Christine has a PhD in biomedical engineering.
What is the Cytoskeleton?
Our skeletons give our bodies structure. But cells need structure too, so they have a cytoskeleton. 'Cyto-' means cell, so like the name implies, the cytoskeleton is the 'bones' of the cell. It is composed of materials that give the cell its shape, and can bear loads and stresses, and even help the cell move just like our skeletons do.
We have many different bones in our bodies that come in all shapes and sizes. In the cell, there are three types of 'bones': microfilaments, microtubules, and intermediate filaments.
However, unlike our bones, the cytoskeleton is always changing. Microfilaments, microtubules, and intermediate filaments are composed of smaller building blocks that can be added or subtracted to make them longer or shorter, whatever the cell needs at the moment. They also act like little highways, and there are special proteins that move along the different filaments carrying cargo around the cell.
Let's go into more detail about each part of the cytoskeleton and its particular role in the cell.
Microfilaments are the thinnest of three parts of the cytoskeleton, measuring in at 5-9 nm in diameter. The building block of the microfilament is a protein called actin, so they are sometimes called 'actin filaments'. The actin molecules stick together in a particular way, similar to the way Legos have to be in a particular position to stick together. This also helps the cell to tell the difference between the two ends of the filament.
One end is called the plus end and one end is called the minus end. Because the two ends are different, a microfilament is polar. Typically, growth or shrinkage of the filament occurs by adding or subtracting actin molecules to the plus end.
Actin has many different jobs in the cell. One of the jobs is to support the structure of the cell. If you were to peel back the cell membrane, you would likely find a lot of microfilaments just below it. This is very important in our tissues because the cells need to stick together, and this requires them to be very strong. There are special proteins in the cell membrane that help attach the cell to surrounding cells; microfilaments inside the cell will then attach to these proteins to reinforce these attachment points.
Microfilaments are also involved in cell movement. Some cells will grow the microfilaments in the direction they want to move so that the microfilaments will push the cell membrane forward. This helps the cell to explore its environment or move in the direction of a particular signal. Microfilaments are also an important part of muscle contraction. They are the backbone of the thin filaments, which act like walkways for myosin proteins. As myosin proteins walk along the thin filaments, the muscle will contract.
Microtubules are the largest structural part of the cytoskeleton. They are fatter than microfilaments, with a diameter of about 25 nm. They are made up of building blocks called tubulin (you can remember this because they look like tubes!). There are actually two types of tubulin that stick together to form a peanut-shaped dimer. The tubulin protein dimers then stick together to form long, hollow tubes. As a result, they are much stiffer than microfilaments. Microtubules are also polar, with a plus end and a minus end.
Microtubules are responsible for:
- cell movement in cells that have cilia or flagella (like the tail on a sperm cell). Microtubules are bundled together inside these structures, and special proteins allow the microtubules to slide past one another and create the whipping motion that propels the cell forward.
- helping the chromosomes to line up in the center of the cell, and then pulling them apart during mitosis (cell division).
- acting like roads for the transport of organelles around the cell. There are special proteins that can walk along the microtubule, sometimes from the plus end to the minus end, some the other way. This helps the cell to move important cargo to its proper destination.
Intermediate filaments are the structural part of the cytoskeleton that is intermediate (hence the name) in size, measuring in at about 10nm. They have building blocks that are shaped like ropes. Several long proteins will coil together to make a dimer; two dimers will stick together to make one tetramer, which is the building block for intermediate filaments. There are actually many types of proteins that can form these building blocks.
You may recognize the name keratin, which is one type of intermediate filament protein. Because intermediate filament proteins don't have a particular orientation for sticking together, the resulting intermediate filament is not polar. Therefore, there is no plus or minus end.
The main role of intermediate filaments is to give extra strength to cells. For example, some of our neurons have very long arms, called axons. Intermediate filaments help to strengthen these long, thin axons. While most cells will have microtubules and microfilaments, there are some cells that don't have intermediate filaments.
Microfilaments, microtubules, and intermediate filaments make up the cytoskeleton. Each type of filament is made up of small protein subunits.
- Microfilaments are made of actin,
- microtubules are made of tubulin, and
- intermediate filaments can be made of a variety of different rope-like proteins.
The cytoskeleton is mainly responsible for giving cells their structure and support. The components of the cytoskeleton are also important in helping cells move and providing tracks for the movement of cargo around the cell.
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