Cytoskeleton: Structure, Components, and Functions

The cytoskeleton is the cell's internal scaffolding, and much more than that. It gives the cell its shape, holds organelles in place, forms the tracks along which materials are transported, and drives movements from a crawling white blood cell to a dividing cell pulling its chromosomes apart. It is built from three kinds of protein fiber, each with a distinct job, and learning those three is the key to the whole topic. This article covers what the cytoskeleton is, its three components and how they differ, and what each one does.

What is the cytoskeleton?

The cytoskeleton is a network of protein fibers that runs throughout the cytoplasm of a cell. Despite the name "skeleton," it is not rigid or permanent: it is constantly built up and broken down as the cell changes shape, moves, and divides. It does the jobs of a skeleton (support and shape), of muscles (movement), and of a road network (internal transport), all at once.

The cytoskeleton is made of three components, defined by the protein they are built from and their thickness:

• Microfilaments (made of actin), the thinnest
• Intermediate filaments, medium thickness
• Microtubules (made of tubulin), the thickest

A useful point to fix straight away, because it confuses students: despite their names, microfilaments are the smallest and microtubules are the largest. The "micro" in both is misleading; go by the second half of the word.

The three components of the cytoskeleton

Microfilaments (actin filaments)

Microfilaments are the thinnest fibers, about 5 to 7 nm in diameter, built from the protein actin. Actin exists as single molecules (globular or G-actin) that link into long chains (filamentous or F-actin), and this constant assembly and disassembly is what lets microfilaments drive movement.

Microfilaments are the cell's movement and shape-change component. They lie just beneath the plasma membrane, where they control the cell surface, and they are responsible for cell crawling, the pinching of the cell in two during division (cytokinesis), cytoplasmic streaming, and the support of small surface projections such as the microvilli of intestinal cells. In muscle cells, actin working with the motor protein myosin produces contraction.

Intermediate filaments

Intermediate filaments are the middle-sized fibers, about 8 to 10 nm in diameter, which is where the name comes from (intermediate between the thin microfilaments and thick microtubules). Unlike the other two, they are built from several different proteins depending on the cell type, and they are the most stable and least dynamic of the three.

Intermediate filaments are the cell's mechanical strength component. They act like ropes that bear tension and resist being pulled apart, giving the cell and its tissue the ability to withstand mechanical stress. They form a network around the nucleus and extend to the cell edge, and they anchor cells to one another at junctions called desmosomes. Different tissues use different intermediate-filament proteins:

• Keratins in epithelial cells (and in hair and nails).
• Vimentin in connective tissue cells such as fibroblasts.
• Desmin in muscle cells.
• Neurofilaments in nerve cell axons.
• Lamins, which line and support the inside of the nuclear envelope.

Microtubules

Microtubules are the thickest fibers, about 25 nm in diameter, and unlike the solid microfilaments and intermediate filaments they are hollow tubes. They are built from the protein tubulin, which comes as pairs (dimers) of alpha-tubulin and beta-tubulin that stack into the tube wall. Microtubules are polar, meaning they have two different ends, which matters for the direction they grow and transport things.

Microtubules are the cell's transport and organization component. They act as tracks along which motor proteins carry vesicles, granules, and organelles across the cell. They also form the mitotic spindle that separates chromosomes during cell division, they make up the core of cilia and flagella (the structures that move cells), and they build the centrioles. They radiate out from a region near the nucleus called the centrosome.

Comparing the three components

Do bacteria have a cytoskeleton?

For a long time the cytoskeleton was thought to be a purely eukaryotic feature. We now know that bacteria have a cytoskeleton too, built from proteins that are the ancestors of the eukaryotic ones:

• FtsZ is the bacterial version of tubulin. It forms a ring (the Z-ring) at the middle of the cell that drives bacterial cell division.
• MreB is the bacterial version of actin. It helps determine the cell's shape, giving rod-shaped bacteria their rods.
• Crescentin is an intermediate-filament-like protein that gives the curved bacterium Caulobacter crescentus its shape.

This is a good example of how a structure once thought unique to complex cells turns out to have deep roots in bacteria, and it is a useful bridge from cell biology into microbiology.

Functions of the cytoskeleton (summary)

• Cell shape and support: the cytoskeleton gives the cell its form and holds it against mechanical stress.
• Movement: it drives cell crawling, muscle contraction, and the beating of cilia and flagella.
• Internal transport: microtubules act as tracks for moving vesicles and organelles around the cell.
• Cell division: microfilaments pinch the cell in two, and microtubules form the spindle that separates the chromosomes.
• Holding organelles in place: it anchors the nucleus and other organelles in position.
• Cell-to-cell connection: intermediate filaments anchor cells together at desmosomes.

How to Remember

Three fibers, smallest to largest: microfilaments, intermediate filaments, microtubules. Actin filaments are thinnest (5 to 7 nm), intermediate filaments are in the middle (8 to 10 nm), microtubules are thickest and hollow (25 nm). Do not let the names fool you: microfilaments are the smallest.

Three jobs: move, strengthen, transport. Microfilaments move (crawling, division, muscle). Intermediate filaments strengthen (resist stress like ropes). Microtubules transport (tracks for cargo, spindle, cilia). Move, strengthen, transport.

Tubulin for tubes, actin for action. Microtubules are made of tubulin and are hollow tubes. Microfilaments are made of actin and drive action (movement). The names match the function.

Intermediate in size and in job. Intermediate filaments sit in the middle for thickness, and their job is the steady, structural one: bearing tension and holding things together.

Bacteria have it too: FtsZ, MreB, crescentin. The bacterial versions of tubulin, actin, and intermediate filaments. The cytoskeleton is not eukaryote-only.

Key exam facts

Where Students Get Confused

Microfilaments are the biggest because of "micro"... no. Microfilaments are the smallest (5 to 7 nm); microtubules are the largest (25 nm). The names are misleading. Judge by the second half of the word: filaments are thin, tubules are thick.

All three are made of the same protein. They are not. Microfilaments are actin, microtubules are tubulin, and intermediate filaments are made of a range of proteins (keratin, vimentin, desmin, and others) depending on the cell type.

The cytoskeleton is a fixed, rigid frame. It is dynamic. Microfilaments and microtubules are constantly assembled and disassembled as the cell changes shape, moves, and divides. Only intermediate filaments are relatively stable.

Only eukaryotes have a cytoskeleton. Not true anymore. Bacteria have cytoskeletal proteins (FtsZ, MreB, crescentin) that are the evolutionary ancestors of the eukaryotic ones.

Microtubules and microvilli are the same thing. They are not. Microtubules are hollow tubulin fibers used for transport and division. Microvilli are small finger-like projections of the cell surface, supported by microfilaments (actin).

References

1. Alberts B, Heald R, Johnson A, et al. Molecular Biology of the Cell. 7th ed. New York: W. W. Norton; 2022.
2. Iwasa J, Marshall W. Karp's Cell and Molecular Biology. 8th ed. Hoboken: Wiley; 2016.
3. Reece JB, Urry LA, Cain ML, et al. Campbell Biology. 12th ed. New York: Pearson; 2021.