Physiology of Cartilage

Question 1

What are connective tissues?

Answer 1

• Connective tissues provide structural support for other tissues/organs throughout body.
• Characteristically they have large amounts of extracellular matrix (ECM) e.g. articular cartilage, bone

Question 2

What are the three principle types of cartilage?

Answer 2

• Hyaline
• Fibro-cartilage
• Elastic

Question 3

What is hyaline cartilage?

Answer 3

On articulating surfaces of moveable joints - adapted to withstand mainly compressive forces (i.e. load-bearing) although surface can withstand tensile (i.e. stretching) forces.

• Collagen fibrils mainly basketweave structure
• Pliable= spreads loads over ends of bones
• Glassy, low friction surface

Question 4

Describe hyaline cartilage

Answer 4

Amorphous, glassy; blue-white, shiny, firm matrix, many collagen fibrils but they form an imperceptible network.

Question 5

What is the function of hyaline cartilage?

Answer 5

Firm support - some pliability; resilient cushioning properties; spread load, good capacity to resist compressive stress; some capacity to withstand tensile stress (superficial zones).
Principal role of hyaline cartilage to absorb shock, distribute load, protect ends of bone, and with synovial fluid, to give very low friction surface for articulating joints (e.g. steel bearings on oil = 1.00, cartilage/synovial fluid/cartilage = 0.01). Synovial fluid (ultrafiltrate of plasma) contains complex macromolecules, esp. hyaluronic acid - (an essential lubricant).

Question 6

Describe the location of hyaline cartilage

Answer 6

Most of embryonic skeleton; covers ends of long bones in joint cavities; Anterior ends of ribs, parts of larynx, trachea, bronchi; most abundant type of cartilage
-Trachea, larynx, articular cartilage, e.m. chondrocytes, cytoskeleton of chondrocytes

Question 7

Describe the cells and ECM of hyaline cartilage

Answer 7

Cell density low (1-10%); chondrocytes situated within lacuna (chondron) and usually solitary. Wide range of collagen types, mainly type II in basketweave pattern. PGs mainly aggrecan.

Question 8

What is fibro-cartilage?

Answer 8

(e.g. intervertebral disc, meniscus, enthesis in tendon) adapted to withstand mainly tensile but also compressive forces. Some connective tissues (e.g. tendon, a dense fibrous connective tissue) have hyaline and fibro-cartilaginous regions. Collagen fibrils have clear orientation aligned parallel to force direction

Question 9

Describe fibro-cartilage

Answer 9

ECM similar to hyaline cartilage, but thick bundles of collagen fibres with clear parallel orientation.
-Cells often in rows, mainly fibroblasts but some chondrocytes

Question 10

What is the function of fibro-cartilage?

Answer 10

Considerable tensile strength and some capacity for withstanding compressive shock. Tendon has properties of fibrocartilage (regions exposed to tensile forces) and hyaline cartilage (regions exposed to compressive forces).
-Supports, prevents bone-bone contact, spread load, limits movement

Question 11

What is the location of fibro-cartilage?

Answer 11

Often found where hyaline cartilage meets true ligament or tendon. Structural intermediate between hyaline cartilage and dense regular connective tissues (e.g. tendon / ligament). Intervertebral discs, menisci of knee joint.
-Tendon, meniscus, intervertebral disc, cytoskeletal elements

Question 12

Describe cells and ECM in fibro-cartilage

Answer 12

Depends on location. In regions of tensile stress, cells are fibroblastic; in regions of compressive stress, cells can be chondrocytic. Both cell types have lacunae. Both present at low density. Collagen mainly type I (tensile regions), but also some type II (compressive regions; some aggrecan, but other smaller PGs also

Question 13

What is elastic cartilage?

Answer 13

has elastin, highly and reversibly deformable, and ‘rubbery’. Some collagen fibril orientation

Question 14

Describe elastic cartilage

Answer 14

Histologically v. similar to hyaline. Many more elastin fibres in ECM – form thread-like network in ECM. Some collagen orientation
-Fibroblasts synthesise elastin, collagens, small PGs

Question 15

What is the function of elastic cartilage?

Answer 15

Maintains shape of structure while allowing great flexibility.

Question 16

What is the location of elastic cartilage?

Answer 16

Supports external ear (pinna); epiglottis, auricle of ear, elastin fibres prominent

Question 17

Describe cells and ECM in elastic cartilage

Answer 17

Cells relatively high density; chondrocytic in appearance within lacunae; synthesise some collagen (type I, II and PGs, but large amounts of elastin

Question 18

Describe synovial fluid

Answer 18

• Ultrafiltrate of plasma- with hyaluronic acid- lubricant
• Produced by synoviocytes of synovial membrane
• Primary source of nutrition and removal of waste for cartilage cells
• Viscous when joint immobile- warming up exercises increases production/ secretion, reduces viscosity (smart lubrication)
• RA- autoimmune disease, attacks synovium
• Phagocytes
• Cod liver oil no benefit but anti-inflammatory

Question 19

Describe the load and role of chondrocytes in maintenance of cartilage health

Answer 19

Close relationship between load and biological properties of connective tissues. They are not inert engineering materials but living, dynamic and responsive to their mechanical environment. Appear simple/homogeneous - but are complex/heterogeneous. In load-bearing connective tissues (e.g. cartilage, tendon), mechanical properties are determined by loading pattern

Question 20

How are mechanical properties determined by loading pattern?

Answer 20

• Load-bearing regions are thicker and mechanically-stronger than non-load-bearing regions.
• Immobilisation causes cartilage thinning and loss however this process is reversible. The application of load to previously immobilised cartilage (e.g. patients in bed rest) might be associated with tissue damage.
• Passive cycling (i.e. no load) does not maintain cartilage health. There are signals that occur with loading that are ‘sensed’ by chondrocytes and alter matrix metabolism via the process of mechanotransduction, but mechanisms complex and not well understood.
• Load-bearing connective tissues must be subjected to physiological loads to be healthy and maintain mechanical properties.
• Loads in excess of ‘physiological levels’ (i.e. impact loads) can cause cartilage damage

Question 21

Describe Wolff’s law

Answer 21

Wolff’s law (originally for bone) but also relevant for connective tissue - that within the physiological loading range, ‘Form Follows Function.’ Impact or excessive load causes matrix damage and chondrocyte death.

• Physiological loads essential for health of musculoskeletal tissues
• Cartilage thickness proportional to prevailing load (thicker where greater)

Question 22

How is cartilage adapted to withstand load?

Answer 22

Has no vulnerable structures; avascular, no neurones, no (aneural) lymphatics, no epithelium at cartilage surface, low cell density (1-10%), resilient and complex extracellular matrix (ECM) - behaves like a fibre-reinforced gel adapted to compressive and tensile forces

Question 23

What are the 4 principal components of articular cartilage?

Answer 23

Collagen
Proteoglycans/ ground substance
Interstitial fluid
Chondrocytes

Question 24

Describe collagen

Answer 24

19 collagen types have been recognised.

• Cartilage contains mainly type II (chondrocytes), fibrocartilage mainly type I (fibroblasts).
• Generally, collagens give rise to fibrillar network of rope-like macromolecules, but many minor and important collagens, (e.g. Type IX - links type II together - forms basketweave structure?).

Question 25

Describe proteoglycans

Answer 25

• PGs have one or more glycosaminoglycan (GAG) chains usually chondroitin sulphate (CS) or keratan sulphate (KS) attached to core proteins, and associated with hyaluronic acid.
• AGGRECAN - (main cartilage PG synthesised by chondrocytes, fibroblasts tend to synthesise more small PGs). Comprises monomers of GAG.
• PGs are highly sulphated and acidic i.e. carry fixed (i.e. immobile) negative charges which are retained within ECM.
• PGs tend to attract cations + water, repel anions and SWELL (if unrestricted can swell to 50x dry volume).
• BUT, in cartilage are only swollen to ~20% of total possible since they are restricted by collagens - like coiled spring (partially inflated balloons). Gives rise to hydrostatic pressure within ECM (23 atmospheres).
• It is possible that a PG synthesised exclusively by chondrocytes within the superficial zone (lubricin) could play a role in reducing joint friction.
• Small PGs (decorin, fibromodulin) tend not to be retained effectively by hyaline cartilage matrix

Question 26

Describe interstitial fluid

Answer 26

• Complex composition - influenced by PGs. More cations (Na+, K+, Ca2+), fewer anions (Cl-, HCO3-) compared to synovial fluid
• Composition of interstitial fluid can alter with sustained static load, which causes fluid expression. This increases PG concentration leading to increased cation concentration and decreased anion concentrations – net effect is increased interstitial fluid osmolarity.
• Hypoxic, hyper-osmotic, acidic, cation concentrations higher, anions lower, changes during static load as fluid extruded

Question 27

Describe chondrocytes

Answer 27

Only living agent, single cell type. Do not divide in healthy cartilage. Totally responsible for synthesis/breakdown i.e. turnover of the ECM, and its repair (if this occurs). Changes to chondrocyte physiology could play a role in the initiation and/or acceleration of the cartilage degeneration in OA – but cellular basis for OA poorly understood.

• Single resident cell type of hyaline cartilage
• Synthesise proteins, growth factors and degradative enzymes
• Chondrocyte proliferation only evident in late stage OA

Question 28

What is matrix permeability?

Answer 28

• The cartilage matrix is selectively permeable.
• Permeation rates are dependent on molecular size, charge and charge distribution of penetrating molecule/ion.
• Some molecules/ions permeate relatively easily (e.g. O2, water, amino acids, anions, cations). Others have extremely low permeability (e.g. proteins in synovial fluid, immunoglobins).
• Permeation of some molecules by reptation? Healthy matrix very tight however matrix becomes ‘loose’ with degeneration (damage to collagen fibrils?)– selective permeability will be lost

Question 29

What is sensing load?

Answer 29

• What are signals during load to which chondrocytes are sensitive? They are complex, and not well understood.
• Depends on type of load, static (tends to depress synthesis) vs dynamic (tends to stimulate synthesis); (a) deformation - a role for ion channels sensitive to membrane stretch? (b) hydrostatic pressure - (200x atmospheric pressure; 20MPa on standing) (c) fluid flow/streaming potentials - as fluid/ions move past cells, (d) changes to ionic/osmotic environment as fluid squeezed out of cartilage during static loading - probably all these signals (and others) involved to differing extents

Question 30

What is matrix turnover?

Answer 30

• Matrix synthesis/breakdown controlled by chondrocytes.
• Usually balanced such that with normal loading; synthesis = breakdown.
• However with increased loading (in the physiological range), synthesis > breakdown; reduced loading (in the physiological range) breakdown > synthesis.
• In cartilage degeneration (e.g. in OA) breakdown&raquo_space; synthesis. Impact can cause permanent cartilage damage. t½ for turnover of PGs, 200-300 days; collagens, many years/decades.
• Small changes can have long term consequences to mechanical properties of cartilage.

Question 31

What are degradative enzymes?

Answer 31

• Matrix breakdown by connective tissue cells occurs continuously with different proteolytic pathways degrading different components of the extracellular matrix.
• Some enzymes might not participate directly in the cleavage of matrix proteins but instead are capable of activating other proteinases which degrade the matrix.
• There is increasing interest in proteinases which are membrane-bound and are not released from the cells. These have been implicated in cytokine processing, control of receptor expression and cell-cell and cell-matrix interactions.
• There is evidence of potent enzymes called ‘aggrecanases’ (specifically ‘metalloproteinases’, MMPs) which cleave at specific site(s) within the proteoglycan molecules.
• Considerable commercial interest has developed recently to identify agents which could block this enzyme to see if it could block the chronic cycle of tissue destruction seen in OA.
• However there are problems since if undiscovered MMPs are involved in normal pathways for matrix turnover, their inadvertent inhibition could lead to undesirable side effects.

Question 32

What is matrix repair?

Answer 32

• Loss of collagen from cartilage is essentially irreversible, and any attempts at repair do not lead to restoration of normal cartilage.
• At the moment most therapeutic approaches are aimed at preventing loss of ECM and hence joint function, and are at the level of counteracting the initial pro-inflammatory stimuli.

Question 33

Describe the adaptions of tendons/ ligaments to mechanical forces

Answer 33

• Tendons transmit load from muscle to bone
• Ligaments transmit load/ give stability from bone to bone (hold skeleton together)
• Cells adapt to prevailing mechanical forces by modifying ECM synthesis

Question 34

Contrast ligaments and tendons

Answer 34

L: joint stabilisation, lower collagen, higher proteoglycans, more random organisation, weaving pattern organisation, 2x elastin, 90% type 1 collagen, 10% type 2
T: force transmission, organised, long axis direction, very little elastin, 98% type 1 collagen

Question 35

What happens in response to excessive in vitro impact?

Answer 35

• Wave of chondrocyte death away from lesion

- Chondrocyte death and cartilage damage from inappropriate mechanical stress

Question 36

What is mosaicplasty?

Answer 36

A method for repairing small areas of degenerate load-bearing cartilage using osteochondral explants

Question 37

Describe matrix synthesis and breakdown

Answer 37

• Totally controlled by chondrocytes
• Normal dynamic loading; synthesis= breakdown
• Greater loading; synthesis> breakdown
• Less loading; breakdown> synthesis
• Cartilage degeneration; breakdown» synthesis
• Impact/ excessive loading can cause permanent cartilage damage
• Loading of connective tissues within physiological limits is essential for cartilage health

Question 38

How does mechanical stress influence biological properties of connective tissues?

Answer 38

• Low intermittent physiological stress decrease properties
• Normal stress= control
• Non-strenuous= increase
• Strenuous= failure of connective tissues, cell/tissue damage

Question 39

What are the signals of load on chondrocytes?

Answer 39

• Depends on type of load, static or dynamic
• Static load depresses synthesis , fluid expression percentage loss increases
• Dynamic load stimulates synthesis fluid expression percentage loss constant
• Fluid flow/ streaming potentials/ ionic composition= static load
• High hydrostatic pressure (above x200 on standing)= dynamic load

Question 40

What is mechanotransduction?

Answer 40

Link between changes to mechanical environment of cells and the cell’s response
-role for integrins and stretch sensitive ion channels

Question 41

How is the ionic/ osmotic environment of chondrocytes unusual?

Answer 41

There is a small positive pressure (2-3 atmospheres) within the matrix arising from the partially-inflated PGs. The partial pressure of oxygen (pO2) in cartilage is ~6% that of blood. (Data from human femoral head cartilage). #Osmotic pressure increased compared to plasma by presence of HA, albumin, proteins.

Question 42

Describe what chondrocytes are

Answer 42

• Exclusively responsible for synthesis/ breakdown of ECM components
• Normally synthesise cartilage-specific ECM components (collagen type 2=strings, aggrecan=balloons)
• Specialised matrix surrounds cells (lacuna or chondron; type 6 collagen)
• Synthesise wide range of degradative enzymes
• Chondrocytes are phenotypically very unstable- role of the cytoskeleton

Question 43

What are the zones of articular cartilage

Answer 43

• Superficial = ellipsoid (8-15 micrometres), 17% cells, high collagen, low GAG, fibro
• Middle (1)= more rounded (12-15 micro), 53% cells, high GAG and collagen, mixed (tangential and basketweave)
• Middle (2)= single/ some pairs (15-25 micro), 53%, higher GAG, lowest collagen, basketweave
• Deep= pairs/ small groups (15-25), 30%. highest GAG, high collagen, basketweave/ perpendicular
• Tide mark
• Calcified cartilage
• Bone

Question 44

Describe the turnover of articular cartilage

Answer 44

• Very slow
• t(0.5) for proteoglycans (200-300 days), and for collagens (decades/ lifetime)
• Cartilage does not repair effectively- connective tissue produced is mechanically incompetent

Question 45

Describe pathogenesis of osteoarthritis

Answer 45

• Complex multifactorial disorder (syndrome) – not really a disease.
• Initiating factor(s) unknown - normal load on abnormal cartilage, or abnormal load on normal cartilage?
• Damage to the basketweave structure of collagen type II – (damage to type IX collagen which links type II).
• This causes increased cartilage hydration (first detectable macroscopic event), decreased PG content and reduced cartilage resilience to load.
• Changes to collagen synthesis – more type I than type II – basketweave structure progressively weakened?
• Synthesis of PGs increases, but this is mostly of small PGs and not the large aggregating PGs (Aggrecan) so the PGs are not effectively retained within the (weakened) collagen network.
• Chondrocyte death increases (cause unknown), and proliferation of the remaining cells increases - an attempted repair response? The increased cell numbers could also be related to the weakening of the ECM resulting in the penetration of growth factors into the cartilage stimulating chondrocyte division.
• There is elevated synthesis and secretion of pro-inflammatory cytokines and mediators (e.g. IL-1, TNF, NO) and degradative enzymes (e.g. proteases, metalloproteinases MMPs, aggrecanases) probably mediated by raised IL-1 levels

Question 46

Describe injuries and occupational risks in OA

Answer 46

• Meniscal injuries, mal-aligned fractures and post-traumatic articular cartilage surface defects are important causes of premature, localised OA in the knee (large joint OA; LJOA) and the risk of developing knee OA is increased more than 3x following major knee surgery.
• Nearly 50% of patients have knee OA 21yrs after open menisectomy and the average time to develop secondary hip OA severe enough for hip arthroplasty, following fracture dislocation is 7yrs.
• Instability of the knee secondary to cruciate ligament injury leads to premature OA in orthopaedic and veterinary practice as well as to experimentally induced OA in the stifle joints of dogs.
• Although previous meniscal injury and surgery are the main predisposing cause of knee OA in professional footballers, the role of sports-related exercise in the absence of defined injury is more controversial.
• Although there is evidence for an increase in the prevalence of knee OA in former elite runners, footballers, weightlifters and tennis players, recreational running seems to be free from this risk and no increase in hip OA has been found after 20yrs of running marathons.

Question 47

Describe sub-chondral bone

Answer 47

• The biomechanical properties of the cortical and sub-chondral bone play a critical role in protecting articular cartilage following impact loading.
• The tissues of the bone and joint capsule are ~30x more effective than articular cartilage in attenuating force.
• Relationship between pathogenesis of OA, and density/stiffness of bone is complex and under-researched and poorly-understood.
• People with OA hip and knee do not have increased bone mass compared to age-matched controls.
• Severe OA was seldom observed in femoral neck fractures.

Question 48

Describe treatment for OA

Answer 48

• Education of the patient about the nature of their condition, its investigation, treatment and prognosis markedly improves outcome.
• Although poorly understood, information access (e.g. via the Internet) and therapist contact both reduce pain and disability, improve self-efficacy (autonomy) and reduce health care costs. Such benefits are modest but long-lasting and safe.
• A ‘prescription of exercise’ including aerobic fitness training and muscle strengthening can reduce OA pain and disability although mechanism unclear.
• Increases the sense of well-being – also benefits common co-morbidity e.g. obesity, diabetes, chronic heart failure and hypertension.
• Reduction of adverse mechanical factors by ‘pacing’ physical activities and ensuring use of appropriate footwear can reduce LJOA symptoms

Question 49

What advice is given on weight loss if the patient is obese?

Answer 49

• Obesity is an important factor for symptomatic and radiographically-defined knee OA affecting all components of the joint.
• Data shows that obesity preceded the development of knee OA with a 40% increase in risk with every 5kg gain in weight. Obesity is a modifiable risk factor.
• Although the relationship between obesity and knee OA is likely to be attributable to changes in the mechanical forces acting across weight-bearing joints, the association with hand OA requires an alternative explanation.
• Relationship between obesity and OA is stronger for women than men suggests that unidentified endocrine factors.
• Joint alignment influences the distribution of load to the articular cartilage and other tissues of weight-bearing joints.
• Intrinsic, as well as acquired, changes in alignment are important independent risk factors for the development, localisation and progression of knee OA with obesity adding markedly to the risk.

Question 50

What are additional options for OA treatment?

Answer 50

• Food products (‘nutriceuticals’) chondroitin and glucosamine are available unlicensed over the counter (OTC).
• The rationale for OA is that as a basic ingredient for some cartilage components they assist in the production of cartilage matrix.
• There is evidence for a modest slow-onset analgesic action in OA lasting 2-4 wks but as yet no convincing data for long-term structural modification of cartilage (or protection of chondrocytes against damage - ‘chondroprotection’) or clinical benefit.
• Such products appear safe and are very popular with patients, however caveats to their use include bovine origin of chondroitin and absence as a food product of the rigorous control required for drugs