The Cytoskeleton
When we extend our hand to touch something and explore the world around us, the bones and muscles within our hands are what allow us to do this. Our bones give our hand and arm their structure, and our muscles and tendons move our bones. While the cells in our bodies don’t have bones and muscles, they do have components that give them structure and allow them to move. When cells “put out feelers” to explore the world around them, they deploy their cytoskeletons. A cell’s cytoskeleton is very complex, but there are three major parts that compose most of it and that all work together: actin filaments, intermediate filaments, and microtubules. But unlike your bony skeleton, which stays rigid, the cytoskeleton is dynamic, always changing, building up, and rearranging to allow cells to carry out their functions and explore their world.
Written by Christina Wilson Bowers; Amherst College; Massachusetts, USA
Background image by Andy Moore, HHMI's Janelia Research Campus, and Erika Holzbaur, University of Pennsylvania; Pennsylvania, USA
Dynamic chains of actin
Actin and tubulin assemble into chains, growing from one end and coming apart at the other. These long chains of actin – known as actin filaments – support cell movement, the creation of specialized structures, and much more. In the video above, you can see actin filaments (purple) and actin cross-linking proteins (gold) growing and retracting specialized structures called microvilli from an intestinal epithelial cell grown in culture.
Video by Matt Tyska; Vanderbilt University; Tennessee, USA
Dynamic chains of microtubules
Microtubules form intracellular “highways,” allowing the organized movement of materials and even whole organelles like mitochondria. They collaborate with actin to support cell mechanics and movement, but they also help relay information between the compartments of a cell and the environment outside of a cell. In the video above, you can see individual how tubulin proteins form into microtubules, how they grow, and then how they disassemble.
Video by Eva Nogales; University of California, Berkeley; California, USA
A bird’s-eye view of intermediate filaments
Intermediate filaments have interchangeable building blocks depending on cell type. In skin cells, they are made of a tough protein called keratin. This image shows a cell filled with intermediate filaments made of a protein called vimentin, shown here stained in blue and green. Cell shape and integrity are supported by this mesh-like network of fibers. The cell in this image is flexible. If you pushed against it, the cell would rebound. Vimentin filaments help cells sense pressure through a process called mechanosensing. In the world of a cell, this could be another cell or a solid surface applying pressure. Intermediate filaments extend throughout the cell from top to bottom.
Image by Andy Moore, HHMI's Janelia Research Campus, and Erika Holzbaur, University of Pennsylvania
The transportation systems of the cell
Actin and microtubules form chains from subunits that stick together or separate in response to changing environmental conditions. Once built, actin chains form twisted filaments, while microtubule chains coil into hollow tubes. Both actin and microtubules have one growth end and one end that falls apart. Regulating the formation of these dynamic structures is crucial to cell survival.
The fine actin threads stained orange in this image appear haphazard, almost like someone sprayed silly string into the cell. Don’t be fooled, though – the layout is highly organized. Actin chains form networks ready to respond at a moment’s notice to support functions like cell movement and division.
Microtubules, stained white in this image, form connected networks that are used to move materials around the cell. The process involves building on one end while disassembling on the other end – like constructing a railroad track just ahead of a moving train, while simultaneously taking the track apart once the train has moved on by.
Image by Andy Moore, HHMI's Janelia Research Campus
An actin sunburst
Actin is the most abundant protein in most eukaryotic cells. Free-floating subunits called monomers assemble rapidly into chains of F-actin (F stands for “filaments”) in response to cell signals. F-actin chains collaborate with many different protein partners, allowing functional flexibility. The streaming orange tips of the cell in this video are filled with F-actin (labeled in orange), which has received a signal telling the cell that it is time to put out feelers and explore.
Video by Andy Moore, HHMI's Janelia Research Campus
Microtubule fireworks
Assembling the leading edge of a growing microtubule requires carefully coordinating the addition of new subunits to 12 parallel filaments at the same time. In this video, the growth, or plus (+), end of the microtubule is labeled in red. Imagine an army of workers carrying buckets of cargo from the interior network to the edge of the cell, following the leader who illuminates the path with a fiery torch.
Video by Andy Moore, HHMI's Janelia Research Campus
Cells in community
This image shows a community of cells resting against each other. The region of the nucleus appears orange in the center of each cell. Microtubules (blue) appear to extend into neighboring cells, an illusion due to invisible membranes. The entire microtubule network can reorganize in an instant in response to the appropriate signal.
Communities of cells rely on the microtubule network to coordinate interactions during important processes like wound healing. When cells divide to fill in a breach in the skin, their signal to stop dividing is contact with neighboring cells.
Image by Andy Moore, HHMI's Janelia Research Campus
Helping neurons navigate
In a developing embryo, connections between neurons build circuits that will eventually form a fully functioning nervous system. Interacting microtubules (blue) and actin (purple) work together to form the growth cone. The growth cone is the part of the developing neuron that expands into the environment, seeking chemical signals. Actin filaments reach beyond the leading edge of the growth cone like tiny fingers, exploring the surface for signals and cues that tell the cell where to move. When a compatible partner is found, the growth cone recedes and a connection between neurons is forged.
Image by Dylan T. Burnette; Vanderbilt University
Sparks flying
Cell division and separation
The cytoskeleton plays a vital role in coordinating the changes that happen during cell division. Errors can have dire consequences, such as cells with too many or too few chromosomes – which can lead to cell death, tumor formation, or problems during embryonic development.
Actin is labeled red in this image, and myosin, a motor protein, in blue. In the view here, the chromosomes shown in yellow have separated and begun returning to their predivision state (decondensing). As the cells prepare to separate, actin (red) and myosin (blue) collaborate to form a contractile ring, much like cinching a belt tight to squeeze the cells in two. This process relies on precise ring placement and contraction timing. If the cells pinch apart too soon or in the wrong place, the resulting daughter cells might end up with the wrong amount of DNA in each daughter cell.
Image by Dylan T. Burnette, Vanderbilt University
The actin wave
The stain used in this video reveals an actin wave sweeping through a cell right before it divides. This actin wave helps distribute organelles across the cell so that the daughter cells each receive at least some of the parent organelles, such as mitochondria. This does not have to be an equal distribution between the daughter cells, as must be the case with the cellular DNA, because the daughter cells can produce additional organelles if they’re needed.
Video by Andy Moore, HHMI's Janelia Research Campus
A cellular superhighway
This image incorporates a technique called depth coding – microtubules closer to the viewer are green and those farther away are blue. This depth coding reveals the three-dimensional positions of the microtubules within the cell, giving researchers important information about how the microtubule network is structured within cells.
Image by Derek Sung, University of Pennsylvania
A cytoskeleton dance party
The cytoskeleton maintains dynamic partnerships with the organelles within the cell. In this video, intermediate filaments are shown in orange and the endoplasmic reticulum in blue. As these cells move, the network of intermediate filaments grows and shrinks to satisfy the structural requirements of the cell as it’s in motion. This also affects the position of the endoplasmic reticulum within the cell, as it moves in concert with the cytoskeleton.
Only the intermediate filaments are labeled here, but the movements and responses you see rely on dynamic interactions between actin, microtubules, and myosin as well.
Video by Andy Moore, HHMI's Janelia Research Campus
Conclusion
Without their cytoskeleton, cells would be little more than flimsy membranous bags. Actin, microtubules, and intermediate filaments provide strength; support a cell’s shape and shape-shifting abilities; and guide cell behavior in community – whether that means coming together or pulling apart. They support exploration and communicate information across cell compartments and beyond. All this and more make the cytoskeleton essential for the survival and proper function of living cells.
Image by Matt Tyska, Vanderbilt University
For suggestions on how to incorporate this journey into your teaching, see our “Implementation Suggestions.”