Bone remodeling !
- Mechanism
- Exam priority: medium
Bone is rebuilt all the time: some cells dig out old bone and others fill the hole with new bone. If digging wins, bone gets weak.
Updated 2026-11-01
Also called: RANKL–OPG, Osteoclasts, Osteoblasts, Bone turnover
See it in 3D: Femur · L1 vertebra
What it is
Adult bone is constantly renewed by small teams of cells called basic multicellular units: osteoclasts resorb a patch of old bone and osteoblasts then fill it with new matrix that later mineralizes. About a tenth of the adult skeleton is replaced each year, trabecular bone much faster than cortical bone. Remodeling repairs microdamage, adapts bone to load and serves as a calcium store. The balance is set by RANKL and osteoprotegerin and by hormones such as PTH, estrogen, cortisol and vitamin D.
How it works, step by step
- Microcracks or mechanical unloading signal through osteocytes, which reduce sclerostin and change RANKL output.
- Lining cells retract and osteoclast precursors (monocyte–macrophage lineage) are recruited.
- RANKL and M-CSF fuse them into multinucleated osteoclasts that seal onto bone with a ruffled border.
- Osteoclasts pump H⁺ (carbonic anhydrase II, proton pump) to dissolve mineral and secrete cathepsin K to digest collagen.
- Released factors (TGF-β, IGF) recruit osteoblasts from mesenchymal stem cells into the cavity.
- Osteoblasts lay down osteoid (type I collagen) and alkaline phosphatase helps it mineralize with calcium phosphate.
- Some osteoblasts become trapped as osteocytes, which sense load and make sclerostin and FGF-23.
Hormone axes and feedback
Calcium regulation (PTH, calcitriol, calcitonin)Lesion None (normal)
Tap a box to see the hormone, or choose a lesion to see the lab pattern.
- Stimulates (+)
- Inhibits / negative feedback (−)
- Level ↑
- Level ↓
Typical figures (adults)
| Parameter | Value | Note |
|---|---|---|
| Skeleton renewed per year | ≈ 10% | trabecular ≈ 25%, cortical ≈ 3% |
| Peak bone mass | around age 30 | then slow loss; faster after menopause |
| T-score osteopenia | −1.0 to −2.5 | DXA, compared with young adults |
| T-score osteoporosis | ≤ −2.5 | or a fragility fracture |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Regulation
- +Continuous PTH, calcitriol (high), thyroid hormone excess, inflammatory cytokines: Increase RANKL and resorption.
- −Estrogen and testosterone: Reduce RANKL, raise OPG, shorten osteoclast life.
- −Calcitonin: Directly inhibits osteoclasts (small role in adults).
- +Mechanical loading, intermittent PTH, Wnt signalling (low sclerostin): Increase bone formation.
- −Glucocorticoids, immobilization: Reduce formation and increase net loss.
Clinical notes
- Postmenopausal osteoporosis: vertebral compression fractures (height loss, kyphosis) and distal radius and hip fractures after minor falls.
- Markers: CTX reflects resorption; P1NP and bone alkaline phosphatase reflect formation.
- Rickets (children) and osteomalacia (adults): vitamin D or phosphate deficiency leaves unmineralized osteoid; alkaline phosphatase rises.
- Immobilization and weightlessness cause rapid bone loss and hypercalciuria.
Test yourself
Think of the answer, then tap the question to see it.
Why does estrogen loss at menopause cause bone loss mainly in the vertebrae and wrist?
Estrogen restrains RANKL and osteoclast lifespan. Without it, resorption outpaces formation, and trabecular bone, with its large surface and fast turnover, loses the most.
How can the same hormone, PTH, both destroy and build bone?
Continuous high PTH raises RANKL and favours resorption; brief daily pulses stimulate osteoblasts more than osteoclasts, so formation wins.
What is the difference between osteoporosis and osteomalacia at the tissue level?
In osteoporosis there is too little bone but it is normally mineralized; in osteomalacia the matrix is there but poorly mineralized, usually from vitamin D or phosphate deficiency.
Why does long-term glucocorticoid excess cause fractures?
It suppresses osteoblast formation and survival, prolongs osteoclast life, reduces calcium absorption and causes muscle weakness, so bone mass and quality fall.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Calcium regulation: PTH, vitamin D and calcitonin
- Phosphate and FGF-23
- Menopause
- Actions of cortisol
- Actions of estrogen and progesterone
Related conditions
- Osteoporosis
- Osteoporotic vertebral compression fracture
- Rickets and osteomalacia
- Primary hyperparathyroidism
- Cushing syndrome
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 6.4 Bone Formation and Development (OpenStax (Rice University), 2022); 6.6 Exercise, Nutrition, Hormones, and Bone Tissue (OpenStax (Rice University), 2022); Osteoporosis in Females (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 80). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.