What is the cytoskeleton?

Definition

The cytoskeleon is an elaborate array of protein fibers that help maintain cell shape, produce locomotion, provide mechanical strength, aid in chromosome separation during mitosis, and facilitate the intracellular transport of organelles. 3 types of protein filaments make up the cytoskeleton: actin filaments (microfilaments), intermediate filaments, and microtubules.

Actin filaments

Actin filaments are formed from the polymerization of actin monomers to form long, thin fibers. These fibers, also known as microfilaments, are about 8 nm in diameter and are the thinnest of the cytoskeletal filaments. Structurally, actin filaments form a layer under the plasma membrane that provides mechanical strength, anchors centromes at opposite poles during mitosis, aids in cytokinesis by pinching the cell into 2 daughter cells, and links transmembrane proteins to cytoplasmic proteins. Other functions of actin filaments include the generation of locomotion and cytoplasmic streaming in some cells, and interactions with myosin in skeletal muscle to creat the force of contraction. The above movie shows the interaction between actin and myosin during muscle contraction.

Intermediate filaments

Intermediate filaments are about 10 nm in diameter, and are larger than actin filaments, but smaller than microtubules which have a diameter of about 25 nm. There are several different types of intermediate filaments including keratins found in epithelial cells, hair and nails, nuclear lamins that stabilize the nuclear envelope, neurofilaments that strengthen axons, and vimentins that strengthen muscle cells. Generally, intermediate filaments provide a structural framework in the cell.

Microtubules

Microtubules are straight, hollow cylinders made up of a 13 protofilament ring. Microtubules tend to be variable in length, but can be their length can be thousands of times as long as their width. These filaments are composed of dimers of alpha tubulin and beta tubulin. Tublin dimer polymerization is powered by the hydrolysis of GTP, generally at the plus end of the filament. Most microtubule activity is involved in motion, provided by protein motors that use energy from ATP to move along the microtubule. There are two major types of microtubule motors. Kinesins move towards the plus end of the microtubule and dyneins move towards the minus end. Choromosomal migration during cell division occurs on microtubules making up the spindle fibers using both kinesins and dyneins.

Cilia and flagella

Cilia and flagella are composed of microtubules and provide locomotion for cells such as sperm, and also aid in moving fluid past the cells. Cilia and flagella are essentially the same structure, and if there are many short ones, they are called cilia and if there are only a few or one, they are called flagella. Each cilium is composed of

a cylindrical array of 9 microtubules and 2 single microtubules pass through the center of the bundle, creating a 9+2 pattern. The complex is surrounded by an extension of the plasma membrane. Motion is created by the sliding of microtubules past one another using dynein and ATP. The movie on the right depicts the construction of a bacterial flagellum.

Microtubules and cell division

The centrosome is an important part of cell division. It is located in the cytoplasm, just outside the nucleus. The centrosome is duplicated during the cell cycle S phase. Before mitosis, the resulting 2 centrosomes move apart to opposite poles of the nucleus. During mitosis, microtubules polymerize from each centrosome forming clusters called spindle fibers. Spindle fibers attach to 1 kinetochore, the chromosomes, and to each other by growing from each centrosome until they overlap. These structures aid in the migration of chromosomes during cell division. Separation of the chromosomes at anaphase involves movement along the microtubules, powered by dyneins. The overlapping spindle fibers move past each other, pushing the poles further apart, allowing the sister chromatids to migrate to opposite poles. The video above shows cell division using differential interference contrast microscopy. The spindle fibers and chromosomes can be clearly seen.

Featured Cytoskeleton Content

• Graph cytoskeleton articles over time - Plot the number of articles that have been published over time in specific areas of cytoskeleton research or in all of basic research. You can gauge changes in interest in certain topics and use this to guide your research. It also makes a good introductory slide for a presentation as it demonstrates the importance of your topic.