The bone matrix It is the mineralized substance in which the different cells that make up the bones are embedded. It is defined more specifically as the intercellular substance of bone tissue, which represents most of its mass and which is composed of two fractions, one organic and the other inorganic..
Bone tissue is that which forms the bones of the skeleton of some animals and is made up of different types of cells, each with specific functions. It is an extremely resistant and hard tissue, but at the same time very dynamic and changing, as it is in a permanent balance of formation and resorption (bone remodeling), for which the different types of cells that form it are responsible..
Generally speaking, this specialized connective tissue is made up of its cells and the bone matrix with its organic and inorganic fractions. The mineralization of those components that are external to the cells (extracellular or intercellular) is what gives the bones strength and resistance..
There are 4 types of bone cells: osteoprogenitor or osteogenic cells, osteoblasts, osteocytes and osteoclasts. Osteocytes are mature bone cells and are mainly responsible for the secretion of the organic fraction of the bone matrix.
These cells are confined inside some "gaps" produced by the mineralization of the matrix that they have secreted, they do not have the capacity to divide later and are derived from osteoblasts.
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As we mentioned earlier, the bone matrix is a complex substance, as it is made up of an organic and an inorganic fraction..
About 30% of the weight of bone tissue corresponds to the organic fraction of the bone matrix, which consists mainly of fibers of a protein known as collagen and other different protein elements such as, for example, different classes of proteoglycans that form the so-called “ground substance” (a kind of homogeneous gel).
The other 70% corresponds to the mineral fraction, which is formed mainly by hydroxyapatite, a crystalline calcium phosphate complex, and it is for this reason that bone tissue is said to be very important for calcium homeostasis in the human body and in that of other animals.
The organic fraction of the bone matrix is mainly collagen, one of the most abundant proteins in the human body. Collagen is a multimeric fibrous protein, whose structure is similar to a rope or rope, since it is composed of several subunits or fibrils.
More than 30 genes in the human genome code for collagen-like proteins and there are more than 20 different types of collagens distributed in the different tissues of the body. The bone matrix is rich in type I collagen (more than 90%), but it also has lower proportions of collagen III, V, X and XII.
Each collagen fiber is formed by the grouping of other "procollagen" fibrils, which are composed of three alpha-helical chains of more than 1,000 amino acid residues and are around 300 nanometers long..
Collagen gives bones some flexibility, while minerals in the inorganic fraction give them rigidity and strength.
Without the inorganic fraction, the bones would be completely flexible, but without collagen they would be fragile like blackboard chalk, so the variations between the proportions and distribution of both fractions give the bones the ability to maintain a “balance” with respect to the flexibility and stiffness requirements.
Some genetic mutations of the genes that code for the different types of collagens in the body, or for the proteins that participate in their assembly, produce anatomical abnormalities that can seriously compromise the integrity of the bone tissue and, therefore, the physical health of who presents them.
Approximately 10% of the organic fraction of the bone matrix is made up of other non-collagenous proteins, among which are:
- Fibronectin
- Osteopontin
- Osteocalcin
- Bone sialoprotein
- Decorin (proteoglycan)
- Biglycan (proteoglycan)
Of this group, the most abundant proteins are bone sialoprotein and osteopontin, although this depends on different factors.
Although these non-collagenous proteins represent only a small portion of the organic fraction, they have important functions in bone tissue, especially related to osteoblast differentiation, mineralization, cell adhesion and bone remodeling..
The inorganic fraction represents a substantial portion of the components of the bone matrix (between 60 and 80%, depending on the type of bone). This is the fraction, as we have already mentioned, that gives the characteristic stiffness and resistance to the bones of the body..
Bone tissue, thanks to the composition of the inorganic fraction of its matrix, is the main reservoir of ions such as calcium (almost 99%), phosphorus (85%), sodium and magnesium (between 40% and 60%). %), which form crystals around and between the collagen fibers of the organic fraction.
The main crystalline compound formed by some of the ions described is calcium hydroxyapatite, which is by far the most abundant compound in the inorganic fraction of the bone matrix. Hydroxyapatite is a calcium phosphate compound (Ca10PO4OH2) whose crystals are around 200 Å.
It is important to note that, although a large amount of the bone matrix is calcified, that is, mineralized (hardened), there is a thin layer of non-calcified bone matrix that surrounds osteoblasts and osteocytes and that forms a kind of interface between the cells and the calcified matrix.
This thin layer is known in the area of osteology and medicine as osteoid and it has different functions. In addition, it can be enzymatically removed or degraded in order to start the bone resorption and remodeling processes..
The bone matrix is responsible for the rigidity and strength of the bones, which is why it plays a fundamental role in regard to the main functions of this tissue as mechanical support for the body, as well as protection and support of the body structures. against the force of gravity.
On the other hand, thanks to the presence of this substance in the structure of each bone, the skeleton serves as a site of attachment of the muscles that allow locomotion and other movements of great importance for animal life and, of course, human life..
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