Stem cells hold the promise of of healing injuries and defects in humans and animals, but scientists are still trying to determine the best way to get the stem cells to the defects, and how to optimize their healing effects once they're there. A recent study shows that stem cells can help with healing bone injuries, but they were indignant about being watched over their shoulders—attaching quantum dots to the stem cells for tracking purposes caused them to become less localized to the site they were applied to and to die off rather quickly.

Scientists know that providing stem cells to injured or damaged areas of the body can help them heal because of the stem cells' ability to develop into many different kinds of tissue. However, they are unsure of how delivery methods affect the ability of stem cells to function; for example, it is unclear whether a systemic introduction of stem cells might be more effective than applying them directly to the defective area.

In the new paper, a group of scientists attempted to study different stem cell application methods by attaching quantum dots to their cells. Quantum dots are fluorescent semiconductor nanocrystals that have a long half-life and typically stay attached to a cell as long as the cell remains alive.

The subjects in the tests were rats with segmental defects, or gaps in the bone, in both their left and right femurs. On one side, the researchers used an acellular scaffolding to bridge the bone together; this acted as a control. On the other leg, they directly applied either human fetal- or bone marrow-derived stem cells. They used both untracked and quantum dot labelled stem cells, and varied the time that the stem cells were incubated with the quantum dots.

A regular 12-week application of stem cells successfully bridged the bone gap in four out of nine cases when bone marrow stem cells were used, but only one in eight cases with fetal stem cells. The scientists attributed this to the relatively large size of the gaps—about 8mm—that they were attempting to bridge. Even when the gap was not successfully bridged, though, the stem cells were at least able to promote bone growth and improve the torsional strength of the leg.

Comparing these trials to ones with quantum dots, scientists found that attaching the quantum dots gave the stem cells a sort of ADD. Instead of remaining at the defect site, a significant portion of the quantum dots would appear at the rat's other leg within one week, where they were found in macrophages, not stem cells. The overall population of stem cells decreased rapidly over three weeks, suggesting the macrophages may be scavenging dead cells. After 12 weeks, the stem cells with quantum dots attached failed to accomplish much of anything—no bridging took place, and bone formation was not enhanced at all.

No negative effects on the rats were observed from the quantum dots, which have a toxic cadmium-selenium core; however, the researchers were stumped as to why the quantum dots, which are generally considered uninvasive, caused the stem cells to perform so poorly.

The paper's authors speculate that stem cells without quantum dots were effective in starting the healing process, but couldn't overcome the large gaps used here. But they seem to be at a loss about what's happening with the quantum dots. (Maybe stem cells get performance anxiety—rimshot—when being observed). The negative impact of this label means it's a poor choice for following stem cells during injury repair, but there is still hope for other tracking methods, such as using stem cells that express fluorescent proteins.

PNAS, 2010. DOI: 10.1073/pnas.0905444107