Immature, fracture, quick - randomly orientated collagen fibres - variable collagen fibre diameter - rapid matrix mineralisation - forms rapidly - rapid turnover (remodelling)

Bone form and function Flashcards by Ellie Roberts

What are long bones?

Bones of the leg, arm, primarily compact bones e.g. the tibia

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What are short bones?

Bones of the wrist and ankle, primarily spongy bones

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What are flat bones?

Most bones of the cranium, ribs, spongy bone between 2 layers of compact bone

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What are irregular bones?

Vertebrae, primarily spongy bones

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What are the two origins of bones?

Long bones - Endochondral ossification

Flat bones - intramembranous ossification

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What are the two histological classifications given to bones?

Woven - primary immature: woven bone, not organised

Lamellar - Secondary mature: lamellar bone

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What is woven bone?

Immature, fracture, quick

randomly orientated collagen fibres
variable collagen fibre diameter
rapid matrix mineralisation
forms rapidly
rapid turnover (remodelling)

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What is lamellar bone?

Mature, adult

regular orientation collagen fibres
regular collagen fibre diameter
delayed matrix mineralisation
forms slowly
slow turnover

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How can bones be classified anatomically?

Spongy (trabecular)
Compact (cortical)
- both are made of the same cells and matrix elements

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What is compact bone?

Very strong

Made of osteons: central canal (haversian) surrounded by rings of bone matrix (concentric lamellae)

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What forms when two osteons merge?

One osteon with radiating canaliculi (remainder of the other osteon)

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What is between osteons?

Interstital lamellae

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How do osteocytes communicate?

Radiating canaliculi

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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

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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

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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

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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