Extracellular Matrix (ECM)
In biology, the extracellular matrix (ECM) is a collection of extracellular molecules secreted by cells that provides structural and biochemical support to the surrounding cells. Because multicellularity evolved independently in different multicellular lineages, the composition of ECM varies between multicellular structures; however, cell adhesion, cell-to-cell communication and differentiation are common functions of the ECM.
- The extracellular matrix of animal cells like the picture above is made up of proteins and carbohydrates.
- Cell communication within tissue and tissue formation are main functions of the extracellular matrix of animal cells.
- Tissue communication is kick-started when a molecule within the matrix binds a receptor; the end results are conformational changes that induce chemical signals that ultimately change activities within the cell.
Source: Boundless. “Extracellular Matrix of Animal Cells.” Boundless Biology. Boundless, 21 Jul. 2015. Retrieved 17 Sep. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/cell-structure-4/connections-between-cells-and-cellular-activities-63/extracellular-matrix-of-animal-cells-324-11461/
The plant ECM includes cell wall components, like cellulose, in addition to more complex signaling molecules. Some single-celled organisms adopt multicellular biofilms in which the cells are embedded in an ECM composed primarily of extracellular polymeric substances.
Terms
Proteoglycans- GAGs are carbohydrate polymers and are usually attached to extracellular matrix proteins to form proteoglycans (hyaluronic acid is a notable exception, see below). Proteoglycans have a net negative charge that attracts positively charged sodium ions (Na+), which attracts water molecules via osmosis, keeping the ECM and resident cells hydrated. Proteoglycans may also help to trap and store growth factors within the ECM.
Described below are the different types of proteoglycan found within the extracellular matrix.
Heparan sulfate- Heparan sulfate (HS) is a linear polysaccharide found in all animal tissues. It occurs as a proteoglycan (PG) in which two or three HS chains are attached in close proximity to cell surface or ECM proteins. It is in this form that HS binds to a variety of protein ligands and regulates a wide variety of biological activities, including developmental processes,angiogenesis, blood coagulation, and tumour metastasis.
In the extracellular matrix, especially basement membranes, the multi-domain proteins perlecan, agrin, and collagen XVIII are the main proteins to which heparan sulfate is attached.
Chondroitin sulfate, Chondroitin sulfates contribute to the tensile strength of cartilage, tendons, ligaments, and walls of the aorta. They have also been known to affect neuroplasticity.
Keratan sulfate- Keratan sulfates have a variable sulfate content and, unlike many other GAGs, do not contain uronic acid. They are present in the cornea, cartilage, bones, and the horns of animals.
Non-proteoglycan polysaccharide Hyaluronic acid Hyaluronic acid (or "hyaluronan") is a polysaccharide consisting of alternating residues of D-glucuronic acid and N-acetylglucosamine, and unlike other GAGs, is not found as a proteoglycan. Hyaluronic acid in the extracellular space confers upon tissues the ability to resist compression by providing a counteracting turgor (swelling) force by absorbing significant amounts of water. Hyaluronic acid is thus found in abundance in the ECM of load-bearing joints. It is also a chief component of the interstitial gel. Hyaluronic acid is found on the inner surface of the cell membrane and is translocated out of the cell during biosynthesis.
Hyaluronic acid- acts as an environmental cue that regulates cell behavior during embryonic development, healing processes, inflammation, and tumor development. It interacts with a specific transmembrane receptor,
Fibers Collagen-Collagens are the most abundant protein in the ECM. In fact, collagen is the most abundant protein in the human body and accounts for 90% of bone matrix protein content. Collagens are present in the ECM as fibrillar proteins and give structural support to resident cells. Collagen is exocytosed in precursor form (procollagen), which is then cleaved by procollagen proteases to allow extracellular assembly. Disorders such as Ehlers Danlos Syndrome, osteogenesis imperfecta, and epidermolysis bullosa are linked with genetic defects in collagen-encoding genes. The collagen can be divided into several families according to the types of structure they form:
- Fibrillar (Type I, II, III, V, XI)
- Facit (Type IX, XII, XIV)
- Short chain (Type VIII, X)
- Basement membrane (Type IV)
- Other (Type VI, VII, XIII)