23 A - Cartilage and Bone Supplement Flashcards
3 types of cartilage
- hyaline
- elastic
- fibrous
Proporties of hyaline cartilage
- most common
- large chondrocytes surrounded by cartilage matrix
- mainly type 2 collagen and chondroitin sulfate (GAG)
- articulating surfaces of joints, nose, larynx, trachea and bronchi
Colour of hyaline cartilage
transparent
Properties of elastic cartilage
- histology and matrix similar to hyaline (large chondrocytes surrounded by cartilage matrix)
- but matrix has elastic fibres/elastin
- chondrocytes arranged between fibres
- ear (pinna and ear canal) and epiglottis
What colour is elastic cartilage>
yellow
Properties of fibrous cartilage
- parallel rows of smaller chondrocytes embedded between type I collagen fibre bundles
- high tensile strength, resists pressure
- intevertebral disks, TMJ, pubic symphysis
Where is hyaline cartilage found?
- articulating surfaces of joints, nose, larynx, trachea, bronchi
Where are elastic cartilage found?
- ear (pinna and ear canal)
- epiglottis
Where is fibrous cartilage found?
- intevertebral disks
- TMJ
- pubic symphysis
3 mechanisms of bone formation
- endochondral ossification
- intramembranous ossification
- sutural ossification
Explain endochondral ossification
- bones made from cartilage model
- condrocytes produce hyaline cartilage that is replaced by osteoid/bone from osteoblasts
- e.g long bones (epiphyseal growth plate), mandibular condyle (secondary cartilage) and base of skull (synchondrosis)
Explain intramembranous ossification
- bones made directly from osteoblasts that have differentiated from mesenchymal stem cells
- like flat skull bones, facial bones, mandible, maxilla
Explain sutural ossification
- similar to intramembranous - bone directly from osteoblasts from mesenchymal stem cells
- but with fibrous connection providing stability during growth
- e.g postnatal growth of skull bones
List embryonic origins of skeleton
- trunk axial skeleton
- appendicular skeleton
- skull bones
Explain the embryonic origins of the skeleton
- trunk axial skeleton from sclerotome of mesodermal somites - endochondral ossification
- appendicular skeleton from lateral plate mesoderm - endochondral ossification
- skull bones - roof and base from mesoderm or neural crest cells (roof - intramembranous ossi, base is endochondral)
- facial bones from neural crest cells - intramembranous ossification
Development of endochondral bones
- early perichondrium is formed by chondroblasts derived from condensed mesenchymal cells
- cartilage model assumes shape of future bone and pericondrium becomes more prominent
- in diaphysis region, perichondrium becomes periosteum. Osteoblasts differentiate from osteoprogenitor cells in periosteum and produce collar of bone (cortical bone - intramembranous)
- cartilage matrix begins to calcify (dots)
- blood vessels invade cartilage model through bone collar and introduce osteoblasts and clasts. get formation of primary ossification centre
- bone trabeculae formed and link to bone collar
- secondary ossification centre established in epiphysis
Growth in length of endochondral bones is … but thickness is …
- epiphyseal growth plate
- periosteum
Cells involved with epiphyseal growth plate
- resting chondrocytes
- proliferating chondrocytes
- prehypertrophic chondrocytes
- hypertrophic chondrocytes
- then calcification zone
What are resting chondrocytes?
resevoir of chondrocytes to replenish lost chondrocytes
What are proliferating chondrocytes?
chondrocytes align in column and divide (secrete collagen matrix, collagen type II)
What are prehypertrophic chondrocytes?
- chondrocytes begin to swell
- increased production of cartilage matrix (collagen type X)
What are hypertrophic chondrocytes?
- fully matured chondrocytes
- eventually die by apoptosis
What happens in the calcification zone?
- cartilage matrix being replaced by osteoblasts
Explain mineralisation of endochondral bones
- matrix vesicles bud off from chondrocytes and induce mineral deposition between collagen II fibres
- first hydroxyapatite crystals catalyse formation of mineralisation foci - calcified cartilage
- osteoblasts surround calcified cartilage and deposit osteoid (bone matrix) that is later mineralised to bone
- mixed spicule contains calcified cartilage and bone - chondroclasts remove cartilage
Development of intramembranous bones
- mesenchymal cells in cellular periosteum differentiate to become osteoblasts which produce irregular bone type (woven bone)
- gradual turnover of woven bone to lamellar bone
- formation of primary osteons by osteoblasts surrounding blood capillary
- continued bone replacement produces highly organised, mature bone
- fewer cells, secondary and tertiary osteons, circumferential lamellae
What colour are osteoblasts?
yelloe
Development of sutural bones
- condensation of mesenchymal cells that form periosteum
- differentiation into osteoblasts that deposit woven bone
What is a suture?
- fibrous joints between skull bones
- enable skull bone growth in response to brain growth
How are sutures organised?
- cambrian layer - cellular for bone growth mediated by osteoblasts
- capsular layer - fibrous for stability mediated by fibroblasts
Histological sequence of sutures
- bone
- cells
- fibres
- cells
- bone
List molecular control methods of skeletal development
- induction of mesenchyme
- condensation of cells
- cell differentiation programme
- endochondral ossification
How is induction of mesenchyme a molecular control of skeletal development?
- notochord to sclerotome cells
- AER to lateral plate mesoderm cells
- neural fold to local environment to neural crest cells
- inducers - WNT, BMP, FGF, SHH
How is condensation of cells a molecular control of skeletal development?
- express N-cadherin (cell adhesion)
- TGF-beta signals stabilise condensation
- differentiation of osteo-chondroprogenitors
How is cell differentiation programme a molecular control of skeletal development?
- express Sox9 to chondroblasts to cartilage (matrix proteins of Collagen II, X) - perm or temp endochondral cartilage
OR - express Runx2 to osteoblasts to bone (matrix proteins of Collagen I, Opn, Ocn) - intramembranous or endochondral bone
How is endochondral ossification a molecular control of skeletal development?
- ordered chondrocyte differentiation
- IHH, PTHrP, BMP regulatory loop
- RUNX2 also induces chondrocyte hypertrophy
OR - VEGF secreted by hypertrophic chondrocytes induces vascular invasion - introduction of osteoblasts and clasts
Molecular control of endochondral bone formation
- PTHrP secreted by perichondrium and periarticular chondrocytes
- induces chondrocyte proliferation and inhibits IHH secretion
- IHH expressed by (pre)hypertrophic chondrocytes that are out of reach of PTHrP signals - IHH directly stimulates chondrocyte proliferation
- IHH stimulates PTHrP expression - negative feedback loop, coordinated chondrocyte differentiation
- IHH also stimulates osteoblasts of bone collar
Molecular control of intramembranous bone formation
- differentiation of mesenchymal stem cells into connective tissue
- osteoblast differentiation (Runx2 induces foramtion of preosteoblasts, osterix induces osteoblast differentiation - express RANKL on cell surface, secrete osteoprotegerin)
- osteoclast differentiation (M-CSF induces formation of preosteoclasts from hematopoietic stem cells - express RANK of cell surface - RANKL/RANK interaction induces fusion of preosteoclasts to form mature osteoclasts - OPG is a decoy receptor blocking this interaction)
- coordination of bone formation and resorption
