Connective Tissues: Matrix Composition and Its Relevance to Physical Therapy
he 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.
Components of Connective Tissues Connective tissuesand 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, whichdepend 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).
Key Words: Connective tissues, Fibers,Function, Proteoglycans. Culav EM, Clark CH, Merrilees MJ. Connective tissues: matrix composition and its relevance to physical therapy. Phys Ther. 1999;79:308 –319.
Elizabeth M Culav C Heather Clark Mervyn J Merrilees
308 Physical Therapy . Volume 79 . Number 3 . March 1999
The 2 most important fibrous components of the ECM are collagen and elastin, both insoluble macromolecularproteins. 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 ECMis 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 theability 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 en-
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 rolesinclude 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...