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Extracellular Matrix

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Extracellular Matrix

We have learned all about cell components and intracellular activities. However, cells interact with their environment, and in multicellular organisms, they must communicate with other cells and the rest of the body. What do we find outside of a cell membrane? What is a tissue composed of? We now move to the extracellular environment, where we see a matrix of proteins and polysaccharides. We discuss the importance of this matrix for cell and tissue properties, functions, and intercellular communication.

Definition of the extracellular matrix

Most cells synthesize compounds and materials destined to be secreted in the outside space of the cell (extracellular space). These materials form an extracellular matrix (ECM) that surrounds the cell and serves functions related to structure and communication with the extracellular environment.

The extracellular matrix is a mesh structure made of water and diverse proteins and carbohydrates, which surround a cell. It serves in functions such as cell support within a tissue, intercellular adhesion and communication, and cell migration.

The name extracellular matrix mainly refers to the components found outside of animal cells. However, fungi, plant cells, and some protists have an extracellular structure called the cell wall that some biologists consider a specialized extracellular matrix. Other sections discuss plant cell walls in more detail; therefore, we focus on animal cells’ extracellular matrix here.

Extracellular matrix structure

The ECM is a network mainly composed of water and diverse proteins and carbohydrates; the abundance of each component depends on the type of tissue they are part of (figure 1). Some tissues are mainly formed by lots of cells with some ECM in between (like in the brain and cardiac muscle), and others, called connective tissues, are primarily ECM with scattered cells suspended within it.

Connective tissues are found in animals and serve mainly to bind and support other tissues. Some connective tissues are cartilage, blood, adipose tissue, and bone tissue. They are predominantly composed of ECM with specific composition and properties for each tissue. Bone tissue, for example, is hard due to the mineralization of the matrix with calcium. In contrast, cartilage’s matrix is rich in collagen and chondroitin sulfate (a rubbery complex of protein and carbohydrate), which makes it a flexible support tissue and very strong at the same time.

The ECM of blood is liquid, called plasma, formed by water, salts, and dissolved proteins. As a liquid matrix, it allows the fast movement of blood cells (red and white) and other components throughout the body.

Extracellular matrix components

As mentioned, animals’ ECM are composed mostly of water, proteins, and polysaccharides. Variation in the way these molecules are organized and their relative amount gives a tissue its specific texture (going from liquid and gel-like to solid), form and function. In the following, we describe the main components found in ECMs.

A glycoprotein is a protein with one or more oligosaccharides (short chains of covalently linked sugars) chains attached.

A proteoglycan is a molecule composed of one or more glycosaminoglycan (GAGs) chains attached to a central protein.

A glycosaminoglycan is a long, linear polysaccharide formed by a repeating pair of sugars (for example, hyaluronic acid, chondroitin sulfate, and heparin). They are mainly found attached to a protein to form a proteoglycan.

The terms glycoprotein and proteoglycans can be confusing, especially because both are proteins with saccharides (sugars) attached. The differences are that proteoglycans are mainly carbohydrates, at least one of the sugar chains must be a GAG, and they can be huge molecules. On the other hand, glycoproteins are smaller, with less abundant carbohydrates in the form of shorter, branched chains. Technically, proteoglycans are a group of glycoproteins.

Collagen

The most abundant component of animal cells’ ECM is the glycoprotein collagen. Glycoproteins are proteins that have carbohydrates attached to them. After exiting the cell, collagen molecules form long fibers called collagen fibrils. Collagen is so abundant that it comprises about 30% of the proteins in animals, and as such, there are many types of collagens. Within a tissue, collagen fibers are a mix of different types, which, depending on the needs of the tissue, one type of collagen typically predominates over others. The collagen fibrils organize differently depending on the tissue.

In human skin, and mammals in general, collagen fibrils form a wickerwork pattern as the skin needs to resist pressure from multiple directions. A parallel pattern, on the other hand, like in tendons, enables the tissue to resist tension in one major axis or direction.

Elastin

The glycoprotein elastin is also common in ECMs and associates with collagen. Elastin forms elastic fibers that can extend, giving flexibility to tissues subjected to repeated stretch. Several tissues that have high amounts of elastin include vasculature (circulatory system) and lung tissues.

Elastin is present in tissues that need to be both strong and elastic (skin, blood vessels, lungs). It is the dominant protein in arteries.

Proteoglycans

Collagen fibers are embedded in a mesh made of proteoglycans complexes. The complexes comprise a long polysaccharide (carbohydrate) chain with hundreds of proteoglycan molecules. The GAG chains in the proteoglycans complexes are able to absorb high quantities of water and gives the gel-like consistency of some connective tissues. It allows the matrix to resist compressive forces

Fibronectins and Integrins

The ECM is connected to the outside of the cell membrane by other types of receptor glycoproteins, like fibronectins. The fibronectins bind to proteins called integrins that are embedded into the plasma membrane. The integrins span the whole plasma membrane width, and their cytoplasmic side binds with proteins attached to microfilaments (elements of the cell cytoskeleton). Therefore, these glycoproteins enable cell adhesion to the ECM.

The importance of fibronectin is highlighted by the fact that mutant mice that cannot produce this glycoprotein end up dying as embryos because endothelial cells do not form blood vessels appropriately.

Summary of extracellular matrix components

The following table summarizes the main components and their functions in an extracellular matrix.

Table 1: summary of the main components of animals' extracellular matrices (ECM) and their main functions.

ECM component

Function

Examples of tissues where it is found

Proteoglycans

They fill most of the extracellular space and gives the hydrogel consistency of tissues, which allows the matrix to resist compressive forces. It may help to regulate the traffic of cells and molecules through the ECM.

Found in all tissues, the amount of water they absorb depends on the function (for example, resistance to compressive forces is important in cartilage).

Collagen (glycoprotein)

Gives mechanical support to tissues and resistance to tensile/stretching forces. The most abundant component of animals’ ECM.

Found in all tissues, and it is the main components in bone and skin.

Elastin (glycoprotein)

Gives flexibility to tissues. Also abundant in ECMs, usually associated with collagen.

Found in many tissues like skin, vasculature (circulatory system), and lung tissues.

Fibronectin (glycoprotein)

Attachment to cells (they attach to integrins in the plasma membrane), cell migration.

Found in all tissues.

Integrin (glycoprotein)

Cell attachment to the ECM, cell communication, signal transmission.

Found in all tissues.

Extracellular matrix diagram

The following diagram shows a typical ECM with its main components.

Extracellular Matrix Diagram of an animal ECM StudySmarterFigure 1. Diagram of an animal extracellular matrix and its main components. Source: CNX OpenStax, CC BY 4.0 , via Wikimedia Commons

Extracellular matrix function

The ECM provides mechanical support to the cells in a tissue and regulates intercellular adhesion and communication. Recent research shows that the ECM is highly dynamic and of vital importance. It determines and controls essential behaviors and characteristics of cells such as proliferation, adhesion, migration, polarity, differentiation, and apoptosis.

Mechanical and structural support

The strong collagen fibers of the ECM mainly give mechanical support throughout tissues. Additionally, the carbohydrate chains that form a proteoglycan molecule are very good for absorbing water, and the amount of water differs for particular tissues and can give the matrix a hydrated gel consistency. Therefore, the ECM also serves for resisting compressive forces due to its gel consistency.

When you walk around, or run and jump, your joints have to withstand significant compressive forces. They can do that because of cartilage, a type of connective tissue whose ECM has a large amount of water. Thus, the resistance of ECM to compressive forces (shock absorbance) is significant in some tissues like cartilage.

Because of its structure, the ECM can work as a physical barrier, an anchorage site, or a movement track for cell migration. The glycoprotein fibronectin functions in cell attachment and migration. Cell migration (movement of cells inside the body or changes in their position) is essential in processes like wound healing and the growth of a fetus.

Regulation of cell behavior and communication

Integrins are called that way because they integrate or connect the outside and inside of the cell. Their extracellular side binds with the fibronectins, and their cytoplasmic side with the cytoskeleton; they serve functions in cellular communication and signal transmission. Matrix components can activate integrins and change their conformation, which is important, because protein function is directly related to their conformation.

The activated integrin then initiates signaling pathways that affect cell proliferation, differentiation, polarity, contractility, and gene expression. On the other hand, intracellular signals can also activate integrins and trigger signaling to the outside.

Cells can sense the biochemical properties of the ECM, including growth factors and bioactive molecules, and interact accordingly with the outside environment. However, cells can also sense the physical properties of the matrix, like rigidity, density, porosity, and insolubility.

For example, stem cells surrounded by a softer matrix usually follow neurogenesis, and when surrounded by a stiffer matrix, they follow osteogenesis, which is the process in which bone cells are generated.

The impact of ECM on stem cell differentiation, tissue repair, and tissue regeneration has been a prolific research topic in recent years. Culturing cells in vitro requires using a mesh that simulates an ECM. Researchers have found that using a matrix homologous or more similar to the specific matrix composition corresponding to the target tissue inside the body can sometimes improve tissue regeneration outcomes compared to using non-homologous matrices.

For example, a liver matrix provides higher adhesion and survival for cultured hepatocytes (liver cells) than non-liver material. Thus, adequately replicating the properties of specific ECMs may enhance regenerative medicine results. Besides, ECM research has implications for embryonic development, wound healing, cancer, diabetes, and others.

Extracellular Matrix - Key takeaways

  • The diverse functions of the extracellular matrix (ECM) depend on the type and proportions of its main components: water, proteins, and carbohydrates.
  • The most abundant component of the ECM is the glycoprotein collagen which is also the most abundant protein found in animals. The function of collagen fibers in an ECM is mainly to provide support to the cells in a tissue.
  • Elastin fibers associate with collagen to give flexibility to tissues that stretch, and the carbohydrate part of proteoglycans can give the ECM a hydrated gel consistency by absorbing water.
  • Integrins connect the outside and inside of the cell, serving in cellular communication and signal transmission when activated by components of the ECM.
  • Both biochemical and physical properties of the ECM are sensed by the cell and are essential in the determination and control of cell characteristics and behavior.

References

  1. Beatrice Yue, Biology of the Extracellular Matrix: An Overview, J Glaucoma, 2014. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4185430/
  2. Turney McKee et al., Extracellular matrix composition of connective tissues: a systematic review and meta-analysis, Scientific Reports, 2019. https://www.nature.com/articles/s41598-019-46896-0
  3. 3. Bruce Alberts et al. Molecular Biology of the Cell. 4th Ed. Garland Science. 2002.

Frequently Asked Questions about Extracellular Matrix

Elastic fibers made up of elastin protein give flexibility to the extracellular matrix when associated with the collagen fibers in the matrix.

The extracellular matrix is composed of different proportions of water, proteins, and carbohydrates. A connective tissue (cartilage, blood, adipose, osseous tissue) is mostly made up of extracellular matrix with some cells suspended within it.

The extracellular matrix is located throughout an animal body, around cells, within tissues, and surrounding organs. 

The extracellular matrix functions in cell support within a tissue, cell adhesion and communication, cell migration, and regulation of cell characteristics and behavior.

The proteins that compose the extracellular matrix are synthesized inside cells as secretory proteins, therefore they are specifically synthesized by bound ribosomes in the rough endoplasmic reticulum.

Final Extracellular Matrix Quiz

Question

The extracellular matrix only serves as mechanical and structural support to cells within a tissue

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False

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Question

The main components of the ECM are:

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Answer

proteins

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Question

The ECM has the same proportion of components throughout the body, regardless of the tissue type


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Answer

False

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Question

Is a plant cell wall an extracellular matrix?

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Answer

Cell walls in plants can be considered a type of extracellular matrix as it is formed by materials secreted by the cell and secreted outside of the membrane.

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Question

The following components can be found in ECMs:


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Answer

collagen

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Collagen fibers are flexible, giving elasticity to tissues

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False

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Which of the following is a membrane component that is activated by ECM components and can attach to the matrix?


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Answer

integrin

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Question

Which of the following associates with collagen to give flexibility to tissues?


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Answer

Integrin

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Which of the following functions in cell attachment and migration?

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Integrin

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Which of the following can absorb water giving the matrix a hydrated gel consistency?


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Integrin

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Which of the following forms part of the connection between the outside and inside of the cell?


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Integrin

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Question

Integrins are involved in the regulation of cell behavior because:

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Answer

They can be activated by matrix components

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Question

What kind of ECM properties represent cues that a cell can sense?

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Answer

Cells can sense ECM biochemical properties (growth factors and bioactive molecules) and physical properties (like rigidity, density, porosity, and insolubility)

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What similarities do an animal ECM have with a plant cell wall?

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They are both composed of a mix of proteins and carbohydrates, and they can give structural and mechanical support to cells within a tissue.

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Question

Why would the ECM be important in cultivation of cells for regenerative medicine?

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Answer

Because the ECM is tissue-specific and its biochemical and physical properties provide cues for cell development, characteristics, and behavior. 

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