MPIO contrast improves MRI's ability to monitor in vivo stem cell tracking

CONTEXT: Cellular MRI is principally performed by labeling cells in culture with an intracellular contrast agent, transplanting the cells into a host, and tracking the migration with MRI. The approach poses potential problems, particularly with respect to stem cells. Cell culture cannot accurately reproduce the chemical and physical niche environment in which stem cells reside. Cell culture might alter the differentiation of stem cells into their natural fate. Erik M. Shapiro, Ph.D., and colleagues at the National Institutes of Health have developed a protocol that avoids the potential problems by means of in vivo labeling of stem cells. Because MRI can detect single-micron-sized particles, poor labeling efficiency does not pose a problem, enabling in vivo stem cell labeling and tracking.

RESULTS: Neural stem cells were labeled by direct stereotactic injection of 5 to 50 microL of 1.63-micron supraparamagnetic iron oxide particles into the lateral cerebral ventricle of six-week-old rats. Other animals received particles presoaked in EGF, a stem cell signaling chemical. Three-D gradient-echo images of live, anesthetized rats were acquired over six weeks. The rats were perfused and fixed, and the brains and intact olfactory bulbs were removed, placed in saline, and imaged for 24 hours, then sectioned for immunohistochemistry (IHC) to determine the types of cells the particles entered. MR images of the EGF-soaked particles revealed a rostral migratory stream (RMS) identical to the one indicated by histologic evaluation.

IMAGE: The images depict sagittal slices of the same rat brain. A: Immediately before injection of the MR contrast agent. White dotted line depicts the location of the RMS. B: Immediately after injection of contrast into the lateral ventricle. C: Five weeks after injection. A dark trail of contrast can be visualized along the RMS into the olfactory bulb (arrows). D: Same brain five weeks following injection, perfused, fixed, and imaged at 50-micron resolution. The dark trail along the RMS contains discrete, punctate dark spots, concentrated only within the RMS and branching out once inside the olfactory bulb (arrows).

IMPLICATIONS: Although IHC remains the gold standard for determining cell type in cell migration models, MRI examines migratory pathways in three dimensions and progression over time. Amplification of the MR signal due to the contrast agent may permit observation of rare cellular events that are difficult to observe by histology. The ability to visualize single cells simplifies the process of cell counting.