18.1: The cytoskeleton

There are three types of fibers within the cytoskeleton:

• Microfilaments,
• Intermediate filaments, and
• Microtubules.

The collection of these fibers plays key roles in structure and support, intracellular transport, contractility and motility, as well as spacial organization (figure 18.2).

Microfilaments thicken the cortex around the cellʼs inner edge. Like rubber bands, they resist tension. There are microtubules in the cellʼs interior where they maintain their shape by resisting compressive forces. There are intermediate filaments throughout the cell that hold organelles in place.

Microfilaments

Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They function in cellular movement, have a diameter of about 7 to 8 nm, and are comprised of two globular protein intertwined strands, which we call actin (figure 18.3). For this reason, we also call microfilaments actin filaments.

Figure 18.2: Spatial organization of the three types of fibers. Microfilaments thicken the cortex around the cell’s inner edge. Intermediate filaments have no role in cell movement. Their function is purely structural. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell.

ATP powers actin to assemble its filamentous form, which serves as a track for the movement of a motor protein we call myosin. This enables actin to engage in cellular events requiring motion, such as cell division in eukaryotic cells. Actin and myosin are plentiful in muscle cells.

Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move.

Figure 18.3: Microfilaments are comprised of two globular protein intertwined strands, which we call actin. For this reason, we also call microfilaments actin filaments.

Intermediate filaments

Several strands of fibrous proteins that are wound together comprise intermediate filaments (figure 18.4). These cytoskeleton elements get their name from the fact that their diameter, 10 to 12 nm, is between those of microfilaments and microtubules.

Figure 18.4: Several strands of fibrous proteins that are wound together comprise intermediate filaments.

Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the cellʼs shape, and anchor the nucleus and other organelles in place (figure 18.1).

The intermediate filaments are the most diverse group of cytoskeletal elements. They are unbranched and rope-like with long fibrous subunits. There is no polarity associated with their assembly. Intermediate filaments are classified by their location and function. The table below summarizes various types of intermediate filaments.

Class	Protein type	Location	Function
1 & 2	Keratins	Epithelial cells	Structural support, stretching
3	Vimentin	Fibroblasts/Epithelial cells	Cage-like structure around the nucleus
	Desmin	Muscle cells	Links myofibrils together
	Glial fibrillary protein (GFAP)	Glial cells	Structural support
4	Neurofilaments	Neurons	Support for axon and dendrites
5	Lamins	Nucleus	Support for inner surface of the nuclear membrane

Table 18.1: Proteins and their functions.

Microtubules

As their name implies, microtubules are small hollow tubes. With a diameter of about 25 nm, microtubules are cytoskeletonsʼ widest components. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell (figure 18.5).

Figure 18.5: Microtubules are hollow. Their walls consist of thirteen polymerized dimers of \(\alpha\)-tubulin and \(\beta\)-tubulin. The left image shows the tube’s molecular structure.

Like microfilaments, microtubules can disassemble and reform quickly using GTP. The tube is formed from polymerized dimers of \(\alpha\)-tubulin and \(\beta\)-tubulin, two globular proteins. These proteins form long chains that comprise the microtubuleʼs walls. The assembly is slow and occurs from the plus end, which is designated by a row of \(\beta\)-tubulin. Disassembly can occur rapidly at the plus end. (Note the minus end has a row of \(\alpha\)-tubulin.)

Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the centrosomeʼs two perpendicular bodies). In animal cells, the centrosome is the microtubule-organizing center.

Flagella and cilia

The flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and enable an entire cell to move. When present, the cell has just one flagellum or a few flagella.

However, when cilia (singular = cilium) are present, many of them extend along the plasma membraneʼs entire surface. They are short, hair-like structures that move entire cells (such as paramecia) or substances along the cellʼs outer surface (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils).

Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet (axoneme) in the center (figure 18.6).

Figure 18.6: This transmission electron micrograph of two flagella shows the microtubules’ 9 + 2 array: nine microtubule doublets surround a single microtubule doublet.

References and resources

Text

Clark, M. A. Biology, 2nd ed. Houston, TX: OpenStax College, Rice University, 2018, Chapter 4: Cell Structure, Chapter 5: Structure and Function of the Plasma Membranes.

Karp, G., and J. G. Patton. Cell and Molecular Biology: Concepts and Experiments, 7th ed. Hoboken, NJ: John Wiley, 2013, Chapter 9: The Cytoskeleton and Cell Mobility.

Le, T., and V. Bhushan. First Aid for the USMLE Step 1, 29th ed. New York: McGraw Hill Education, 2018, 48–49.

Figures

Dartmouth Electron Microscope Facility, Dartmouth College. Figure 18.6 This transmission electron micrograph of two flagella shows the microtubules’ 9 + 2 array: nine microtubule doublets surround a single microtubule doublet. Scale bar data from Matt Russell. Public domain. From Wikimedia Commons.

Grey, Kindred, Figure 18.1 Summary of the three major types of structural filaments. 2021. https://archive.org/details/18.1_20210926. CC BY 4.0. Adapted from Figure 3.18. CC BY 4.0. From OpenStax.

Grey, Kindred, Figure 18.2 Spatial organization of the three types of fibers... 2021. https://archive.org/details/18.2_20210926. CC BY 4.0. Adapted from Figure 4.22. CC BY 4.0. From OpenStax.

Grey, Kindred, Figure 18.3 Microfilaments are comprised of two globular protein intertwined strands, which we call actin. For this reason, we also call microfilaments actin filaments. 2021. https://archive.org/details/18.3_20210926. CC BY 4.0. Adapted from Figure 4.23. CC BY 4.0. From OpenStax.

Grey, Kindred, Figure 18.4 Several strands of fibrous proteins that are wound together comprise intermediate filaments. 2021. https://archive.org/details/18.4_20210926. CC BY 4.0. Adapted from Figure 4.24. CC BY 4.0. From OpenStax.

Grey, Kindred, Figure 18.5 Microtubules are hollow. Their walls consist of 13 polymerized dimers of \(\alpha\)-tubulin and \(\beta\)-tubulin. The left image shows the tube’s molecular structure. 2021. https://archive.org/details/18.5_20210926. CC BY-SA 4.0. Adapted from Microtubule structure esp by Posible2006. CC BY-SA 4.0. From Wikimedia Commons.