Composition of Bones

Human Physiology

Lesson 10 : Bones Classification

Composition of Bones

The intercellular matrix of bone is composed of collagen with calcium phosphate deposited in it. The organic matrix of bone is elastic rather than rigid. Abnormal loss of bone mineral (mainly calcium and phosphate) makes bones to become soft and bendy. The structure of healthy bone is like that of reinforced concrete, results from the combined effect of the hard minerals (representing concrete) and the flexible organic matrix (representing reinforcing rods). Alone, the mineral is too brittle, as in senile bone, to get broken easily. The high calcium content of bone is responsible for its radio-opacity, and bone is clearly seen on radiographs as white areas. An important fact frequently overlooked is that bone is a living tissue; it contains cells (osteocytes) and has a blood supply; fractured bones often bleed profusely. A prepared skeleton consists of only bones in which all the surrounding soft tissues i.e. muscles, tendons, ligaments and blood vessels have been removed.

Calcium phosphate Ca3 (PO4)2 constitute two third of weight of bones. Ca3(PO4)2 combines with calcium hydroxide Ca(OH)2 to form crystals of hydroxyapatite Ca10(PO4)6(OH)2. Other compounds such as CaCO3, salts of sodium, magnesium and fluoride are present in very small quantities. Collagen constitute remaining one third of bone mass and cells accounts of nearly 20% of bone mass.


Dried bone consists of organic and inorganic constituents in 1:2 ratio. Organic matter mainly composed of gelatin that provides elasticity and toughness to the bone and inorganic matter contains various universals for providing hardness to the bone.

Gelatin 33.3%
Calcium phosphate 57.35%
Calcium carbonate 3.85%
Magnesium phosphate 2.05%
Sodium chloride/carbonate 3.45%

Cell mass includes:

Osteoblasts: - located in the matrix of bones and are responsible for osteogenesis or synthesis of bones and its mineralization.

Osteoclasts: - located inside the bone towards medullary space and brings about ; demineralization or osteolysis. They are also known as giant cells.

Osteoprogenitor Cells: - are located both on outer surface of bone along periosteum and also towards medullary cavity i.e. endosteum. These cells are precursors of osteoblasts.

Last modified: Tuesday, 10 April 2012, 6:13 AM