Locomotor - Tendons

Question 1

What are the components of tendon? What is the mix of water, collagen, elastin and other proteins?

Answer 1

Tendon is has a very high collagen content (Type I) and very little elastin or proteoglycans as it’s not really anticompressive but meant to be tensile.

Water: 65%
Collagen: 80-95% of dry mass
Type I Collagen: ~60%, Type III 3% + can include Types IV, V, VI, XII & XIV)
Elastin: <3% dry mass
Proteoglycans: ~0.5% dry mass eg. decorin, aggrecan, versican, lumican
fibroblasts arranged in parallel rows
- proteins & macromolecules (proteoglycans) in ECM
- Glycoproteins: 5% eg., tenascin, COMP, fibronectin (the more COMP, the stronger the tendon)

Question 2

What is the blood supply to tendon?

Answer 2

Vessels in perimysium-periosteal insertion & surrounding tissues

Question 3

What makes up the cellular populations of tendon?

Answer 3

Cells only make up 1.5% of tissue volume - it’s mostly water and collagen fibrils:

95%: Tenocytes - long, thin; long nucleus; lie between fibrils; synthesise & remodel ECM. Have cytoplasmic extensions for cell-cell communications

5%: Endotenon cells: small, rounded; lie in dense groups between fascicles & **Chondrocyte-like cells **– small, rounded; found in compressed regions (joints, @ bone attachment); round nucleus; synovial cells, vasculature cells

Question 4

What is the microscopic structure of tendon?

Answer 4

Remember it will have a lot of collagen fibres, very few tenocyte cells and a lot of water:

Triple-helical collagen molecules → crimped fibril→ fibres (w/ tenocytes) → 1° fibre bundle (subfascicle) → 2° fibre bundle (fascicle) → 3° fascicle (1-3 mm) → tendon unit (2-12 mm)

See illustration.

Question 5

What are the difference between the makeup of tendon and ligaments?

Answer 5

Ligaments connect bone to bone, so they don’t need to be as stretchy but they need to be more anti-compressive, so there is more proteoglycans in ligaments. They also tend to be stiffer, to limit movement, such as the ligaments around the femorotibial joint, but they also need to be elastic to bear load.

• *Water**: 70-75%
• *Collagen:** 80-95% of dry mass
• *Type I Collagen:** ~60%; Type III 10% + can include Types IV, V, VI, XII & XIV)

Elastin: 10-15% dry mass
Proteoglycans: 5%
- Higher percentage of proteoglycan matrix than tendon (for more anti-compressive & stiffer properties)
- Fibroblasts

Question 6

Explain strain and stress and how they relate to the tendon.

Answer 6

Strain = % change in tendon length under load
Strain rate = time dependent
Stress = Load experienced by the tendon per unit cross-sectional area

At low strain rates -the tendon is easily deformed but absorb more energy, so less effective in transmitting loads.
At high strain rates - tendons become stiffer and less deformable, so more effective in moving large loads

Question 7

What are three factors that define the viscoelasticity of tendons and ligaments?

Answer 7

• Stress-relaxation - the amount of load required to result in a specific tissue strain (or constant elongation) decreases with time. Tissue extends due to load, but when load removed, tissue remains extended from original length; to get it back to longer length, you need smaller load
• Creep - increasing deformation (strain) under constant load. Time dependent. Compare to an elastic material: does not exhibit increase in strain with time of load applied
• Hysteresis - the unloaded curve does not return along the loaded curve