Connective Tissues: Matrix Composition and Its Relevance to Physical Therapy

The purposes of this update are to provide an overview of the composition, structure, and function of the connective tissue (CT) matrix and to illustrate how recent research has contributed to an improved understanding of the ways in which CT responds to mechanical forces. The overview is not exhaustive, but rather seeks to illustrate the complexity of these tissues, tissues once regarded as relatively simple structures within a mechanical system. Specific tissues and their special features, such as those of cartilage and bone, are not discussed in depth; instead, the overview emphasizes general principles that apply across the CT spectrum. Connective tissues and their matrix components make up a large proportion of the total body mass, are highly specialized, and have a diversity of roles. They provide for mechanical support, movement, tissue fluid transport, cell migration, wound healing, and—as is becoming increasingly evident—control of metabolic processes in other tissues.1,2 Unlike the properties of epithelial, muscle, or nerve tissues, which depend primarily on their cellular elements, the properties of CT are determined primarily by the amount, type, and arrangement of an abundant extracellular matrix (ECM). The ECM consists of 3 major types of macromolecules—fibers, proteoglycans (PGs), and glycoproteins—each of which is synthesized and maintained by cells specific to the tissue type (Fig. 1). Principal components of connective tissues. The 2 most important fibrous components of the ECM are collagen and elastin, both insoluble macromolecular proteins. Collagen has a variety of forms but is perhaps best exemplified by the prominent aligned fibers of tendons and ligaments. Other collagen fibers, which are far less prominent, include the small reticular fibers of soft organs such as the liver and the submicroscopic fibrils found in basement membranes. The striking feature of the most prominent collagens is their ability to resist tensile loads. Generally, they show minimal elongation (less than 10%) under tension; a proportion of this elongation is not the result of true elongation of individual fibers, but of the straightening of fibers that are packed in various 3-dimensional arrays.3,4 In contrast, elastic fibers may increase their length by 150%, yet still return to their previous configuration.3 The second major component of the ECM is the PGs, a diverse group of soluble macromolecules that have both structural and metabolic roles.5,6 They occupy, along with collagen, the interstitial spaces between the cells, form part of basement membranes, and attach to cell surfaces where they function as receptors.5,6 Important mechanical functions of PGs include hydration of the matrix, stabilization of collagen networks, and the ability to resist compressive forces, an ability best exhibited by the PGs of articular cartilage.5 Hyaluronan (HA), which is technically not a PG because it lacks a protein core, is particularly important because it readily entrains large amounts of water and is abundant in hydrated soft loose tissues where repeated movement is required (eg, tendon sheaths and bursae).7,8 The third group of matrix molecules, the glycoproteins, are ubiquitous in all CTs and, as with the PGs, have both structural and metabolic roles. Their mechanical roles include providing linkage between matrix components and between cells and matrix components. An important concept is that the mechanical properties of CT, such as the ability to resist tension, compression, extensibility, and torsion, are determined by the proportions of the matrix components. In turn, the maintenance of these matrix components and their organization depend on the nature and extent of loading these tissues experience. Generally, tissues with a high collagen-fiber content and low amounts of PG resist tensile forces, and those tissues with a high PG content, combined with a network of collagen fibers, withstand compression (Tab. 1). Trauma or pathology may affect normal movements and lead to changed mechanical stresses placed on the CT. This, in turn, produces changes in the ECM and at the level of gene expression, as will be discussed below. Major Extracellular Matrix Components and Mechanical Properties of the Common Connective Tissues1,7,a PG=proteoglycan, GAG=glycosaminoglycan. Major Extracellular Matrix Components and Mechanical Properties of the Common Connective Tissues1,7,a PG=proteoglycan, GAG=glycosaminoglycan. Nineteen distinct types of collagens are recognized, all with individual characteristics that serve specific functions in a variety of tissues.9 The common structural feature that identifies all collagens, however, is a triple helix region within the molecule. This section of the molecule provides the characteristic mechanical properties of tendons and ligaments (ie, the ability to withstand tensile loads). The triple helix is made up of 3 polypeptide chains folded to form a ropelike coil. Each chain, known as an α-chain, is characterized by repeating sequences of 3 amino acids, glycine-X-Y (Fig. 2). Because glycine is the smallest amino acid and occupies the central core of the triple helix, the repetition of glycine as every third amino acid is essential for the correct folding of the 3 α-chains into the helical conformation.10,11 Specific collagen types are formed by a variety of α-chains and by variations in the combination of different α-chains: in some collagens, all 3 α-chains are identical; in other collagens, 2 α-chains may be identical; and in some collagens, all 3 α-chains are different. Alteration of the glycine-X-Y sequence of amino acids usually results in dysfunction of the collagen molecule and loss of its mechanical properties (eg, osteogenesis imperfecta).12 The helical complex, which inherently resists tension, is further strengthened by inter-molecular bonds between the α-chains of adjacent molecules.13 Portion of a collagen molecule showing individual alpha chains coiled to form a triple helix. Within each chain, the amino acids are similarly arranged in a helix, with glycine (G) facing the center of the triple helix. The other amino acids are represented by the dots. The extremities or terminals of the collagen molecule are nonhelical but are important for the formation of collagen fibrils and for other nontensile functions, including interactions with other extracellular components. The α-chains of the principal collagens are synthesized with relatively long extremities, and, after formation of the triple helix, this newly formed collagen molecule (called procollagen) is emitted from the cell into the extracellular space where most of the nonhelical ends are enzymatically removed. Removal allows the shortened molecules, now called tropocollagen, to associate with each other and form fibrils, which are visible under the electron microscope and characterized by distinct cross-bands. These fibrils then aggregate to form fibers, which are visible under the light microscope, and bundles of fibers, which are visible to the eye14 (Fig. 3). Representation of collagen synthesis, secretion, and assembly. Adapted with permission from Kielty CM, Hopkinson I, Grant ME. Collagen: the collagen family, structure, assembly, and organization in the extracellular matrix. In: Royce PM, Steinmann BS, eds. Connective Tissue and Its Heritable Disorders: Molecular, Genetic, and Medical Aspects. New York, NY: Wiley-Liss; 1993:113. Modifications, variations, and additions to the basic triple-helix conformation give rise to 6 classes of collagens (Tab. 2).9,10 Of most relevance to physical therapists are the fibril-forming collagens that are found in tissues (ie, tendons, ligaments) where their primary function is to resist tensile forces and in tissues where there is a requirement for resisting tensile loads (ie, dermis, articular cartilage, intervertebral disks [IVDs], bone). The other 5 classes of collagen, which are much less abundant but nevertheless essential to CT functions throughout the body, have a variety of roles.9,10 These classes of collagen and their roles are summarized in Table 2. Collagen Types, Location, and Functions9,10 Collagen Types, Location, and Functions9,10 Fibrilforming collagens account for over 70% of the total collagen found in the body.10 Type I collagen predominates in tissues such as bones, tendons, ligaments, joint capsules, and the annulus fibrosus of the IVD. Type II collagen is located principally in articular cartilage and the nucleus pulposus of the IVD. Type III collagen appears to play a role in the extensibility of tissue and is found especially in embryonic tissues and in many adult tissues, such as arteries, skin, and soft organs, where they form reticular fibers.11,15 The prevalence of type III collagen is also an indicator of tissue maturity and is also prominent in the initial stages of healing and scar-tissue formation, where it provides early mechanical strength to the newly synthesized matrix.14 As fetal development proceeds and as healing tissue gains in strength, type III fibers are replaced by the stronger type I fibers.16–18 Generally, type I fibrils have a large diameter, a feature that correlates with the ability to carry a greater mechanical load. In young, growing tendons, exercise increases fibril diameter and ultimate tensile strength, but, in the adult, the effect of exercise is minimal. Nevertheless, continued tension is necessary to maintain tendon structure because immobilization leads to a loss of tensile strength.19 Fibrils may also be formed of more than one type of collagen. Types V and XI combine with type I and II collagen, respectively, to form heterotypic fibrils, an arrangement that is thought to play a role in determining fibril diameter and thereby influence mechanical properties. In general, the greater the fibril diameter, the smaller the percentage of type V and type XI collagen.11 The tension-resisting property of the fibril-forming collagens is the principal means of limiting the range of motion of joints, transmitting forces generated by muscle, imparting tensile strength to the bony skeleton, and resisting extension by the surface layers of articular cartilage. The arrangement and alignment of the collagen fibers reflect the mechanical stresses acting on the tissues. In tendons, the majority of fibers are aligned in parallel, enabling them to resist unidirectional forces and to efficiently transmit forces generated by muscles to bones.4 In comparison, type I fibers in ligaments are often positioned in slightly less parallel arrays, reflecting the need to resist multidirectional forces. For example, in ligaments associated with joints, there is a need to both limit motion and provide for joint stability. Collagen also plays an important role in attaching tendons and ligaments to bone. At these junctions, tendons and ligaments usually widen and give way to fibrocartilage, a transformation where the aligned fibers originating from the tendon or ligament are separated by other collagen fibers arranged in a 3-dimensional network surrounding rounded cells.20 This arrangement helps to transmit tensile forces onto a broad area and reduces the chance of failure under excessive loading. The type I collagen fibers of bone have a more complex arrangement. Generally, the fibrils are arranged in orthogonal arrays, similar to the way the wood fibers in plywood are arranged in alternating sheets. This arrangement, especially when configured as small cylinders, such as in osteons, imparts a great deal of multidirectional tensile strength. A combination of type I and type II collagen is found in the IVD and in tendons with fibrocartilaginous pressure pads.21 In the annulus fibrosus of the IVD, alternating layers of type I fibers link adjacent vertebral bodies and surround the central nucleus pulposus. The fibrous bands are generally aligned at angles of about 45 degrees from the vertebral axis, an arrangement that provides a mechanism for spinal flexibility and for increasing resistance to excessive motion near the limits of movement. In the nucleus pulposus, type II collagen predominates and there are high levels of HA and sulphated PG that function in association with the type II fibers to provide a hydrated and pressure-resistant core.22 In articular cartilage, the principal collagen fibers are type II, which are arranged to form a network of bands between the cells. Superficially, these fibrous bands are mostly tangential to the articular surface, but, with increasing depth, they become more radial and pass between columns of cells. Immediately around the cells, other type II collagen fibers combine with types VI, IX, and XI in a dense capsule arrangement. These fibrous bands provide both the tensile properties of cartilage and, in conjunction with large sulphated PG, a mechanism for resisting compression. The capsular collagen is thought to protect the chondrocytes from these external fibers in the ECM tissues such as skin, the and to withstand repeated and and to return to a The arrangement of and on the strength and of forces on the The fibers may be into (eg, as small individual fibers (eg, skin, or as a 3-dimensional network of fibers (eg, elastic fibers are of an core and located mostly around the (Fig. The which are made up of as a on which is but once the core is the majority of are to the of the 2 amino acids (ie, and that form between adjacent chains and are important in imparting the elastic properties to The mechanism of extensibility is not but the of found within the tissue usually the of mechanical on it and the requirement for a of and Representation of elastic showing core and by Adapted with permission from fibers are and found in most organs to They are found throughout the of the and are for pressure The in the elastic at the of is with the of the the that is found in the of the by and the stresses that the of on the In the dermis, the elastic fibers provide the characteristic of is a with coiled fibers at angles to of tension and in a that allows for greater of the a changed conformation and general loss of elastic fibers with increasing the ability of the to fibers are relatively in ligaments, with 2 the in the region of the vertebral and the the of adjacent The elastic in these ligaments in the and thereby the on the muscles of the The of of elastic fibers in is a major and, once this ability to is the of normal function is not however, is synthesized by adult tissues in to and and by tissues in a of including however, the elastic and function is not In general, there is a of about the of of elastic The PGs are characterized by a core protein to one or more sulphated The core are generally specific to each of the PG types and show in there are various The chains are of repeating with the type and of determining the properties of the of make up the in 6 major and 6 also known as and Hyaluronan is because it is not to a protein core, is it is usually under a of PG, however, because it is the most abundant and ubiquitous of the and it plays an important role in to other PGs to form are and have a to an that results in the water from surrounding This helps maintain the hydration of the the of hydration on the of chains and on the placed on PG by the surrounding collagen The percentage of within CT with mechanical load. to high compressive forces (eg, articular have a large PG content of the of the in tension-resisting tissues such as tendons and ligaments, PGs are found in relatively small of the proportions of PG with the mechanical in such a way that the is in tissues to compression and in tissues that resist be into and The that between these 2 are their ability or to aggregate with HA and the of chains that to the protein PGs to A large complex results when many PG link to a of The linkage is by a known as link protein that helps the PG to the Because the chains to the PG core are and from the core protein the of a a high is This an in the movement of water into the matrix. the PG will to but the collagen fibers and the of the chains to of on collagen fibrils limits the of PGs to of their This provides the of the matrix and, where PG content is the tissue with the ability to resist compressive forces. of PGs are and is the and is the PG in articular cartilage and plays a major role in normal joint function and in A large of chains and a smaller of chains are to the protein core of the (Fig. has chains to its core but it also with HA and to resistance of compressive is found in many tissues, including the and some tendon that are to compressive along with also functions as an molecule and cell Representation of an with and chains to the protein The is to and is at this region by link attach to to form the large Adapted with permission from matrix proteins. In: Royce PM, Steinmann eds. Connective Tissue and Its Heritable Disorders: Molecular, Genetic, and Medical Aspects. New York, NY: Wiley-Liss; The PGs not to HA and a small of chains of and They to play a role in compression, but they with other matrix components and to mechanical with collagen. which has one chain, is one of the smallest PGs and functions, in to link adjacent collagen The core at specific on the surface of fibrils, and the to form an with a from an adjacent is also small and is found in the matrix between bundles of collagen The mechanical and other functions of are not but both and play a role in cell most the of specific and on the core (Fig. Representation of and with their similar core proteins. Adapted with permission from matrix proteins. In: Royce PM, Steinmann eds. Connective Tissue and Its Heritable Disorders: Molecular, Genetic, and Medical Aspects. New York, NY: Wiley-Liss; The PG, is to the cell and plays a role in cell such as basic and acting as a is found to cell surfaces and to the structure of basement membranes. In to providing support, it in cellular Hyaluronan is an important component of the complex, but it also as a molecule. Hyaluronan entrains water and is prominent where the matrix is highly such as in loose A relatively of HA is found in the of the the and the fluid of where its properties are for is to show that the of normal tissue normal mechanical loading and that CTs to changes in stresses by their PG content and motion is important for the normal maintenance and of PG in articular cartilage. joint immobilization or results in of the articular cartilage because of a loss of PG from the this PG loss joint immobilization is with a is to maintain PG content in articular The of both and movement, however, in a large loss of PG over a of by or processes also lead to a in and and in the of the to to HA and form large As a cartilage may to resist compression The IVD also has a high PG content, with the PG mostly in the nucleus pulposus and the annulus where the tissue is under increasing the region of the annulus however, has a PG content than major tension-resisting structures such as tendons and ligaments, reflecting the need to resist both tension and of the IVD may in from the of the and HA to form a complex because of the of the link In tendons that are around a bony the of the tendon to tension has a low PG content, with a high proportion of In contrast, the part of the tendon that is the bony surface has a high PG content, with a high proportion of also In the region under tension, the cells are In the pressure they are rounded and similar to cells. the of the compressive forces by of the tendon results in 2 and loss of PG from the the of tension, total PG content but with a rise in the proportion of The return of the tendon to its results in a increase in PG it has that compression of fetal tendons leads to changes in specific PGs and at the level of the and acids a in or type I collagen these changes to be by of a specific (ie, that is known to be a for and but not a but proportion of the total matrix components. They are they not have prominent mechanical functions, they are to the surrounding matrix and the matrix to the They are with the of many functions, including changes in cell cell and cell and the are link and is in the ECM of most CTs and plays a role in cell to matrix components for example, also in cell is in embryonic tissues and in adult tissues including the and to basement as discussed is required to the PG in the cartilage matrix, with various matrix components and collagen fibril formation, and tissue and plays a role in cell normal the maintenance of fibers, PG, and is and a between and This is maintained by and in to the matrix and the tissue of The and of and is similarly by an network of and The of the between and normal tissue and changes the mechanical properties of the tissues. As a general of matrix components in and increases in over leads to of ECM in such as interstitial liver and the Trauma to CT also A or of CT excessive tensile loading in ligaments and tendons and at As a general the loss of tensile or compressive loading in the of articular cartilage in a leads to tissue The and of these structures is usually many but a generally the initial stages of healing, are by newly synthesized type III collagen, but, as increasing amounts of type I collagen and provide greater exercise also appears to have a effect on the strength of normal tendons and ligaments, the results are This may be because normal tendons and ligaments are in an on is also thought to collagen and the by the collagen fibrils to parallel to the of than for that are not to The of tension on healing however, is more as tension leads to where sulphated PGs a In the 2 the understanding of CT structure and function has is now that the cells of the various CTs a variety of ECM components that not to the specific and properties of tissues, but also to a variety of cellular for the physical and as discussed CTs are to changes in the mechanical both and The proportions of collagens and PGs the mechanical properties of The between the fibril-forming collagens and PG is Connective tissues to resist high tensile forces are high in collagen and low in total PG content CTs to compressive forces have a greater PG content Hyaluronan has roles and not provides tissue hydration and of and movements but also forms an component of large PG in tissues. The smaller to and link collagens and PGs to the cell The result is a complex network of matrix (Fig. which both the mechanical properties and the metabolic of tissues. Representation of components of the extracellular matrix and their interactions with each other and with on the cell Components are not to with CT movement are and by physical A of the CT matrix and its to the properties of these tissues, particularly the to mechanical forces, an to provide a for The complexity of the the however, that further research be to more the of on the structure and function of of of The of for with of the