Bone form and function Flashcards
What are long bones?
Bones of the leg, arm, primarily compact bones e.g. the tibia
What are short bones?
Bones of the wrist and ankle, primarily spongy bones
What are flat bones?
Most bones of the cranium, ribs, spongy bone between 2 layers of compact bone
What are irregular bones?
Vertebrae, primarily spongy bones
What are the two origins of bones?
Long bones - Endochondral ossification
Flat bones - intramembranous ossification
What are the two histological classifications given to bones?
Woven - primary immature: woven bone, not organised
Lamellar - Secondary mature: lamellar bone
What is woven bone?
Immature, fracture, quick
- randomly orientated collagen fibres
- variable collagen fibre diameter
- rapid matrix mineralisation
- forms rapidly
- rapid turnover (remodelling)
What is lamellar bone?
Mature, adult
- regular orientation collagen fibres
- regular collagen fibre diameter
- delayed matrix mineralisation
- forms slowly
- slow turnover
How can bones be classified anatomically?
Spongy (trabecular)
Compact (cortical)
- both are made of the same cells and matrix elements
What is compact bone?
Very strong
Made of osteons: central canal (haversian) surrounded by rings of bone matrix (concentric lamellae)
What forms when two osteons merge?
One osteon with radiating canaliculi (remainder of the other osteon)
What is between osteons?
Interstital lamellae
How do osteocytes communicate?
Radiating canaliculi
What is trabecular bone?
Forms mesh like structures made of plates and rods
- holes formed are the trabecule
Large surface area but less dense
Very metabolically active due to being highly vascularised
What is the periosteum?
Fibrous membrane of connective tissue that covers all bones but not articular surfaces
- has an important role in bone growth in length but mainly in diameter
- also important in fracture repair
Describe the macroscopic organisation of bone
From cranial to caudal - Cartilage - Growth plate -- both in the epiphysis - trabecular bone surrounded by the compact bone -- in the metaphysis region - bone marrow cavity - bone marrow - blood vessel -- all in the diaphysis region Periosteum surrounds all the structures on the outside of the compact bone layer
What are the blood vessels within the bone?
Perforating canals (Volkman's canals) - blood vessels from periosteum penetrate bone Vessels of the central canal
What does a blood supply to the bone provide?
Nutrients to cells
Route for migration for stem cells
Regenerative ability
How does a bone function determine its form?
Bone adapts its shape dependant on the mechanical force applied to it with 2 targets:
- minimum weight with maximum mechanical strength
Why is no bone straight?
Deformation due to the force applied
- dependant on the material properties of the bone
What material is bone?
Composite material
- hydroxyapatite crystals, collagen orientation
Describe the microscopic organisation of bone
Bone matrix organic (proteins)
- collagen fibres type 1 (90% of the total protein)
- proteoglycans
- non - collagenous proteins
Mineral (inorganic): Crystals of hydroxyapatite (Ca10(PO4)6(OH)2)
- Cells: osteoblasts, osteocytes, osteoclasts
What happens if mineral is removed from bone?
Too bendable
What happens if collagen is removed from bone?
Too brittle
When is the organic matrix mineralised?
Immediately after secretion in case of rapidly forming (woven) bone (fetal, fracture callus)
- in adults m
normally a delay of several days before osteoid mineralises
What is the role of hydroxyapatite in bone formation?
Tiny crystals surround collagen fibres
- provides rigidity, resistance to compression
What is the mineralisation of osteoid dependant on?
Hormonally active form of vitamin D (1,25 (OH)2 D3)
What does a deficiency in hormonally active vitamin D cause?
Results in a failure to mineralise
- rickets in children
- osteomalacia in adults
What are mineralisation promoters?
- tissue non-specific celeceline phosphate (TNAP)
- phosphate orphan 1 (PHOSPHO1)
What are mineralisation inhibitors?
- inorganic pyrophosphate (PPi)
- organic non-collagenous proteins
What is NPP?
NPP = ectonucleotide pyrophosphatase/phosphodiesterase-1
- NPP1 key regulator of skeletal and soft tissue mineralisation
How does NPP work?
NPP
ATP →→→→→ AMP + PPi
What are osteoblasts?
Cells responsible for bone formation
What are osteoclasts?
Cells responsible for bone reabsorption
What are osteocytes?
Cells that mediate effect of mechanical loading and regulate phosphate homeostasis
Where do osteocytes originate from?
Hematopoietic cells in bone marrow
Which bone cells originate from mesenchymal stem cells?
Osteoblasts
Osteocytes
How do osteoblasts work?
- Secrete organic matrix of unmineralised bone (osteoid) and mineralise the matrix with Ca10 (PO4)6 (OH)2
- Express high levels of alkaline phosphatase, secrete and respond to many growth factors, cytokines and hormones
- Active or inactive lining cells (non-active osteoblast) cover most bone surfaces
How do osteocytes work?
Most abundant cell found in bone (90-95% of all bone cells)
- communicate with each other via canaciuli
- mechanically responsive cells
- express high levels of sclerostin (inhibitor of bone formation), FGF23 (regulates phosphate metabolism in the kidney) and rank ligand (acts on osteoclasts to stimulate bone reabsorption)
How do osteoclasts work?
Polarised, multi-nucleated, motile, bone-reabsorbing cells
- formed by fusion of promonocytic precursors present in marrow and circulation
- specialised organ of reabsorption (“ruffled border”) adjacent to bone surface
- express high levels of carbonic anhydrase II, tartrate - resistant acid phosphatase (TRAP) and cathepsin K
What does osteoclastic differentiation depend on?
Cytokines
M-CSF
RANK 2
How is osteoclastic bone reabsorption performed?
2 steps:
- initiated by the dissolution of the mineralised matrix followed by enzyme degredation of the organic matrix
Osteoclasts dissolve minerals by a process of acidification due to secretions of of protons by the vacuolar type proton pump V-ATPase
- acidifies enclosed canunae - pH 4.5
- this leads to de-calcification of hypoxyapatate leading to exposure of organic matrix consisting largely of type one collagen
