2.1.2 Bone turnover

Both osteoblasts and osteoclasts are continually active in a process called remodelling. Old or damaged bone is removed by osteoclasts (resorption) and replaced by osteoblasts (bone formation). The balance or ‘coupling’ of this activity is crucial to bone strength. Where resorption exceeds formation, bone strength is reduced and osteoporosis may occur.

Some key factors influence this balance of remodelling:

• Hormones
  • Parathyroid hormone (PTH)

    PTH is responsible for the calcium balance of the body and depending on secretion patterns, PTH will either promote bone formation or bone resorption. PTH is elevated when vitamin D or calcium are low in order to draw calcium from the bones into the blood.

  • Sex hormones

    Oestrogen and testosterone both promote bone formation.

  • Thyroid hormone

    Thyroid over-activity reduces bone and muscle strength.

• Mechanical loading

  Osteocytes respond to mechanical stress on bone to promote bone formation.

• Cytokines (cell signalling molecules)

  RANK-Ligand and other substances secreted by osteoblasts and osteocytes control osteoclast activity.

Please complete the following quiz. This is formative. It is really helpful in consolidating your learning, but there is no pass mark.

So far we have looked at the structure and function of healthy bone. But if the balance of remodelling is disturbed, bone may lose strength and become osteoporotic.