Bone tissue is dynamic and plastic tissue; it modulates its structure following organic and mechanical stimuli. It is made up of an organic part and an inorganic part. The organic part is made up of own cells of the bone tissue and extracellular matrix (amorphous substance and type I collagen fibers). The inorganic part consists of numerous mineral salts such as calcium and magnesium phosphates and the citrates of Na, Mn, and K. The organic component of the extracellular matrix represents 35% of the dry weight of the bone and determines its strength and elasticity. While the component mineralized inorganic represents 65% of the dry weight and gives the bone tissue compactness and hardness. Bone tissue is subject to numerous structural and functional changes due to the individual's age, nutrition, and general condition.
In addition to collagen, the organic components include proteoglycans, some non-collagen proteins, cytokines, and growth factors. The most abundant element type I collagen, which is organized into fibers, which act as a support. The other protein components have the function of modulating this process of formation, mineralization, and adhesion between the cells and the bone matrix.
As mentioned, the collagen fibers do not randomly arrange themselves, but align themselves regularly, giving rise to an organic matrix known as osteone. The osteon gives the bones remarkable resistance and compactness. Collagen, like the other components of the organic matrix, is secreted by osteoblasts.
Among the inorganic components, we recognize minerals such as calcium, phosphorus, fluorine, and magnesium, which give 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 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 the teeth). The presence of minerals gives the bones a degree of hardness lower than that of the enamel only.
They are cells of mesenchymal origin with stem properties; they can proliferate and differentiate into osteoblasts. They are found in the periosteum and endosteum reactivated. They provide the formation of new bone tissue.
The osteoblasts are the precursors of osteocytes; they are voluminous, highly polarized cells, with an ovoid nucleus slightly displaced in the periphery and with an intensely basophilic cytoplasm. Osteoblasts provide for the production of both the organic matrix (called osteoid) and the deposition of the inorganic ones; therefore, they have osteogenic functions. They produce type I collagen, osteocalcin, osteopontin, and bone sialoprotein.
When osteoblasts have finished forming bone and become trapped within gaps in the matrix they produce, they become osteocytes. Osteocytes are irregularly shaped cells, with a clearly visible nucleus and a cytoplasm that has several extensions. They are kept in the bone gaps from which numerous microscopic canaliculi depart in every direction. Through these channels, the cytoplasmic extensions of different osteocytes make contact with each other through communicating junctions and with blood capillaries present in the bone channels. Thus, it allows metabolic exchanges between the osteocytes themselves and between osteocytes and blood. Osteocytes maintain the bone extracellular matrix.
The osteoclasts do not belong to the osteoprogenitor line but derive from the fusion of numerous monocyte precursors (up to 30) and are responsible for the destruction (reabsorption) and rearrangement of bone tissue. They are very large cells, being able to even exceed 100 µm in diameter, and have numerous nuclei. Osteoclasts are also highly polarized cells; when activated, they have a cytoplasmic face near the bone with very mobile ripples and adhere to the bone surface, creating a microenvironment isolated from the surrounding one (sealed area). It is acidified for subsequent activation of lysosomal (proteinase and phosphatase) and non-lysosomal (metalloproteinase) derivative enzymes.
The bone tissue of non-lamellar or trabecular represents primary bone tissue and is present during prenatal life and in adults in cases of new bone deposition. The lamellar bone tissue constitutes the vast majority of bone tissue in adult mammals and is organized in lamellae.
In particular conditions, the function of the bone tissue is not so much to fulfill the task of strong resistance to pressure or traction, but rather to be as light, elastic, and plastic as possible. Non-lamellar bone tissue is divided into non-lamellar bone tissue with interwoven fibers and non-lamellar bone tissue with parallel fibers (mainly present in birds).
In non-lamellar woven bone fibers collagen fibers are intertwined to form a dense network, the fundamental substance, arranged irregularly, is poorly represented both in its organic and inorganic part, the bone gaps have a globular shape and tend to be larger than in the lamellar bone tissue. The non-lamellar bone tissue with intertwined fibers is also present in the adult at the level of sutures due to fractures, in the ligamentous and tendon insertions, on the surfaces close to the periosteum, in all the new bone depositions in general. The bone tissue of non-lamellar with parallel fibers is rather rare in mammals; it can be found in areas of insertion of the tendons.
The lamellar bone tissue, due to its chemical composition and its particular structural organization, has a strong resistance to traction, pressure, and mechanical stress in general. Due to its organization in lamellae, in fact, this tissue guarantees good resistance to stress, even if it does not particularly weigh down the skeleton.
The lamellar bone tissue divides into compact lamellar bone tissue if it is composed mainly of complete concentric lamellae, as for example, in the diaphyses and in spongy lamellar bone tissue. Instead, it is composed of incomplete lamellae that form many small fragments embedded between them (trabeculae bones), as for example, in the epiphysis. The compact bone tissue is very hard, crossed by numerous channels containing blood vessels and lymphatic ducts visible only under a microscope.
The spongy bone tissue looks like a three-dimensional lattice of bone trabeculae that delimits a labyrinthine space filled with bone marrow.
In the long bones, the epiphyses are distinguished, short and rounded, located at the ends and consisting mainly of spongy bone, and an elongated, central part, shaped like a hollow cylinder called the diaphysis. In flat bones, however, we distinguish two surfaces of compact bone tissue
, called internal and external planking.
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