Extracellular matrix

Definitions & Key takeaways

The extracellular matrix (ECM) refers to the mixture of proteins and carbohydrates that surrounds cells and provides structural and biochemical support. ECM components can be put into three categories: adhesive proteins, structural proteins, and proteoglycans.

Adhesive proteins are needed to stick individual cells together, forming an organized tissue. They include integrins and cadherins. Next, structural proteins provide the tissues with structural support, including tensile strength. These proteins include keratins, and elastin. There are also proteoglycans which fill up the space between cells, helping to trap water molecules thanks to their negative charges.

The ECM also plays an important role in the communication between cells, mediating cell-to-cell interaction and cell-matrix interaction. Dysfunction of the ECM can lead to diseases such as cancer, arthritis, and fibrosis.

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Cells live within an environment called the extracellular matrix, and it’s a bit like how homes have yards and streets that surround them.
Also, just like how lots of homes form a community, lots of cells form a tissue. And there are different types of tissues - epithelial, connective, muscular, and nervous tissue.
Each tissue has an extracellular matrix that’s got a unique composition that’s adapted for each tissue’s unique needs. Having said that, all of the different types of extracellular matrix are made up of three major molecules - adhesive proteins, structural proteins, and proteoglycans.
First, there’s adhesive proteins, which help to stick the individual cells together and organizes the tissue into a neat structure.
Now, adhesive proteins - like integrins and cadherins - are found on the cell’s surface and they’re like molecular velcro.
Cells use adhesive proteins to anchor themselves to other cells and to molecules in the extracellular matrix. Adhesive proteins also help communicate messages from the extracellular matrix to the cell.
For example, signals relayed by integrins can help a cell decide when it’s time to grow, divide, differentiate, or even die - like in apoptosis.
Next, there are structural proteins which give our tissues their tensile and compressive strength. Some examples are collagens, elastins, and keratins.
Collagen is the most common type of structural protein in the human body, mostly because it resists tension and it can also stretch.
When collagen is made and released into the extracellular space, it’s in the form of a precursor called procollagen. Procollagen is a protein made up of three polypeptide strands, each coiled into a left-handed helix, and then all three are twisted together into a right-handed triple helix or "superhelix" with three loose strands at each end.
Once procollagen is in the extracellular space, it encounters a tiny band of enzymes called collagen peptidases that cleave the loose strands at both ends of the procollagen protein, turning procollagen into tropocollagen.
Once there are enough tropocollagen molecules, they bind with one another to form a stack of collagen fibrils. Collagen fibrils are arranged in different ways depending on the tissue, and when there’s a bunch of collagen fibrils, they can join up to form a bigger tube called a collagen fiber.
The four most common types of collagen are I, II, III, and IV. And you can use the mnemonic Be So Totally Cool, Read Books, Bro, to help you remember where each type of collagen is located in the body.
So, Type I is found in the Bone, Skin, and Tendons. Type II is in our cool Cartilage.
Type III is in Reticulin – a type of fiber found in the Liver and the Lymphatic System - and Blood vessels. Type IV is found in the Basement membrane – a thin sheet of protein that separates different types of cells like nerve cells from muscle cells.
Another type of structural protein is elastin, which is an elastic protein that allows connective tissues to regain their original shape after stretching.
Elastin is found in ligaments, blood vessels, skin, and lungs. And the last structural protein is keratin - a pretty tough protein that’s found in our hair and nails.
Next, are the proteoglycans which are a group of molecules that have a protein core that’s attached to chains of sugars called glycosaminoglycans.
Lots of glycosaminoglycans attach perpendicular to the core protein, so it looks a bit like the fibers of a test tube brush.
Examples of glycosaminoglycans include chondroitin, keratan sulfates and hyaluronic acid. Now, proteoglycans mostly help to fill up the space between cells and the negative charge of glycosaminoglycans helps to trap water molecules.
The result is that the proteoglycans plus water form a gel-like substance that hydrates and cushions the cells, and it’s the main reason that it’s so abundant in cartilage.
Alright, as a quick recap, the extracellular matrix is largely composed of adhesive proteins, structural or fibrous proteins, and proteoglycans.
Adhesive proteins like integrins and cadherins help anchor the cell in place and link it to other cells. Collagen is the main structural protein in our body, and it gives tissues their firmness and resistance to pressure.
Other structural proteins are elastin - that allows tissues to stretch - and keratin, which forms the hair and nails. Finally, there are the proteoglycans - which keep cells hydrated and cushioned.