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Markers of bone formation and resorption
Last reviewed: 05.07.2025
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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 minerals. Cells account for only 3% of the volume of bone tissue.
Bone tissue is formed by osteoblasts. The main function of osteoblasts is the synthesis of osteoid (protein matrix), which consists of 90-95% 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 osteoid. They contain alkaline phosphatase, carry receptors for parathyroid hormone and calcitriol, and are capable of proliferation. Osteoblasts, surrounded by mineralized organic matrix, turn into osteocytes (mature, non-proliferating cells that are located in the cavities between the layers of newly formed bone).
Bone tissue resorption is carried out by osteoclasts. By secreting proteolytic enzymes and acid phosphatase, osteoclasts cause collagen degradation, hydroxyapatite destruction, and mineral removal from the matrix. Newly formed, poorly mineralized bone tissue (osteoid) is resistant to osteoclastic resorption.
Collagen type I is the main protein that makes up 90% of the organic matrix of bone. It is synthesized by osteoblasts as a precursor, procollagen type I, which is a large molecule containing carboxy- and amino-terminal propeptides (N- and C-terminal propeptides of collagen type I). These propeptides are separated from the main molecule by specific peptidases after procollagen is released from the cell.
Non-collagen proteins account for approximately 10% of the organic bone matrix. They give the bone matrix its unique structure. The deposition of hydroxyapatite largely depends on the correct ratio of matrix proteins, the synthesis of which is carried out by osteoblastic cells.
The mineral part of 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 hydroxyapatite crystals is determined primarily by the orientation of the collagen fibers of the matrix.
The vital activity of the skeletal system is based on two interconnected and mutually substituting processes: the process of formation of new bone and the process of destruction - resorption of old bone. Normally, the formation and resorption of bone tissue (bone remodeling) are balanced.
Osteoclasts continuously resorb old bone tissue, and osteoblasts form new bone by synthesizing osteoid (a protein matrix), which is subsequently mineralized with calcium and phosphate from the extracellular fluid. These complexes of cells involved in the local process of bone resorption and formation are called the basic multicellular remodeling units.
Disturbances in bone remodeling sites occur due to a change in the balance between the formation and resorption processes towards the predominance of the latter, which leads to bone loss. The intensity and severity of bone loss depend on the rate of "bone turnover". The predominance of bone tissue formation processes and its increased mineralization lead to an increase in bone mass and density - osteosclerosis.
To designate clinical, laboratory and radiological manifestations of bone loss, a collective term is used - osteopenia. Causes of osteopenia are osteoporosis, osteomalacia, primary hyperparathyroidism, myeloma, mastocytosis, renal osteodystrophy.
An increase in bone mass and density is called osteosclerosis. Osteosclerosis is characterized by foci of increased formation of organic matrix, which subsequently mineralizes, resulting in an increase in bone mass and density. Osteosclerosis usually occurs in the late stages of chronic renal failure.
Markers of bone tissue metabolism (markers of bone tissue formation) include bone isoenzyme of alkaline phosphatase, osteocalcin, and C-terminal propeptide of type I collagen.
The main biochemical parameters used in clinical practice as criteria for bone resorption include urinary calcium excretion, N-terminal propeptide of type I collagen, and pyridine bonds of collagen.
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