Cytoskeleton

When we learn about all the organelles, molecules, and other components floating in the cytoplasm of a cell, we might imagine them randomly located and moving around the cell freely. Biologists noticed early on in cell research that there was an internal organization and nonrandom movement of intracellular components. They did not know how this was accomplished until more recent improvements in microscopy revealed a network of filaments extending throughout the cell. They called this network the cytoskeleton. Contrary to what the name might suggest, the cytoskeleton is far from static or rigid, and its function goes beyond cellular support.

Cytoskeleton definition

The cytoskeleton gives both support and flexibility to the cell. It performs diverse functions in maintaining and changing cell shape, intracellular organization and transport, cell division, and cell movement. In eukaryotic cells, the cytoskeleton is composed of three types of protein fibers: microfilaments, intermediate filaments, and microtubules. These fibers differ in structure, diameter size, composition, and specific function.

Cytoskeleton structure and function

The cytoskeleton is composed of a number of components that all play a role in providing the cell with structural support, cellular transport, the ability to move, and the ability to function appropriately. In the following section, we will cover multiple cytoskeleton components, including their makeup and function.

Microfilaments

Microfilaments are the thinnest of the cytoskeletal fibers, composed of only two intertwined protein threads. The threads are made up of chains of actin monomers, thus, microfilaments are commonly called actin filaments. Microfilaments and microtubules can be quickly disassembled and reassembled in different parts of the cell. Their primary function is to maintain or change the cell shape and to aid in intracellular transport (Figure 1).

Figure 1. Left: an osteosarcoma cell (cancerous bone cell) with DNA in blue, mitochondria in yellow, and actin filaments in purple. Right: mammal cell in the process of dividing. The chromosomes (dark purple) have already replicated, and the duplicates are being pulled apart by microtubules (green). Source: both images from NIH Image Gallery from Bethesda, Maryland, USA, Public domain, via Wikimedia Commons.

Actin filaments form a dynamic mesh in the portions of the cytoplasm that are adjacent to the plasma membrane. This microfilament mesh is connected to the plasma membrane and, with the bordering cytosol, forms a gel-like layer all around the internal side of the membrane (note how in figure 1, left, the actin filaments are more abundant at the edge of the cytoplasm). This layer, called the cortex, contrasts with the more fluid cytoplasm in the interior. In cells with outward extensions of the cytoplasm (like microvilli in nutrient-absorbing intestinal cells), this microfilament network forms bundles that enlarge into the extensions and reinforce them (Figure 2).

Figure 2. micrograph shows microvilli, the fine extensions in intestinal cells that increase the cellular surface to absorb nutrients. The core of these microvilli is composed of bundles of microfilaments. Source: Louisa Howard, Katherine Connollly, Public domain, via Wikimedia Commons.

Muscle contractions

In muscle cells, thousands of actin filaments interact with thicker filaments of myosin that are located in between the microfilaments (figure 3). The myosin filaments have “arms” that attach to two continuous actin filaments (the filaments are placed end to end without contact). The myosin “arms” move along the microfilaments dragging them closer to each other, causing a muscle cell to contract.

Figure 3. Extensions of the myosin filaments pull actin filaments closer to each other, resulting in muscle cell contraction. Source: modified from Jag123 at English Wikipedia, Public domain, via Wikimedia Commons.

Ameboid movement

Unicellular protists such as Amoeba move (crawl) along a surface by projecting cytoplasmic extensions called pseudopodia (from the Greek pseudo = false, pod = foot). The formation of the pseudopod is facilitated by the rapid assembly and growth of actin filaments in that region of the cell. Then, the pseudopod drags the rest of the cell towards it.

Cytoplasmic streaming

Localized contractions of actin filaments and the cortex produce a circular flow of the cytoplasm inside the cell. This cytoplasm movement can occur in all eukaryotic cells but is particularly useful in large plant cells, where it accelerates the distribution of materials through the cell.

Actin filaments are also important in cytokinesis. During cell division in animal cells, a contractile ring of actin-myosin aggregates forms the segmentation groove and keeps tightening until the cell’s cytoplasm divides into two daughter cells.

Intermediate filaments

Intermediate filaments have an intermediate diameter size between microfilaments and microtubules and vary in composition. Each type of filament is made up of a different protein, all belonging to the same family that includes keratin (the main component of hair and nails). Multiple strings of fibrous protein (like keratin) intertwine to form one intermediate filament.

Due to their sturdiness, their main functions are structural, such as reinforcing the shape of the cell and securing the position of some organelles (for example, the nucleus). They also coat the interior side of the nuclear envelope, forming the nuclear lamina. The intermediate filaments represent a more permanent support frame for the cell. Intermediate filaments are not disassembled as commonly as actin filaments and microtubules.

Microtubules

Microtubules are the thickest of the cytoskeletal components. They are composed of tubulin molecules (a globular protein) that are arranged to form a tube. Thus, unlike microfilaments and intermediate filaments, microtubules are hollow. Each tubulin is a dimer made of two slightly different polypeptides (called alpha-tubulin and beta-tubulin). Like actin filaments, microtubules can be disassembled and reassembled in different parts of the cell. In eukaryotic cells, microtubule origin, growth, and/or anchorage are concentrated in regions of the cytoplasm called microtubule-organizing centers (MTOCs).

Microtubules guide organelles and other cellular components’ movement (including the movement of chromosomes during cell division, see figure 1, right) and are the structural components of cilia and flagella. They serve as tracks that guide vesicles from the endoplasmic reticulum to the Golgi apparatus, and from the Golgi apparatus to the plasma membrane. Dynein proteins (motor proteins) can move along a microtubule transporting attached vesicles and

organelles inside the cell (myosin proteins can also transport material through microfilaments).

Flagella and Cilia

Some eukaryotic cells have extensions of the plasma membrane that serve in cell movement. Long extensions used to move an entire cell are called flagella (singular flagellum, like in sperm cells, or unicellular organisms like Euglena). Cells only have one or a few flagella. Cilia (singular cilium) are numerous, short extensions used to move the entire cell (like unicellular Paramecium) or substances along the surface of a tissue (like the mucus that is moved out of your lungs by the ciliated cells of the trachea).

Both appendages have the same structure. They are composed of nine pairs of microtubules arranged in a ring (forming a bigger tube) and two microtubules in its center. This design is called a “9 + 2” pattern and forms the appendage that is covered by the plasma membrane (Figure 4). Another structure called the basal body anchors the microtubule assembly to the rest of the cell. The basal body is also made of nine groups of microtubules, but in this case, they are triplets instead of pairs, with no microtubules in the center. It is called a “9 + 0” pattern.

Figure 4. Flagella and cilia are composed of ring of nine pairs of microtubules with two more in its center. Left: diagram representing the “9 + 2” structure of a cilium/flagellum, and the “9 + 0” pattern for the basal body. Source: LadyofHats, Public domain, via Wikimedia Commons. Right: micrograph showing a cross section of numerous cilia in bronchiolar cells. Source: Louisa Howard, Michael Binder, Public domain, via Wikimedia Commons.

How do cilia and flagella move?

Dyneins are attached along the most external microtubule of each of the nine pairs that form a flagellum or cilium. The dynein protein has one extension that grabs the outer microtubule of the adjacent pair and pulls it forward before releasing it. The dynein movement would cause the sliding of one pair of microtubules over the adjacent one, but as the pairs are secured in place, it results in the bending of the microtubule.

Dyneins synchronize to be active only at one side of the flagellum (or cilium) at a time, to alternate the direction of bending and producing a beating movement. Although both appendages have the same structure, their beating movement is different. A flagellum usually undulates (like snake-like movements), while a cilium moves in a back-and-forth motion (a powerful stroke followed by a recovery stroke).

Cytoskeleton in animal cells

Animal cells have some distinctive cytoskeletal features. They have a main MTOC commonly found near the nucleus. This MTOC is the centrosome, and it contains a pair of centrioles. As mentioned above, centrioles are composed of nine triplets of microtubules in a “9 + 0” arrangement. Centrosomes are more active during cell division; they replicate before a cell divides and are thought to be involved in microtubule assembly and organization. Centrioles help pull the duplicated chromosomes to opposite sides during cell division. However, as other eukaryotic cells lack centrioles and are capable of cell division, their function is not clear (even removing the centrioles from most cells does not stop them from dividing).

The structural support and maintenance of cell shape given by the cytoskeleton are probably more important in animal cells compared to plant cells. Remember that cell walls are mainly responsible for support in plant cells.