Vibrational imaging of lipid droplets in live fibroblast cells with coherent anti-Stokes Raman scattering microscopy.

A new vibrational imaging method based on coherent anti-Stokes Raman scattering (CARS) has been used for high-speed, selective imaging of neutral lipid droplets (LDs) in unstained live fibroblast cells. LDs have a high density of C-H bonds and show a high contrast in laser-scanning CARS images taken at 2,845 cm-1, the frequency for aliphatic C-H vibrations. The contrast from LDs was confirmed by comparing CARS and Oil Red O (ORO)-stained fluorescence images. The fluorescent labeling processes were examined with CARS microscopy. It was found that ORO staining of fixed cells caused aggregation of LDs, whereas fixing with formaldehyde or staining with Nile Red did not affect LDs. CARS microscopy was also used to monitor the 3T3-L1 cell differentiation process, revealing that there was an obvious clearance of LDs at the early stage of differentiation. After that, the cells started to differentiate and reaccumulate LDs in the cytoplasm in a largely unsynchronized manner. Differentiated cells formed small colonies surrounded by undifferentiated cells that were devoid of LDs. These observations demonstrate that CARS microscopy can follow dynamic changes in live cells with chemical selectivity and noninvasiveness. CARS microscopy, in tandem with other techniques, provides exciting possibilities for studying LD dynamics under physiological conditions without perturbation of cell functions.