Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Bone tissue
Chemical composition of bone tissue

Studying The chemical composition of Bone Tissue involves considerable challenges, as isolating the organic matrix requires prior bone decalcification. Furthermore, both the content and COMPOSITION OF THE organic matrix undergo significant changes depending on the degree of bone tissue mineralization.

It is well established that prolonged Treatment of bone with dilute acid solutions dissolves its mineral components, leaving behind a flexible, soft organic residue (the organic matrix) that retains the shape of the intact bone. The intercellular organic matrix of compact bone accounts for about 20%, inorganic substances for 70%, and Water for 10%. In spongy bone, organic components predominate, making up over 50%, while Inorganic Compounds account for 33–40%. The water content remains within the same range as in compact bone (Yu.S. Kasavina, V.P. Torbenko).

According to A. White et al., Inorganic Components make up about ¹/₄ of the bone volume, with the remainder occupied by the organic matrix. Due to differences in the relative specific gravity of organic and inorganic components, insoluble minerals account for half of the bone mass.

Inorganic composition of bone tissue. More than a century ago, it was hypothesized that bone tissue crystals have an apatite Structure, a theory largely confirmed in subsequent research. Indeed, bone crystals are classified as hydroxyapatites, have a plate- or rod-like shape, and possess the following chemical composition: Са₁₀(РО₄)₆(ОН)₂. Hydroxyapatite crystals make up only a fraction of the mineral phase of bone tissue, with the remainder represented by amorphous calcium phosphate Са₃(РО₄)₂. The content of amorphous calcium phosphate varies considerably with age: it predominates in early life, whereas crystalline hydroxyapatite becomes dominant in mature bone. Amorphous calcium phosphate is generally regarded as a labile reserve of Са²⁺ and phosphate ions.

The adult human body contains over 1 kg of calcium, almost entirely localized in the bones and Teeth, where it forms insoluble hydroxyapatite together with phosphate. The majority of calcium in bones is constantly undergoing turnover, with the skeletal bones losing and regaining approximately 700–800 mg of calcium daily.

The mineral phase of bone includes a significant amount of ions typically absent in pure hydroxyapatite, such as sodium, magnesium, potassium, and chloride ions, among others. It has been suggested that within the crystal lattice of hydroxyapatite, Са²⁺ ions can be substituted by other divalent cations, whereas anions other than phosphate and hydroxyl are either adsorbed onto the crystal surface or dissolved in the Hydration shell of the crystal lattice.

Organic matrix of bone tissue. Approximately 95% of the organic matrix consists of Collagen. Alongside mineral components, Collagen is the primary factor determining the mechanical properties of bone. The collagen fibrils of the bone matrix are formed by type I collagen. While this collagen type is also found in tendons and Skin, bone collagen possesses certain distinctive features. Evidence indicates that bone collagen contains slightly more hydroxyproline than tendon and skin collagen. Another characteristic feature of bone collagen is its high content of free ε-amino groups in Lysine and hydroxylysine residues. An additional hallmark of bone collagen is an elevated phosphate content compared to other Tissues, with the majority of this phosphate bound to Serine residues.

The dry demineralized bone matrix contains about 17% non-collagenous Proteins, which include the protein components of Proteoglycans. Overall, The amount of proteoglycans in mature dense bone is relatively small.

The organic matrix of bone tissue contains glycosaminoglycans, with chondroitin-4-sulfate being the principal representative. Chondroitin-6-sulfate, keratan sulfate, and hyaluronic acid are present in minor quantities.

It is generally accepted that glycosaminoglycans are directly related to ossification*. Research has shown that Bone Formation is accompanied by changes in glycosaminoglycans, wherein sulfated compounds give way to unsulfated ones. The bone matrix also contains Lipids, which represent an intrinsic component of bone tissue rather than contaminants resulting from incomplete removal of lipid-rich Bone Marrow. Lipids participate in the mineralization process and are believed to play a crucial role in forming nucleation sites during bone mineralization.

Biochemical and cytochemical studies have demonstrated that osteoblasts—the primary Cells of bone tissue—are rich in RNA. The high RNA content in bone cells reflects their high metabolic activity and continuous biosynthetic function (Table 22.1).

Class="center">Table 22.1. Chemical composition of the human Tibia (in grams per 100 g of dry defatted bone) (L.I. Slutskiy)

* Ossification (osteogenesis) is the physiological process of impregnating the intercellular substance of Cartilage or Connective Tissue with mineral salts, occurring during The formation of bone tissue.

A unique feature of the bone matrix is its high concentration of citrate: about 90% of the body's total citrate is localized in bone tissue. Citrate is widely considered essential for bone mineralization, likely by forming complex compounds with Calcium and phosphorus salts that allow their tissue concentration to reach the threshold required for crystallization and mineralization to begin.

In addition to citrate, succinate, fumarate, malate, lactate, and other organic acids have been detected in bone tissue.