bone remodelling
Summary
Bone remodelling is the continual turnover of bone: osteoclasts resorb old matrix and osteoblasts deposit new matrix. It repairs bone, adapts it to load and exchanges calcium with blood. PTH raises blood Ca²⁺ by promoting resorption; calcitonin opposes it.
Detail
Meaning: Bone is living tissue that is continually broken down and rebuilt. Osteoclasts secrete acid and enzymes that dissolve the mineral (hydroxyapatite, a calcium phosphate salt) and the collagen matrix. Osteoblasts deposit new collagen-rich matrix, which then mineralizes. Osteocytes, former osteoblasts trapped in lacunae, sense mechanical stress. Remodelling repairs microdamage, adapts bone to load, and exchanges calcium with the blood.
On the MCAT: Passages tie remodelling to calcium homeostasis. Low blood Ca²⁺ triggers parathyroid hormone (PTH), which increases osteoclast activity (through signals from osteoblasts) and releases Ca²⁺ and phosphate from bone. Calcitonin from the thyroid opposes this by inhibiting osteoclasts. Questions ask you to predict net bone change: weight-bearing exercise favors gain; immobilization or weightlessness favors loss; low estrogen after menopause lets resorption outpace formation.
Common confusion: Students swap the two cell types. Osteoblasts build and osteoclasts break down (clasts cleave). Remodelling is also not limited to fracture healing. It goes on in adult bone without injury, and bone mass depends on the balance between resorption and formation.
Sources
- Campbell Biology
- Human Physiology: An Integrated Approach
- Principles of Anatomy and Physiology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.