Download Composition of Bone

Composition of Bone
• Composed of mineralized calcium phosphate (specifically, the calcium
phosphate phase called hydroxyapatite (HAP)  Ca10(PO4)6(OH)2) within a matrix of
collagen fibrils (~1/3 dry weight, 50% of bone by volume, 90% of protein content) and
200 other proteins (e.g., acidic glycoproteins, proteoglycans & carboxylated proteins
comprising the other 10% by weight)
• The HAP of bones and teeth is not compositionally pure
• it’s composition can actually be better represented as
(Ca, Sr, Mg, Na, H2O, [])10(PO4, HPO4, CO3P2O7)6(OH, F, Cl, H2O, O, [])2
where [] represent crystal lattice defects
• HAP is a ‘living mineral’ that is continually grown, dissolved & remodeled
in response to signals of internal (e.g., pregnancy) and external (e.g., gravity,
exercise) origin
Composition of Bone
• Bone stiffness (i.e., Young’s modulus) is controlled by varying its mineral content
• Increasing stiffness causes increasing brittleness, therefore, natural
selection favors the production of mineralized long bone structures that have
maximal energy absorption (i.e., toughness)
• Specialized bones often have different degrees of mineralization that are
adapted to their function (e.g., auditory bones with high mineral content)
Composition of Bone
•
The integration of organic and inorganic materials, as well as their
sculpting into unique microstructures, creates a final object that is tougher
that either HAP or collagen alone
Composition of Bone
•
Collagen is a protein containing approximately 1000 amino acids
•
A repeating sequence of Glycine-Proline-Hydroxyproline in Type I
collagen produces a helical twisting that packs most tightly as a triple
helical coiled coil (i.e., a superhelical cable called tropocollagen)
Glycine, Proline & Hydroxyproline
Amino Acid Structures
•
tropocollagen triple helix
The tropocollagen cables are lined up head-to-tail in
rows that are staggered by 64 nm along their long axes
and crosslinked at their ends to improve strength
Composition of Bone
•
The mismatch created between the staggered tropocollagen cables
creates a regular array of “hole zones” that are 40 nm long & 5 nm wide
•
The hole zones are sites where HAP crystals are nucleated and grown
•
Crystal growth is initially confined to the hole zone, producing HAP
plates 45 nm long, 20 nm wide & 3 nm thick
•
The HAP crystals grow in a parallel fashion across individual collagen
fibrils and are oriented within the hole zone
•
This long range ordering is believed to be responsible for the unusual
fracture properties of bone
Composition of Teeth
•
Teeth contain tooth enamel and dentine
•
Enamel is 95% HAP and is consequently
less tough than bone
•
Enamel gains mechanical strength by
interweaving HAP crystals
•
Enamel initially starts with a high protein
content, but these are removed and the
voids backfilled with HAP as the tooth
matures
•
Dentine contains collagen and is similar in composition to bone
Other Functional Forms of Biominerals
Navigation Aids
Fe3O4 (magnetite) Crystal
 Magnetosphere Sensor
BaSO4 Crystal
 Gravity Sensor
Other Functional Forms of Biominerals
Structures for Mechanical Strength
CaCO3
 Armor
(sea shell ‘mother-of-pearl’)
CaCO3
 Rigid Skeleton
(deep sea enabled cuttlefish)
Side view
Pillar-supported layer structure of cuttlebone
Inorganic component
Top view
Organic matrix
Structural Hierarchy in Biomineralized Materials
SiO2-based deep sea sponge skeleton  a mechanically-rigid structure made of glass!
100 m
5 mm
1 cm
10 m
5 m
20 m
1 m
J. Aizenberg et al., Science 2005 309, 275
25 m
500 nm