Despite a lack of exoskeleton, isolated animal cells normally exhibit stable, nonspherical shapes with excess membrane area. To maintain cell shape, the plasma membrane conforms to a cytoskeleton located inside the cell, but the membrane surface area to cell volume ratio determines how extensively the membrane can conform. In a few cases, such as endothelial cells in capillary tubes, the cytoskeleton alone can support the cell shape. In most cases, however, the cytoskeleton is dynamic, and defines the cell shape in concert with the cell volume, and the effect of contracting against external contacts, or adhesions. Modulation of these adhesions, whether they are between adjacent cells, or cells and matrices, provide the stimuli to initiate cytoskeleton dynamics. These dynamics, which include filament assembly and disassembly, typically determine any changes in the shape of the cell. Actin filaments are often assembled from adhesion sites or sites of membrane extension, both of which lie primarily at the cell periphery. They are then pulled inward and disassemble in a process known as ‘actin treadmilling.’ Actin filaments are contracted by myosin and this creates tension in the cytoskeleton and is responsible for tissue organization and the cohesive property of cells. Paradoxically, the rapid loss of contractile force increases filament dynamics, and cytoskeleton assembly. Cohesion of cells over longer periods is stabilized by intermediate filaments. These appear to be the safety net that prevents acute fracture of cells in epithelia under tension. Microtubules, on the other hand, help stabilize and define cell polarity and structure. Microtubules originate from the microtubule-organizing center (MTOC) and provide directional pathways on which motors will transport membranous organelles and other cellular components. During mitosis, the dynamic microtubules of the mitotic spindle play critical roles in the transport of chromosomes to the daughter cells. Thus, the cytoskeleton is a dynamic, mechanically active component of the cell that is responsible for shaping the cell and its environment as well as for organizing the intracellular organelles.

The cytoskeleton is comprised of three long filament systems; specifically, microtubules, actin filaments, and intermediate filaments. These filaments are crosslinked by motor proteins or multivalent binding proteins, which serve to stabilize the cytoskeleton, and provide it with dynamic qualities including the ability to disassemble, reassemble, and contract. As evidenced by photobleaching recovery experiments, filament subunit exchange occurs on the timescale of minutes for actin and microtubules.