The cell. 2.Extracellular matrix
EXTRACELLULAR MATRIX EXAMPLES
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Different molecular compositions make different extracellular matrices
Plant cell wall is mainly composed of carbohydrates, most of them as cellulose.
Basal lamina is a 3D net made up of collagen and laminin.
Cartilage contains a large amount of glycosaminoglycans to counteract mechanical loads.
Bone contains type I collagen, which provides elasticity, whereas stiffness relies on calcium and phosphate crystals.
Many tissue functions rely on the properties of the extracellular matrix, which show distinct molecular compositions. This is true for both plant and animal tissues.
The extracellular matrix of plant tissues is known as cell wall. It is quite different from any animal extracellular matrix. Cell wall provides structural support to plant cells, and therefore to the entire plant body, and also determines the shape and size of the cells. Cell wall replaces some functions of cytoskeleton, particularly intermediate filaments, which is much less developed in plant cells than in the animal cells. The supporting role of cell wall in the plant body continues even when cells are dead, for example wood is mostly cell walls. The most prominent component of cell wall is cellulose, which is the more abundant molecule in nature. Depending on the plant tissue and differentiation state of the cell, cell wall may show different molecular composition.
Layers of cell wall showing secondary growth.
Layers. The thickness of the cell wall depends on the type and age of the cell. Up to 3 layers can be found: middle lamella, primary cell wall, and secondary cell wall. Cells synthesize these layers starting with middle lamella, then primary cell wall, and finally secondary wall, that is not always present. The most recent layer is the nearest to the plasma membrane. All plant cells have middle lamella and primary cell wall, with variable thickness, but only some cells develop secondary cell wall. Primary cell wall allows cell to grow in size because it can be stretched by hydrostatic pressure, which is called turgency. Secondary cell wall is synthesized by cells that withstand heavy mechanical loads and by some vascular cells. Secondary cell wall may be divided in sublayers. The synthesis of secondary cell wall usually means that cell is not going to grow any more, because cell elongation is not possible. Secondary wall is no permeable to water but there are many pores that allow water to cross the secondary cell wall of adjoining cells.
Components. Cellulose is the main component of the cell wall. It is a polysaccharide made up of D-glucose, linked together with β (1-4) bonds. Long chains of D-glucose are joined to form cellulose microfibrils, with a variable diameter, which in turn are associated together to form cellulose fibers. These fibers are visible with a light microscope and are very resistant to mechanical loads, similar to steel. Cellulose fibers also contain other saccharides like hemicellulose and pectins, as well as glycoproteins. Hemicellulose is a family of polysaccharides with differente members depending of the cell type, being xyloglucans the most abundant ones. Pectins are also a diverse type of polysaccharides that can be found in primary cell wall, but are absent from secondary cell wall. Hemicellulose and pectin are highly hydrophilic molecules. Calose is a polysaccharide located between cell membrane and cellulose fibers. It is synthesized and released in response to wounds and pathogens, and stops the communication between adjoining cells. Lignin is a polyphenol that prevents water difusion through the secondary cell wall. In the cell wall of epidermis there are cutin and suberin, which are lipid deposits that prevent water evaporation from underlying tissues.
Basal lamina is a thin layer of extracellular matrix located at the basal part of epithelia, and around muscle cells and peripheral nerve cells. It works as a physical support and as a selectively permeable barrier. For example, in kidney glomeruli it is a key player in filtering the blood. Basal lamina is composed of molecules arranged in an ordered 3D net, such as type IV collagen, laminin, perlecan proteoglycan, nidogen protein, and some more. Basal lamina is linked to the membrane of the basal domain of epithelial cells by connections between integrins and laminins.
Extracellular matrix of loose and dense connective proper tissues.
Loose connective proper extracellular matrix shows low protein density, with a high proportion of hyaluronan and proteoglycans, and low amount of collagen and elastic fibers. Its main role is to fill and hydrate intercellular spaces and provide the environment through which many cells travel from one site to another. However, in the dense connective proper tissue there are many collagen molecules, either arranged parallel to the tensile forces, such as in tendons, or disorganized as can be found in the dermis or in the intestine. In this type of extracellular matrix, elastic fibers may be abundant, as in the artery wall.
Cartilage extracellular matrix is synthesized by chondrocytes, and is responsible for both stiffness and elasticity of cartilage. It is mainly composed of type II collagen, up to 25 % of dry weight, but type IX and XI collagen are also present in lower proportion. The second most abundant molecules are glycosaminoglycans. such as hyaluronan and aggrecan, which join together to form macromolecular complexes. Collagen counteracts tensile loads and glycosaminoglycans dampen mechanical pressures. Elastic cartilage contains abundant elastic fibers, which are necessary for the elasticity of structures such as, pharynx, epiglottis, and pinna.
Bone extracellular matrix contains type I collagen intermingled with a matrix of calcium phosphate crystal (up to 70 % of the dry weight). Collagen allows bone to be elastic enought to avoid fragility and not to be easily broken, and calcium phosphate crystal provides stiffness and hardness. There are also proteoglycans and glycoproteins, which are less abundant but very important for the organization of collagen fibers, mineralization and resorption of bone.
Blood serum as a type of extracellular matrix is no accepted by everyone, but here it is considered as a highly specialized extracellular matrix, containing up to 90 % of water. It is the environment of blood cells. The most abundant protein of blood serum is albumin, which keep a physiological osmotic pressure between blood vessels and tissues, for example it prevents edemas. Other proteins are γ-immunoglobulins, which are immune system antibodies, α- and β-globulins, which are carrier of elements such as cupper and iron. Fibronectins can move between tissues and blood serum. Fibrinogen is essential for blood coagulation. The rest of blood serum molecules show low molecular weight and they can be found in the extracellular matriz of other tissues, since they can freely cross endothelium.
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Updated: 2016-03-23. 16:22
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo