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MSK > Tendons Composition > Flashcards

Tendons Composition Flashcards

1
Q

What are tendons?

A

Tendons are soft, fibrous tissues that connect muscle to bone.

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2
Q

What are some characteristics of tendon?

A

They are relatively passive, inelastic structures, able to resist high forces

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3
Q

Do all muscles have a tendon?

A

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.

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4
Q

What is the function of tendons?

A

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.

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5
Q

What are tendons composed of?

A

Water (65%)
Fibrous component: Collagen (30%) , elastin
Cells
Ground substance (5%): Glycoproteins, Proteoglycans

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6
Q

What is the water content of tendon?

A

50-70%

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7
Q

What is the percentage of the dry weight of collagen in tedon?

A

75%

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8
Q

What is collagen?

A

Collagen is a structural protein providing tissues with tensile strength

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9
Q

What is the main kind of collagen in tendon and how much of total tendon collagen does it make up?

A

Type I (90-95%)

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10
Q

What other collagens are found in tendon?

A

Types III, V, VI, XII and XIV

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11
Q

What is the second most abundant kind of collagen in tendon?

A

Type III (~10%)

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12
Q

What is the role of type III collagen?

A

Plays important role in collagen fibrillogenesis by regulating the size of type I collagen fibrils

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13
Q

Where is type III collagen localised?

A

Often localised in intrafascicular matrix but the role it has in this tendon compartment is still unclear

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14
Q

Where is type V collagen found?

A

Found in the centre of collagen-I fibrils

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15
Q

What is the role of type V collagen in tendons?

A

Thought to provide a template for fibrillogenesis

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16
Q

Which collagens in tendon are not fibrillar?

A

Types VI, XII, and XIV

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17
Q

Where is type VI collagen found?

A

Pericellular matrix

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18
Q

What are type XII and XIV collagens?

A

Fibril associated collagens

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19
Q

What is the purpose of fibril associated collagens?

A

provide a molecular bridge between type I collagen and other matrix molecules

Play important roles in tendon development

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20
Q

Do tendons have many cells?

A

No, tendons often classified as hypocellular

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21
Q

What cells can you find in tendons?

A

Interfascicular cells, tenocytes, fibroblasts, proigenitor cells, and vascular cells

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22
Q

Where can you find interfascicular cells?

A

IFM between fascicles

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23
Q

What are some characteristics of interfascicular cells?

A

have rounded morphology

More metabolically active than intrafascicular tenocytes

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24
Q

Where are tenocytes found?

A

between the collagen fibres within the fascicles

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25
Q

Describe the structure of a tenocyte

A

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.

26
Q

What is the role of tenocytes?

A

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.

27
Q

What are two important glycoproteins in tendons?

A

Cartilage oligomeric matrix protein (COMP)

Tenascin-C

28
Q

What is COMP?

A

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

29
Q

Where is COMP localised in tendon?

A

High concentrations in the inter-fibrillar matrix but absent from the IFM

30
Q

What is the function of COMP in tendon?

A

Unknown. No effect on knockout mice

31
Q

What is Tenascin-C?

A

Glycoprotein found at low levels in tendon

Composed of six subunits bound together by N-terminal interchain cross-linking domains

32
Q

Where is tenascin-C found?

A

Predominates in regions where tendons experience high forces

33
Q

What are the levels of tenascin-C modulated by?

A

mechanical loading

34
Q

What is the function of tenascin-C?

A

Unknown

May contribute to elasticity

35
Q

What percentage of tendon dry weight are elastic fibres?

A

1-10%

36
Q

Where are elastic fibres localised?

A

Mainly localised to the IFM but also presents in fascicles particularly around cells

37
Q

Describe the structure of elastic fibres

A

Central core of elastin surrounded by a sheath of fibrillins 1 and 2 and other associated proteins

38
Q

What are the properties of elastic fibres?

A

Highly elastic
Fatigue resistant
Able to store and return energy

39
Q

What are the functions of elastic fibres in tendons and ligaments?

A

In ligaments elastin resists transverse and shear deformation but this has not yet been established for tendons

40
Q

What are proteoglycans?

A

Molecule composed of a core protein covalently bound to one or more glycosaminoglycan chain

41
Q

What percentage if tendon dry weight is proteoglycan?

A

1-5% if tendon dry weight - most abundant non-fibrous protein

Primary component of the matrix interspersed between collagen units

42
Q

Describe GAGs

A

Negatively charged, attracting water into the tendon

Play an important role in mechanics (viscoelastic properties) via their association with water.

43
Q

What are the main proteoglycans?

A

decorin and biglycan

44
Q

What do decorin and biglycan have in common?

A

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.

45
Q

What percentage of total proteoglycan content is decorin?

A

80%

46
Q

Describe the structure of decorin

A

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.

47
Q

Where is decorin found?

A

Found both within fascicles ad IFM

48
Q

What is the difference between decorin and biglycan?

A

Biglycan has several dermatan or chondroitin sulphate chains while decorin only has one

49
Q

What more minor proteoglycans are found in tendon?

A

Lumican and Fibromodulin

50
Q

What are lumican and fibromodulin?

A

Class II SLRPS

51
Q

Describe Class II SLRPS

A

Contain keratan sulphate side chains

Share a fibril binding site which is distinct to that used by decorin and biglycan

52
Q

What do SLRPs have a role in?

A

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

53
Q

How might SLRPs contribute directly to the mechanical properties of tendon?

A

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

54
Q

What is an alternative theory for what decorin does?

A

Alternative: Decorin actually promotes sliding between fibrils rather than force transfer

55
Q

What is Lubricin also known as?

A

superficial zone glycoprotein or PRG4

56
Q

Where is lubricin localised

A

Localised on tendon surface, IFM, and compressive regions of the tendon

57
Q

What is the function of lubricin?

A

Provides lubrication allowing gliding at the tendon surface and may also facilitate sliding between adjacent fascicles

58
Q

What is versican?

A

Large aggregating proteoglycan

59
Q

Where is versican found?

A

Preferentially expressed in IFM (especially pericellular region)

60
Q

What does versican do?

A

Interacts with elastic fibres and so may contribute to the structural properties of the IFM (precise role not yet determined)

61
Q

How does proteoglycan distribution vary along the length of a tendon?

A

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.

MSK (22 decks)

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