Chemical composition of bone
The unique mechanical characteristics of bone depend on a strictly calibrated chemical balance. The bulk of the tissue is formed by mineral salts, which account for 60 to 70% of the total weight. These inorganic compounds are distributed in two main forms in equal proportions (50% each):
- Amorphous calcium phosphate. Present in the tissue in the form of the smallest granules.
- Hydroxyapatite crystals. This is a more complex spatial lattice consisting of calcium phosphate and hydroxide.
The most important physicochemical feature of the tissue is the extremely low content of water: fluid makes up only 6–20%. The remaining 10–20% of the mass consists of organic components, which include cellular elements and a complexly organized intercellular matrix.
Organic matrix and mineralization
The organic intercellular substance is not just a framework, but an active environment that plays a key role in initiating and regulating the mineralization process.
The absolute leader in the composition of the matrix is collagen (predominantly type I), which forms up to 95% of all organic mass. The structural feature is that mineral granules and growing crystals are sorbed directly in the spaces between tropocollagen molecules.
The remaining 5% of organic matter is represented by non-collagenous macromolecules. Despite their small volume, they are vital for the proper hardening of the bone, as each group of molecules performs its specific task:
- Phosphoproteins are responsible for the initial attraction and concentration of calcium ions in the required areas.
- Enzymes, primarily alkaline phosphatase, catalyze the reactions of inorganic mineral compound formation.
- Proteoglycans act as a linking element, attaching the forming minerals to collagen fibers.
- Specific proteins (e.g., osteonectin) control and direct the growth of hydroxyapatite crystals. Glycoproteins and osteocalcin are also present.
Cell lineages
The bone cell pool is formed from two completely different sources that provide opposite yet interrelated processes—the creation and destruction of the matrix.
- Mechanocyte differon (bone-forming lineage). This is a sequential chain of maturation of cells responsible for matrix synthesis. Development begins with an immature osteogenic cell (preosteoblast), which then differentiates into an actively synthesizing osteoblast. At the final stage, the cell becomes entrapped within its own secretion and turns into a mature osteocyte.
- Hematogenous lineage (bone-destroying lineage). Represented by giant cells—osteoclasts. Unlike mechanocytes, they are of hematopoietic origin: osteoclasts develop directly from blood monocytes and biologically belong to the unified macrophage system.
Briefly: answers to topical questions
What is the structure of an osteon (Haversian system)?
An osteon is formed by concentric layers of bone lamellae having the shape of hollow cylinders nested inside each other around a central canal with a blood vessel. The boundary of the osteon is delineated by a resorption (cement) line. Between the canal wall and the vessel lies the perivascular space containing tissue fluid, osteogenic cells, osteoblasts, and osteoclasts. The bodies of osteocytes lie in bone lacunae between the lamellae, and their processes pass through radially oriented bone canaliculi, ensuring the transport of substances from the vessel.
How does woven (reticulofibrous) bone tissue differ from lamellar bone tissue?
Woven (reticulofibrous) and lamellar (finely fibered) bone tissues are two types of bone tissue differing in matrix structure. Woven tissue is formed during the development of primary spongy bone and lacks an ordered lamellar organization; in early developmental stages it is the main tissue, and every bone passes through this stage. In adults, it is rare and found in small volumes: at bone tubercles (tendon attachment sites) and in cranial suture fusion zones. During remodeling, woven tissue is partially destroyed by osteoclasts and replaced by mature lamellar tissue: trabeculae of secondary spongy bone form in the inner part, bone lamellae organize around vessels to form osteons in the middle layer, and general lamellae form externally.
Which hormones regulate the activity of osteoclasts and osteoblasts?
Bone cell activity and calcium metabolism are regulated by parathyroid hormone, calcitonin, and calcitriol (the active form of vitamin D3).
- Parathyroid hormone: increases blood calcium concentration; in bone tissue, it stimulates osteoclasts, accelerates bone matrix resorption, and releases Ca2+ into the blood. Osteoblasts have parathyroid hormone receptors; under its action, osteoblasts release the osteoclast-activating factor.
- Calcitonin: is an antagonist of parathyroid hormone, lowers blood Ca2+ levels, enhances calcium influx into bone tissue, stimulates osteoblast activity, and decreases osteoclast activity.
- Calcitriol: the active form of vitamin D3; its production in the kidneys is stimulated by parathyroid hormone. Calcitriol enhances intestinal Ca2+ absorption and promotes calcium salt deposition in bone tissue. It is also noted for vitamin D that osteoblasts have receptors for it, and the ruffled border of osteoclasts enlarges upon vitamin D activation.
How does the mineralization of the bone matrix occur with the participation of matrix vesicles?
Mineralization of the bone matrix with the participation of matrix vesicles (the physical pathway) begins with the secretion of membrane vesicles into the matrix (osteoid) by osteoblasts. With the participation of the enzyme alkaline phosphatase, calcium phosphate accumulates inside these vesicles, from which hydroxyapatite crystals are formed. As a result, the vesicles rupture, and the crystals emerge onto the collagen fibers, where they serve as nuclei (centers) for the further mineralization process.
What are the functions and morphological features of mature osteocytes?
Osteocytes are the main cell type of formed bone tissue, originating from osteoblasts. They lie strictly as single cells within bone cavities (lacunae) of the mineralized matrix. Osteocytes have numerous thin cytoplasmic processes running through bone canaliculi, through which they contact blood vessels and neighboring cells. The main function of osteocytes is trophic: they ensure the transport and exchange of substances between the vascular bed and the bone tissue, and also maintain the state of the bone matrix.
To memorize the functions of non-collagenous matrix proteins, use the mnemonic rule: Phosphoproteins — Focus (concentrate) calcium, Proteoglycans — Pin down (bind) minerals to collagen, Osteonectin — Provides crystal growth.
Class and exam questions
Which type of collagen predominates in bone tissue?
The organic bone matrix contains predominantly type I collagen, which accounts for about 95% of all organic components.
What is the origin of osteoclasts?
Osteoclasts are of hematogenous origin. They develop from blood monocytes and functionally belong to the macrophage system.
Which enzyme ensures the formation of mineral compounds?
The enzyme alkaline phosphatase plays a key role in initiating mineralization and forming mineral complexes.
What else to study on this topic
- Spatial arrangement of tropocollagen molecules and granule sorption
- Mechanism of action of alkaline phosphatase during mineralization
- Differences between preosteoblasts and mature osteoblasts
- Functions of osteocalcin and glycoproteins in the matrix
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