Video: Cytoskeleton

Home sweet home, but it's a bit empty. Let's get to work. I want a large kitchen and dining room on the ground floor. The bedroom goes just on top of it where the big window is. Bathroom on the side -- oh, that's a problem. Some loadbearing pillars will do the trick. Great! Now I need a way to get upstairs and a fast way to get down to the kitchen for my midnight snacks. Ah, perfect.

Just like houses have internal walls, cells have a structure that keeps things organized, avoid structural collapses, and ensures efficient transport. Wait, what? Oh, right. I forgot. This structure also makes the cell move. Let's talk about the cytoskeleton.

The cytoskeleton is a complex threadlike network of fibrous proteins that spans the cytoplasm. This non-membranous organelle maintains the structure of the cell, helps cells move, resists mechanical forces, anchors the organelles, transports substances around the cell, and assists with cell division.

The cytoskeleton is also a key component of cellular extensions like microvilli, cilia, and flagella. These functions are performed by three types of fibrous proteins: microfilaments, intermediate filaments, and microtubules. These are the filaments of the cytoskeleton.

Microfilaments are the thinnest cytoskeletal filaments, measuring only 5 to 9 nanometers in diameter. They're made up of actin monomers arranged as a double-stranded helix. Microfilaments in the periphery of the cytoplasm form the cell cortex, which provides structural support to the cell membrane and regulates changes in cell membrane shape.

For example, actin microfilaments split the parent cell during cell division, help form vesicles for endocytosis, and facilitate the crawling motion of cells. Actin filaments play an important role in muscle cells where they work with myosin to enable muscle contraction.

Intermediate filaments are thicker than microfilaments, typically measuring 8 to 12 nanometers in diameter. These filaments are made up of proteins like keratin, desmin, or vimentin, and generally form a mesh spanning the cytoplasm.

The cable-like structure of the intermediate filaments makes them flexible and durable contributing to the cell's resistance to mechanical stress. They also anchor the other organelles, maintaining their shape and size. Some intermediate filaments are made up of lamin and form the nuclear lamina, a structure primarily dedicated to supporting the nuclear envelope.

At 25 nanometers, the microtubules are the thickest filaments of the cytoskeleton. They resemble hollow tubes and are made of tubulin subunits. Microtubules help maintain the shape of the cell, function as highways along which motor proteins like dynein and kinesin transport organelles and genetic material, organize the mitotic spindle, and form the motile core of cilia and flagella.

Microtubules extend from the centrosome, a region that functions as the microtubule organization center in human cells. Centrosomes consist of gel-like material surrounding two centrioles which are short cylindrical structures with 27 microtubules bundled in nine groups of three. These structures regulate the dynamic assembly and disassembly of microtubules.

Other microtubules originate from the basal bodies, which are structures similar to centrioles but located near the cell membrane, underneath cilia and flagella.

Speaking of cilia and flagella, it's time to dive deeper into cellular extensions.

Cellular extensions are protrusions that help cells perform different functions including absorption and movement. There are three main types of cellular extensions: microvilli, cilia, and flagella. Microvilli are short, finger-like cellular extensions with a core of actin microfilaments. They cannot move and their main function is to increase the surface area of epithelial cells involved in absorption.

Cilia are longer hair-like extensions mainly present in epithelial cells. Some cilia are unable to move while others, like the motile cilia in the respiratory and reproductive tracts, move rhythmically to sweep substances along the epithelium. Ciliary motion is possible because the axoneme of motile cilia is a structure with two individual microtubules surrounded by nine pairs of microtubules. Motor proteins like dynein crawl along pairs of microtubules, making them slide against each other. This makes the microtubules bend, generating the characteristic sweeping motion in the case of cilia.

The axoneme is also the core of the tail-shaped flagella, which are the longest cellular extensions. The only cells in the human body to have this cellular extension are free-floating sperm cells, which use their flagella's whip-like motion to move around.

And that concludes our tutorial on the cytoskeleton.

To get a solid foundation on the cell structure, check out the quiz and other learning materials in this study unit. See you next time!