Optical trapping of nucleolus reveals viscoelastic properties of nucleoplasm inside mouse germinal vesicle oocytes

We propose a technique of controlled manipulation with mammalian intracellular bodies by means of optical trapping in order to reveal viscoelastic properties of cell interior. Near infrared laser in the spectral range of tissue transparency was applied to study dynamics of the nucleolus-chromatin complex inside the thermodynamically non-equilibrium system of a mouse oocyte. A nucleolus of germinal vesicle (GV) oocyte as spherical probe was displaced from the equilibrium and its relaxation dynamics was observed. We developed software for subdiffraction tracking of a nucleolus position with lateral resolution up to 3 nm and applied it for different GV-oocyte chromatin configurations. We showed differences in viscoelastic properties within nucleoplasm of NSN-oocytes, visualized by Hoechst 33342 staining. Also, we demonstrate that in germ cells basic biophysical properties of nucleoplasm can be obtained by using optical trapping without disruption and modification of cellular interior.