Experimental Soft Condensed Matter Group
Harvard University, Prof. D. A. Weitz

Local Mechanical Properties of Cells and Cytoplasm

Cells are complicated microstructures --- how can we study local mechanics???

Cells are complicated! They can support huge stresses and strains even though they are mostly water, they can change shape and move, and have sophisticated transport mechanisms that allow them to move proteins, DNA and other molecules inside the cell and in and out of the cell membrane. We use novel particle tracking methods to measure the microscopic mechanical and rheological properties of these complex systems.

XENOPUS EGG EXTRACTS

We study extracted cellular cytoplasm from Xenopus (AKA FROG) Egg Extracts. This work is done in collaboration with Zach Perlman of Tim Mitchison's group at the Harvard Medical School. Frogs' eggs are pretty big -- 1 mm in diameter -- and they contain lots of cytoplasm. Using a high speed centrifugation technique, we can separate the cytoplasm from the lipid structures and organelles. We can then use micro- and macro-rheology techniques to study the mechanics of the cytoplasm. Using drugs and other chemical techniques we can try to isolate contributions of the different filaments to mechanical response. Additionally, the extracts retain a large portion of their biological activity, so we can also probe the impact of rheology and mechanics on mitotic spindle formation, transport mechanisms, and sol/gel transitions in the cell.

On the right, we show the MSDs of an ensemble of particles moving in actin-poisoned mitotic extracts; each curve represents a different particle size. An actin mediated contraction prevents measurements of native state extracts in this stage of the cell cycle. The frequency-dependent viscoelastic moduli are shown on the left. G’ is the elastic modulus and G’’ is the viscous modulus. G’’ dominates over the entire frequency range, characteristic of a viscoelastic fluid. The elastic response is extremely weak, and we are still investigating its origin.

Also, check out our new Cell Culture Microscope Facility.

Also see our work on Active Transport Mechanisms in Fibroblasts and Microinjected Colloids in Cells. Other people who have worked on this project in the past: Andreas Bausch, Hallam Stevens, and Heather Rose.

This page is maintained by:

Megan Valentine
Department of Physics
Division of Engineering and Applied Science
Harvard University
9 & 15 Oxford Street, McKay Laboratory
Cambridge, MA 02138
617-495-3705

valentin@fas.harvard.edu