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Grades 9–12 · Organisms

How can a hard bone keep rebuilding itself?

A bone looks solid, but its tissue is continuously maintained. In remodeling, cells remove a small region of old bone and other cells replace it. This helps renew tissue throughout life. Remodeling is distinct from simply making a growing bone larger.

O72 visual explanationAn invented region starts with 100 material units, loses 2 through resorption and receives 2 replacement units: 100 remain, net change 0. Osteoclasts remove old bone; osteoblasts produce replacement matrix that mineralizes. Osteocytes help coordinate remodeling but are not drawn. No density, strength, time or health forecast is calculated.Living cells remove and replace bone tissueOne enlarged surface region • invented material unitsOsteoclastRemoves old tissueOsteoblastsMake new matrix100 − 2 + 2 = 100 material unitsNew matrix mineralizes; no strength is calculated.

In the diagramLiving cells remove and replace bone tissueOne enlarged surface region • invented material unitsOsteoclastRemoves old tissueOsteoblastsMake new matrix100 − 2 + 2 = 100 material unitsNew matrix mineralizes; no strength is calculated.

Resorption opens a small region; new matrix and mineral replace the removed tissue.

Osteoclasts resorb bone: they break down mineral and organic material at a surface. Osteoblasts then lay down new protein-rich matrix, including collagen. Mineral is deposited into this matrix to form new bone. Osteocytes, long-lived cells within bone, help coordinate these activities.

Removal and formation need not balance exactly. In the invented record shown here, a region begins with 100 material units. Removing 2 leaves 98. Replacing 2 restores 100; replacing only 1 leaves 99. These are accounting examples, not physiological amounts or a measure of bone strength.

Real remodeling happens at many sites and is influenced by loading, hormones and other signals. The new matrix takes time to mineralize. The drawing separates removal and replacement so the mechanism is visible; it does not forecast density, disease, fracture risk or treatment effects.

About this explanation

Starting knowledge: Know that bone is living tissue and materials can be removed or added.

Sources

  • NIAMS — Health Lesson: Learning About Bones
  • NIAMS — What Is Bone?
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