The basic substance of bone tissue is two-thirds inorganic mineral salts and one-third organic substances (living cells). The collagenous connective tissue fibers are responsible for the high bending strength of bone, while the mineral salts create the enormous compressive strength (15 kPa/mm2). The mineral salts are mainly calcium phosphate in the form of hydroxyapatite crystals. Apatites are crystallized molecules that were once easily confused with other crystals, hence the name (apatan = they deceive).
Bone develops in two ways: embryonic connective tissue is converted directly into bone substance by the deposition of bone-forming cells (osteoblasts), or cartilage tissue forms first as a precursor that later develops into bone tissue through calcification. The first type is known as membrane or dermal bone and the second as replacing or cartilage bone.
Constituents of bone
In bone, a distinction is made between three constituents that together form a functional unit: periosteum, bony substance, and bone marrow (Figs 2-10 to 2-12).
Periosteum is a tight envelope of connective tissue that completely encases the bone, except at the joints. It contains blood vessels and nerves and is made up of an outer fibrous layer and an inner germ layer of bone-forming cells, the osteoblasts. The periosteum nourishes the bone and bone marrow. In mature bone, the osteoblasts are in a resting state; they become active and regenerate the bone if there is a fracture.
Isolating organic and inorganic components of bone
The organic substance can be removed from the bone by heating until red hot, which renders the bone noticeably brittle. The calcium salts can be removed from the bone by acid treatment, which leaves only the flexible substances. In response to both treatments, the bone retains its original form, but the mechanical properties are exactly reversed; whereas the organic part of the bone is so rubbery and elastic that it can be coiled into a knot, the inorganic part of the bone remains brittle and hard.