Bone composition

Bone composition

Dried bone consists of roughly 1/3 organic matter and 2/3 inorganic salts (CaPO4

57%, CaCO3 4%, MgPO4 2%, NaCl and Na2CO3 3%). Most of the inorganic salt is a

form of calcium phosphate called calcium hydroxyapatite.

Bone derives its hardness from the deposition of mineral salts within the soft organic

matrix. It has a compressive strength of 20,000 lbs/in2 and a tensile strength of 15,000

lbs/in2. In behaviour, bone is similar to reinforced concrete: the organic matter

(collagen fibres, equivalent to steel girders) resist tension and the inorganic matter

(mineral salts equivalent to concrete) resist compression.

Mineralization of bones is a compromise between increasing strength and increasing

stiffness (making them more brittle). The combination of lightness and compressional

strength is achieved by internal sculpturing with trabeculae (from the Latin: little

beams) orientated parallel to compressional cortices.

Bone is ALIVE: it requires oxygen and nutrients, it can grow, change shape, erode,

become infected and die.

Bone development

Bones develop either in fibrous tissue (membranous or dermal bones – flat bones;

mainly cranial or facial) or in cartilage (cartilage bones – long bones). Some bones eg

the ethmoid and temporal bones of the skull, have components of both.

Membranous ossification

The loose mesenchyme in the region of the future bone is invaded by osteoprogenitor

cells, which develop into osteoblasts. These lay down calcium salts on a randomly

arranged framework of collagenous fibres to form woven bone.

Osteoclastic erosion of and osteoblastic remodelling coverts this weaker woven bone

into lamellar bone. This takes the form either of a continuous latticework of trabeculae

i.e. cancellous (spongy) bone, or compact bone with Haversian systems, according

to the stresses each region of the bone is required to endure.

Osteoblasts in the periosteum lay down trabeculae in dense parallel sheets to form

circumferential lamellae of compact bone.

Blood vessels carry stem cells which colonize the marrow forming haemopoietic tissue.

Figure 1-M: Ossification in a long bone

Endochondral ossification (see Fig. 1-M)

This occurs by replacement of a pre-existing cartilage model with bone. Ossification

begins in the mid-diaphyseal region (primary ossification centre), when osteoprogenitor

cells of the perichondrium differentiate into osteoblasts, forming a periosteal collar of

woven bone. This is remodelling into lamellar bone, as described above. Trabeculae of

cancellous bone and compact bone Haversian systems are formed, and haemopoietic

cells colonise the marrow.

Increase in length of the bones takes place by the continued growth of either end of the

cartilage model. Eventually secondary ossification centres begin to form in each

epiphysis, but a collar of epiphyseal cartilage (the growth plate) is retained and continues

to grow between the primary and secondary ossification centres. It is steadily

replaced by endochondral ossification on the diaphyseal side of the growth plate

The chondrocytes of the epiphyseal cartilage multiply (zone of hyperplasia) to form

columns, and then increase in size (zone of hypertrophy). The hyaline matrix thins out

and mineral crystals appear in it (zone of calcification), until the nutrient supply of the

chondrocytes is cut off and they die (zone of regression). Osteoprogenitor cells migrate

along the scaffold of the calcified matrix and differentiate into osteoblasts which secrete

collagen fibres and the matrix material of bone (osteoid) which becomes mineralised

(zone of ossification). Finally, remodelling by osteoclasts and osteoblasts forms compact

bone.

Increase in diameter occurs by the periosteal deposition of compact bone around the

spongy core. Finally,there is a breakdown of spongy bone to leave a hollow interior

filled with marrow.