004 Cartilage and endochondrial ossification

Question 1

what are some characteristics of cartilage?

Answer 1

• structural and load-bearing connective tissue
• maintains form and resists deformation
• flexible

Question 2

what structural properties mean that cartilage can hold loads?

Answer 2

• composite matrix
• fluid flow that dissipates forces

Question 3

what are the 2 ways cartilage can grow?

Answer 3

• interstitial growth
• appositional growth

Question 4

describe interstitial growth of cartilage

Answer 4

• expansion from within the matrix due to cell division

Question 5

describe appositional growth of cartilage

Answer 5

• ( when young )
• formation of new cells, chondroblasts –> chondrocytes, and matrix at the cartilage boundary (outside)
• precursors at edge

Question 6

what are the structural properties of cartilage that make it unique?

Answer 6

• no nerves or blood vessels
• only cells and ECM
• cells are not interconnected
• it is primitive

Question 7

what are the 2 parts that makeup cartilage?

Answer 7

• cells = chondrocytes
• extracellular matrix ( made by the chondrocytes )

Question 8

what is cartilage ECM made from?

Answer 8

• fibrous component = collagen type 2 ( not visible), elastin and collagen type 1
• ground substance = glycosaminoglycans, proteoglycans

Question 9

give 3 properties of cartilage and why it has the properties due to its components

Answer 9

• permeable = stiff in compression due to proteoglycan aggregates
• resist high tensile stress = fibrous collagen network and fluid-structure
• high swelling pressure = matrix is swollen with water due to proteoglycan aggregates

Question 10

what is type 1 collagen?

Answer 10

• most common (90%)
• fibrils –> fibres
• forms bone, skin, tendons, ligaments, cornea, internal organs, connective tissue

Question 11

what is type 2 collagen?

Answer 11

• fibrils form a meshwork
• not visible with a light microscope
• forms cartilage, intervertebral disc, vitreous humour

Question 12

what is type 3 collagen?

Answer 12

• fibrils –> fibres, form reticular coarse mesh
• forms skin, blood vessels, internal organsa
• reticular fibres

Question 13

what is type 4 collagen?

Answer 13

• sheet-like network
• basal lamina (basement membrane, epithelium sits on top)

Question 14

what is type 5 collagen?

Answer 14

• embryonic collagen

Question 15

what are glycosaminoglycans (GAGs)?

Answer 15

• ground substance part of ECM
• repeating disaccharide units to form very large molecules
• up to 25,000 residues per chain
• stronger negative charge due to COO- group
• hydrophilic = polar = high water binding
• forms hyaluronic acid which is synovial fluid = joint lubricant

Question 16

what are proteoglycans?

Answer 16

• ground substance part of ECM
• glycosaminoglycans (GAGs) on core proteins which are stuck onto a hyaluronic acid core to form aggregates (only in cartilage)
• water binding capacity

Question 17

what are the 2 types of cartilage?

Answer 17

• permanent cartilage
• temporary cartilage

Question 18

give some examples of permanent cartilage

Answer 18

• trachea, bronchioles, ear cartilage, articular cartilage

Question 19

give some examples of temporary cartilage

Answer 19

• epiphyseal growth plate, Meckel’s cartilage, base of skull

Question 20

what are the 3 types of cartilage joints?

Answer 20

• synovial
• symphysis
• synchondrosis

Question 21

describe synovial cartilage joints

Answer 21

• free moveable
• articular cartilage
• e.g. knee joint

Question 22

describe symphysis cartilage joints

Answer 22

• slight movement
• bone - cartilage - fibrous tissue - cartilage - bone
• secondary
• e.g. pubic symphysis

Question 23

describe synchondrosis cartilage joints

Answer 23

• no movement
• bone - cartilage - bone
• primary
• e.g. skull in babies

Question 24

what is mineralisation of cartilage?

Answer 24

• mineralisation at the junction of articular cartilage and bone-tidemark zone
• mineralisation = where cartilage is replaced with bone during growth (endochondral ossification) and is calcified
• mineralisation within hyaline cartilage is part of ageing process

Question 25

what is the tidemark and how does it change with age?

Answer 25

- front of mineralised cartilage - area between mineralised (hard) and unmineralised (soft) cartilage - tidemark advances with age, so unmineralised layer decreases and more cartilage becomes hard and mineralised

Question 26

what are the 2 types of bone formation (ossification)?

Answer 26

- endochondral ossification = long bones - intamembranous ossification = flat bones

Question 27

describe what endochondral ossification is

Answer 27

- occurs in long bones that form initially as a soft cartilage framework which is then replaced by bone - osteoblasts start depositing in the center in the middle of the cartilage (primary ossification) - chondrocytes undergo hypertrophy (increase in size) and then become mineralised which sends a signal to trigger angiogenesis - blood vessels destroy the cartilage as they break through and clear out the bone marrow cavity - secondary ossification centres form at each end of the long bone ( 3 ossifying sections) - epiphyseal growth plates lie between these primary and secondary ossification centres, where the chondrocytes are undergoing hypertrophy and mineralising - post puberty the growth plates disappear and osteoblasts can lay down bone on the mineralised cartilage remanents (destroyed chondrocytes) - only cartilage left is a thin layer on each end

Question 28

where is the primary ossification centre?

Answer 28

centre of the bone

Question 29

where are the secondary ossification centres?

Answer 29

- ends of bone

Question 30

what lies between primary and secondary ossification centres?

Answer 30

- epiphyseal growth plates

Question 31

when do the epiphyseal growth plates disappear?

Answer 31

- after puberty

Question 32

describe intramembranous ossification

Answer 32

- flat bone formation - no cartilage framework - progenitor cells in cartilage differentiate into osteoblasts which release osteoid (uncalcified bone matric) - osteoblasts become osteocytes and matrix is mineralised to form bone

Question 33

what are the 3 locations where mineralisation of cartilage occurs?

Answer 33

- tidemark zone, junction of articular cartilage and bone - endochondral ossification, where cartilage is being replace by bone during growth - hyaline cartilage as part of ageing

Question 34

what 3 things does vitamin D deficiency cause?

Answer 34

- defective mineralisation of cartilage into bone - rickets in children, both cartilage and bone are affected, the growth plate widens and fails to mineralise and be replaced by bone - osteomalacia in adults, newly formed bone matrix fails to mineralise, so bone is soft and weak

Question 35

describe briefly osteoarthritis

Answer 35

- localised condition due to trauma, wear and tear - abrasion of collage meshwork and loss of proteoglycan aggregates = cartilage erosion and bony outgrowths (osteophytes) = deformity - 50% of over 35s have OA at least in 1 joint - advancing irregular tidemark that damages soft cartilage

Question 36

describe briefly rheumatoid arthritis

Answer 36

- a complex, systematic, autoimmune inflammatory condition - chronic with flare-ups - synovium primarily affected joints with the release of inflammatory cytokines like TNFa and IL-1 - morning stiffness, joint pain/deformity, low-grade fever, weight loss bone loss, skin nodules, increased risk of CVD, linked to inflammatory vasculitis - 1% of population affected - smokers, females, and genetic increased risk - treatment = TNFa or B cell antibodies, NSAIDs, glucocorticoids