The cytoskeleton is a network of protein filaments and tubules found within the cytoplasm of eukaryotic cells. This internal framework acts as a dynamic scaffolding that provides structural support and maintains a cell’s internal organization. The existence of these microscopic fibers was only confirmed with the advent of the electron microscope, which revealed their complex nature.
The Three Main Filaments of the Cytoskeleton
The cytoskeleton is constructed from three types of protein filaments: microfilaments, intermediate filaments, and microtubules. Each type is characterized by its size, protein composition, and arrangement within the cell. This diversity allows the cytoskeleton to perform a wide array of tasks.
Microfilaments, also known as actin filaments, are the thinnest of the three, with a diameter of about 7 nanometers. They are composed of two intertwined strands of the protein actin, forming a flexible but strong double helix. These filaments are most highly concentrated in a network just beneath the plasma membrane, called the cell cortex. This positioning is important for their role in defining cell shape and enabling certain types of movement.
Intermediate filaments are intermediate in diameter, measuring between 8 and 12 nanometers. Their structure is analogous to a rope, composed of various fibrous protein subunits wound together, which gives them great tensile strength. The proteins that make up intermediate filaments are a diverse group; for example, they include keratin in epithelial cells and neurofilaments in the axons of nerve cells. These stable structures form a cellular skeleton, anchoring organelles like the nucleus in place.
Microtubules are the largest of the cytoskeletal components, appearing as hollow tubes approximately 25 nanometers in diameter. They are constructed from repeating protein subunits of alpha- and beta-tubulin, which join to form a long, rigid cylinder. Microtubules extend from a central organizing center near the nucleus, called the centrosome, and radiate outwards toward the cell periphery. This arrangement establishes a coordinate system within the cell that is used for various transport processes.
Primary Roles of the Cytoskeleton
The cytoskeleton’s collective network provides mechanical support, allowing cells to adopt and maintain complex shapes and resist external forces. The intermediate filaments are particularly noteworthy for their contribution to this structural integrity, providing a framework that withstands stretching and tension. This ensures that cells can maintain their form within tissues and organs.
A highly organized system of intracellular transport operates along cytoskeletal tracks. Microtubules serve as highways for motor proteins, like kinesins and dyneins, which actively move cellular cargo. These motor proteins bind to vesicles and organelles, and use energy to walk along the microtubule network, delivering their contents to specific destinations within the cell. This transport system is active in processes like nerve cell function and general cellular maintenance.
The cytoskeleton is also central to the process of cell division. During mitosis, microtubules completely reorganize to form the mitotic spindle. This bipolar structure attaches to the chromosomes and segregates them into two identical sets. This ensures that each new daughter cell receives a complete copy of the genome.
The cytoskeleton also powers cellular motility. The dynamic assembly and disassembly of actin filaments at the leading edge of a cell allow it to crawl across a surface. In other instances, microtubules form the core of cilia and flagella, which are whip-like appendages that propel cells through fluid environments.
A Constantly Changing Cellular Structure
The cytoskeleton is not a fixed or static entity; rather, it is in a state of constant flux and capable of rapid reorganization. This dynamic nature is a property of microfilaments and microtubules, which undergo continuous assembly and disassembly. This process involves the addition (polymerization) and removal (depolymerization) of their respective protein subunits, actin and tubulin.
This ability to quickly build and break down components allows the cell to adapt its structure and function in response to internal and external cues. For example, a cell can change its shape or move from one location to another. During cell movement, actin filaments polymerize at the front of the cell, pushing the membrane forward, while they depolymerize at the rear.
The controlled breakdown and reconstruction of the microtubule network is what enables the formation of the mitotic spindle during cell division. The cell can dissolve its interphase microtubule array and rapidly reassemble the tubulin subunits into the spindle structure needed to separate chromosomes.
The Cytoskeleton’s Role in Human Disease
Defects in the proteins that form or regulate the cytoskeleton can lead to a variety of human diseases. Its disruption can have severe consequences for cellular health, impacting tissues and entire organ systems.
In neurodegenerative disorders like Alzheimer’s disease, a protein called tau, which normally stabilizes microtubules in neurons, becomes dysfunctional. This leads to the destabilization of microtubules, disrupting the transport of materials along the axon and contributing to neuronal death. The loss of this transport network is a factor in the disease’s progression.
The cytoskeleton is also a major target in cancer therapy. Many chemotherapy drugs, such as Taxol, work by interfering with the dynamics of microtubules. These drugs can stabilize the microtubules, preventing them from disassembling, which halts the formation and function of the mitotic spindle. Additionally, changes in the actin cytoskeleton are known to facilitate the spread, or metastasis, of cancer cells.
Genetic disorders can also arise from mutations in genes that code for cytoskeletal proteins. For instance, certain forms of muscular dystrophy and cardiomyopathy (a disease of the heart muscle) are caused by defects in cytoskeletal components that weaken the structure of muscle cells.