High‐ and low‐frequency mechanical properties of living starfish oocytes

We studied the mechanical properties of living starfish oocytes belonging to two species, Astropecten Auranciacus and Asterina pectinifera, over a wide range of timescales. We monitored the Brownian motion of microspheres injected in the cytoplasm using laser particle‐tracking (LPT) and video multiple‐particle‐tracking (MPT) techniques, to explore high‐ and low‐frequency response ranges, respectively. The analysis of the mean‐square‐displacements (MSD) allowed us to characterize the samples on different timescales. The MSD behavior is explained by three power‐law exponents: for short times (τ < 1 ms) it reflects the semiflexible behavior of the actin network; for intermediate timescales (1 ms < τ < 1 s) it is similar to that of a soft‐glass material; finally for long times (τ > 1 s) it behaves mainly like a viscous medium. We computed and compared the viscoelastic moduli using a recently proposed model describing the frequency response of the cell material. The large fluctuations found in the MSD over hundreds of trajectories indicate and confirm the significant cytoplasm heterogeneity. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)