Bone, Cartilage (growth, repair, anatomy)

What are the needs for a skeleton?

• structural - supports the body

• protection - of vital organs against trauma

• locomotion - provides rigid rods and levers through which muscles can act to cause movement

• mineral reservoir - of organic ions e.g calcium, phosphorus important for bone itself and rest of body

What are the limitations of bone?

• rigid

• hard/brittle - relatively easy to break

• unable to expand from within - limited growth potential

What is bone tissue composed of?

• matrix (organic, inorganic)

• cells (osteocytes, osteoblasts, osteoclasts)

• vascular spaces

What makes up the organic component of the bone matrix and what is it called?

Osteoid = organic component.

consists of: type 1 (mostly) collagen fibres embedded in ground substance

What synthesises osteoid?

osteoblasts - secrete osteoid as liquid, but can form new bone (solid)

what does the ground substance consist of?

• water

• glycoproteins - binds collagen and minerals

• proteoglycans - bind growth factors

• bone sialoproteins - associated with cell adhesion

What makes up the inorganic component of bone matrix?

bone minerals - calcium + phosphorus. make up 60-70% of dry weight. contributes to the hardness and rigidity. composed mostly go crystals.

What is the process of bone formation?

• osteoid secreted by osteoblasts

• mineralisation begins as soon as osteoid secreted (can take years to reach full mineral capacity)

• collagen fibres deposited in different organisation structures. haphazard collagen organisation forms woven bone (quick and easy), as bone matures, lamellar bone is formed which has a collagen fibres laid down parallel (structurally superior, more strength)

when is woven bone formed?

• young growing animal

• fracture repair

What is osteonal bone?

a type of mature lamellar bone, consisting of primary and secondary osteons.

What are primary osteons and when are they formed?

formed during appositional bone growth, when the bone increases in diameter. formed during modelling of the bone (rather than remodelling). they run parallel to the long axis of the bone, contain one or more vascular canals and are always surrounded by woven bone (gives flexibility for osteons to be created).

What are secondary osteons and when are they formed?

formed during remodelling (replace bone, repair fractures). brach off primary osteons out towards site of bone remodelling (e.g. site of fracture). can die back again if not needed.

What are the components of an Osteon/Haversian System?

• Haversian Canal - in the centre, contains nerves, blood vessels, lymphatic vessels

• lamella - concentric rings/layers of bone surrounding Haversian Canal

• osteocytes - found within lacunae

• lacunae - spaces between lamella

• canaliculi - channels carrying nutrient fluid, connect the lacunae, communicate with central Haversian canal, processes which extend off osteocytes lie in the canaliculi

What are interstitial lamellae?

layers of bone that fill the space between adjacent Haversian Systems

What are Volkman’s canals?

join the various Haversian Canals between different systems. contain nerves, blood and lymphatic vessels.

What is the difference between Volkman’s canals vs Haversian canals?

• Volkmans tend to be smaller than Haversian canals

• volkmans canals tend mostly to be found in secondary osteons rather than primary osteons, while Haversian canals tend to be found mostly in primary osteons (not always)

What are osteogenic cells?

stem cells found within the inner aspect of the periosteum, the endosperm and in bone marrow. they divide to form osteoblasts.

what are osteoblasts?

bone building cells, secrete the osteoid. active in mineralisation process. once trapped in the organic matrix, and can no longer secrete the osteoid, osteoblasts become osteocytes.

What are osteocytes?

mature bone cells that maintain the normal cellular activity of bone, such as exchange on nutrients, maintain the matrix. formed from osteoblasts. long dendritic processes from osteocytes lie in the canaliculi allowing osteocytes to contact and communicate with each other, forming gap junctions. are found scattered between the matrix.

What are osteoclasts?

bone-consuming cells (bone resorption), release strong enzymes/proteases to dissolve the bone matrix. can release protons to acidify the environment leading to demineralisation. important for remodelling, growth and repair of bone. they are larger, multi-nucleated cells with a ruffled border. derived from bone marrow.

Where are osteoclasts mainly found?

mainly found in the endosteum (thin, vascular membrane that lines the inner surface of bones). often sit within a Howship’s lacuna (open spaces/areas of bone that osteoclasts have eroded away).

Outline the process of bone remodelling.

• bone excavated/ bone resorption- osteoclasts remove a portion of bone

• bone replaced/ bone formation - osteoblasts release osteoid, which undergoes mineralisation to form bone.

When is bone remodelling needed?

• change in bone shape

• change in bone material

• repair of damaged bone

• release of mineral ions

What are the differences between bone and cartilage?

• cartilage is more pliable than bone

• cartilage able to undergo interstitial growth (internal expansion)

• matrix is different in biochemistry, microstructure, metabolism and cell types

• avascular (no blood vessels) and aneural (no nerves), no lymphatic system

what are the main roles of cartilage?

• joints - flexible but strong interface between bones, smooth bearing surface

• flexible support - outer ear, sternum, larynx, cartilage rings of trachea

• withstand and distribute load

• act as a shock absorber

What are the different types of cartilage?

• Hyaline cartilage

• Elastic Cartilage

• Fibrocartilage

What is hyaline cartilage composed of and where is it found?

• mainly composed of type II collagen, highly hydrated ground substance

• found on articular surfaces of bone (joints), walls of respiratory system, epiphyseal plates/growth plates

What is articular cartilage?

a specialised type of hyaline cartilage that covers the end of bones at synovial joints.

What is elastic cartilage composed of and where is it found?

• mainly composed of elastic fibres and type II collagen.

• found where flexibility required, e.g external ear and auditory tubes, epiglottis (in the throat)

What is fibrocartilage composed of and where is it found?

• mainly composed of type I and type III collagen, a mixture of hyaline and dense regular connective tissue. combines tensile strength of collagen fibres with resistance to compression.

• found where tendons attach to bones, menisci and intervertebral discs. areas which require tough support or great tensile strength.

What type of cartilage does the embryonic skeleton start as?

hyaline cartilage

What are chondrocytes, what are their properties and function?

• (pretty much) the only cells found in the cartilage

• produce and maintain the cartilage matrix

• exist in low density

• obtain nutrition and oxygen by diffusion

• chondrocytes continue secreting new matrix when embedded in matrix, internal expansion/interstitial growth (bone does not do this)

• capable of division within the matrix

• chondrocytes in different zones of cartilage have different biochemical, biomechanics ad physiological properties.

What are the different zones found in hyaline cartilage?

• superficial

• middle

• deep

• calcified (closest to bone)

What is hyaline cartilage composed of (detail)?

• 3D lattice of type II collagen fibres - orientation of fibres divided into zones, deeper layer calcifies

• hydrated gel - contains chondrocytes, no blood vessels, lymphatics or nerves, deeper portion calcifies, fills space between collagen fibres. the hydrated gel is held together by other structural molecules.

What is a stress fracture?

a syndrome involving localised bone injury associated with fatigue damage subsequent to repetitive loading. a combination of load, repetition and inadequate recovery = stress fracture.

describe stress fractures in racehorses/

• underlying cause of lameness in wide range of conditions in racehorses

• behind most catastrophic fractures in racehorses - stress fractures can build up over time to lead to catastrophic fractures

stress

force per unit area

strain

percentage of elongation

elastic region

measure of elasticity - structure will return to its original shape

yield point

point at which the structure will no longer return to its original shape

plastic region

structure deformed and moving towards failure

how do fractures make bone more likely to fracture again?

high strain, micro damage, remodelling, increased porosity (bone weaker)

what history can indicate a stress fracture?

• intense training

• none - incidental finding

• none - spontaneous catastrophic fractures

• subtle loss of performance

• acute-onset lameness associated with work

how can stress fractures be prevented?

change intensity of exercise, type of exercise, ensure adequate rest periods.

What is connective tissue composed of?

• fibroblast/cytes

• extracellular matrix - composed of fibres and ground substance

What fibres are found in connective tissue (within the ECM), what are their properties and where are they found?

• collagen fibres - tensile strength, tend to keep shape, crimp (wavy) arrangement, found in tendons, fascia, capsules, cartilage, bones

• elastic fibres - stretch up to 150%, found in capsules, lungs, arteries

• reticular fibres - forms storm - provide a supportive scaffolding, help keep shape of an organ. found in liver, kidney, spleen, lymph nodes, bone marrow.

What is collagen composed of?

• polypeptide triple helix - rope-like structure = v. strong

• amino acids - glycine, proline, hydroxyproline, hydroxylysine

• Lysyl oxidase - catalyses cross links formation (covalent bonds = v.strong)

What are the different types of collagen fibres and where are they found?

• type I - skin, tendons, organs, mature scar tissue, artery walls, cornea, endomysium, fibrocartilage, surrounding muscle fibres, organic part of bones and teeth

• type II - hyaline cartilage, vitreous humour

• type III - reticular fibres (organ stroma), granulation tissue

• type IV - basal lamina, eye lens, filtration system of capillaries and glomerular

• type V - interstitial tissue, placenta

What does ground substance (within the ECM) consist of?

• interstitial fluid - transport of small molecules between cells and blood supply

• ground substance - GAG (glycosaminoglycans), Proteoglycans, Glycoproteins

What are glycosaminoglycans and which are found in ground substance?

• unbranched polysaccharide chains composed of repeating disaccharide units (one of them an amino sugar)

• huge molecules that occupy large amounts of extracellular space

• highly negatively charged

• good at withstanding compressive forces

• found in ground substance: Hyaluronic acid, Chrondroitin sulphate, Keratin sulphate

how can glycosaminoglycans be used clinically?

• reduce inflammation

• enhances function

• not sure how they work/not understood

What are the characteristics and functions of hyaluronic acid?

• resists compression - major component of hyaline cartilage and synovial fluidity.

• major component of skin - involved in tissue repair

• facilitates cell migration and proliferation

outline the importance of hylauranic acid in synovial joints.

allows cartilage and bone to slip past without damage by reducing friction. as animals get older, hyaluronic acid decreases in joints, leading to joint issues.

What are proteoglycans and what do they do?

• large aggregates - composed of a core protein and 1 or more covalently bound GAGs

• lend plasticity and elasticity to tissues

• trap water to influence tissue consistency

• maintains shape of tissue

• e.g. aggrecan - major component of cartilage ECM

What are glycoproteins, their role and which are found in ground substance?

• proteins that contain oligosaccharide chains convalently bonded to polypeptide side chains

• play a role in cell-cell interactions

• bind together fibres, cells and ground substance in connective tissues.

• found in ground substance: laminin, fibronectin

What is fibronectin secreted by and what is its functions?

• secreted by many cells, primarily fibroblasts

• involved in cell adhesion, growth, migration and differentiation, wound healing, embryonic development

Where is laminin a major protein, and what are its functions?

• major proteins in the basal lamina

• functions: cell differentiation, migration, adhesion, phenotype, survival

What does loose connective tissue consist of?

• interstitium - supporting nerves and vessels

• stroma - structural tissue of organ, framework supporting parenchyma

• fluid reservoir

• defence and regeneration

• type III collagen

What does dense connective tissue consist of/structure?

• fibre-rich

• disordered bunches of parallel fibres found in sclera, skin and dura matter

• parallel ordered bundles sound in tendon and ligaments

• sparse cellularity

• type I collagen

What does tendon/ligament consist of?

• parallel collagen fibres - strength needed in one direction

• surrounded by peritenon (protective layer)

• fibrocytes/tenocytes

Where is supportive connective tissue found and what are its properties?

• bone and cartilage (types of supportive connective tissue)

• sparse cellularity

• collagen fibres give tensile strength

• composition of ground substance give characteristic mechanical properties

What are the different types and subgroups joints?

• fibrous joint - subgroups: sutures e.g skull, syndesmoses e.g. splint bones, gomphosis e.g. teeth-alveoli

• cartilaginous joint - synchondroses e.g. between skull and hyoid bone, symphysis e.g. 2 halves of pelvis

• synovial joint

What are fibrous joints?

bones are directly united by dense fibrous connective tissue without a joint cavity, allowing little or no movement.

what are cartilaginous joints?

bones are united by hyaline or fibrocartilage, without a joint cavity, allows only slight or temporary movement.

What are synovial joints?

2 or more opposing bone surfaces covered by articular cartilage and enclosed by a fluid-filled (synovial fluid) joint capsule

What are the features of a synovial joint?

• articular cartilage

• subchondral bone - sits just below the articular cartilage

• epiphyseal bone

• synovial membrane

• synovial fluid

• joint capsule and ligaments

What are the characteristics of subchondral bone?

• forms joint surfaces

• distributes loads to epihyseal, metaphysical cortical bone

• variable thickness

• undulating junction with cartilage

• rich blood supply, lymphatics and nerves - due to cartilage not having any vessels

• relatively compliant - absorbs impact loads

What are the characteristics of synovial fluid and what does it do?

• clear, slightly yellow, slippery, viscous fluid

• fills joint space

• an ultrafiltrate of plasma

• provides lubrication - joint slides past itslef

• provides cartilage with oxygen and nutrients

what does an increased volume of synovial fluid cause, when does it occur?

• distended joint

• reduced joint stability

• increased distance for nutrients to diffuse

• occurs when joint is experiencing disease

What are some clinical benefits of synovial fluid?

• can analyse fluid to determine if there is evidence of disease

• intra-articular injection is used for diagnostic and therapeutic purposes

What is the normal vs abnormal appearance of synovial fluid?

normal:

• clear/pale yellow

• no cloudiness

• viscous - forms a string between your fingers or from end of syringe

abnormal:

• yellow/orange/red

• cloudy

• watery - does not form a string

Describe the cytology of normal vs abnormal synovial fluid.

normal:

• usually low cell count

• some neutrophils, lymphocytes and mononuclear cells

abnormal:

• increased neutrophils for bacterial and fungal infections

• increased eosinophils for autoimmune disease

what biomarkers can be found in synovial fluid?

• markers of increased metabolism (bone and cartilage)

• markers of increased matrix desrtuction

• markers of inflammation

What is the synovial membrane and its characteristics?

• lines all inner surfaces of joint (except cartilage)

• highly vascularised

• ruffled or smooth surface - synovial villi increase surface area and secret fluid.

• 2 main cell types (synoviocytes) - A (macrophage-like, eat debris) and B (fibroblast-like, synthesise macromolecules)

What is the function of the synovial membrane?

• produce synovial fluid - filter plasma and synthesise essential molecules e.g collagen and hyaluronan

• nutrition - brings blood vessels close to cartilage surface

• clear debris from joint

What is the joint capsule?

• supports synovial membrane/synovium

• stablises joint

• contains nerves which are important in proprioception - monitor joint position and movement

What are collateral ligaments and their function?

• provide stability between bones forming a joint

• tendon-like but higher ratio of elastin to collagen - allows some more stretch

What do tendon sheaths do at a synovial joint?

ease passage of tendons around angles

What do bursae do at a synovial joint?

cushion and protect soft tissues passing over hard points

What factors are important in joint homeostasis?

• load-bearing/movement - without movement, new synovial fluid is not passed around joints so cartilage not fed oxygen and nutrients

• ‘normal’ forces - ensures even load-bearing across the joint surface, distributes weight evenly through peri-artiular tissues

• cartilage turnover - continual slow process of remodelling and repair of collagen and proteoglycans

• low concentrations of cytokines - peptide regulatory fibres

What is the function of cytokines?

• released by cells for intercellular communication

• important in regulation, metabolism and disease

What are important cytokines found in joints?

• Interleukin 1

• Tumor Necrosis factor

• Insulin-like Growth Factor

• Transforming Growth Factor

How can cytokines in joints be used clinically?

• normally, cytokines present in low concentrations in normal joints, but concentrations can change which can be used to indicate disease e.g. cancer. so can be used as biomarkers.

What are the different types of synovial joint, with examples?

• ball and socket e.g. hip, shoulder

• hinge e.g. elbow, knee, fetlock

• gliding e.g. carpal (wrist) and tarsal (ankle)

how is an equine shoulder joint different to a human shoulder joint and what are the possible reasons?

horse shoulder joint is technically a ball and socket joint but only allows flexion and extension rather than movement in all directions (as in humans). horses of not have a clavicle (collar bone), while humans do.

humans shoulder prioritises dexterity and positioning of hand while horse shoulder made for weight-bearing, high speed propulsion and impact adsorption rather than fine motoring tasks.

Where are synovial joints found on a horse?

shoulder joint, elbow joint, knee/carpal, fetlock, pastern, coffin, hock, stifle, hip

What forces act on a synovial joint?

• compression - minimised by articular cartilage and synovial fluid

• tension - resisted by ligaments and joint capsule (primarily)

• shear

• gravitaitional

• muscular

• frictional resistance - minimised by articular cartilage and synovial fluid

What is joint pathology?

study and diagnosis of diseases, disorders and structural changes affecting joints.

What is a normal biomechanical force?

force that is balanced, evenly distributed loads - maintain joint and tissue health during movement

What is an abnormal biomechanical force?

misaligned, unstable or concentrated loads that damage cartilage or bones - anatomic derangements e.g. hip dysplasia, focus high forces onto small localised areas of cartilage.

What minerals are stored in bone?

calcium and phosphorus

What are the roles of calcitonin, calcitriol and parathyroid hormone (PTH)?

• calcitonin - acts on the bone to decrease elevated blood calcium levels

• PTH - acts on bone to increase low blood calcium levels to normal (PTH results in rapid mobilisation of calcium from bone)

• calcitriol - increases the intestinal adsorption of calcium (and phosphate)