Magnetic Resonance Imaging for tracking cellular patterns obtained by Laser-Assisted Bioprinting

Abstract

Recent advances in the field of Tissue Engineering allowed to control the three-dimensional organization of engineered constructs. Cell pattern imaging and in vivo follow-up remain a major hurdle in in situ bioprinting onto deep tissues. Magnetic Resonance Imaging (MRI) associated with Micron-sized superParamagnetic Iron Oxide (MPIO) particles constitutes a non-invasive method for tracking cells in vivo. To date, no studies have utilized Cellular MRI as a tool to follow cell patterns obtained via bioprinting technologies. Laser-Assisted Bioprinting (LAB) has been increasingly recognized as a new and exciting addition to the bioprinting's arsenal, due to its rapidity, precision and ability to print viable cells. This non-contact technology has been successfully used in recent in vivo applications. The aim of this study was to assess the methodology of tracking MPIO-labeled stem cells using MRI after organizing them by Laser-Assisted Bioprinting. Optimal MPIO concentrations for tracking bioprinted cells were determined. Accuracy of printed patterns was compared using MRI and confocal microscopy. Cell densities within the patterns and MRI signals were correlated. MRI enabled to detect cell patterns after in situ bioprinting onto a mouse calvarial defect. Results demonstrate that MRI combined with MPIO cell labeling is a valuable technique to track bioprinted cells in vitro and in animal models.

Figure 1

Determination of the optimal MPIO concentration for cell incubation. (a) Pictures of MSC pellets after their incubation with decreasing concentrations of MPIO (d:100, d:200, d:500 and d:1000 from the stock solution at 4.5 mgFe/mL) and the corresponding MR T2*-weighted image (spatial resolution: 137 × 137 × 141 µm) after their bio-printing in a line pattern. (b) Graph showing the Contrast-to-Noise Ratio (CNR) of the lines with background measured on T2*-weighted images in function of the MPIO dilution factor. *means significantly different with d:500 and d:1000.

Figure 2

Determination of the optimal cell density. (a) MR T2*-weighted images of the line pattern (spatial resolution: 137 × 137 × 52 µm) after the bioprinting of MPIO-labeled MSC with increasing density (laser energy of 27 µJ, 28 µJ and 29 µJ). (b) Quantification of the above MR images through the measurement of the amount of voxels per line in function of the density.

Figure 3

MR and fluorescence detection of three different patterns of MPIO- and GFP- labeled MSC immediately post-printing in vitro. The two left columns show MR T2*-weighted images of three different printed patterns and their magnifications (spatial resolution: 137 × 137 × 52 µm). The last three columns show fluorescence microscopy images of the corresponding printed patterns, with respectively FlashRed-MPIO signal, GFP signal, and the merged images. The white inserts show the high magnification view of a GFP- and MPIO-labeled cell.

Figure 4

In vitro longitudinal MR follow-up of the bioprinted cells. (a) MR T2*-weighted images (spatial resolution: 137 × 137 × 52 µm) acquired at day 1 (D1), 3 (D3) and 7 (D7) of the same live MPIO-labeled MSC bio-printed in a line pattern (arrows). The corresponding magnified GFP-fluorescence images are shown below. The far right MR image was acquired at D7 but with longer acquisition time. (b) Quantification of the surface covered by the labeled MSC over time after their bio-printing in a 2 mm-disk (the corresponding magnified MR T2*-weighted images are shown on the top of each graph bar).

Figure 5

Post-mortem MR and fluorescence images of a mouse calvaria bone defect filled or not with MPIO-labeled cells bio-printed in a ring pattern. A scheme of the position of the reconstructed curved MR slice (red) is shown on top. The left column shows MR curved slice (spatial resolution: 97 × 94 × 94 µm) from mice bioprinted or not (control) with cells. Dashed circles represent the position of the circular bone defect. Labeled cells were bioprinted in a ring pattern between the dashed and plain circles. The middle column is identical to the left one without any indication to better visualize the signal void generated by the labeled cells. The corresponding fluorescence images are also shown on the right column. Scale bar represents 1 mm.

Figure 6

MR set up for the in vitro MR imaging of Petri dish containing MPIO-labeled cells bioprinted in different patterns. The coil is represented as the two golden rings, where the petri dish is attached on. Two tubes filled of water are positioned under the coil for the MR adjustments. Bioprinted cells are shown in green and are immersed in cell culture medium.