Cytoskeleton

Learning Goals

By the end of this lesson, you will be able to:

• Identify and describe the structure of cytoskeletons
• Explain function and role of a cellular cytoskeleton and its parts

Key Vocabulary

Cytoskeleton – a network of filaments and tubules found in the cytoplasm that give the cell its shape.

Microfilaments – protein (actin) filaments found in eukaryotic cells. They assist in cell membrane motility, endocytosis, exocytosis, secretion, and vesicle transfer.

Intermediate Fibers – filaments that provide structure and support for cells. They are also essential in anchoring the cell to other cells and to the extracellular matrix.

Microtubules – rigid, hollow rods that provide structure, help to move cells, organize genetic material during cellular division, and help with intracellular transport. Cilia and flagella are common types of microtubules.

The Cytoskeleton 

The cytoskeleton is a complex network of protein fibers that provides the cell with shape, structure, and mechanical support. One of the key functions of the cytoskeleton is to facilitate the movement of materials within the cell. The cytoskeleton is composed of three main types of fibers: microfilaments, intermediate fibers, and microtubules. 

Microfilaments

Microfilaments, or actin filaments, are thin, flexible fibers made of the protein actin. They create a constantly changing network in cells that is essential for many functions. Microfilaments help the cell move by forming temporary extensions called pseudopodia. They are also important for moving organelles and other materials within the cell, which is called cytoplasmic streaming. Additionally, microfilaments can bundle together to help the cell keep its shape. Think of them like the flexible pipes that run throughout a building, allowing water and other materials to flow where they need to go.

Microfilaments also play a very significant role during cellular replication and division, most notably during cytokinesis, when the cell splits into two daughter cells. During this process, microfilaments contract and form a furrow in the middle of the parent cell. The microfilaments form a ring that gets smaller as they contract.  The cytoplasm is pinched until the original cell is split into two daughter cells. You’ll learn more about this process in the Mitosis lesson later in this course.

Did you notice that microfilaments are also called actin filaments? Have you heard of actin filaments before? If so, you’ll recall they play a main role in the movement of our muscles. Each muscle movement is driven by a muscle contraction. During muscle contraction, the actin filaments within the muscle fiber are pulled toward the center of the sarcomere, which is the basic unit of muscle contraction. This occurs due to the action of the protein myosin, which forms cross-bridges with the actin filaments and uses ATP energy to “walk” along the filaments and pull them toward the center of the sarcomere. You can learn more about muscle contraction by visiting our Musculoskeletal System lesson in this course. 

Intermediate Fibers

Intermediate fibers are thicker and more rigid than microfilaments (but not as thick as microtubules), and they provide the cell with mechanical strength and support. They are made of a variety of proteins, including keratin, desmin, and vimentin. While intermediate fibers are not directly involved in the movement of materials within the cell, they provide a stable framework that allows for the proper organization of other cytoskeletal elements.

Microtubules

Microtubules are long, tubular structures that play a critical role in maintaining the shape and organization of cells. They are a key component of the cytoskeleton, which is a network of protein fibers that provides structural support and helps with cell division, movement, and communication.

One important aspect of microtubules is that they are made up of dimers, which are pairs of protein subunits called alpha-tubulin and beta-tubulin. These dimers are arranged in a head-to-tail fashion to form long chains, which then assemble into the cylindrical shape of a microtubule.

The formation of dimers is essential for microtubule stability and function. For example, when a microtubule is assembled, the dimers are positioned in a specific way that allows for the microtubule to grow and shrink in a controlled manner. The dimer formation allows proteins to travel along the microtubules. Two of these traveling proteins are motor proteins called kinesins and dyneins. Kinesins move, or “walk”, toward the positive end of the microtubule, while dyneins travel toward the negative end. These proteins typically carry vesicles to their destinations. This dynamic behavior is important for many cellular processes, such as cell division, movement, and organization. 

In addition, the binding of certain drugs or proteins to microtubules can disrupt the formation of dimers or cause them to be misaligned, which can alter the stability and function of the microtubule. This has important implications for medical research, as drugs that target microtubules can be used to treat diseases such as cancer and Alzheimer’s.

Microtubules are also major components of the mitotic spindle fibers, which separate sister chromatids during mitosis. One end of the spindle fibers attaches to the center (kinetochore) of each sister chromatid, while the other end of the fiber attaches to one of the two centrosomes at each end of the cell. The spindle fibers begin to shorten and pull the sister chromatids apart to opposite ends of the cell. You’ll learn more about this process in our mitosis lesson later in this course!

QUIZ