locomotor Flashcards
(29 cards)
bone
A mineralized collagen-rich matrix which is very rigid and strong while still retaining some degree of flexibility
bone functions
• Resistance to compression : inorganic content • Resistance to tension : organic matrix
- Houses bone marrow
- Calcium homeostasis
- Protects vital organs
bone cells
– Osteoblasts bone forming – Osteocytes a mature osteoblast surrounded by bone matrix – Bone lining cells – Osteoclasts resorption and degradation of existing bone -osteoprognitor cells osteoblast precursors
bone structure and function
• Bone matrix / mineralisation • Bone remodelling • Bone development – Intramembranous – Endochondral
long bone anatomy
epiphysis metaphysis diaphysis metaphysis epiphysis
epiphyseal growth plate
- Specialised zone of cartilage
- Lies between epiphysis and metaphysis
- Site of longitudinal growth
- ‘Closes’ at /after puberty
- Long bone growth stops
bone - composition
• Cortical (70%) – compact • Trabecular (30%) – cancellous – medullary – spongy bone
bone - macroscopic organisation
• Proportion of cortical / cancellous bone varies in different parts and types of the bone • Mid bone / diaphysis – most cortical little cancellous bone • End of bone / epiphysis – predominantly cancellous bone
compact/ cortical bone
• Provides most structural support • Resists bending and torsion stresses – Thicker in mid part of bone
microscopic structure of cortical bone
• Osteons / Haversian canals – Main structural unit of cortical bone – Bone cylinders 2-3mm long – 8-15 concentric lamellae 0.2mm wide – Axis parallel to long axis of bone – Central cavity with blood vessels and nerve • Volkmann’s canals – Carry blood vessels from periosteum to Haversian system
microscopic structure of cancellous/ trabecular/ spongy bone
• Found inside cortices • Forms interconnecting network of plates / trabeculae • Provides large surface area for metabolic functions
Cancellous / Trabecular Bone
• Provides strength without disadvantage of weight • Organisation of trabecular plates is purposeful • Arranged along lines of maximum mechanical stress – Allows transmission of loads – Support areas of maximum stress • More metabolically active than cortical bone – Larger surface area
osteoid
• Unmineralised bone matrix – produced by osteoblasts • Type I collagen ( 90%) • Non collagenous proteins – Osteocalcin • Marker of bone formation – Osteonectin – Osteopontin – Growth factors
bone matrix - microscopic organisation
• Lamellar bone – Type I collagen fibres laid down in parallel sheets / lamellae – structurally very strong • Woven bone – collagen fibres randomly arranged – Mechanically weak – Formed when bone is being produced rapidly e.g. foetus or fracture
whats needed to from osteoblast
transcription factors Runx2 and osterix
osteoblast function
• Produce and deposit osteoid
• Regulate osteoclast differentiation / function – RANKL – RANK interactions [RANK(L)] = Receptor Activator of Nuclear Factor κ B [(ligand)]
osteoblast fate
• Life span 6 months – Osteoid production – 10 –15% entombed in bone – differentiate into osteocytes – Others die by apoptosis or differentiate into lining cells on quiescent bone
osteocytes
• Most common cell in bone • Reside in lacunae in cortical and trabecular bone – Connect to other osteocytes, osteoblasts and osteoclasts via long cytoplasmic processes
osteocyte function
Regulation of bone remodelling
• in response to local (biomechanical) or systemic
e.g. parathyroid hormone (PTH) signals
• increases osteoclast formation by increased
expression of RANKL = bone resorption
• inhibits osteoblast formation by production of
Sclerostin = decreased bone formation
• Sclerostin production inhibited by PTH and
mechanical loading = increased bone
formation
Calcium homeostasis
• Responds to increasing PTH levels by inducing
rapid calcium release (osteocytic osteolysis)
RANK-RANKL interaction
– induce precursor cell fusion and increase osteoclast
activity
– Regulated by Osteoprotegerin (OPG) a decoy receptor
that binds RANKL and inhibits osteoclast formation
preventing excessive bone resorption
– OPG secreted by osteoblasts and stromal cells
osteoclasts - 1
Monocyte / macrophage derived
multinucleate giant cells
Formation regulated by growth factors and
interactions between RANK (expressed by
osteoclast lineage) and RANKL expressed by
stromal cells / osteoblasts /osteocytes
osteoclasts 2
• Bind to mineralised bone surface • integrins • Resorb bone by production of – Acid to release calcium – Proteases to breakdown organic matrix • By-products of bone breakdown and osteoclast enzymatic activity are used as markers of bone resorption – Detected in blood or urine – Type I collagen fragments – N- and C-terminal cross-linked telopeptides – Tartrate-resistant acid phosphatase – Expressed by osteoclasts
mechanisms of bone formation and skeletal development
• Intramembranous ossification – Osteoid laid down by osteoblasts within loose fibroconnective tissue of a fibrous membrane • Endochondral ossification – Osteoid deposited on cartilage scaffolds
intramembranous ossification
• Formation of skull, maxilla
parts of clavicle/mandible
• Subperiosteal bone growth
• Fracture repair
