimeter) and is commonly used for macroscopic imaging to determine vessel graft patency and flow patterns or to grossly monitor ECM production [80], whereas X-ray CT scans provide greater resolution but may require longer scanning time to visualize at the cellular level [80].…”

“…Since TEBV maturation involves complex interplays between cells and their changing environment, a noninvasive imaging system that provides information at a cellular level would allow for optimization of TEBV preparations. MRI can provide cellular-level imaging with the proper labeling but is limited by depth of penetration and the unknown effect of labeling agents [78]. Optical coherence tomography can provide information about ECM changes in vascular graft but is limited in resolution [81].…”

The Pharmacology of Regenerative Medicine

“…Injectable delivery materials (left): the delivery systems described in the center panel have multiple applications to regenerative medicine when delivered either systemically or locally. (E) Shown is the in vivo tracking of implanted scaffolds containing cells loaded with ultrasmall superparamagnetic iron oxide nanoparticles (Reprinted with permission from Harrington et al, 2011). (F) The use of cationic liposomes to deliver DNA encoding for IGF-1 (and Lac-Z for imaging purposes) is shown at left (Reproduced with permission of BENTHAM SCIENCE PUBLISHERS LTD; Jeschke MG, Herndon DN, Baer W, Barrow RE, and Jauch KW (2001) Possibilities of non-viral gene transfer to improve cutaneous wound healing.…”

“…Such techniques are also finding applications in improving imaging modalities important to regenerative medicine (see above). Ultrasmall paramagnetic iron oxide nanoparticles (Harrington et al, 2011) or other imaging contrast agents encapsulated in nanocarriers can be used to improve nondestructive imaging modalities (i.e., theragnostics). Specifically, iron oxide is useful for magnetic resonance imaging (Xu et al, 2012a), iodine, or gold nanoparticles for computed tomography (Kao et al, 2003), and as noted above, quantum dots (de Mel et al, 2012) are also being used for imaging modalities in regenerative medicine.…”

1984 : on monitoring cell fate in three-dimensional polymeric scaffolds for tissue engineering applications

“…Upon endocytosis, accumulation of the agent leads to a darker or brighter signal, which will either reduce or increase the contrast between the labeled cells and the scaffolds depending on the type of contrast agent used [249]. Although MRI seems to be very promising in non-invasive tissue construct quality assessment, this technology has not yet become part of standard laboratory equipment [250]. To increase the signal-to-noise ratio in a voxel, and consequently increase the spatial resolution, the use of high-field MRI and long scanning times are required.…”