Bone and cartilage Flashcards
Describe cartilage
Specialised connective tissue with a support function (often the shock absorbers of the body, can be tough or flexible depending on composition of matrix)
Cells: chondrocytes
Matrix: Type II collagen (car”two”lage)and proteoglycans ; hydrophilic properties - retains water allowing shock absorption
describe the embryonic derivations of cartilage and how growth occurs
Structure
Derived from embryonic mesenchyme (spindle)
Structure - clusters of chondroblasts surrounded by a layer of perichondrium (mesenchyme derived fibroblastic cells and collagen)
Growth of cartilage is by interstitial (growth from middle) and appositional growth (new chondroblasts from perichondrium; growth from edge ) .
After matrix deposition cells become less active and become maintaining cells (chondrocytes) (lacuna is an artefact)
Describe the ECM of hyaline cartilage
Collagen II + ground substance
70% water
proteoglycan aggregates- GAGs attached to a core protein and hyaluronin ; they are hydrophilic —> compressive strength
Proteoglycans are woven with collagen
What are the 3 types of cartilage and where are they found?
Hyaline - type II collagen ; articulation surface (joints, rib cartilage, tracheal rings, endochondral bone formation)
Elastic - type II + elastic fibres (pinna of ear, auditory canal, laryngeal cartilages, epiglottis)
Fibrocartilage - type I (intervertebral discs, pubic symphysis, joint capsules, ligaments and tendons)
Describe the properties and functions of bone
Specialised form of connective tissue ; composed of cells and ECM
Matrix specialised to be max strength and low weight
Reservoir for calcium and phosphate (role calcium homeostasis)
Involved in haematopoiesis (bone marrow)
Damage to hyaline cartilage
Reversible but very slow as most is a vascular and few perichondrium (stem cells)
Nutrition is dependent on diffusion which limits the thickness
Bone organisation
Dense outer shell - called the compact bone; makes up the majority of the skeleton; consist of lamellae in concentric rings and haversian system of canals (allow passage of blood vessels and nerves)
Spongy inside (makes it lighter) - interconnecting trabeculae with spaces for bone marrow ; also called cancellous bone
Periosteum - fibrous CT layer around the outside ; carries blood supply and osteoprogenitor cells; not present at the joint ends of long bones
Endosteum lines the interior of bones
Describe osteoporosis
Thinning of both the compact/spongy bone (loss of bone mass)
Common in post-menopausal women due to the lack of oestrogen ; also caused by disuse of bone (wheelchair or reduced mobility)
Prone to fracture
Describe the 2 parts of the bone matrix
Organic (living)
Made by osteoblasts, collagen type 1, tensile and compressive strength, non-collagenous proteins mediate mineral deposition
Inorganic (non-living)
Calcium phosphate (hydroxyapatite)
Deposited in organic matrix
Make up the majority of dry weight of bone
Provide hardness to bone
Describe osteogenesis imperfecta (OI)
Congenital disease causing defecting collagen
Fragile skeleton
Type II fatal in uterine
Type I increase childhood fractures
Extra skeletal symptoms - lax(hyperflexible) joint/skin and blue sclera
Describe the bone cell differentiation
Derived from mesenchymal stem cells —> osteoprogenitor cells or chondroblasts
Osteoprogenitor cells —> osteoblasts
Osteoblast: lays down organic bone matrix
And mediates mineralisation of osteoid
Osteoblast becomes osteocyte when surrounded by mineralised bone; their function is to maintains matrix
Describe the mineralisation of osteoid
Osteoblasts secrete type 1 collagen and matrix vesicles
Matrix vesicles contain enzymes and proteins to control availability of calcium and phosphate so that mineral is precipitated.
Immature: Woven bone. haphazard fibre arrangement, mechanically weak – found in foetal development/fracture repair
Mature: Lamellar bone: remodelled woven bone –regular parallel collagen, strong: all adult bone. arranged as individual units called osteons (aligned with the direction of force)
describe osteoclasts and osteocytes and the communication between them
Osteoclasts break down bone (bone resorbing) to provide source of Ca2+ during starvation ; they work with osteoblasts to regulate bone turnover and remodelling ; originate from macrophage cell line
Osteocytes are mature osteoblasts are surrounded by mineralised matrix; long cytoplasmic processes allow connection to other cells via gap junctions ; allows response to stress from bone deformity
Hormones affecting osteoclast activity
Calcitonin reduces osteoclast activity
Oestrogen and parathyroid hormone increase osteoclast activity
Describe the factors affecting bone remodelling
Slower bone remodelling in adults vs children
Can be increased by increased activity, repair of fractures, Paget’s disease
Osteopetrosis
Cause
Mechanism
Symptoms
Treatment
Rare group of inherited conditions
Reduced osteoclast activity - defective bone remodelling
Osteoclasts cannot excrete H+ ions to dissolve bone mineral
Dense stone bone - brittle and easily fractures
Symptoms - fractures, spinal nerve compression from excesss bone, recurrent infections due to reduced bone marrow cavity ; hepatosplenomegaly due to haemoatopoiesis outside bone
Bone marrow transplant can be effective
Embryonic bone development - describe intramembranous ossficiation
occurs when mesenchymal cells ensheathed in membranous tissue directly undergo ossification
sheets of mesenchymal cells differentiate into osteoblasts in centres of ossification ; these merge to form trabecular bone that is remodelled. Remaining mesenchymal cells make the bone marrow and periosteum
Skull, mandible and maxilla are made this way
Contrast bone growth at the epiphyseal end and diaphyseal end
Epiphyseal end: Proliferation
Diaphyseal end: chondrocytes mature and die and are replaced by bone
Achondroplasia
Congenital bone disease
Commonly known as dwarfism
Caused by mutation causing activation of fibroblasts growth factor receptor 3 ; this inhibits chondrocyte proliferation - affects growth plates
Disorganised and hypoplastic growth plates - long bone growth is stunted
Lordosis/bowed legs/stunted extremities
Describe the relationship between osteoblasts and osteoclasts
PTH binds to receptors on osteoblasts
Osteoblasts activate osteoclast precursors
Osteoclasts precursors stimulated to differentiate into osteoclasts = bone resorption occurs
Osteoblasts can also inhibits resorption by making osteoprotegrin
Bone resorption regulated by balance of PTH action and osteoprotegrin production
Describe the growth of curved bones and increased circumference of long bones
Coordinated appositional growth at the periosteum and resorption on the inside
(Bone grows outwards whilst the inside is being the removed)
Role of PTH
1st effect is to increases blood [Ca2+] to meet metabolic demand :
Releasing calcium from bone via osteoclasts
Stimulating vit D - increased Ca 2+ uptake in gut
Increased Ca2+ kidney reabsorption
2nd effect is to decrease blood [phosphate]:
Reduced reabsorption of phosphate by kidneys
3rd effect is to remove bone material from areas of bone that are under little stress (from sedentary lifestyle)
Describe how bone fractures are repaired
Periosteum is breached = haematoma = blood clot
Break in periosteum is replaced by granulation tissue which becomes fibrous
Eventually fibrous granulation tissue become a provisional callus made of hyaline cartilage
Bony callus formed by newly layed down woven none by osteoblasts
Bony union is slowly remodelled to form oriented lamellar bone
embryonic bone development - describe endochondral ossification
occurs when mesenchymal cells first differentiate into cartilage models before undergoing ossification
Cartilage template ; gets laid down first as blood supply not sufficient
When blood supply arrives, osteoblast differentiation occurs ; chondroblasts replaced by osteocytes (cartilage is replaced by bone) ; growth occurs outwards from centres of ossification (clusters of osteoblasts)
Long bones and base of skull made this way
what is osteoid?
osteoblasts secrete a matrix material called osteoid, a gelatinous substance made up of collagen, a fibrous protein, and mucopolysaccharide, an organic glue
long bone of the limbs have how many centres of ossification ?
one at the centre then one at each end
describe how bone deposition occurs in response to stress (e.g lifting weights)
calcitonin released = activation of osteoblasts = osteoblasts deposit osteoid = phosphate and calcium ions move into bone from blood = mineral deposition in osteoid = calcification
