Living cells are formidably complex systems that perform highly coordinated tasks which relate multiple biochemical and biophysical inputs to cell activities. Cell tasks may include not only adhesion and spreading, receptor-ligand mediated signal transduction, division, growth and programmed death, but also cell-type-dependent functions such as the environmental barrier provided by skin cells. These various cellular activities, often performed simultaneously or in a hierarchical order, involve hundreds of membrane, cytoplasmic, and extracellular proteins, ions, and small molecules, which interact with one another by means of regulated forces.
For instance, cell migration requires the coordination of membrane extension and retraction, cytoskeletal gelation-contraction-dissolution, the formation of focal adhesions at the front of the cell, and detachment of these adhesions at the rear of the cell. These processes involve not only cytoskeletal polymers and motor proteins (which provide the cell with the necessary motor forces and passive mechanical resistance to sustain both cell movement and cell integrity), but also specific membrane proteins (“cell receptors”) to promote intimate contact between the cell and its extracellular milieu. Nevertheless, despite the critical function of cell migration in wound healing, immune response, cancer metastasis, and embryogenesis, the fundamental mechanisms of this phenomenon are not well understood.
One of the steps limiting our understanding of cellular activities such as cell migration has been the lack of fundamental theory, backed by experimental methods, to monitor and characterize cellular processes quantitatively, noninvasively, and in real time.