Supporting/connective tissues

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Supporting/connective tissues



Introduction


Supporting/connective tissue is the term applied to tissues which provide general structure, mechanical strength, space filling (sculpting body shape), and physical and metabolic support for more specialised tissues.


Connective tissues usually have three structural properties with corresponding construction materials:



The combined mix of fibres and ground substance is called extracellular matrix and this determines the physical properties of the tissue. Matrix is produced and assembled under the control of support cells, most commonly fibroblasts. The cells of supporting tissue are derived from precursor cells in primitive (fetal) supporting tissue called mesenchyme.


Supporting tissues occur with diverse physical properties. In most organs, loose connective tissue (also known as areolar tissue) acts as a biological packing and wrapping material. Tissue with a greater density of fibres provides a structural framework. Dense forms of supporting tissue provide tough physical support in the dermis of the skin, comprise the robust capsules of organs such as the liver and spleen, and the specialised high–tensile strength ligaments and tendons. Cartilage and bone, both major skeletal components, are specialised forms of connective tissue that are considered separately in Ch. 10.


Specialised fat storage is a further function, with adipose tissues having important metabolic roles. White adipose tissue also provides a structural fill and forms part of shock-absorbing padding. Highly metabolically active brown adipose tissue helps in the regulation of body temperature and body weight.


In addition, supporting tissues usually contain blood vessels, lymphatic vessels and associated nerves. Repair of tissue damage, especially wound closure and scar formation, is also largely a function of supporting tissues, involving both the support cells and blood and lymphatic vessels.





Fibres of Connective Tissue


The fibrous components of connective tissues are of two main types: collagen (including reticulin, which was formerly considered a separate fibre type) and elastin.



Collagen


Collagen is the main fibre type found in most supporting tissues and is the most abundant protein in the human body. Its notable function is the provision of tensile strength to resist pulling, stretching and tearing.


Collagen is secreted into the extracellular matrix by connective tissue cells (e.g. fibroblasts) in the form of a tropocollagen monomer. This consists of three polypeptide chains (each called an alpha chain and not necessarily all identical), bound together to form a helical protein structure 300 nm long and 1.5 nm in diameter. In the extracellular matrix, these tropocollagen molecules polymerise longitudinally and also side-to-side, forming collagen fibrils which are cross-linked by the enzyme lysyl oxidase.


At least 28 different types of collagen (designated by Roman numerals I to XXVIII) have now been delineated in the collagen super-family on the basis of morphology, amino acid composition and physical properties. Collagens can be fibre forming, mesh/network forming or cell membrane–associated proteins



• Type I collagen is the main structural collagen and is found in fibrous supporting tissue, skin (dermis), tendons, ligaments and bone. The tropocollagen molecules polymerise longitudinally and also side-to-side to form fibrils, and these are strengthened by numerous intermolecular bonds. Parallel collagen fibrils are further arranged into strong fibre bundles 2 to 10 µm in diameter, which confer great tensile strength to the tissue. These collagen fibres are visible with the light microscope, staining pink with H&E, with fibres in varying patterns of orientation, size and density according to the mechanical support required in the tissue.


• Type II collagen is the main structural collagen of hyaline cartilage and consists of fibrils in the cartilage ground substance.


• Type III collagen forms the delicate branched ‘reticular’ supporting meshwork which is prominent in highly cellular tissues such as the liver, bone marrow and lymphoid organs. This fibre was initially recognised by its affinity for silver salts and was (and often still is) called reticulin.


• Type IV collagen is a network/mesh-forming collagen and is an important constituent of basement membranes.


• Type VII collagen forms special anchoring fibrils that link extracellular matrix to basement membranes.


The remaining collagen types are present in various specialised situations.







Elastin


Elastin is arranged as fibres and/or discontinuous sheets in the extracellular matrix where it confers the properties of stretching and elastic recoil. Elastin is a protein synthesised by fibroblasts in the form of a precursor monomer known as tropoelastin. The monomers are polymerised in the extracellular matrix by the enzyme lysyl oxidase, with extensive cross-linking of lysine amino acid side chains. Deposition of elastin in the form of fibres requires the presence of a template of microfibrils of the structural glycoprotein fibrillin and associated glycoproteins. These become incorporated around and within the ultimate elastic fibre.


Elastin is the name of both the fibre and the polymerised protein. There are also two named related fibres, oxytalan and elaunin, which have more fibrillin and less polymerised tropoelastin than generic elastin.


Elastin is found in varying proportions in most supporting tissues, conferring elasticity to enable recovery of tissue shape following normal physiological deformation. Elastin is present in large amounts in tissues such as lung, skin and urinary bladder. It is an important constituent of the wall of blood vessels; in arteries, elastin provides the stretch and recoil to smooth and transmit the pulse pressure generated by each heartbeat. In the lung, the stretch and recoil of the elastin is basic to that organ’s function.


Elastic fibres are eosinophilic and when large they are slightly refractile, meaning they bend light differently to other tissue components. This may enable their recognition; however, special elastin stains are usually needed.









Ground Substance


Ground substance derived its name from being an amorphous transparent material with the physical character of semi-solid gel. It is a mixture of glycoproteins and complex carbohydrates with profound water-binding ability. Extracellular fluid, both water and salts (particularly sodium), are bound to these molecules, providing volume and compression resistance to the tissue and its tissue turgor (i.e. the internal pressure). They form the physical milieu and indirectly control the passage of both molecules and cells through the tissue and the exchange of metabolites with the circulatory system.


The carbohydrates are long, unbranched polysaccharide chains of seven different types, each composed of repeating units of two sugar derivatives, usually a uronic acid and an amino sugar such as N-acetyl glucosamine. This gives rise to the term glycosaminoglycan (GAG).


Hyaluronate, also known as hyaluronic acid, consists of repeating D-glucuronate (β1,3)-N-acetyl-D-glucosamine units and is the predominant GAG, forming huge unbranching linear molecules of 100,000 to 10,000,000 molecular weight.


The other GAGs include chondroitin-4-sulphate, chondroitin-6-sulphate, dermatan sulphate, keratan sulphate, heparan sulphate and heparin sulphate. Each of these molecules contains sulphated N-acetyl groups substituted onto galactosamine sugars in the repeating carbohydrate units, making them highly negatively charged (acidic). These charged groups prevent the carbohydrate chains from folding into globular aggregates, causing them to remain in an expanded linear form, thereby occupying a large volume for a small mass. The charged side groups also render them extremely hydrophilic, attracting a large volume of water and positive ions, particularly sodium.


These GAGs (other than hyaluronate) exist as carbohydrate chains covalently linked to various protein molecules, forming a range of molecular structures containing up to 90% to 95% carbohydrate. These are called proteoglycans. There are numerous specific proteins, including perlecan, syndecan, decorin, lumican and aggrecan. Proteoglycans have various specific functions. Some bind to hyaluronic acid producing massive quaternary structures, others interact with collagens or bind to various other matrix molecules including remodelling enzymes, enzyme inhibitors, growth factors, cytokines and cell surface receptors.


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Aug 22, 2016 | Posted by in HISTOLOGY | Comments Off on Supporting/connective tissues

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