A very interesting use of fluorescence polarized light microscope by the Marine Biological Laboratory to see internal forces inside cells by observing transmembrane proteins : Integrins.

How do cells move in a certain direction in the body—go to a wound site and repair it, for example, or hunt down infectious bacteria and kill it? Two new studies from the Marine Biological Laboratory (MBL) show how cells respond to internal forces when they orient, gain traction, and migrate in a specific direction. The research, which began as a student project in the MBL Physiology Course and was developed in the MBL Whitman Center, is published in Proceedings of the National Academy of Sciences (PNAS) an

Source: Internal forces directing cell migration are revealed by live-cell microscopy