Bone is a specialized connective tissue for the supporting function. It consists of a very hard, mineralized extracellular matrix and a component of cells, called osteocytes. The matrix, rich in calcium, forms a complex three-dimensional network that encloses the same cells that produced it within small gaps.
EXTRACELLULAR MATRIX, also called bone matrix or intracellular matrix
Like all types of connective tissue, the extracellular matrix consists of an amorphous component (lacking its own form), very small and essentially of a proteoglycan nature, and an abundant fibrous component, consisting mainly of type I collagen fibers.
Inside the bone tissue and the extracellular matrix itself we can recognize organic (30-35%) and extra-organic (65-70%) components. The organic components are often enclosed under the single term ossein .
Collagen fibers are responsible for bone flexibility,
while the inorganic fraction is associated with their hardness.
ORGANIC COMPONENTS OF THE MATRIX
Among the organic components we recall, in addition to collagen, proteoglycans, some non-collagen proteins, cytokines and growth factors. The most abundant element is type I collagen, which is organized into fibers, which act as a support (matrix) for the sedimentation of salts during the mineralization process. The other protein components (osteocalcin, osteonectin, osteopontin) have the function of modulating this process of formation, mineralization and adhesion between the cells and the bone matrix.
As mentioned, the collagen fibers are not randomly arranged, but align on a regular basis, giving rise to an organic matrix known as an osteone.
The osteone gives the bones considerable resistance and compactness (it is said that the bone has a lamellar structure, see below).
Collagen, like the other components of the organic matrix, is secreted by osteoblasts.
INORGANIC COMPONENTS OF THE BONE TISSUE
Among the inorganic components we recognize minerals such as calcium, phosphorus, fluorine and magnesium, which give the bones the characteristic hardness, well known to all.
Calcium is found as calcium diphosphate, deposited in the form of crystals similar to hydroxyapatite and anchored on a fibrous collagen support.
The hydroxyapatite crystals are arranged along the collagen fibers in an orderly manner.
There are also other salts, such as calcium carbonate (a component of marble) and traces of magnesium phosphate and calcium fluoride (also important in teeth).
The presence of minerals gives the bones a lower degree of hardness than that of enamel.
FUNCTIONAL CORRELATIONS and CHARACTERISTICS OF THE BONE FABRIC
The presence of minerals, as well as the abundance and the particular distribution of collagen fibers, gives the bone marked mechanical properties of hardness and resistance to pressure, traction and torsion. In particular:
collagen gives the bones a certain degree of elasticity, which translates into a considerable resistance to traction (elongation), ie to a load that is distributed along its longitudinal axis.
The mineral component of the bone confers tissue hardness, stiffness and a certain resistance to compression forces.
