Cytoskeleton and intracellular motility

The cell is the basic unit of life, that can replicate on its own.

The human body alone has over 200 different cell types - from long skinny neurons that can grow over 1 meter long to myocytes or heart muscle cells that contract to let you to flex your muscles.

But despite their differences, they share many features, including the cytoskeleton.

The cytoskeleton is a network of proteins within the cell.

The cytoskeleton gives each cell its shape and anchors organelles in place - keeping everything sturdy - a bit like the frame for a house.

But it’s also a dynamic network, which can change shape when the cell wants to move, contract, divide, or pull in or push out molecules. Imagine if your house could do that - perhaps it would get up and walk away during an earthquake!

So the cytoskeleton is pretty special and it’s made up of three proteins: actin filaments, intermediate filaments, and microtubules.

Actin filaments are the thinnest of the three proteins, so they’re also called microfilaments.

They’re made up of two strands of actin proteins arranged in a long twisting chain like a twisted necklace.

The actin filaments connect to one another to form a network - like a spider's web - that’s located just below the cell membrane.

The actin filaments slide closer together and further apart, allowing the cell to change shape during muscle contraction.

Not surprisingly, muscle cells have plenty of actin, as well as another protein called myosin.

Myosin filaments bind to actin filaments, and that’s what allows the actin to slide closer together and further apart.

And ultimately, that makes the muscle cells shrink and stretch during muscle contraction and relaxation.

Similarly, sometimes these networks change their shape and that allows cells to move.

White blood cells like neutrophils use extensions called pseudopodia, or false feet, to crawl in and out of blood vessels - a process called diapedesis.

The way that works is that the neutrophil’s actin filaments grow rapidly through the polymerization of many actin monomers in one direction, to push out the cell membrane and create a foot.

This newly created foot wedges between the endothelial cells that make up the blood vessels.

The neutrophil then begins to squeeze through, until it reaches the other side.

It’s a bit like squeezing between two fence poles to sneak into an amusement park, rather than paying admission.

Not saying that you should do that. Now, actin filaments also play a role in mitosis - or cell division.

At the end of mitosis, the cell has two sets of chromosomes that each sit in their own nucleus.

A ring of actin filaments then forms around the center of the cell between the two nuclei.

This ring uses the sliding movement of actin and myosin to help constrict or squeeze the cell so that it pinches off into two new cells.

Next, are the microtubules which are the thickest and largest of the protein structures in the cytoskeleton.

Microtubules are made of alternating round proteins called α- and β-tubulins - which form long strands called protofilaments.

Thirteen of these protofilaments come together to form a single microtubule.

Microtubules stretch to and from every corner of the cell, which allows them to be used like railroads for intracellular transport.

For example, proteins like kinesin and dynein which pick up vesicles full of proteins, lipids, or hormones, and carry them on microtubules either to or from the cell membrane to specific organelles - like the Golgi apparatus.

Microtubules can also resist a lot of compression force and still maintain their shape, so they’re like the steel beams that support and give a building its shape.

Microtubules also play a role in cell division.

During cell division there are two centrosomes, and each centrosome is made out of two centrioles.

Each centriole is in turn made up of nine sets of microtubule triplets, and each microtubule triplet is at a slight angle with respect to the next microtubule triplet.