The Cell as a Machine

The shape of a eukaryotic cell or tissue is determined by the actions of the dynamic cytoskeleton pulling or pushing on the ECM or neighboring cells through a set of complex functions. In the previous chapters, we have considered the important elements of cytoskeletal filament dynamics and myosin activation as well as how cell–matrix and cell–cell interactions might affect the cytoskeleton. However, these motile systems must be orchestrated spatially and temporally to produce the correct types of motility for the needed functions of individual cells or of cells in tissues. Because of the dramatic differences in the behavior of individual cells and cells in tissues, we will discuss each type of behavior separately, making this a two-part chapter. In the first part, we will describe individual cell migration and motility, whereas in the second part, we will consider the cooperative processes that occur in tissue formation and remodeling. In both cases, the actin cytoskeleton is dynamic and the major active forces are generated by myosin II. However, there is a different organization of motile functions through the activation of different sites of actin polymerization and myosin activation in the different motile processes. At the individual cell level, we will consider the mechanisms of the few characterized types of motility and how the different elements of the cytoskeleton might be coordinated to produce migration or matrix remodeling. In tissues, the types of motility are dramatically different and the subcellular forces are difficult to measure. Thus, there is a lot of speculation, but some important principles such as the cohesion of the cells in the tissue have emerged. Modeling of the motility processes has started and such models will be very useful in prioritizing experiments to focus on the critical elements that will control each type of motility selectively. The goal in this area is to define the steps in motile functions, how they are coordinated as well as altered by forces or mechanics, and the roles of specific molecules involved.

Cell and tissue shape is critical for the survival of the organism. Because eukaryotic cells form without an exoskeleton, organism structure and shape must be developed using the tools that we have discussed in previous chapters; namely, the cytoskeleton, motor proteins, and extracellular matrices.