Markers of bone formation and resorption
Last reviewed: 23.04.2024
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Bone tissue forms a dynamic "depot" of calcium, phosphorus, magnesium and other compounds necessary to maintain homeostasis in mineral metabolism. Bone consists of three components: cells, organic matrix and mineral substances. The share of cells accounts for only 3% of the volume of bone tissue.
Bone tissue is formed by osteoblasts. The main function of osteoblasts is the synthesis of the osteoid (protein matrix), which consists of 90-95% of collagen, small amounts of mucopolysaccharides and non-collagen proteins (osteocalcin, osteopontin), and is subsequently mineralized by calcium and phosphate from the extracellular fluid. Osteoblasts are located on the bone surface and are in close contact with the osteoid. They contain alkaline phosphatase, carry the receptors of parathyroid hormone and calcitriol and are capable of proliferation. Osteoblasts, surrounded by a mineralized organic matrix, are transformed into osteocytes (mature, non-proliferating cells that are located in the cavities between the layers of the newly formed bone).
Resorption of bone tissue is carried out by osteoclasts. By isolating proteolytic enzymes and acid phosphatase, osteoclasts cause degradation of collagen, destruction of hydroxyapatite, and elimination of minerals from the matrix. The newly formed slightly mineralized bone tissue (osteoid) is resistant to osteoclastic resorption.
Collagen type I - the main protein, accounting for 90% of the organic matrix of bone. It is synthesized by osteoblasts in the form of a precursor - procollagen type I, which is a large molecule containing carboxy- and amino terminal propeptides (N- and C-terminal propeptides of type I collagen). These propeptides are separated from the base molecule by specific peptidases after procollagen release from the cell.
The proportion of non-collagen proteins accounts for approximately 10% of the organic matrix of bone. They give the bone matrix a unique structure. From the correct ratio of matrix proteins, the synthesis of which is carried out by the cells of the osteoblastic line, the deposition of hydroxyapatite largely depends.
The mineral part of the bone consists of hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ] and amorphous calcium phosphate, which are non-covalently bound to proteins of the organic matrix. The orientation of the crystals of hydroxyapatite is determined primarily by the orientation of the collagen fibers of the matrix.
At the core of the life of the bone system are two interrelated and mutually replacing processes: the process of formation (formation) of a new bone and the process of destruction - the resorption of the old bone. Normally, the formation and resorption of bone tissue (bone remodeling) is balanced.
Osteoclasts continuously resorb the old bone tissue, and osteoblasts form a new way of osteoid synthesis (protein matrix), which is subsequently mineralized by calcium and phosphate from the extracellular fluid. These complexes of cells participating in the local process of resorption and bone formation are called the main multicellular units of remodeling.
Disturbances in places of bone remodeling arise due to a change in the balance between the formation process and the process of resorption towards the predominance of the latter, which leads to bone loss. The intensity and severity of bone loss depends on the speed of "bone turnover." The predominance of bone formation processes and its enhanced mineralization lead to an increase in bone mass and density - osteosclerosis.
To denote clinical, laboratory and radiologic manifestations of bone loss, a collective concept is used - osteopenia. The causes of osteopenia are osteoporosis, osteomalacia, primary hyperparathyroidism, myeloma, mastocytosis, renal osteodystrophy.
The increase in the mass of bone tissue and its density is called osteosclerosis. When osteosclerosis occurs, foci of enhanced formation of the organic matrix, which subsequently mineralizes, as a result, the mass of bone tissue and its density increase. Osteosclerosis occurs usually in the late stages of chronic renal failure.
Markers of bone tissue metabolism (markers of bone tissue formation) include alkaline phosphatase bone isoenzyme, osteocalcin, C-terminal propeptide of type I collagen.
The main biochemical indicators used in clinical practice as criteria for bone resorption include urinary calcium excretion, type I collagen N-terminal propeptide, pyridine collagen bonds.
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