CARS microscopy for the monitoring of fat deposition mechanisms in a living organism

We introduce near-infrared Coherent Anti-Stokes Raman Scattering (CARS) microscopy as a method for the monitoring of fat deposition in a living organism by directly probing the CH 2 vibration of the lipids without the need for staining or labeling. This study nicely brings forward all the advantages of the technique: deep probe depth, low excitation powers, high 3-dimensional resolution, and visualization without the interference of exogenous label molecules, or fixation and staining procedures. Differences in fat deposition during the life cycle of the nematode Caenorhabditis elegans were evaluated quantitatively from the CARS microscopy images, showing that the technique can be used to study mechanisms that regulate lipid storage. Beside the wild type nematode, the feeding-deficient mutant pha-3 was studied. It was shown that the embryonal accumulation of energy stores is enough for the development of a full-sized pre-adult larva, being possible also for the mutant. However, the volume density of lipid stores at the fourth and last pre-adult development stage seems to determine its adult body size. Whereas the wild type larva maintains its size when becoming adult, though at the cost of reduced lipid density, the feeding deficient mutant instead has to reduce its body size in order to reach the same volume density of lipid stores. Both strains start off their adult life with a volume fraction of lipid stores corresponding to 6-7%; the wild type with a radius of 24±2 µm and the pha-3 mutant with a significantly smaller radius of 16±3 μm.