Harvard University, Prof. D. A. Weitz

Local Mechanical Properties of Cells

We are also studying cellular mechanics using a microinjection technique to shoot nanobeads directly into the cytoplasm of living cells and use the thermal motion of the beads to probe the biopolymer networks.

This schematic shows how we get inject the beads into cells. We use special needles with an opening only 500nm in diameter. We inject a red dye along with the beads so that we can identify injected cells and be sure that the beads are truly within the cytoplasm of the cell. This work is being done in collaboration with Don Ingber’s group at the Harvard Medical School.

Click here to view a 5 second movie of the bead motion from the boxed region. They don’t move much!

At the top left is a phase contrast image of an endothelial cell that has been microinjected. At the top right is a fluorescence image showing more than ten beads have been injected into the cell. Note that the beads are dispersed throughout the cell and do not aggregate.

Above is a plot of the mean squared displacement of injected beads vs. time. Because the cell is a viscoelastic material and not just a simple viscous liquid the slope of this curve is less than one. The cell is not just a bag full of liquid – it’s a complex material!

In this movie we can see that some beads move much more than others. The beads that move a lot may be within watery micro-compartments of the cell. It appears that some cell types have more of these compartments than others – we are just beginning to sort this out!

At the right is a fibroblast cell that has been injected with 100nm green beads and then fixed and stained. The red is the filamentous actin structure within the cell – this biopolymer is very important in giving the cell its shape, exerting forces on neighboring cells, and regulating biochemical reactions. The actin structures provide the stiffness that the cell needs to function within the tissue. The blue is a dye that binds to DNA and therefore shows the cell’s nucleus.

Also, check out our new Cell Culture Microscope Facility.

Other people who have worked on this project in the past: Andreas Bausch, Hallam Stevens, and Heather Rose.

This page is maintained by:

Cliff Brangwynne
Division of Engineering and Applied Science
Harvard University
9 & 15 Oxford Street, McKay Laboratory
Cambridge, MA 02138
617-496-9562

brangwyn@fas.harvard.edu