Local Mechanical Properties of Cells and Cytoplasm

Right are pictures of bovine capillary endothelial cells, another cell line we sometimes study.
Here are fluorescence pictures of the 3T3 fibroblasts obtained by Heather an undergraduate student who worked with us a few summers ago, with a confocal microscope. The left picture shows the actin cytoskeleton and the right picture show the microtubule skeleton.

We introduce small polystyrene beads inside the cells to measure the local viscoelastic and mechanical properties, as shown at right. It's easy to get the beads inside - we just add them to solution and the cells eat them up (it's called endocytosis and it's complicated -- but it works!) The particles contain a fluorescent dye, which allows us to image particles as small as 20 nm with conventional fluorescence microscopy.

With this technique, we have measured a variety of different types of behaviors with our multiple particle tracking technique. Some particles appear to be moving randomly for at least part of their trajectory. From these particles, we can extract a local viscosity, and have measured a wide distribution of values, from 50 cp to over 10000 cp (the viscosity of water is 1 cp). We can look for spatial co-localization of similar viscosities as shown below.

The most mobile particles experience a low viscosity, as the viscosity increases, the particles move less and less. Other particles appear to be caged - they are unable to move out of a local constraining volume. This may be due to the elastic nature of the medium, or due to local structure which physically prevents the particles from slipping past.

Still other particles appear to be actively transported through the cell. We think that there are molecular motors which attach to the lipid membrane that surrounds the bead and these motors direct the beads at velocities of up to a micron per minute -- that's fast! Here is an example of a trajectory for a actively transported particle:

The particle is intermittently transported along specific directions. At times, the motor appears to stall or the particle "falls off its track" and the motion becomes more randomized. From these data we can measure the forces exerted on the particles by the motors and also characterize the local mechanics; we hope to better understand the transport of cargo through the cell.