Bone and cartilage

Question 1

Describe cartilage

Answer 1

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

Question 2

describe the embryonic derivations of cartilage and how growth occurs
Structure

Answer 2

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)

Question 3

Describe the ECM of hyaline cartilage

Answer 3

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

Question 4

What are the 3 types of cartilage and where are they found?

Answer 4

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)

Question 5

Describe the properties and functions of bone

Answer 5

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)

Question 6

Damage to hyaline cartilage

Answer 6

Reversible but very slow as most is a vascular and few perichondrium (stem cells)
Nutrition is dependent on diffusion which limits the thickness

Question 7

Bone organisation

Answer 7

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

Question 8

Describe osteoporosis

Answer 8

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

Question 9

Describe the 2 parts of the bone matrix

Answer 9

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

Question 10

Describe osteogenesis imperfecta (OI)

Answer 10

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

Question 11

Describe the bone cell differentiation

Answer 11

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

Question 12

Describe the mineralisation of osteoid

Answer 12

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)

Question 13

describe osteoclasts and osteocytes and the communication between them

Answer 13

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

Question 14

Hormones affecting osteoclast activity

Answer 14

Calcitonin reduces osteoclast activity

Oestrogen and parathyroid hormone increase osteoclast activity

Question 15

Describe the factors affecting bone remodelling

Answer 15

Slower bone remodelling in adults vs children

Can be increased by increased activity, repair of fractures, Paget’s disease

Question 16

Osteopetrosis

Cause
Mechanism
Symptoms
Treatment

Answer 16

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

Question 17

Embryonic bone development - describe intramembranous ossficiation

Answer 17

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

Question 18

Contrast bone growth at the epiphyseal end and diaphyseal end

Answer 18

Epiphyseal end: Proliferation

Diaphyseal end: chondrocytes mature and die and are replaced by bone

Question 19

Achondroplasia

Answer 19

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

Question 20

Describe the relationship between osteoblasts and osteoclasts

Answer 20

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

Question 21

Describe the growth of curved bones and increased circumference of long bones

Answer 21

Coordinated appositional growth at the periosteum and resorption on the inside

(Bone grows outwards whilst the inside is being the removed)

Question 22

Role of PTH

Answer 22

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)

Question 23

Describe how bone fractures are repaired

Answer 23

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

Question 24

embryonic bone development - describe endochondral ossification

Answer 24

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

Question 25

what is osteoid?

Answer 25

osteoblasts secrete a matrix material called osteoid, a gelatinous substance made up of collagen, a fibrous protein, and mucopolysaccharide, an organic glue

Question 26

long bone of the limbs have how many centres of ossification ?

Answer 26

one at the centre then one at each end

Question 27

describe how bone deposition occurs in response to stress (e.g lifting weights)

Answer 27

calcitonin released = activation of osteoblasts = osteoblasts deposit osteoid = phosphate and calcium ions move into bone from blood = mineral deposition in osteoid = calcification