Figure 1.

Principles of indirect and direct cell labeling used for cell tracking. (A) Indirect cell labeling. Cells are genetically modified with a reporter gene, enabling them to express a reporter protein, which allows binding or uptake of the imaging label in vivo. The cells can then be administered into the subject and imaged over time by repeated injections of imaging label that binds specifically to cells expressing the reporter gene. In principle, the gene expression persists over the lifespan of the cell and can be passed on to daughter cells. (B) Direct cell labeling. Cells are isolated from the subject, donor or culture and labeled in vitro. The labeled cells are then administered into the subject and can be imaged repeatedly for as long as the half-life of the imaging label allows (from hours to days).