The role of the non-collagenous matrix in tendon function

Abstract

Tendon consists of highly ordered type I collagen molecules that are grouped together to form subunits of increasing diameter. At each hierarchical level, the type I collagen is interspersed with a predominantly non-collagenous matrix (NCM) (Connect. Tissue Res., 6, 1978, 11). Whilst many studies have investigated the structure, organization and function of the collagenous matrix within tendon, relatively few have studied the non-collagenous components. However, there is a growing body of research suggesting the NCM plays an important role within tendon; adaptations to this matrix may confer the specific properties required by tendons with different functions. Furthermore, age-related alterations to non-collagenous proteins have been identified, which may affect tendon resistance to injury. This review focuses on the NCM within the tensional region of developing and mature tendon, discussing the current knowledge and identifying areas that require further study to fully understand structure-function relationships within tendon. This information will aid in the development of appropriate techniques for tendon injury prevention and treatment.

Keywords: ageing; glycoprotein; interfascicular matrix; proteoglycan; structure-function.

Figure 1

Schematic showing the hierarchical structure of tendon, in which collagen molecules assemble to form subunits of increasing diameter. At each level, the collagen is interspersed with a small amount of non-collagenous matrix. Adapted from Thorpe et al. (2010a) with permission from Wiley.

Figure 2

Schematic showing interaction between two collagen molecules from adjacent fibrils via decorin core proteins (white horseshoe shaped) and their glycosaminoglycan side chains (black lines). (a) Transverse view of collagen fibrils showing connections formed by decorin. (b) Longitudinal view of collagen fibrils showing decorin binding in the gap region of the collagen fibrils. (c) 3D representation of the decorin protein collagen molecule complex within a microfibril. Reprinted from Vesentini et al. (2005) with permission from Elsevier.

Figure 3

Images showing distribution of elastin and interfascicular connections in the equine superficial digital flexor tendon (SDFT). (a) Longitudinally sectioned SDFT stained with Miller's stain showing distribution of elastic fibres (black lines) between collagen bundles (E.G. Laird, unpublished data). (b) Nomarski differential interference contrast optical microscopy image of longitudinally cryosectioned SDFT showing interconnecting fibres (indicated by arrows) between adjacent fascicles (F) that have been teased apart (K.D. Smith, unpublished data).