Structure And Function Of Connective Tissue Pdf
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- 4.3 Connective Tissue Supports and Protects
- Biology of Bone Tissue: Structure, Function, and Factors That Influence Bone Cells
- 4.3 Connective Tissue Supports and Protects
4.3 Connective Tissue Supports and Protects
Collagens are a large family of triple helical proteins which are found extensively throughout the body and are necessary to perform various functions such as tissue scaffolding, cell adhesion, cell migration, angiogenesis, tissue morphogenesis, and tissue repair.
They are a group of fibrous proteins that occur in vertebrates as the chief constituent of connective tissue fibrils and in bones. Collagen is a complex molecule, the structure of which has been revised over the years.
Astbury and Bell stated that collagen is a single extended polypeptide chain which has amide bonds, while Pauling and Corey stated that it was made up of three polypeptide chains held by hydrogen bonds. This model was called the Madras model as it was first described in Madras. According to Ramachandran and Karta, each triplet consists of two hydrogen molecules.
The length of the stagger is quarter of the length of the molecule and this quarter stagger arrangement gives the 65 nm banding characteristic under the electron microscope. The structure of collagen also has non- collagenous domains which are important for the structural stability of the molecule. These domains are seen flanking the central part of the molecule and are called -C and -N terminals of the molecule.
The -C terminal is involved in initiation of the polypeptide chain formation while the -N terminal is associated with the regulation of the fibril diameter [ Figure 1 ]. The three polypeptide chains coiled together have a pitch of 0. Collagen is not synthesized from fibroblasts alone but by various other cells such as cementoblasts, odontoblasts, chondroblasts, osteoblasts, muscle cells, epithelial cells, endothelial cells and Schwann cells.
The formation of collagen starts in the nucleus where various exons of a gene are joined to form messenger RNAs mRNAs of different types of collagen. These chains move into the cisternae of the RER where hydroxylation of the proline and lysine residues occurs. This step occurs under the influence of Vitamin C and the enzyme prolyl hydroxylase and lysyl hydroxylase.
In addition, in the RER, galactosyltransferase brings about glycosylation of some of the hydroxylysine residues. The triple helix chains are joined by disulfide bonds in the presence of disulfide isomerase, which help to keep the chains properly aligned to each other, following which a molecule called procollagen is formed. The length of procollagen is many times longer than the final product collagen. Twisting of this large molecule occurs before it passes into the golgi complex.
Within the Golgi complex, a final glycosylation takes place by the addition of glucose at the O-linked galactose residues. This molecule then moves out of the trans face of the Golgi complex by being contained within secretory granules. Extracellularly, the -C terminal and part of -N terminal of this helical structure are cleaved by -C and -N proteinases, respectively, leading to the formation of tropocollagen — a 5-unit quarter stagger microfibril. The remaining part of the N-terminal is cleaved by procollagen peptidase.
In this way, the large collagen molecule is trimmed. The stabilizing of the collagen molecule occurs through the cross-linking of the molecule by oxidation of the lysine and hydroxylysine residues by lysyl oxidase [ Figure 2 ].
In normal tissues along with the synthesis of collagen, its degradation, followed by replacement with new fibers, is also necessary for constant remodeling of the connective tissue. The degradation is necessary in physiological processes such as development and tissue repair and pathological processes such as tumorigenesis and metastasis.
Collagen is cleaved in two locations, intracellular and extracellular. Intracellular degradation is the most important mechanism for remodeling of the connective tissue. It occurs in the following manner: a Recognition of the fibrils by binding to fibroblast integrin receptors, b partial digestion into smaller fibrils, c formation of phagolysosome, and d digestion of the fibrils by lysosomal enzymes [ Figure 2 ].
The inhibition of degradation through collagenases and other MMPs is brought about by tissue inhibitors of metalloproteinases TIMPs , which bind to the active site of these enzymes to inhibit them.
The various types of collagen are mentioned in Table 2 , along with their function and distribution in different orodental tissues. Based on these findings, they can be classified as fibril associated collagens with interrupted triple helices, microfibril forming, anchoring fibril, network forming, multiplexin, transmembrane and miscellaneous.
Hematoxylin and eosin is the routinely used stain in histopathology and is generally sufficient to make visible a number of components of a tissue under the light microscope.
However, in terms of evaluating collagen, sometimes, this stain may fall short in helping the clinician differentiate collagen fibers from other fibers such as keratin and muscle. Hence, in these instances, various special stains can be used which specifically stain collagen, as given in Table 3. These stains are differentiated based on color to aid in better understanding. Apart from light microscopy, polarizing microscopy is also used to identify and analyze collagen.
Electron microscopy provides information on the size, height and shape of the fibrils. Transmission microscopy helps to study the protein and peptide in collagen. The maxillofacial region is formed by a number of hard and soft tissues where the collagens form a principal component. These are basically bone, connective tissue, muscles, tendons, cartilage and oral mucosa. Among the dental tissues excluding enamel, the collagens are found in dentin, cementum, pulp and periodontal ligament PDL.
The combination of hard mineral and flexible collagen makes bone harder than cartilage but lacks brittleness. In tendon, ligaments, and cartilage, collagen is present in the form of elongated fibrils. Tendons are chiefly made up of Type I collagen. Collagen acts as a scaffold for the mineral components of dentin. The pulp is a loose connective tissue that is highly vascularized and innervated. They are present in the extracellular matrix ECM along with the ground substance. As age increases, the collagen content of the pulp increases, leading to fibrosis.
The amino acid analysis reveals that the collagen of dentin, alveolar bone, and cementum in human teeth is similar. The PDL has also the capacity to adapt to functional changes. When functional demand increases, the width of PDL increases and so does the fiber bundles thickness. PDL fibroblasts contract and transmit a contractile force to the extracellular environment; this permits summation of contractile forces.
They also exhibit fibronexuses by which such forces can be transmitted to the collagen fiber bundles. Although the in vitro observations conclude the myofibroblastic nature of the fibroblast and the existence of fibronexus, in vivo findings do not support the migratory nature or features of myofibroblast and the existence of fibronexus between fibroblast and fibers. Therefore, these cells would not be able to transmit a tractional force required to pull the tooth in eruption.
The surface of the oral cavity is bounded by mucous membrane or oral mucosa. The two main tissue components of the oral mucosa are stratified squamous epithelium, called the oral epithelium, and an underlying connective tissue layer called the lamina propria.
Lamina propria consists of ground substance, collagen fibers, and different cells. The interface between the connective tissue and the epithelium in light microscopy appears thick and it includes the reticular fibers. It consists of both lamina and the fibers. The lamina densa consists essentially of a network of polymers of Type IV collagen and laminins. The lamina lucida essentially contains proteins that attach the cell to the basal lamina, that is, the interacting portions of hemidesmosome- associated membrane proteins collagen XVII, integrins and laminin Anchoring fibrils, consisting of collagen Type VII, insert into the lamina densa and form a flexible attachment between the basal lamina and subjacent connective tissue.
With aging, Type III collagen synthesis reduces resulting in changes in skin tension, elasticity and healing. Apart from their presence in the normal tissues in the maxillofacial region, the collagens also play a key role in different physiologic mechanisms like wound healing. During the healing of wounds on skin or in the oral cavity, around the 3 rd day fibroblasts invade the tissue.
These cells originate either from undamaged fibroblasts at the periphery of the wound or from undifferentiated connective tissue cells and around the 5 th day collagen is formed which causes contraction of the wound and an increase in the tensile strength of the wound.
Authors have attributed this scarless healing to ECM content of fetal wounds. One of the ECM contents, which was considered as a contributing factor toward scarless healing, is the type of collagen. Fetal wounds have more of Type III collagen fibers, which help in keeping the wound less rigid and also allow better cell migration and regeneration.
In addition, the ECM of fetal wounds has fibroblasts that have a faster rate of collagen secretion, wherein as they proliferate they simultaneously secrete collagen. The most common wound healing occurring in the oral cavity is tooth extraction wound. Here, the wound heals primarily by the invasion of the clot tissue by osteogenic cells, leading to the formation of bone while the overlying epithelium heals like any other cutaneous wounds but without scar formation.
As collagen is an integral part of the oral cavity, both in its soft tissue and in its hard tissue, it is very important to know about its structure, function and distribution. Any aberration in its formation and its structure can alter its function which ultimately leads to various pathologies in the body and the oral cavity.
Skip to content. Share Tweet Share. Review Article. Collagen — structure, function and distribution in orodental tissues. Sonawane , Anuradha Sinha. J Global Oral Health ;2 2 Abstract Collagens are a large family of triple helical proteins which are found extensively throughout the body. They form the basic framework of the extracellular matrix providing support and form to cells and tissues.
They are important for various functions such as angiogenesis, morphogenesis, cell adhesion, repair, and regeneration. In this article, we have focused our discussion to the structure, the synthesis, and the degradation of collagen followed by its distribution and function in various oral tissues.
Show Related Articles from PubMed. Figure Triple helical structure of collagen. Export to PPT. Figure Synthesis and Degradation of Collagen. Synthesis: 1 Genetic transcription initiated by various growth factors and cytokines; 2 mRNA formed by joining of exons of a gene followed by splicing of introns of a gene; 3 polypeptide chain formation on ribosomes and hydroxylation of lysine and proline residues prolyl hydroxylase and lysyl hydroxylase , glycosylation galactosyltransferase of lysine residues and disulfide bond formation disulfide isomerase between the chains, leading to the formation of procollagen in the cisternae of the rough endoplasmic reticulum; 4 final glycosylation of the O-linked galactose residues; 5 moves extracellularly; 6 cleavage of the -C and part of -N terminals by proteinases; 7 5-unit quarter stagger fibril formed called tropocollagen; 8 cross-linking of the fibrils at lysine and hydroxylysine residues.
Degradation: 11 Intracellular degradation by ingestion of collagen fibrils, 12 extracellular degradation by secretion of matrix metalloproteinases by the cells such as fibroblasts and leukocytes. Table Enzymes for collagen degradation. MMP: Matrix metalloproteinase. Table Types of collagen. Forms core of Type I 2. Table Identification of collagen. Staining technique Color a.
Biology of Bone Tissue: Structure, Function, and Factors That Influence Bone Cells
Bone tissue is continuously remodeled through the concerted actions of bone cells, which include bone resorption by osteoclasts and bone formation by osteoblasts, whereas osteocytes act as mechanosensors and orchestrators of the bone remodeling process. This process is under the control of local e. An imbalance between bone resorption and formation can result in bone diseases including osteoporosis. Recently, it has been recognized that, during bone remodeling, there are an intricate communication among bone cells. For instance, the coupling from bone resorption to bone formation is achieved by interaction between osteoclasts and osteoblasts. Moreover, osteocytes produce factors that influence osteoblast and osteoclast activities, whereas osteocyte apoptosis is followed by osteoclastic bone resorption. The increasing knowledge about the structure and functions of bone cells contributed to a better understanding of bone biology.
Connective tissue fills the spaces between organs and tissues, and provides structural and metabolic support for other tissues and organs. Connective tissue is made up of cells and extracellular matrix. The extracellular matrix is made up of fibres in a protein and polysaccharide matrix, secreted and organised by cells in the extracellular matrix. Variations in the composition of the extracellular matrix, determines the properties of the connective tissue. For example, if the matrix is calcified, it can form bone or teeth.
4.3 Connective Tissue Supports and Protects
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NCBI Bookshelf. Trevor A. Nezwek ; Matthew Varacallo. Authors Trevor A. Nezwek 1 ; Matthew Varacallo 2.
Describe the structural characteristics of the various connective tissues and how these characteristics enable their functions. Connective tissues perform many functions in the body, most importantly, they support and connect other tissues: from the connective tissue sheath that surrounds a muscle, to the tendons that attach muscles to bones, and to the skeleton that supports the positions of the body. Protection is another major function of connective tissue, in the form of fibrous capsules and bones that protect delicate organs. Specialized cells in connective tissue defend the body from microorganisms that enter the body. Transport of gases, nutrients, waste, and chemical messengers is ensured by specialized fluid connective tissues, such as blood and lymph.
Collagens are a large family of triple helical proteins which are found extensively throughout the body and are necessary to perform various functions such as tissue scaffolding, cell adhesion, cell migration, angiogenesis, tissue morphogenesis, and tissue repair. They are a group of fibrous proteins that occur in vertebrates as the chief constituent of connective tissue fibrils and in bones. Collagen is a complex molecule, the structure of which has been revised over the years.