Extracellular matrix

Extracellular matrix (MEC). It is the set of extracellular materials that are part of a tissue . The MEC is a physiological integration medium, of a complex biochemical nature, in which the cells are “immersed” . Thus the MEC is the substance of the interstitial (intercellular) medium.

It is a network of molecules , proteins and carbohydrates that are arranged in the intercellular space and that is synthesized and secreted by the cells themselves.

Summary

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• 1 Functions
• 2 General
• 3 Main macromolecules that make up the extracellular matrix
  • 1 Collagen
  • 2 Elastin
  • 3 Glycosaminoglycans
  • 4 Proteoglycans
  • 5 Glycoproteins.
• 4 Binding complex
• 5 types
• 6 Sources
• 7 External links

Features

• It provides mechanical properties to tissues (both in animals and plants ).
• It maintains the cellular form.
• It allows the adhesion of cells to form tissues.
• It is used for intercellular communication.
• It forms pathways through which cells move.
• Modulates cell differentiation and cell physiology .
• It hijacks growth factors.

General

The amount, composition and disposition of the extracellular matrix depends on the type of tissue considered. There are some like the epithelial and the nervous that lack or have very little extracellular matrix, while in others like the connective tissue it is the most important element in volume.

In plant tissues, the cell wall can be considered, although there is not always agreement, as a specialized extracellular matrix with characteristics very different from that of animal tissues. Its role is crucial to stiffen the cells and by extension to the plant , it is a barrier to permeability, it protects against pathogenic or mechanical aggressions, among other functions.

Main macromolecules that make up the extracellular matrix

All of them are found in an aqueous medium together with other smaller molecules , in addition to the ions . It is the quantity, proportion, and type of each of these macromolecules that distinguishes some extracellular matrices from others.

Collagen

Collagen is a family of proteins that is very abundant in the body of animals . The molecules of collagen can represent from 25 to 30% of all body protein.

Its main mission in tissues is to form a framework that supports tissues and resists mechanical tensile forces. The organization of collagen molecules in three-dimensional macromolecular structures is varied. They can form parallel fibers to resist unidirectional stresses, as occurs in tendons and ligaments, or mesh-oriented fibers to withstand stresses that can come from all directions, such as occurs in bone , cartilage, and connective tissue.

Cells are “attached” to collagen molecules by various adhesion proteins such as integrins, immunoglobulins, annexins, etc. Collagen molecules are characterized by: a) A rare composition of amino acids . Collagen molecules abound with the amino acid glycine , which is very common, and other less common ones such as proline and hydroxyproline . b) They can organize themselves forming fibers, meshes or specialize in forming bonds between molecules. All this depends on the chemical composition of its α subunits and the types of subunits that form it.

Elastin

Elastin molecules are linked to one another by links between regions rich in the amino acid lysine . It is an abundant protein in many extracellular matrices and appears as a component of the so-called elastic fibers , which are insoluble protein aggregates.

Elastic fibers have the ability to stretch in response to mechanical stresses and to contract to regain their initial length at rest. The elasticity of the tissues depends on the elastic fibers. They are found mainly in the dermis , on the walls of the arteries , in the elastic cartilage and in the connective tissue of the lungs . In addition to elastin, which represents 90%, elastic fibers are formed by so-called fibrillin microfibrils and by other glycoproteins and proteoglycans in a smaller proportion.

Other functions of elastic fibers are to provide support to tissues or to regulate the activity of fibrillin-mediated growth factors TGF-β.

Elastin has a long chain of amino acids in which there are numerous sequences with hydrophobic amino acids , separated by other sequences that contain pairs of glycine and other small amino acids such as lysine. It is this composition of amino acids that confers the elastic properties, since the hydrophobic amino acids allow the arrangement in coiled structures and the lysine the formation of α-helices, which are the points where two neighboring elastin molecules are linked. Elastin appears to be a vertebrate invention since it has not been found in invertebrates .

Glycosaminoglycans

They are unbranched polymers of sugars that can form very long chains. They are made up of repeats of monosaccharide pairs where one of the sugars has an amino group (N-acetylgalactosamine or N-acetylglucosamine) and the other is normally galactose or glucuronic acid.. These sugars have carboxyl groups (COO-) and sulfate groups (SO3-), whose negative charges allow a strong and abundant association with water molecules, providing great hydration to the extracellular matrix. Glucosaminoclucans are inflexible molecules, so they occupy a large volume and, thanks to their strong hydration, make the extracellular matrix behave like a gel. This allows tissues that have a high proportion of glycosaminoglycans to resist strong mechanical pressures and also favors a high rate of diffusion of substances between cells. The most common types of glulcosaminoglycans are hyaluronic acid and sulfated glycosaminoglycans: chondroitin sulfate , dermatan sulfate , kerathan sulfate and heparan sulfate .

Proteoglycans

A proteoglycan is a molecule made up of the covalent link between an amino acid chain and one or more sulfated glycosaminoglycans.

It is a ubiquitous family of molecules. Proteoglycans are synthesized inside the cell. The protein part is synthesized in the endoplasmic reticulum , where the addition of carbohydrates also begins . However, elongation of glycosaminoglycan repeat chains and sulfation occurs on the trans side of the Golgi apparatus .

Most proteoglycans are exocyted into the intercellular space, but some will form part of the plasma membrane thanks to the fact that their protein part contains hydrophobic amino acid sequences that are inserted between the fatty acid chains of the membrane. Other molecules such as type IX, XII and XVII collagen also contain glycosaminoglycan chains.

Proteoglycans differ mainly in sequence and in the length of the amino acid chain (from 100 to 4000 amino acids). They also differ in the number and type of glycosaminoglycan molecules that are bound together. For example, decorin has a single molecule while aggrecan contains more than 200.

The function of proteoglycans depends on their glycosaminoglycan molecules: hydration, resistance to mechanical pressures, lubricants, affect differentiation, mobility and cellular physiology, etc. Its mechanical action is essential in cartilage and joints . But they are also anchoring points of the cells to the extracellular matrix that surrounds them, either because of their direct action as they are integral molecules of the plasma membrane, because they form junctions with phospholipids of the membrane or because they are recognized by adhesion proteins present in the plasma membranes such as integrins.

Glycoproteins.

The different molecules that make up the extracellular matrix are linked together to form a cohesive framework. Similarly, the cells are attached to the molecules of the extracellular matrix. Most of these linkages are between proteins, but also between proteins and sugars, and are mainly mediated by glycoproteins. There are three types of junctions that give tissues cohesion: between molecules of the extracellular matrix, between cells and the extracellular matrix, and between neighboring cells.

Junction complex

Binding complexes are classified according to their shape, the adhesion molecules that compose them, the elements to which they bind, and their interactions with the cytoskeleton .

• The tight junctions or zonula occludensare found in the apical parts of the epithelia and in the cardiac muscle tissue . They establish such strong and tight junctions between neighboring cells that they leave virtually no intercellular space between their plasma membranes, limiting the diffusion of soluble extracellular substances. The tight junctions form a kind of belt that surrounds the entire cell perimeter, in the case of epithelial cells. In addition to holding cells tightly together, they perform other functions.
• The adherens junctions or zonula adherensare binding complexes formed in the epithelial cells and that are located close to the basal and tight junctions. Its mission is to unite neighboring cells. They are the first junction complexes that are formed during the development of the epithelia, they appear before the narrow junctions, so they seem to act in morphogenetic processes during embryonic development .
• The desmosomes or macula adherens, unlike the two previous junction complexes, establish point-shaped disc connections between neighboring cells, as if they were rivets. They are very abundant between epithelial and muscle cells, but also in other tissues such as the nervous. The junctions between cells are mediated by molecules of the cadherin type called desmogleins and desmocholines . The intracellular domain of these cadherins contacts intermediate filaments such as keratins , thanks to intermediate proteins.
• The Hemidesmosome and focal unionsestablish strong bonds between cells and the extracellular matrix. In both cases the unions are established by integrins. Hemidesmosomes link epithelial cells to the basal lamina thanks to the extracellular domain of the integrin, while the intracellular domain contacts the cytosolic intermediate filaments. Although hemidesmosomes look like desmosomes without one of their parts, molecularly they are different. Focal junctions unite cells with various types of extracellular matrices thanks to other types of integrins that in their intracellular domain contact actin filaments.

Types

• The cell wall: it contains many sugars , especially glucose forming cellulose .
• The basal lamina: contains collagen and laminin forming a mesh.
• The cartilage : contains a lot of glycosaminoglycans to withstand pressures and mechanical forces.
• The bone : it has type I collagen that provides elasticity and calcium precipitates that provides hardness.