Cytoskeleton and intracellular motility

Definitions & Key takeaways

The cytoskeleton is a mesh-like structure that spans the entire cell and provides structural support. It is made of three main protein filaments: microtubules, actin filaments, and intermediate filaments. These filaments can assemble and disassemble to allow the cell to change shape and move.

The cytoskeleton is responsible for many cellular processes, including intracellular motility (the movement of organelles within the cell). This motility is accomplished through the actions of motor proteins, which bind to the cytoskeleton and use it as a track to move along. This movement allows cells to respond quickly to their environment and carry out essential functions like replication and division.

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The cell is the basic unit of life, that can replicate on its own. The human body alone has over 200 different cell types - from long skinny neurons that can grow over 1 meter long to myocytes or heart muscle cells that contract to let you to flex your muscles.
But despite their differences, they share many features, including the cytoskeleton. The cytoskeleton is a network of proteins within the cell.
The cytoskeleton gives each cell its shape and anchors organelles in place - keeping everything sturdy - a bit like the frame for a house.
But it’s also a dynamic network, which can change shape when the cell wants to move, contract, divide, or pull in or push out molecules.
Imagine if your house could do that - perhaps it would get up and walk away during an earthquake! So the cytoskeleton is pretty special and it’s made up of three proteins: actin filaments, intermediate filaments, and microtubules.
Actin filaments are the thinnest of the three proteins, so they’re also called microfilaments. They’re made up of two strands of actin proteins arranged in a long twisting chain like a twisted necklace.
The actin filaments connect to one another to form a network - like a spider's web - that’s located just below the cell membrane.
The actin filaments slide closer together and further apart, allowing the cell to change shape during muscle contraction.
Not surprisingly, muscle cells have plenty of actin, as well as another protein called myosin. Myosin filaments bind to actin filaments, and that’s what allows the actin to slide closer together and further apart.
And ultimately, that makes the muscle cells shrink and stretch during muscle contraction and relaxation. Similarly, sometimes these networks change their shape and that allows cells to move.
White blood cells like neutrophils use extensions called pseudopodia, or false feet, to crawl in and out of blood vessels - a process called diapedesis.
The way that works is that the neutrophil’s actin filaments grow rapidly through the polymerization of many actin monomers in one direction, to push out the cell membrane and create a foot.
This newly created foot wedges between the endothelial cells that make up the blood vessels. The neutrophil then begins to squeeze through, until it reaches the other side.
It’s a bit like squeezing between two fence poles to sneak into an amusement park, rather than paying admission. Not saying that you should do that.
Now, actin filaments also play a role in mitosis - or cell division. At the end of mitosis, the cell has two sets of chromosomes that each sit in their own nucleus.
A ring of actin filaments then forms around the center of the cell between the two nuclei. This ring uses the sliding movement of actin and myosin to help constrict or squeeze the cell so that it pinches off into two new cells.
Next, are the microtubules which are the thickest and largest of the protein structures in the cytoskeleton. Microtubules are made of alternating round proteins called α- and β-tubulins - which form long strands called protofilaments.
Thirteen of these protofilaments come together to form a single microtubule. Microtubules stretch to and from every corner of the cell, which allows them to be used like railroads for intracellular transport.
For example, proteins like kinesin and dynein which pick up vesicles full of proteins, lipids, or hormones, and carry them on microtubules either to or from the cell membrane, from or to specific organelles - like the Golgi apparatus.
Microtubules can also resist a lot of compression force and still maintain their shape, so their like the steel beams that support and give a building its shape.
Microtubules also play a role in cell division. During cell division there are two centrosomes, and each centrosome is made out of two centrioles.
Each centriole is in turn made up of nine sets of microtubule triplets, and each microtubule triplet is at a slight angle with respect to the next microtubule triplet.
Now, during cell division, each chromosome is made up of a pair of sister chromatids which are joined together at a region called the centromere.
Now, at the centromere there’s a specific protein complex called the kinetochore. To help the two sister chromatids separate from one another, microtubules from the centriole polymerize in the direction of the kinetochores.
It looks a bit like a fishing line being cast out to it’s target, and then the line gets reeled in pulling the kinetochore and it’s attached sister chromatid away from it’s partner chromosome.
Ultimately, this gives rise to two sets of single-chromatid chromosomes, one for each of the daughter cells. Microtubules also help with cell movement - and that’s because centrioles also form at the base of flagella and cilia.
Flagella form the sperm cell’s tail – which wiggles to help sperm move. Cilia, on the other hand, are shorter than flagella and can be non-motile or primary cilia or they can be motile or secondary cilia.
Nonmotile cilia are found on photoreceptor cells in the human eye and they help with the absorption of photons, which then the brain converts into a color image.
Motile cilia, on the other hand, can be found on cells like the ones that line the fallopian tubes, which slowly sway and drag the egg cell, fertilized or not, towards the uterus.
Finally, there are the intermediate filaments, which fall in between actin filaments and microtubules in terms of size. Intermediate filaments are made of interlaced threads of protein - like a rope.
This makes intermediate filaments stronger than actin filaments, but also less dynamic. Intermediate filaments mostly fasten various organelles like the nucleus in place within the cell.
Intermediate filaments also help anchor cells to neighboring cells, through cell to cell junctions. They also help anchor the cells to their surrounding environment, the extracellular matrix.
Intermediate filaments are less dynamic than actin filaments and microtubules, so they tend to degrade slowly and last longer within the cell.
Now, unlike actin filaments and microtubules, intermediate filaments are made of many types of proteins depending on the cell type.
They are classified by their molecular similarities into type I, II, III, IV, V, and VI. Types I and II are the keratins and they make up the main structural component of hair, nails, and the outermost layer of the skin.
Type III includes the most common of all intermediate filaments - vimentin. Vimentin is found in cells like smooth muscle cells, fibroblasts, and white blood cells.
Another type III protein is desmin and it forms the basic unit of the skeletal and cardiac muscle cells - the sarcomere that makes muscles contract.
A third type III intermediate filament protein called glial fibrillary acidic protein, and it’s found in supporting cells in the central nervous system, like astrocytes.
A fourth type III intermediate filament protein called peripherin is found in neurons of the peripheral nervous system. Type IV intermediate filaments include synemin, also called desmuslin, which provides resistance to mechanical stress in cells - so while they are found in all cells, they’re especially abundant in skeletal muscle cells.
Another muscle-specific type IV intermediate protein is syncoilin. There are also some neuron specific type IV filaments, like alpha-internexin, and a group simply called neurofilaments which provide structural support for axons.
Finally, there’s type V and type VI intermediate filaments which are found in almost all of our cells: type V intermediate filaments are lamins, which make up the nuclear lamina around the nucleus of a cell.
Then there are Type VI intermediate filaments which include nestin, which helps assemble and disassemble other intermediate filaments.
Alright, as a quick recap, the cytoskeleton is formed by actin filaments, intermediate filaments, and microtubules. Actin filaments are the thinnest and help with muscle contraction, cell division, and even cell movement.
Intermediate filaments fall between actin filaments and microtubules when it comes to diameter and they provide strength to the structure of the cell.
And finally, there are microtubules, which are the thickest of the three. Microtubules, give cells their overall shape, form centrioles, and helps in the transportation of vesicles.