Cytoskeleton, Centrosomes, Centrioles, Cilia, and Flagella, oh my!

Whew, that was a mouthful, wasn't it?

Before we start, here's a sneak peek of what this blog post will be about:

Let's go over the definition of each organelle:

Cytoskeleton: A network of fibers, including microtubules, microfilaments, and intermediate filaments, that extend throughout the cytoplasm of a cell

Centrosome: A structure in the cell that is designated for organizing microtubules

Centriole: An organelle composed of triplets of microtubules located inside of a centrosome

Cilia: Short attachments to the cell made with microtubules, that can be both motile and immotile

Flagella: Long attachments to the cell similar to motile cilia.

Cytoskeleton:

The cytoskeleton is literally the backbone of the cell. Its main functions are supporting the cell, keeping the cell's shape, and helping with cell motility. Another function, though not as "mainstream" as the ones listed above, is that the cytoskeleton can ben the plasma membrane inward to form food vacuoles and other phagocytic vesicles.

Through the following fibers, the cytoskeleton is able to perform its duties: Microtubules, microfilaments, and intermediate filaments. Even though the three may seem similar from afar, each has a unique structure and function it contributes to the cell.

Microtubule

Another name for microtubules is "tubulin polymers." Its latter name is probably derived from the fact that microtubules are made from a globular protein called tubulin. Microtubules are hollow rods (or you could say tube) with a diameter of 25 nm and can be easily modified; More tubular proteins, or dimers, can be added to make a microtubule longer, or can be disassembled to make a new microtubule elsewhere.

Microtubules have several functions. An obvious one is shaping and spurning the cell, which was mentioned earlier as we went over the cytoskeleton as a whole. Besides this, microtubules can see as paths for motor proteins to travel the cell on and are involved in splitting chromosomes during cell division. Not to mention, this particular fiber makes up centrioles, flagella, and cilia, and allow flagella and cilia to be motile.

Microfilaments

Also known as an actin filament, this twisted double chain of actin subunits has the smallest diameter of the three fibers with a diameter of 7 nm. (No wonder it's called the MICROfilament) It is able to form structural networks when certain proteins bind with microfilaments. After the proteins are blinded, new filaments can attach to the side and form branches, sort of like this magnet toy below:

The silver magnet spheres are like the proteins and the long colorful magnets are like the actin 

filaments.

The microfilament's ability to form such structural networks mainly helps it execute its duties. Actin filaments are the fibers that bear any tension in the cell, and help with cell motility. A common example would be that when microfilaments interact with myosin, muscle cells can contract. Even outside the cytosol, the structural networks are needed; There are some in the the plasma membrane, which keeps a cell's shape.

Intermediate Filaments

With diameters between 8-12 nm, intermediate filaments are coiled rods with diameters in between microfilaments and microtubules. Along with its diverse set of diameters, different kinds of proteins compose intermediate filaments.

Besides having inconsistent diameters and compositions, intermediate filaments differ from the other two fibers because they are not found in all eukaryotic cells and are sturdier. Chemical treatments that remove microfilaments and microtubules can't remove intermediate filaments, which remain to keep the cell's original shape. Often times, the intermediate filaments will remain even after a cell dies. On top of reinforcing the shape of the cell, these filaments fixate or anchor organelles.

Centrosome

Centrosomes are located near the nucleus and are surrounded by formless mass of protein, or pericentriolar material. Microtubules grow out from centrosome, and there is one pair of centrioles per centrosome that are arranged perpendicular with each other.

Its most important functions are forming the cytoskeleton and cell division. The centrosome is in charge of forming microtubule networks and organizing individual microtubules. Here's a quick overview of what the centrosome does during cell division: First, it duplicates during S-phase, then migrating to the opposite ends of the cell during prophase. Microtubules grow and form spindle fibers , leading chromosomes to be assembled and separated. After that, each daughter cell gets one chromosome, and each daughter cell has a centrosome.

Centriole

The centriole has a cylindrical shape, made up of 9 sets of triplet microtubules. As mentioned before, there are two centrioles per centrosome per cell.

Centrioles are key for cell division. They produce spindle fibers during cell division, which helps the cell actually split in to two. For cells that don't divide, the mother centriole can form a basal body, a structure that develops cilia and flagella.

Cilia & Flagella

Similarities
Both cilia and flagella have a "9 + 2" pattern. This means that nine pairs of microtubules are arranged in a ring with two single strands of microtubules in the center. Also, non-motile cilia and flagella have basal bodies, which anchor microtubules in centrosomes, and have the same structure as a centriole (triplets in a "9+0" pattern).

As for motility, large motor proteins called dyneins, attach to each outer microtubule doublet, and the two "feet" adjacent to the doublet allows the cilia and flagella to bend. All movements are driven by ATP.

Differences
Although there are many similarities between flagella and cilia, they have some differences as well. Cells either have one or few flagella(flagellum) or many cilia. Flagella are longer than cilia, with a length of 20 µm opposed to 1-10 µm. Although both can be found in eukaryotic cells, only flagella can be found in bacteria.

Also, cilia move like the oars of a boat, alternating between power and recovery strokes, while flagella have undulating motion like the tail of a fish. Lastly, a function cilia have that flagella do not have is receiving signals.

Disease

As mentioned before, the centrosome is in charge of cell division. Thus one common disease associated with this organelle is cancer. When a cell has excess centrosomes, cancer can form as a result of the cell reduplicating, fusing with other cells, or even failing to divide. Usually, cancer spreads because cells divide uncontrollably due to dysfunctional centrosomes.

Clearly, cancer cells look a LOT different from regular cells!

A Brief Timeline of the History of the Five Cell Components

1676: Anton von Leeuwenhoek first discovers cilia and flagella while studying protozoa.

1883: Edouard Van Beneden is the first to discover the centrosome

1888: Theodor Boveri describes and names the centrosome, and finds out about centrioles in the process.

1903: Nikolai Koltsov first proposes the existence of the cytoskeleton and is the first to use the term, "cytoskeleton".

1968: H. Holtzer discovers the intermediate filament under an electron microscope. (Before this, only microtubules and microfilaments were thought to exist)

1992-1998: Discovery of cytoskeleton in bacteria

Thanks for reading this blog post! :)