Connective Tissue Adaptation

Question 1

List the molecular components of the ECM and discuss their structural and mechanical properties.

Answer 1

ECM = network of macromolecules which maintain cell/tissue integrity (everything that is not cellular).

ECM contains-
- fibrous proteins (eg: elastin / collagen)
- ground substance - less structured (viscous) and contains glycosaminoglycans
- glycosaminoglycans (GAGs) - polysaccharides that link with proteins to form proteoglycans

GAGs-
- hydrophilic - attract and trap water
- not easily compressed
- lubricative
- important for tissue repair
eg- hyaluronan / keratan sulphate

Question 2

Describe and compare the macro- and microscopic structure of different musculoskeletal tissues (bone, tendon, ligament, cartilage, muscle).

Answer 2

Cells + ECM = tissue

Fibrous proteins:
Elastin-
- protein (elastic protein fibres)
- coiled structure (when not under stress)
- cross-links between molecules (maintains strength/integrity when under stress)
- elastic
Collagen-
- major structural component of ECM
- triple helix of polypeptide chains to make collagen microfibrils (microfibrils -> fibrils -> fibres)
- high tensile strength
- stretchy / crimped

Structure of tendon:
- collagen (predominantly)
- proteoglycans
- elastin
- tenocytes

Structure of ligament:
- fibrocytes (+ECM)
- collagen (less than tendon)
- proteoglycans (more than tendon)

Structure of cartilage:
- collagen (type II)
- proteoglycan
- hyaluronan
- water (predominant) - GAGs give it the lack of compressibility
- regional variation

Structure of bone:
Inorganic ECM component - hydroxyapatite-
- mechanical rigidity
- load-bearing strength
Organic ECM component-
- collagen type I (predominantly)
- elasticity / flexibility - less brittle
- structural organisation
Cells-
- osteocytes - support and maintain bone structure (on the membrane)
- osteoblasts - synthesise bone (within bone matrix)
- osteoclasts - resorb bone (largest cells)

Question 3

Explain how the gross structure and molecular and cellular composition of musculoskeletal tissues relate to their function and mechanical properties.

Answer 3

Bone-
- cell type: osteoblast/clast/cyte
- function: support / strength

Hyaline cartilage-
- cell type: chondrocyte
- function: shock absorption

Muscle-
- cell type: myocyte
- function: contractile movements

Tendon-
- cell type: tenocyte
- function: resisting tensile forces / energy storage

Question 4

Describe the influence of mechanical loading on ECM turnover and tissue adaptation processes.

Answer 4

Structural role of ECM:
- provides extracellular scaffold structure for the tissue
- bears majority of mechanical stress

Supporting role of ECM:
- transmits chemical signals to the cells to regulate their migration, growth and differentiation

Chondrocytes:
Spheroid in shape - except surface that is flattened to allows smooth gliding movement
Synthesis and maintain matrix
Cells survive on low oxygen - depend on anaerobic respiration
Mechanical joint loading influences the functions of chondrocytes

Cartilage adaptation by region:
Superficial (tangential) zone-
- collagen predominant
- collagen fibres orientated tangential to surface
- resists shear stresses
Middle (transitional zone)-
- proteoglycan predominant
- collagen arranged obliquely
- transition between shearing forces of surface to compression forces in deep layer
Deep (radial) zone-
- proteoglycan predominant
- collagen fibres attached radially (vertically) to tidemark - tidemark = where deep zone and calcified cartilage are anchored together
- distributes loads and resists compression

ECM relationship with cells:
Matric can influence cell behaviour, shape, survival and proliferation
ECM is continually renewed
Cells secrete ECM AND enzymes for the ECM
Cells increase matrix turnover (synthesis/degradation) in response to mechanical load put on the tissues
ECM transmits force to cells

Types of mechanical stress:
- Compression
- Tension
- Shear
- Bending
- Torsion

Stiffness of tissue - how much is performed (stretches/compresses) after a load has been applied (gradient on graph = stiffness)

Stress has a direct link to how the tissue develops and behaves

Trabecular spaces in bone align well with the directional forces of stress - more stress creates more stress lines found in trabecular bone

Athletes have stiffer tendons due to the cross sectional area of the Achilles tendon being larger than average - bigger tendon = less stretchy

Phase of plastic deformation = permanent loss of organised structure