004 Cartilage and endochondrial ossification Flashcards
what are some characteristics of cartilage?
- structural and load-bearing connective tissue
- maintains form and resists deformation
- flexible
what structural properties mean that cartilage can hold loads?
- composite matrix
- fluid flow that dissipates forces
what are the 2 ways cartilage can grow?
- interstitial growth
- appositional growth
describe interstitial growth of cartilage
- expansion from within the matrix due to cell division
describe appositional growth of cartilage
- ( when young )
- formation of new cells, chondroblasts –> chondrocytes, and matrix at the cartilage boundary (outside)
- precursors at edge
what are the structural properties of cartilage that make it unique?
- no nerves or blood vessels
- only cells and ECM
- cells are not interconnected
- it is primitive
what are the 2 parts that makeup cartilage?
- cells = chondrocytes
- extracellular matrix ( made by the chondrocytes )
what is cartilage ECM made from?
- fibrous component = collagen type 2 ( not visible), elastin and collagen type 1
- ground substance = glycosaminoglycans, proteoglycans
give 3 properties of cartilage and why it has the properties due to its components
- 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
what is type 1 collagen?
- most common (90%)
- fibrils –> fibres
- forms bone, skin, tendons, ligaments, cornea, internal organs, connective tissue
what is type 2 collagen?
- fibrils form a meshwork
- not visible with a light microscope
- forms cartilage, intervertebral disc, vitreous humour
what is type 3 collagen?
- fibrils –> fibres, form reticular coarse mesh
- forms skin, blood vessels, internal organsa
- reticular fibres
what is type 4 collagen?
- sheet-like network
- basal lamina (basement membrane, epithelium sits on top)
what is type 5 collagen?
- embryonic collagen
what are glycosaminoglycans (GAGs)?
- 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
what are proteoglycans?
- 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
what are the 3 types of cartilage joints?
- synovial
- symphysis
- synchondrosis
describe synovial cartilage joints
- free moveable
- articular cartilage
- e.g. knee joint
describe symphysis cartilage joints
- slight movement
- bone - cartilage - fibrous tissue - cartilage - bone
- secondary
- e.g. pubic symphysis
describe synchondrosis cartilage joints
- no movement
- bone - cartilage - bone
- primary
- e.g. skull in babies
what is mineralisation of cartilage?
- 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
what is the tidemark and how does it change with age?
- 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
describe what endochondral ossification is
- 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
where is the primary ossification centre?
centre of the bone
where are the secondary ossification centres?
- ends of bone
what lies between primary and secondary ossification centres?
- epiphyseal growth plates
describe intramembranous ossification
- 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
describe briefly osteoarthritis
- 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
describe briefly rheumatoid arthritis
- 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
