10.9:
The Bone Matrix
Bone is a supportive connective tissue with the major bulk made up of matrix components, and only 2 to 3 percent of mass contributed by cells.
About two-thirds of the matrix comprises inorganic salts, and one-third is made of the organic component called osteoid.
The osteoid is mainly composed of the fibrous protein collagen, along with glycoproteins such as proteoglycans.
While collagen provides structure and elasticity that prevents the bones from becoming brittle; proteoglycans help absorb and retain water in the matrix, making the bones resistant to compression.
The inorganic matrix is mainly composed of calcium phosphate, which combines with calcium hydroxide to form hydroxyapatite crystals.
These crystals form in between and around the triple-helical collagen fibrils, conferring hardness to the bone tissue.
Arrays of such mineralized fibrils form the collagen fibers that are arranged parallelly in the lamellae of the bone.
Thus, the different bone matrix components together form a hard tissue that can withstand both compressive and tensile forces.
10.9:
The Bone Matrix
Bone contains a relatively small number of cells entrenched in a matrix of collagen fibers that provide an adherent surface for inorganic salt crystals. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. This can be observed by an experiment: when the minerals of a bone are dissolved by soaking the bone in acid or vinegar, the result is a flexible, rubbery bone.
The calcification process begins with the crystallization of minerals in the microscopic spaces between the collagen fibers. These salt crystals form when calcium phosphate and calcium carbonate combine to create hydroxyapatite, which further incorporates other inorganic salts such as magnesium hydroxide, fluoride, and sulfate as it crystallizes, or calcifies, on the collagen fibers.
The bone matrix also acts as a reservoir for many physiologically important minerals, especially calcium and phosphorus. When required, the bone cells called osteoclasts, secrete enzymes and acids to break down the matrix and release these minerals back into the bloodstream to maintain optimal levels in the body. Calcium ions, for example, are essential for muscle contractions and controlling the flow of other ions involved in the transmission of nerve impulses; thus, it is essential to maintain the optimal concentration in the body.
This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 6.3: Bone Structure