Collagen self-assembly and the development of tendon mechanical properties
Introduction
Elastic energy storage in tendons in the legs and feet of many animals is an important mechanism that saves substantial quantities of muscular energy during locomotion (Alexander (1983), Alexander (1984)). During normal gait, potential energy is stored as strain energy in the muscles and tendons that are stretched by impact with the ground (Alexander (1983), Alexander (1984)). Elastic recoil, primarily by the tendons, converts most of the stored energy back to kinetic energy. Elastic energy storage in tendons has been studied in several animal models. In the pig, the digital flexor tendons are involved in the elastic storage of strain energy (Shadwick, 1990); the amount of elastic energy stored in the digital flexor tendons decreases with age after the animal reaches maturity. In the turkey, direct measurement of force and fiber length in the lateral gastrocnemius muscle reveals that the active muscle produces high force but little work while the tendon produces much of the work because of elastic deformation and recovery (Roberts et al., 1997).
The mechanism by which elastic energy is stored in tendon during locomotion is not well understood. It has been shown that both the axial rise per amino acid residue and the D period increase with increased tendon mechanical deformation suggesting that molecular stretching and slippage occur during deformation (Mosler et al., 1985; Sasakai and Odajima (1996a), Sasakai and Odajima (1996b)). The purpose of this review is to attempt to describe the molecular and supramolecular steps associated with collagen self-assembly and tendon development, and to analyze how these steps relate to the ability of tendons to store energy elastically.
Section snippets
Molecular structure of collagens
The mechanical properties of tendon are a direct consequence of the constituent components and how these components are arranged. Tendon is primarily made up of cells, collagen fibers, proteoglycans and water. Collagen is the most abundant protein in tendon and forms the essential mechanical building blocks in the musculoskeletal system. It can be found in both fibril and non-fibril forming forms. The fibril-forming collagens provide the structural framework of tissues; they include types I,
Collagen assembly in developing tendon
The ability of collagen molecules to assemble into crosslinked fibrils is an important requirement for the development of tissue strength. Although the process is under cellular control, the tendency for collagen molecules to form crossstriated fibrils is a property of the molecular sequence, as discussed in more detail in the section on self-assembly.
Tendon is a multi-unit hierarchical structure that contains collagen molecules, fibrils, fibril bundles, fascicles and tendon units that run
Role of proteoglycans (PGs) in tendon development
Tendon contains a variety of proteoglycans (PGs) including decorin (Scott, 1993) a small leucine-rich PG that binds specifically to the d band of positively stained type I collagen fibrils (Scott and Orford, 1981) as well as hyaluronan, a high molecular weight polysaccharide. Other small leucine-rich PGs include biglycan, fibromodulin, lumican, epiphycan and keratocan (Iozzo and Murdoch, 1996). In mature tendon the PG(s) are predominantly proteodermochondran sulfates (Scott, 1993). PGs are seen
Mineralization of tendon
The major leg tendons of the domestic turkey, Meleagris gallopavo, including the Achilles or gastrocnemius tendon, begin to naturally calcify when the birds reach about 12 weeks of age (Landis, 1986). This appears to be an adaptation in response to external forces, but the relationship between skeletal changes and such forces is still not understood (Landis et al., 1995). The gastrocnemius is a relatively thick tendon at the rear of the turkey leg which passes through a cartilaginous sheath at
Mechanical properties of developing tendons
The mechanical properties of developing and adult tendons have been studied extensively previously (see Silver et al., 1992 for a review). The purpose of this section is to attempt to relate changes in mechanical properties of tendon to structural changes that are observed at the microscopic and gross levels.
The mechanical properties of developing tendons rapidly change just prior to the onset of locomotion. McBride et al. (1988) report that the ultimate tensile strength (UTS) of developing
Mechanical properties of mineralizing tendons
The viscoelastic behavior of mineralizing turkey leg tendon has been reported during the period between 12 and 16 weeks (Silver et al., 2000b). Results of these studies suggest that the elastic modulus for type I collagen is between 5 and 7.75 GPa, which is similar to that found for rat tail tendon (Silver et al., 2000b). Fibril lengths obtained from the viscous component of the stress–strain behavior are between 414 and 616 μm, which is slightly smaller than those found for rat tail tendon, but
Collagen self-assembly in vitro
Approximately 50 years ago it was first observed that purified collagen molecules in solution in vitro would spontaneously self-assemble at neutral pH at room temperature to form fibrils that appeared to be identical to those seen in vivo. Self-assembly of collagen to form rigid gels was observed by Gross and coworkers (1952) and Jackson and Fessler (1955). In their experiments, collagen was solubilized and then heated to 37°C in a buffer containing a neutral salt solution. Under these
Mechanical properties of self-assembled collagen fibers
Studies on the mechanical properties of self-assembled fibers composed of type I collagen fibrils have provided much insight into the development of matrix mechanical properties. Using this system, changes in parameters such as fibril diameter, length and extent of crosslinking can be correlated with changes in the viscoelastic properties. Danielsen first noted that incubation of self-assembled collagen solutions for periods between 0 and 104 days resulted in a gain in mechanical strength (
Effects of physical forces on cell–extracellular matrix interactions
We have implied in this paper that the mechanical properties of tendon are to a first approximation due to the behavior of the collagen fibril network; this is an oversimplification. The mechanical properties of tendon are dynamically dependent on the properties of the crosslinked collagenous network and also on cell–extracellular matrix interactions. It has been hypothesized that forces exerted by the extracellular matrix on cells may be in equilibrium with forces exerted by cells on the
Summary
Collagen assembly in developing tissues is controlled by the synthesis of procollagen within the cell, deposition of procollagen in extracytoplasmic compartments, cleavage of the N and C propeptides extracellularly, and lateral fusion of fibril intermediates that results in linear and lateral growth of fibrils. All of these processes can occur in the absence of crosslinks between collagen molecules; however, the development of strength required for locomotion by skeletal tissues requires the
Acknowledgements
The authors would like to thank Dr. David L. Christiansen for allowing us to use the diagram of the hierarchical structure of tendon and Istvan Horvath, a doctoral candidate in Biomedical Engineering at Rutgers University, for assistance in constructing the flexibility profile for type I collagen.
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