Magnetic particles in the liver: a probe for intracellular movement.

Previous studies have used magnetic particles to estimate the viscosity of cell cytoplasm in vitro 1-4. Here we describe how magnetic Fe2O3 particles can be used to estimate non-invasively the motion of organelles in hepatic macrophages in intact animals. We report that when these particles are injected intravenously (i.v.), most are phagocytosed by hepatic macrophages (Fig. 1)5. When an external magnetic field is applied to the rabbit, these particles become magnetized and aligned. After removal of the field, the particles collectively produce a remanent magnetic field which can be measured at the body surface. This field decreases with time due to particle rotation (relaxation) 6,7. As the particles are contained in phagosomes or secondary lysosomes, we conclude that motions of these organelles are responsible for the particle rotation and relaxation.