Molecular Biology of the Cell - Volume 2 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1993

Cytoskeleton

The ability of Eukaryotic Cells to maintain a defined shape and perform directed, coordinated movements is due to the presence of the Cytoskeleton—a complex network of protein filaments permeating the Cytoplasm. The cytoskeleton could equally well be called "cytomusculature," for it is directly responsible for such forms of motion as Cell crawling on a substrate, Muscle fiber contraction, and the diverse morphogenetic processes in developing vertebrate embryos. In addition, it drives The Active Transport of cellular Organelles within the cytoplasm. Since Bacteria apparently lack a cytoskeleton, it is reasonable to assume that this Structure played a critical role in the evolution of eukaryotic cells.

The diverse Functions of the cytoskeleton rely on three MAIN TYPES OF protein filaments: Actin filaments, microtubules, and Intermediate filaments. These Three types of filaments are constructed from distinct structures depending on the accessory Proteins with which they are associated. Some of these proteins link the filaments to one another or to other cellular components, such as The Plasma Membrane. Others dictate the timing and Location of actin filament and microtubule assembly by regulating the rate and extent of their polymerization. Finally, certain proteins interact with filaments to generate movement; the best-studied Examples include Muscle contraction, which depends on actin filaments, and ciliary beating, which relies on microtubules.

We begin this chapter by examining structures built from actin filaments, ranging from the specialized myofibrils of muscle fibers to the ubiquitous actin-rich cortex underlying the plasma membrane of every animal cell. Next, we turn to microtubules—first those assembled into bundles responsible for ciliary beating, and then those permeating the entire cytoplasm to control organelle movement and determine cell polarity. Following a Discussion of the extensive family of intermediate filaments, which provide tensile strength to The Cell and form the nuclear lamina, we conclude by examining the cytoskeleton as a unified network that dictates and coordinates the motility and Morphology of individual cells and whole Tissues.