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Connective Tissue 3 Connective tissue 3 CHAPTER CONTENTS of connective tissues are determined by the composition of the Structural composition 29 matrix, and their classification is also largely based on its characteristics. Connective tissue cells . 29 Extracellular matrix . 29 Connective tissue cells Structures containing connective tissue 33 Trauma to soft connective tissue 40 Cells of general connective tissues can be separated into the Introduction . 40 resident cell population (mainly fibroblasts) and a population of migrant cells with various defensive functions (macrophages, Inflammation . 40 lymphocytes, mast cells, neutrophils and eosinophils), which Repair . 41 may change in number and moderate their activities according Remodelling . 41 to demand. Self-perpetuating inflammation . 41 Fibroblasts, the majority of cells in ordinary connective Effects of immobilization on healing . 42 tissue, arise from the relevant undifferentiated mesenchymal 1 Effects of mobilization on healing . 43 stem cells and are involved in the production of fibrous ele- ments and non-fibrous ground substance (Fig. 3.1). During Treatment of traumatic soft connective tissue wound repair they are particularly active and migrate along lesions 43 strands of fibrin by amoeboid movements to distribute them- Muscular lesions . 44 selves through the healing area to start repair. Fibroblast activ- Tendinous lesions . 45 ity is influenced by various factors such as the partial pressure Ligamentous lesions . 48 of oxygen, levels of steroid hormones, nutrition and the 2 Capsular lesions . 49 mechanical stress present in the tissue. The other cell types are migrant cells and only occasionally present, such as: macrophages, lymphocytes, mast cells, and granulocutes (Table 3.1).3,4 Structural composition Extracellular matrix (ECM) Together with muscle, nerve and epithelium, connective tissue is one of the basic components in the human body. It The extracellular matrix is composed of insoluble protein binds structures together, helps in mechanical and chemical fibres, the fibrillar matrix and a mixture of macromolecules, protection and also plays a principal role in reparative the interfibrillar matrix. The latter consists of adhesive glyco- processes. proteins and soluble complexes composed of carbohydrate Connective tissues are defined as those composed predomi- polymers linked to protein molecules (proteoglycans and nantly of the extracellular matrix and connective tissue cells. glycosaminoglycans), which bind water. The extracellular The matrix is made up of fibrous proteins and a relatively matrix distributes the mechanical stresses on tissues and also amorphous ground substance. Many of the special properties provides the structural environment of the cells embedded © Copyright 2013 Elsevier, Ltd. All rights reserved. General Principles hyaluronan glycosaminoglycans chain (GAGs) proteoglycans Fig 3.2 • The complex shape of proteoglycans. Polysaccharide molecules (GAGs) bound to a central protein core: proteoglycans are in turn bound to a long central chain of hyaluronan to form a proteoglycan aggregate. This arrangement is typical of cartilage. In other types of connective tissue hyaluronan chains may be absent. Redrawn from Walker PS6 with permission. in it, forming a framework to which they adhere and on which they can move.5 Non-fibrous ground substance The interfibrillar ground substance is composed of proteogly- cans (a family of macromolecules) which bind a high propor- tion of water (60–70%) and glycoproteins. The latter have a complex shape and are soluble polysaccharide molecules (gly- cosaminoglycans) bound to a central protein core. In cartilage, the proteoglycans are in turn bound to hyaluronan (a long chain Fig 3.1 • Electron micrograph of a fibroblast in human connective of non-sulphated disaccharides) to form a proteoglycan aggre- tissue, surrounded by bundles of finely banded collagen fibrils gate – a bottlebrush three-dimensional structure (Fig. 3.2).6 (shown at high magnification in the insert) which they secrete. From Glycoprotein secures the link between proteoglycan and Standring, Gray’s Anatomy, 40th edn. Churchill Livingstone, Edinburgh, 2008 with hyaluronan and also binds the components of ground substance permission. and cells. The three-dimensional structure of the proteoglycan aggre- gates and the amount of water bound gives ground substance its high viscosity. A semi-fluid viscous gel is formed within which fibres and fibroblasts are embedded, so facilitating Table 3.1 Connective tissue cell types normal sliding movements between connective tissue fibres. In structures subject to high compression forces (e.g. articular Connective Resident cells Fibroblasts tissue cells (adipocytes) cartilage), there is a large amount of proteoglycans but the (mesenchymal stem cells) content is relatively small in tissues such as tendons and liga- Migrant cells Lymphocytes ments exposed to tension forces. Mast cells Granulocytes Fibrous elements Macrophages The fibrous elements are collagen and elastin – both insoluble Extracellular Fibrillar matrix Collagen macromolecular proteins. Collagen is the main structural matrix Elastin protein of the body with an organization and type that varies Interfibrillar Proteoglycans from tissue to tissue. Collagen fibres are commonest in ordi- matrix Glucoproteins nary connective tissue such as fascia, ligament and tendon. The Water fibrillar forms have great tensile strength but are relatively inelastic and inextensible. By contrast elastin can be extended to 150% of its original length before it ruptures. Elastin fibres return a tissue to its relaxed state after stretch or other con- siderable deformation. They lose elasticity with age when they 30 Connective tissue C H A P T E R 3 1 Amino acids 2 Assembly of 3 Hydroxylation of 4 Assembly of three including glycine, polypeptide chain proline and lysine in hydroxylated polypeptide chains proline and lysine polypeptide chain into one procollagen molecule synthesis of 2 3 mucopolysaccharides in 1 Golgi apparatus and addition to protein 4 Ground substance Fibroblast 8 5 6 7 8 Aggregation of 7 Aggregation of 6 Passage of 5 Addtion of collagen fibrils to tropocollagen to procollagen to carbohydrate form collagen fibres form collagen fibrils extracellular space moiety and bundle of fibres Fig 3.3 • The successive steps in collagen synthesis by fibroblasts. Box 3.1 Components of connective tissue Cells • fibroblasts → fibrous connective tissue • chondrocytes → cartilage • osteoblasts and osteocytes → bone Extracellular matrix (ECM) Fig 3.4 • Crosslinking and interspaces between head and tail of neighbouring tropocollagen molecules. They overlap each other by • fibres: collagen → framework of the ECM a quarter of their length. Molecules in the same parallel row are • elastin → extensible element of ECM separated from each other by small interspaces. • proteoglycans: hydrators, stabilizers and space fillers of ECM • glycoproteins: stabilizers and linkers of ECM tend to calcify. Box 3.1 outlines the components of connective • fluid tissue. The basic molecule of collagen is procollagen, synthesized in the fibroblast, illustrated in Figure 3.3, steps 1–4. It is formed makes up a whole structure such as a ligament or a tendon. of three polypeptide chains (α-chains). Each chain is character- The individual bundles are in coils, which increases their struc- ized by repeating sequences of three amino acids – glycine, tural stability and resilience, and permits a small physiological proline and lysine joined together in a triple helix. The helical deformation before placing the tissue under stress, and in molecules are secreted into the extracellular space where they consequence permits a more supple transfer of tractive power slowly polymerize and crosslink (Fig. 3.4). They overlap each in the structure itself and at points of insertion (Fig. 3.5). The other by a quarter of their length, lie parallel in rows and are process of collagen synthesis is stimulated by some hormones collected into large insoluble fibrils. The fibrils unite to form (thyroxine, growth hormone and testosterone), although cor- fibres, finally making up a bundle. An aggregate of bundles ticosteroids reduce activity. 31 General Principles * * Fig 3.6 • Dense regular connective tissue in a tendon. Thick parallel (a) bundles of type 1 collagen (asterisks) give tendon its white colour in life. The elongated nuclei of inactive fibroblasts (tendon cells) are visible between collagen bundles. From Standring, Gray’s Anatomy, 40th edn. Churchill Livingstone, Edinburgh, 2008 with permission. Regular types Highly fibrous tissues such as ligaments, tendons, fascia and aponeuroses are predominantly collagenous and show a dense and regular orientation of the fibres with respect to each other. The direction of the fibres is related to the stress they experi- ence. Collagen bundles in ligaments and tendons are very strong and rupture usually takes place at the bony attachments rather than by tearing within their substance (Fig. 3.6). Irregular types (b) The irregular types consist of collagen and elastin interlacing in all directions. It is loose, extensible and elastic and found Fig 3.5 • (a) Unloaded collagen fibres in a human knee ligament. between muscles, blood vessels and nerves. It binds partly (b) Physiological deformation after stress. From Kennedy et al7 with together, although allowing a considerable amount of move- permission (http://jbjs.org/). ment to take place. In the sheaths of muscles and nerves and the adventitia
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