Tendons Composition Flashcards
What are tendons?
Tendons are soft, fibrous tissues that connect muscle to bone.
What are some characteristics of tendon?
They are relatively passive, inelastic structures, able to resist high forces
Do all muscles have a tendon?
No, those that do primarily provide motion at a joint or need to provide force either over a distance or in a tightly confined space.
What is the function of tendons?
transfer of force from muscle to bone
Tendon provide a mechanical advantage by focusing/redirecting force, lengthening a lever arm, or acting around a pulley.
What are tendons composed of?
Water (65%)
Fibrous component: Collagen (30%) , elastin
Cells
Ground substance (5%): Glycoproteins, Proteoglycans
What is the water content of tendon?
50-70%
What is the percentage of the dry weight of collagen in tedon?
75%
What is collagen?
Collagen is a structural protein providing tissues with tensile strength
What is the main kind of collagen in tendon and how much of total tendon collagen does it make up?
Type I (90-95%)
What other collagens are found in tendon?
Types III, V, VI, XII and XIV
What is the second most abundant kind of collagen in tendon?
Type III (~10%)
What is the role of type III collagen?
Plays important role in collagen fibrillogenesis by regulating the size of type I collagen fibrils
Where is type III collagen localised?
Often localised in intrafascicular matrix but the role it has in this tendon compartment is still unclear
Where is type V collagen found?
Found in the centre of collagen-I fibrils
What is the role of type V collagen in tendons?
Thought to provide a template for fibrillogenesis
Which collagens in tendon are not fibrillar?
Types VI, XII, and XIV
Where is type VI collagen found?
Pericellular matrix
What are type XII and XIV collagens?
Fibril associated collagens
What is the purpose of fibril associated collagens?
provide a molecular bridge between type I collagen and other matrix molecules
Play important roles in tendon development
Do tendons have many cells?
No, tendons often classified as hypocellular
What cells can you find in tendons?
Interfascicular cells, tenocytes, fibroblasts, proigenitor cells, and vascular cells
Where can you find interfascicular cells?
IFM between fascicles
What are some characteristics of interfascicular cells?
have rounded morphology
More metabolically active than intrafascicular tenocytes
Where are tenocytes found?
between the collagen fibres within the fascicles
Describe the structure of a tenocyte
Elongated morphology with complex network of cytoplasmic processes that link adjacent cells via gap junctions
They have long cellular processes that envelop and interdigitate with the collagen fibrils.
What is the role of tenocytes?
Recent evidence connects tenocytes with highly defined role in maintenance of tendon structure
Tenocytes maintain the extracellular milieu by secreting ECM, synthesizing collagen and proteoglycans
They are responsive to mechanical loading, can communicate with adjacent tenocytes via gap junctions at their interface and by production of inflammatory mediators under stress.
What are two important glycoproteins in tendons?
Cartilage oligomeric matrix protein (COMP)
Tenascin-C
What is COMP?
Most abundant glycoprotein
Consists of a cylindrical core surrounded by 5 subunits
Each subunit is able to bind to type I collagen. Thus, one COMP can link between 5 collagen molecules
Where is COMP localised in tendon?
High concentrations in the inter-fibrillar matrix but absent from the IFM
What is the function of COMP in tendon?
Unknown. No effect on knockout mice
What is Tenascin-C?
Glycoprotein found at low levels in tendon
Composed of six subunits bound together by N-terminal interchain cross-linking domains
Where is tenascin-C found?
Predominates in regions where tendons experience high forces
What are the levels of tenascin-C modulated by?
mechanical loading
What is the function of tenascin-C?
Unknown
May contribute to elasticity
What percentage of tendon dry weight are elastic fibres?
1-10%
Where are elastic fibres localised?
Mainly localised to the IFM but also presents in fascicles particularly around cells
Describe the structure of elastic fibres
Central core of elastin surrounded by a sheath of fibrillins 1 and 2 and other associated proteins
What are the properties of elastic fibres?
Highly elastic
Fatigue resistant
Able to store and return energy
What are the functions of elastic fibres in tendons and ligaments?
In ligaments elastin resists transverse and shear deformation but this has not yet been established for tendons
What are proteoglycans?
Molecule composed of a core protein covalently bound to one or more glycosaminoglycan chain
What percentage if tendon dry weight is proteoglycan?
1-5% if tendon dry weight - most abundant non-fibrous protein
Primary component of the matrix interspersed between collagen units
Describe GAGs
Negatively charged, attracting water into the tendon
Play an important role in mechanics (viscoelastic properties) via their association with water.
What are the main proteoglycans?
decorin and biglycan
What do decorin and biglycan have in common?
Small leucine-rich proteoglycan family (SLRPs)
Function: Bind collagen to and mediate fibril assembly in tendon
Decorin shares fibril binding site with biglycan
Some researchers have suggested a role for these in tendinopathy.
What percentage of total proteoglycan content is decorin?
80%
Describe the structure of decorin
Consists of a horseshoe shaped core protein attached to which is a single chondroitin or dermatan sulphate side chain.
Interfibrillar bridge: The core protein has a specific binding site on the collagen fibril and is able to interact with other decorin molecules bound to adjacent collagen fibrils via its side chain to form an interfibrillar bridge.
Where is decorin found?
Found both within fascicles ad IFM
What is the difference between decorin and biglycan?
Biglycan has several dermatan or chondroitin sulphate chains while decorin only has one
What more minor proteoglycans are found in tendon?
Lumican and Fibromodulin
What are lumican and fibromodulin?
Class II SLRPS
Describe Class II SLRPS
Contain keratan sulphate side chains
Share a fibril binding site which is distinct to that used by decorin and biglycan
What do SLRPs have a role in?
tendon development (regulating collagen fibrillogenesis)
distinct, time-dependent roles in the process:
Biglycan and lumican: Modulate fibrillogenesis during early development
Decorin and fibromodulin: Play a primary role at later stages of development and maturation
How might SLRPs contribute directly to the mechanical properties of tendon?
By transferring load between discontinuous collagen fibrils via interfibrillar bridges
Individually interfibrillar bonds = weak; but combined they are strong enough to allow force transfer
Controversial bc it is uncertain whether collagen fibrils are discontinuous
What is an alternative theory for what decorin does?
Alternative: Decorin actually promotes sliding between fibrils rather than force transfer
What is Lubricin also known as?
superficial zone glycoprotein or PRG4
Where is lubricin localised
Localised on tendon surface, IFM, and compressive regions of the tendon
What is the function of lubricin?
Provides lubrication allowing gliding at the tendon surface and may also facilitate sliding between adjacent fascicles
What is versican?
Large aggregating proteoglycan
Where is versican found?
Preferentially expressed in IFM (especially pericellular region)
What does versican do?
Interacts with elastic fibres and so may contribute to the structural properties of the IFM (precise role not yet determined)
How does proteoglycan distribution vary along the length of a tendon?
Tensional mid-substance region: SLRPs in highest abundance - Probably related to organisational role in fibrillogenesis
Areas of tendon subjected to compression: Predominance of cartilage associated proteoglycans (aggrecan, biglycan, lubricin) - Probably in order to attract water into the tendon to enable resistance to compression.
